Dog-dung should never be allowed to lie exposed to the air, or it

putrefies and turns black, the bating ingredients are destroyed, and it

will not puer the goods which turn black and putrid without softening.

Dung should, therefore, be mixed to a paste with water and kept in

tanks, so as to be but little exposed to the air, when it will retain

its puering properties for a long time unaffected. Fresh dung should be

allowed to ferment for at least a week before use. No accurate statement

can be made as to the quantities required. Eitner states that 1-1¹⁄₂

pails of dung-paste (say 14-20 litres) is sufficient for 200 medium to

large lamb-skins for glove-kid. It should be sufficient to make the

water quite turbid, but not thick or soupy. For lamb-skins a temperature

of 18°-20° C. is suitable, which may be raised in very cold weather to

25° C., to allow for cooling. The time required is from two hours for

the thinnest slink skins, to 12-14 hours for strong ones. It is well to

use wooden, and not iron, utensils for handling the dung, and it should

be strained through a coarse cloth after diluting with water. As has

been remarked, it is not desirable to keep the skins in constant motion

in the puer; they should be stirred or paddled for the first 20-30

minutes, and then for 10 minutes every hour for five or six hours, after

which they can be allowed to lie for a longer period without injury.

Puering is sufficient when the skins feel quite soft and flaccid,

hanging in folds in any direction and allowing the flesh to be scraped

off with the finger-nail.

Wood recommends that, for the puering of sheep-skins, dung should be

allowed to ferment one month before use, and states that it deteriorates

if kept over three months. The puering products are the result of the

successive action of many sorts of bacteria, and Wood is of opinion that

those actually concerned in puering originate from the air, or from the

vessels in which the dung is stored, and are not present in it when

excreted. Borgman[111] advises that the dung should be kept in a dry

condition, and only made into a paste between a fortnight and three

weeks before use, by covering in a clean cask with cold water, and on

the following day mixing to a smooth paste with a clean wooden

“poss-stick,” made from wood free from tannin. The cask should then be

covered up, and allowed to rest undisturbed till required. Clean

extract-casks are very suitable for the purpose, if carefully and

repeatedly steamed out, and Borgman advises that a regular series should

be arranged, so as to supply the dung required, the date of mixing being

carefully marked on each cask. Throughout the process the utmost

cleanliness should be observed, and the casks should be carefully

steamed out as soon as emptied. Immediately before use the dung-paste

should be heated by steaming nearly but not quite to boiling point, care

being taken to avoid the introduction of condensed water containing

iron, and the dung thoroughly mixed with a large quantity (say 100

gallons) of water at 45°-50° C., allowed to settle, and drawn off

through a basket, and strained into the puering paddle through a second

basket lined with coarse open canvas (such as is used by plasterers to

cover windows while the plaster is drying). A further quantity of warm

water should be poured on the residue in the mixing tub, and used for

diluting that in the paddle to the proper volume. The temperature of the

liquor may reach 42° C. before the skins are introduced. The liquor

should be of a light colour, greenish to brownish yellow; if darker, it

indicates decomposition of the dung by improper storing, or too long

fermentation, and will be liable to cause staining and injury to the

skins. About 33 liters of dry dung is required per 100 kilos. of wet

skin prepared for puering (33 gallons per 1000 lb.). Dry dung should be

of yellow to brown colour, dark brown or black dung is spoiled and

unsuitable for use. Wet dung is more difficult to judge, but very dark

brown or black should be rejected, as well as that with a very strong

smell, indicating that it has already fermented. Borgman’s directions

bear the stamp of experience and common sense, and the book as a whole

repays study.

[111] ‘Die Feinleder-Fabrikation,’ Berlin, 1901, p. 69.

Borgman recommends that the skins should be warmed by paddling for some

time in water of about 40° C. to which a couple of pails of puer-paste

have been added, before bringing them into the puer, the temperature of

which they should reduce to perhaps 38° C. The puered skins should feel

silky on the grain, and even somewhat slippery, and when pressed

between the finger and thumb a dark impress should be left, and the

flesh should be tender and easily scraped off. The requisite condition

will, however, vary somewhat with the kind of skins, and the purpose for

which they are intended. After puering, the skins may be paddled for

half an hour in water of about the same temperature as the puer.

CHAPTER XIV.

_ALUM TANNAGE OR TAWING._

We have now followed the raw material up to the final stage of

preparation for its actual conversion into leather, and it remains to

consider the means by which that important change is produced. Though as

yet the vegetable tanning process is most largely used, and possesses

the greatest commercial importance, the use of mineral salts has long

been known, and, through the advent of chrome tanning, has placed the

permanent supremacy of the vegetable tannins in considerable doubt. Not

only the importance of mineral tanning processes, but their greater

simplicity from the scientific side, justify their consideration before

those of vegetable origin.

In the previous chapters it has been shown that to produce a permanent

leather, it is not only necessary to dry the fibres in a separate and

non-adherent condition, but so to coat them or alter their chemical

character that they are no longer capable of being swelled and rendered

sticky by water. All salts which produce a contraction or dehydration of

the fibre similar to that caused by alcohol are capable of the first

effect in a greater or less degree. Many sulphates, and particularly

those of sodium and magnesium, though they will not alone produce

leather, will so far contract the fibres as to greatly hasten tanning by

vegetable tanning materials, and they are therefore capable of useful

application in quick tanning processes, especially where tough and

light-weighing leathers are aimed at, which may be subsequently weighted

and solidified by further treatment. Strong solutions of ammonium

sulphate are almost as strongly dehydrating as alcohol, and will produce

white leathers very similar to those formed by pickling, a fact which is

certainly of considerable commercial importance. None of these salts,

however, can form a complete leather in themselves, but require the

assistance of metallic salts which will permanently fix themselves in

the fibre, and diminish or destroy its attraction for water. Many

substances have this power in a greater or less degree, but all those of

commercial importance belong to the group of which aluminium, iron and

chromium are representative, and which are capable of producing

salt-forming oxides of the formula M₂O₃ (e.g. alumina, Al₂O₃).

Manganese, of which the salts of this type are very unstable, has very

slight tanning power, while titanium, which in many ways is allied to

the group, though it does not strictly belong to it, has recently been

patented as a tanning agent. For the present, however, we may limit our

attention to the three metals first named.

Alumina and its salts demand the first attention, not only as having

been used for leather manufacture in very early times, but as being

still important commercially. The metal aluminium is now well known, and

its oxide, alumina, Al₂O₃ is abundant in nature, combined with silica in

the form of clay and bauxite, as fluoride in combination with sodium

fluoride in cryolite, and in some cases as a native sulphate.

Alum-shale, which was formerly the principal source of alum, is a

bituminous clay containing much iron sulphide, and which when calcined

yields aluminium sulphate. As aluminium sulphate does not crystallise

readily, and was difficult to free from iron, potassium sulphate was

added to the liquor obtained by leaching the calcined shale, from which,

after concentration by boiling, potash-alum, a double sulphate of

potassium and aluminium, Al₂(SO₄)₃,K₂SO₄,24Aq, was easily crystallised

out. Alum is now usually made by decomposing clay or bauxite with

sulphuric acid, and ammonium sulphate is generally substituted for the

potassium salt, yielding ammonia-alum, a double sulphate of aluminium

and ammonium of similar constitution to potash-alum. Ammonium alum is

easily distinguished from the potassium salt, by the strong smell of

ammonia which it evolves on the addition of caustic soda or lime. So far

as is known, there is no practical difference in tanning effect between

the two salts, and ammonium alum is cheaper, and slightly stronger, its

molecular weight being 906, as against 948 for the potassium salt.

Either alum dissolves readily in cold water to the extent of about nine

parts in 100 of water, and more easily, and to a much larger extent in

hot water, from which the excess crystallises on cooling. It is said

that for purposes of leather manufacture, alum solutions should not be

boiled, and, though it is improbable that this produces any change, it

must be remembered that chrome alum on boiling really does undergo

decomposition to free acid and a more basic salt, indicated by change of

colour from violet to green, from which it slowly returns to the violet

form on cooling.

Alums are only valuable in leather manufacture in proportion to the

aluminium sulphate which they contain, the potassium or ammonium

sulphate taking no part in the reaction, and since improved methods have

rendered possible the production of aluminium sulphate practically free

from iron, it has largely taken the place of alum, than which it is both

cheaper and stronger. Crystallised aluminium sulphate, Al₂(SO₄)₃, 18Aq,

has a molecular weight of 666, which is of equal value to 906 of

ammonia-alum, and 948 of potash-alum. Iron is the most objectionable

impurity in both alums and aluminium sulphate, and may be detected by

the addition of potassium thiocyanate, which will produce a red colour,

or potassium ferrocyanide (yellow prussiate of potash), which will

produce a blue. As the iron may be present in the ferrous condition, it

is safer first to boil the alum solution with a few drops of nitric acid

or bromine water. For more accurate determination of iron see L.I.L.B.,

pp. 20, 136.

No satisfactory leather can be produced with a solution of alum or

aluminium sulphate alone, the skin drying horny, and incapable of

softening by stretching. In practice, salt is always used in addition,

the proportion being very variable, but averaging about half the weight

of alum, or two-thirds the weight of sulphate of alumina employed. The

mode of action of the salt has long puzzled chemists, and it has been

supposed that its use was to convert the aluminium sulphate into

chloride, a reaction which takes place to some extent, but which fails

to explain the production of a soft leather, since aluminium chloride,

though freely taken up by the skin, produces no more satisfactory

leather than aluminium sulphate. The real explanation is found in

Chapter IX. Alumina is a weak base, which readily gives up its acid to

the pelt, becoming converted into a basic salt (see p. 187). The acid

not only swells the pelt, and renders it incapable of producing a soft

leather, but the swollen pelt is less ready to absorb the alumina salt,

and so remains undertanned. The addition of salt prevents the swelling

effect of the acid, and produces a partial pickling of the skin (p.

89), which, in conjunction with the tanning effect of the basic alumina

salt formed, yields a satisfactory leather, though one which is readily

affected by washing. If instead of adding common salt to the alum

solution, an alkali such as soda is added, it combines with a portion of

the acid, forming sodium sulphate, while the alumina remains in solution

as a “basic salt.” As the term “basic salt” must be frequently employed

in connection with mineral tannage, it may here be explained. Basic

salts are compounds intermediate between the normal salt, in which the

whole of the base is combined with acid, and the hydrated oxide in which

the whole is combined with OH groups. Thus aluminium chloride, Al₂Cl₆,

is a normal salt, in which the whole of the combining powers of the

aluminium are saturated with chlorine: aluminium hydrate, Al₂(OH)₆, is

the hydrated oxide, and Al₂Cl₅OH, Al₂Cl₄(OH)₂, and so on are basic salts

in which successively more of the Cl is substituted by OH. Generally, as

a salt becomes more basic, its solution in water becomes more unstable,

and very basic salts are either insoluble, or are precipitated from

their solutions by very trifling causes, such as boiling, dilution, or

the attraction of animal or vegetable fibres; separating into free acid

and either hydrate or a still more basic and insoluble salt. On this

property depends their importance in tanning and dyeing, many of the

metallic mordants being solutions of basic salts. Basic salt solutions

are formed in various ways, the most common being the direct solution of

a hydrated oxide in a solution of the normal salt, or the neutralisation

of a part of the acid of the normal salt by the addition of a stronger

base. This is what takes place on the addition of soda to an alum

solution. If the soda is added in excess, the whole of the alumina is

precipitated as hydrate, or as an insoluble basic salt, but if a

proportion not exceeding about four parts of crystallised sodium

carbonate be dissolved separately, and added _slowly with constant

stirring_ to the ten parts of alum dissolved in water, no precipitation

will take place. In this solution leather can be tanned, either with or

without addition of salt, the alumina is taken up more freely than from

the normal alum, and the leather is more easily softened, and more

resistant to water. In fact such leather bears a strong resemblance to

the chrome tannages, standing a great deal of washing, and considerable

temperatures without returning to a pelty condition. The more basic the

solution that is used, the fuller and softer is the leather produced.

The alumina-salt taken up by the skin from such basic solutions is

always basic, while that absorbed from alum or alumina sulphate is

_apparently_ the normal aluminium sulphate. It is probable however that

the actual tanning salt is in both cases basic, and that the acid is

fixed as free acid, as in the pickling process, as the proportions of

acid and base found in the residual liquor are somewhat variable.

Basic alumina solutions have hardly taken the place in practice which

they deserve, though they were described by Knapp in 1858[112] and have

since been patented by Hunt, but the patent (probably invalid) has been

allowed to lapse. A good stock solution for practical use is made by

dissolving 10 lb. of sulphate of alumina in 10 gallons of water, and 4

lb. of washing soda in 4 gallons, and gradually mixing the latter with

the former. Salt can be used in addition if desired, and flour and

egg-yolk may also be added.

[112] ‘Natur und Wesen der Gerberei,’ Braunschweig, 1858.

In curing small skins, where it is not desirable for the fur to come in

contact with the liquid, or in the tawing of wool rugs, it is often

convenient, after freeing the skin as much as possible from blood and

dirt, and adhering flesh, to stretch it on a frame, or nail it out on a

board, and apply a strong alum-and-salt solution, as hot as the hand

will bear, with a sponge, repeating the operation till the skin is

struck through. About 1 lb. of alum and ¹⁄₂ lb. of salt per gallon is a

suitable strength. In place of applying the solution, powdered alum and

salt is sometimes rubbed into the wet skin. Alumed goods should

generally be dried out rapidly, and finally at a good temperature, as

this tends to fix the tannage, which is also made more permanent and

resistant to water by keeping the skins for a month or more in the

alumed condition, an operation known as “ageing.” When first dried,

alumed goods are invariably stiff and horny, and, to give them softness,

must first be damped back to a half-dry condition, and then gradually

softened by mechanical means. “Staking,” and “perching” are the usual

methods, the first consisting in drawing the goods vigorously over a

bluntish blade fixed on the top of a post, and the second in fixing the

skins on a horizontal pole (the “perch”), and working them with the

“crutch stake,” a tool formed somewhat like a small shovel with a

semicircular blade, in place of which a “moon-knife” (a round blade

somewhat like a broad thin quoit) is often fixed in a wooden crutch. The

tools, and mode of using them are shown in Figs. 36 and 37.[113]

Machines, described on p. 192, are now generally used for these

operations. After the first staking or softening, the skins are allowed

to become nearly dry, and are then staked a second time. Some judgment

is required as to the precise degree of moisture in each case: in the

first instance the skins must be sufficiently damp to yield without

injury to the mechanical stretching, but in this state they retain

sufficient moisture to enable the fibres again to adhere on drying; and

at the second staking or perching, they must be damp enough to allow

these fibres to be again loosened without violence, and dry enough to

prevent their again adhering.

[113] The process shown in Fig. 37 is not actually “perching,” but

“grounding,” in which a moon-knife with a sharp turned edge is used to

reduce the thickness of the skin on the perch, at the same time as it

stretches and softens it.

[Illustration: FIG. 36.--Staking White Leather.]

[Illustration: FIG. 37.--Grounding with the Moon-knife.]

The following slight sketch of the manufacture of calf-kid will serve to

illustrate the practical manufacture of the finer alumed or “white”

leathers. The raw material is in England mostly large market-calf,

though salted and dried skins are sometimes employed. After sufficient

soaking or washing in water, they are limed without arsenic or other

sulphides, in limes which must not be allowed to grow stale or putrid,

until the hair can be easily removed. After unhairing and fleshing in

the usual way, they receive a few days in a pretty fresh lime, in order

to plump them, and are then freed from lime gradually but as completely

as possible, by successive steepings and washings in water softened by a

mixture of that already used on other goods and by working on the beam.

This acts as a partial substitute for puering with dung, which is now no

longer used on calf-kid. The goods are next drenched in the ordinary

way, 3-4 % of bran being used, and the goods allowed to rise two or

three times in the drench, which should be conducted with the usual

precautions (p. 167) to avoid the danger of butyric fermentation in hot

weather. The goods should come out of the drench free from lime, and

unswollen by acid, but full, white, and soft. The tanning (or “tawing”

as it is usually called in the case of alumed goods) is done in a

rotating drum with a mixture of alum or sulphate of alumina, salt,

flour, egg-yolk, and olive oil. About 5 per cent. of flour, 2·5 per

cent. of alum, 1 per cent. of salt, the yolks of 25 eggs, or 1¹⁄₂ lb. of

preserved egg-yolk, 2 oz. of olive oil, and 1¹⁄₄-1¹⁄₂ gallon (12-15 lb.)

of water are required per 100 lb. of wet pelt. The flour is first made

into a smooth paste with a little water, the egg-yolk, somewhat diluted

with warm water and strained, is mixed in together with the oil, and

finally the alum and salt solution is added at such a temperature as to

bring the whole mixture to blood-heat (38° C.). The length of drumming

depends on the thickness of the skins, several hours being required for

very thick ones, but care must be taken to stop and ventilate the drum

at frequent intervals, so as to prevent the skins becoming hot by

friction. This part of the process was formerly accomplished by treading

with bare feet in a tub. After tawing, the goods are allowed to lie in

piles over-night, or are sometimes laid in tanks for a day or so with

any that remains of the tawing paste, to complete the absorption of the

salt and alum, and are then frequently split with the band-knife

machine, though it would be better, as is often done on the Continent,

to split them before tawing, the materials of which are not only costly,

but unfit the splits for many purposes for which they might be employed.

The drying should be rapid, but is best done first at a moderate

temperature, or in the open air, and then in a rather hot stove. They

may now be allowed to “age” from one to three months, but it is usually

better before ageing to do the first part of the finishing process,

consisting of damping back, staking, and if necessary, shaving. Machines

are now almost invariably used for the staking, the principle of which

may be described as that of a pair of tongs, carrying one or generally

two staking blades on one limb, and a roller on the other which closes

on the skin, and presses it against and between the blades, while the

tongs are drawn backwards, allowing it to slip through. Fig. 38

illustrates the Slocomb, one of the most popular machines of this type.

After staking and ageing, the skin is soaked in water till thoroughly

wet in all parts. This not only softens the skin, and prepares it for

dyeing, but takes out the superfluous alum and salt, and at the same

time a good deal of flour and egg. To replace these, “re-egging” is

necessary, and while some manufacturers give egg-yolk, or egg-yolk and

flour only, many add a proportion of salt, and sometimes also of alum.

This is done before dyeing, if the skins are to be blacked on the table,

but as tray-dyeing (see p. 406) would again wash out the egg, the

re-egging is deferred till after dyeing if this process is resorted to.

Before dyeing, the skins receive an alkaline mordant to overcome

greasiness, and enable them better to take the colour. In former times

this was usually stale urine, but this has mostly been superseded by

solutions of “hydroleine” (a washing powder), or of soap rendered more

or less alkaline with ammonia. Eitner gives the following recipe, viz.

¹⁄₂ lb. Marseilles soap dissolved in boiling water, 5 or 6 egg-yolks

added, and the whole made up to 4 gallons with water and ¹⁄₄ lb. potash

bichromate. The colour used is infusion of logwood or its extract, or

two-thirds logwood and one-third fustic, which is best extracted without

alkali, a small quantity of soda or ammonia being afterwards added. It

is fixed and darkened by a wash of iron-liquor or a solution of 1 of

ferrous sulphate in 75 of cold water. After being again dried, the skins

are sometimes grounded with the moon-knife, softened again by staking or

perching, for which a machine with inclined or spiral blades attached to

a drum and working on a sort of leather apron is often preferred to

machines of the Slocomb type, and rubbed over on the grain with a

composition containing oil, wax, etc., and are finally ironed with a

heavy flat-iron, to give them a fine and smooth surface. Eitner gives a

recipe for the gloss:--1 kilo gum arabic, ¹⁄₂ kilo yellow wax, ¹⁄₂ kilo

beef-tallow, ³⁄₄ kilo Marseilles soap, 1 liter strong logwood infusion,

and 5 liters water. The water is brought to a boil in an earthen pot,

and then the soap, wax, gum, and tallow are added successively, each

being stirred till dissolved before adding the next, and lastly the

logwood. After boiling for an hour, it is allowed to completely cool,

being incessantly stirred during the whole process. After ironing the

goods are rubbed over with a final gloss, for which Eitner gives the

following recipe:--8 liters olive oil, 500 grm. tallow, 500 grm. yellow

wax, 500 grm. rosin, 500 grm. gum arabic. (No water is given in the

recipe, but the gum arabic is presumably softened in water.) The

mixture is cooked for two hours in an earthen pot till the water is

evaporated, and allowed to cool with constant stirring. The skins are

then rubbed with a flannel with a very small sprinkling of French chalk,

and are ready for sale.

[Illustration: FIG. 38.--The Slocomb Staking Machine.]

The manufacture of glove-kid is quite similar in principle to that just

described, but varied in detail to suit the softer and more delicate

skins employed, to give greater softness, and especially the quality of

stretching in any direction without springing back, which is so

characteristic of the leather. Lamb-skins are the principal raw

material, though genuine kid is also employed for the best qualities.

The manufacture varies much with the quality and character of the goods.

The skins, which are mostly dried, are soaked in clean and cool water

for three to four days, according to age and thickness. Common qualities

(small imported slink lambs) are often unhaired by dipping in or

painting with a paste of gas-lime, lime and sulphide of sodium, or lime

and red arsenic, so as to destroy the wool. Better skins are sometimes

unhaired by painting on the flesh with lime alone or in mixture, and in

other cases ordinary lime-pits are used, with limes, which are most

usually strengthened with red arsenic, which is added to the lime while

hot from slaking (cp. p. 142).

The calcic sulphydrate (and perhaps sulpharsenite) thus formed hastens

the unhairing, and preserves the gloss of the grain. Well conducted

glove-kid establishments avoid as much as possible the use of old limes,

which produce a loose, porous leather, with a rough, dull grain. The

liming lasts on the average ten days, and is of the greatest importance.

It is essential that the inter-fibrillary substance should be dissolved,

that the leather may have the quality known in Germany as _Stand_, that

is to say, may be strongly stretched in either length or breadth without

springing back. It also depends upon the liming (and this is of special

importance in the case of lamb-skins), whether the tissue of the

fat-glands is well loosened, so that the fat, either as such, or as

lime- or ammonia-soap, may be readily and completely worked out. Skins

in which this is neglected can never be properly dyed.

When the hair (or wool) is well loosened, the skins are rinsed in water,

and then unhaired on the beam with a blunt knife. The water employed in

washing should not be much colder than the limes, or it will prevent

the hair from coming away readily. The wool or hair is washed and dried

for sale. The skins are thrown into water, to which a little lime-liquor

has been added, to prevent precipitation of the lime in the skins by the

free carbonic acid of the water, which would have the effect of making

them rough-grained.

Next comes the first fleshing (_Vergleichen_) or “levelling.” By this,

the loose cellular tissue on the flesh-side is removed, together with

the head, ears, and shanks; and the flanks are trimmed. The skins are

then again thrown into water softened with lime-liquor as above

described, and then into a puer of dogs’-dung. This is prepared by

stirring up white and fermented dogs’-dung with boiling water, and

straining it through a sieve or wicker basket. The puer must be used

tepid, and not too strong. The skins “fall” (lose their plumpness) in it

rapidly, and become extremely soft and fine to the touch; and the

fat-glands, remaining hairs, and other dirt, can now be very readily

scudded out.

Too strong puers, or too long continuance in them, produce evident

putrefactive effects on the skins. (See also p. 181.)

When the skins come out of the puer, they are stretched and worked on

the flesh with a sharp knife, and any remaining subcutaneous tissue is

removed. This constitutes the second fleshing. They are then rinsed in

warm water, and beaten with clubs in a tub, or worked in a tumbler-drum,

in either case with a very little water only; and finally brought into a

tank of water, not too cold, and kept in constant motion with a

paddle-wheel.

The skins are next cleansed on the grain-side by working on the beam

with plates of vulcanite set in wooden handles, so as to remove fat,

lime- and ammonia-soaps, and other lime compounds, together with all

remaining hair or wool. The skins are now a second time washed in the

“paddle-tumbler,” first in cold, and then in tepid water; and after

allowing the water to drain from them, they are transferred to the

bran-drench.

This is prepared by soaking wheaten bran in water at about 50° C., and

diluting with warm water. Sometimes the mixture is strained, and the

bran-water only used, to save the trouble and cost of removing adhering

particles of bran from the delicate skins. Sufficient of the liquid must

be employed to well cover the skins, and the temperature may range from

50° F. (10° C.) to 68° F. (20°C.). These conditions are favourable to

bacterial activity, which comes into play, and, on the one hand, evolves

acetic and lactic acids, which dissolve any remaining traces of lime,

and on the other, loosens and differentiates the hide tissue, so as to

fit it to absorb the tawing solution. Much care is required in the

management of the bran-drench, especially in summer, since the lactic

readily passes into the butyric fermentation (see also p. 167). The

tawing mixture is composed (like that employed in the fabrication of

calf-kid, q.v.) of alum, salt, flour and egg-yolks, in a quite thin

paste. A small quantity of olive oil is also generally used. The skins

are either trodden in it with the feet, or more generally put into a

tumbler-drum with it. Kathreiner pointed out, some years ago,[114] that

a mixture of olive-oil and glycerine might be partially substituted for

the egg-yolks, in both the tanning and dyeing of glove-kid leather.

[114] Gerber, i. (1875) p. 170; ii. (1876) p. 664.

The tawed skins are now dried by hanging on poles, grain inwards. Rapid

drying in well-ventilated, but only moderately heated rooms is essential

to the manufacture of a satisfactory product.

The dry leather is rapidly passed through tepid water, and after being

hung for a very short time, to allow the water to drain off, is trodden

tightly into chests, and allowed to remain in them for about 12 hours,

so that the moisture may be uniformly distributed. It is then trodden on

hurdles (German _Horden_) composed of square bars of wood, joined corner

to corner, so as to make a floor of sharply angular ridges. The next

operation is stretching with the “moon-knife”; after which the leather

is dried nearly completely, and staked again.

This completes the tawing process. The goods are now “aged” as in

calf-kid manufacture. Before dyeing they are washed with tepid water to

remove part of the tawing mixture, and especially, superfluous alum and

salt, and are re-egged much like calf-kid, before dyeing if the latter

is done by brushing, and after if in the dye-tray or paddle. Aniline

colours are more used than formerly, especially for topping and

brightening the natural colours, but the dyewoods and other mordant

colours are still largely employed. The leather is first prepared with

an alkaline mordant (stale urine, ammonia, etc.) (cp. p. 413), then

repeatedly brushed with or dipped in the dyewood liquor, and a wash

(“striker,” German _Ueberstrich_) containing some metallic salt is

generally applied, with the object either of bringing out the special

tone required, or of making the colour more lively and permanent. The

striker is usually a solution of one of the so-called “vitriols”: “white

vitriol” (zinc sulphate), “blue vitriol” (copper sulphate), “green

vitriol” (iron sulphate), or occasionally other salts.

After the dyeing, the skins, if dipped, are wrung out and re-egged; if

brush-dyed, sleeked out with a brass or ebonite sleeker to get rid of

superfluous water. They are then dried in an airy room. Before staking

(stretching), the skins are laid or hung in a damp cellar, or in moist

saw-dust. They are staked twice: once damp, and once nearly dry; and are

finished by glassing or ironing.

Skins which are much damaged on the grain, or otherwise faulty, are

smoothed with lump pumice on the flesh side, or fluffed with fine emery

on the fluffing wheel. They are then dyed on the flesh side, mostly by

dipping, but occasionally with the brush, in which case, the method

described is slightly modified.

Tawing with alum and salt is frequently employed for commoner and

stronger leathers, such as aprons (of sheep-skin), leather for

whip-lashes, laces for belts, and “skivers” for capping chemists’

bottles. The process is practically the same as for calf-kid, except

that no egg, and little flour is used. Often flour is entirely omitted,

and the goods may then be alumed in tubs, in which they are merely

handled, as the alum solution penetrates quickly. Goods which are

required white are frequently handled or tumbled with a milk of

“whitening,” both to improve the colour, and to neutralise any acid

present, and fix the alum by rendering it more basic. Alumed goods can

be stuffed with greases, either by hand or in the drum, after thorough

softening by staking.

Alum, and other salts of alumina are frequently used in

combination-tanning with vegetable materials (see Chap. XVII.). “Green”

leather for laces, “dongola,” and “dog-skin” glove-leathers are made in

this way. Glazed kid for ladies’ shoes must be slightly vegetable-tanned

on the surface, or it will not glaze, but this is frequently

accomplished by the use of materials in the dye-liquor containing

tannins.

CHAPTER XV.

_IRON AND CHROME TANNAGES._

_Iron tannages_ may be very shortly dismissed, as their practical

interest is at present either historical or prospective, but iron salts

enter in so many ways into the chemistry of leather manufacture, that

their properties must be briefly considered. Iron exists in salts in two

states, the ferrous, and the ferric, in the first of which it is

divalent, and in the second trivalent. Thus ferrous chloride is FeCl₂;

ferrous oxide, FeO; ferrous sulphate, FeSO₄; ferrous hydrate, Fe(OH)₂.

The compounds of ferrous iron are mostly green, like ferrous sulphate

(“green vitriol,” “copperas”): exposed to air and moisture, they easily

absorb oxygen, and pass into the ferric form. Ferric chloride is FeCl₃

(or, as it is sometimes written without much reason, Fe₂Cl₆), ferric

hydrate Fe(OH)₃, ferric oxide Fe₂O₃, ferric sulphate Fe₂ (SO₄)₃, and so

on. The atomic weight of iron is 56. Ferric salts are mostly yellow or

orange, ferric hydrate is yellow-brown, and on ignition is converted

into deep red ferric oxide, which is very difficultly soluble in acids.

Ferric salts in contact with more easily oxidisable matters, readily

give up oxygen, and pass into the ferrous state; and especially does

this happen in the presence of organic matter, under the influence of

sunlight. Thus iron-salts often act as carriers of oxygen, and oxidisers

of organic matter, absorbing oxygen from the air, and giving it up again

under the influence of light or heat. There are several other oxides of

iron which do not form salts, and there is a ferric acid, apparently

corresponding to chromic acid, which is so unstable that it has been

very imperfectly investigated.

Ferric salts correspond in structure to those of alumina, and like these

are powerful tanning agents, and readily form basic salts, while the

ferrous salts have no tanning effect until they become oxidised, when

they form basic ferric salts. Ferric salts are characterised by giving

blue-black or green-black compounds with tannins, and with many other

allied bodies, while the corresponding ferrous compounds are mostly

colourless, though they rapidly oxidise and darken.

Ferric iron, like alumina, forms an “alum,” a double sulphate of iron

and potassium, Fe₂(SO₄)₃K₂SO₄, 24Aq, forming fine pale-violet crystals,

but dissolving to a yellow-brown solution. (It must be distinctly

understood that iron-alum and chrome-alum contain no alumina, but are

simply called alums because of their similarity of constitution, iron or

chrome taking the place of the aluminium. Iron-alum, in conjunction with

salt, can be used for tanning, giving a pale buff-coloured leather very

similar to an ordinary alum leather. Thus the presence of a small

quantity of iron in an alum used for tawing is of no consequence, except

as affecting the colour of the leather. In impure sulphate of alumina

such as “alumino-ferric,” it, however, generally exists in the green

ferrous state, and only acquires tanning properties on oxidation.

Without common salt iron-salts are still less satisfactory tanning

agents than those of alumina under the same conditions, as the acid is

yet more loosely held, and though basic ferric salts are taken up in

considerable quantities by hide, the leather produced is thin, and

usually brittle. Professor Knapp devoted much study to the production of

a commercial sole-leather by basic iron-salts; and took several patents,

which did not prove practically successful, though the brittleness was

to some extent overcome by the incorporation of compounds of iron with

organic materials such as blood and urine, of iron-soaps, and of rosin

and paraffin in the leather. Like most mineral tannages, the process was

far more rapid than that with vegetable materials. Knapp’s basic tanning

liquor was made by the oxidation of ferrous sulphate with a small

quantity of nitric acid. Patents have also been taken for the oxidation

of ferrous sulphate by peroxide of manganese in presence of sulphuric

acid, which produces basic ferric sulphate in mixture with manganese

sulphate, which has also some tanning properties. Attempts have also

been made to tan by treatment of the hide with solutions of ferrous

sulphate, and subsequent exposure to the air, in order to oxidise the

iron on the fibre and convert it into a basic ferric salt, but have not

proved of any commercial value.

The principal use of iron at present in leather manufacture is in dyeing

blacks (see p. 413), but in this case, its feeble hold upon acids in the

ferric state, and its tendency to act as an oxidising agent, or oxygen

carrier, renders the blacks somewhat unstable, and is frequently

injurious to the leather. There is also little doubt that the presence

of ferric salts in leather blacks has a great tendency to cause the

resinification of the oil, known as “spueing,” by promoting its

oxidation.

_Chrome tannages_, from a practical point of view, stand on a very

different footing to those which have just been mentioned; having

established their position in the manufacture of almost all sorts of

light leathers, in competition with all the older methods, and making a

serious claim to a share in the production of belting and even of sole

leathers.

Chromium is a grey, and very infusible metal, which chemically much

resembles iron in its compounds, and has an atomic weight of 52, or a

little over. Like iron, it possesses a divalent and a trivalent form,

but the divalent has so strong an affinity for oxygen, and passes so

readily into the trivalent form, that until easier means are found for

its preparation, it is of little practical interest. Its salts are blue.

On the other hand, salts of the trivalent form, corresponding to the

ferric salts of iron, are very stable, and powerful tanning agents. They

are mostly green, but violet modifications are known, corresponding to

the violet crystals of iron-alum, but of a much deeper tint. There is

also a hexavalent form, probably corresponding to that of iron in the

unstable ferrates, but in the case of chromium, of considerable

stability. Its oxide is chromic anhydride, CrO₃, commonly called chromic

acid, which combines with bases, and especially with the alkalies to

form yellow or orange-red salts, and the anhydride itself is almost

crimson in the solid form, though dissolving to orange or yellow

solutions. Chromic acid though it hardens and preserves animal tissues,

has no tanning properties till it becomes reduced to chromic oxide.

There is also a higher, but very unstable oxide, perchromic acid,

possibly corresponding to persulphuric acid, which is soluble in ether

to an intensely blue solution. The name chromium is derived from the

intense colour of many of its compounds.

Our supplies of chromium are derived from chrome-iron-ore, a mineral

which contains oxides both of chrome and iron. This is furnaced with a

mixture of lime, and soda or potash, when it absorbs oxygen from the

air, the chromium becoming converted into chromic acid which combines

with the alkali present, while the iron remains undissolved as ferric

oxide. Lixiviating the mass, and evaporating the solution, lime and

potassium or sodium chromates are obtained, according to the alkali

used, and on adding sufficient sulphuric acid to combine with half the

base, potassium or sodium dichromate (or as it is commonly called

“bichromate”) can be crystallised out. Potassium dichromate is most

commonly made, because it crystallises well, and is not deliquescent,

but sodium dichromate is somewhat cheaper, though less convenient.

Dichromates, at least in the crystallised state, are not hydric salts

like bisulphates, but anhydrochromates corresponding to the potassium

anhydrosulphate obtained by fusing ordinary bisulphate, and to fuming

sulphuric acid. Thus the formula of potassium dichromate is

{ CrO₂OK

{ O ,

{ CrO₂OK

or Cr₂K₂O₇ and its molecular weight is 294, while that of sodium

dichromate, which is similar in constitution, but crystallises with 2Aq,

is 298. The molecular weight of CrO₃ is 100. Chromic acid, and acidified

potassium dichromate are powerful oxidising agents, and are used as such

in many processes, and especially in the manufacture of alizarine. If

sulphuric acid be used in molecular proportions, the product of the

reaction is chrome-alum: 4H₂SO₄ + Cr₂K₂O₇ = 3O + 4OH₂ + K₂Cr₂(SO₄)₄.

This, like ordinary alum, crystallises with 24Aq, and hence has a

molecular weight of 998. It forms dark purple, almost black crystals,

which are a fine garnet-red by transmitted light. In cold water it

dissolves to a violet solution, which becomes green on boiling, but very

slowly resumes the violet condition when cold. This change, which is not

uncommon in chrome solutions, is probably due to a partial decomposition

into free acid and a basic salt, the basic salts of chromium being

generally green. It has been noticed that raw pelt swells much more in

the green, than in the violet solution. Being derived from waste

products, chrome-alum is often a cheap and valuable source of chromium

for chrome tanning.

For the analysis of chrome compounds see L.I.L.B., p. 141 _et seq._

Chrome oxide, and basic chrome salts, when strongly ignited, become

insoluble even in concentrated acids, and their analysis is therefore

attended with some difficulty. If, however, the ignited residue (for

instance a leather-ash) be finely powdered, and intimately mixed with a

fusion-mixture consisting of equal parts of pure calcined magnesia and

pure dry sodium carbonate, and ignited (preferably over a Teclu burner),

in a platinum crucible, in which it is occasionally stirred with a

platinum wire, it will be quantitatively converted into chromate, which

may be dissolved in acid, and estimated with potassium iodide and

thiosulphate in the usual way. If it is desired at the same time to

estimate sulphuric acid, it is sometimes preferable to substitute lime

or calcium carbonate for the magnesia, which is apt to be contaminated

with sulphates.

Chrome is not only of importance in tanning, but in dyeing; on account

of its power of forming insoluble colour-lakes with many mordant

colouring matters. For this purpose normal or basic chromic salts are

sometimes used, sometimes chromic acid or dichromates, the latter acting

not only by yielding chrome-oxide on reduction, but as oxidising agents

to the colouring matters. Most of the colours produced with chrome

mordants are of dark shades, that with logwood being deep violet or

black. The mordanting power of chromium is important in the dyeing of

chrome leather. Bichromate of potash is often used in dilute solution

for darkening the shade of leather dyed with other materials, but is not

to be recommended on account of its destructive action on the leather.

Numerous patents have been taken for processes of chrome tannage. The

first practical method was described by Professor Knapp in 1858 (see p.

210), though he did not recognise its value. Some of the patents have a

historical interest, though of no importance. Among these may be

mentioned that of Cavallin, a Swedish apothecary, whose object was

dyeing rather than tanning, but who treated raw hide with a solution of

bichromate, which was afterwards reduced on the fibre by one of ferrous

sulphate. The leather produced is dark reddish brown, and tender from

the amount of basic ferric salt formed at the same time. Mr. J. W. Swan,

well known in connection with photographic processes, and electric

lighting, also patented a process of chrome tannage (as an addendum to a

patent on carbon printing), in which the chromic acid first fixed in the

pelt was reduced by “oxalic, or other suitable acid.” Although it is

possible to produce leather within the lines of the patent, the

strongly acid reaction of the reducing agent renders it unsuitable for

practical use. The first chrome tanning process which made any show of

practical success, was that patented in 1879 by Heinzerling, which was

acquired in this country by the Eglinton Tanning Company, and also

worked under their license for a short time by the Yorkshire Tanning

Company at Leeds. Though the process was not commercially successful on

any considerable scale, it possesses points of interest which make a

brief description desirable. The hides or skins, after preparation in

the usual way, were treated in a mixed solution of salt, alum (or

aluminium sulphate), and potassium bichromate, but no systematic attempt

was made to reduce the chromic acid to a tanning form, the product

being, at first at least, merely an alum tannage, coloured, and perhaps

somewhat hardened with chromic acid, though on keeping for a length of

time, reduction gradually took place at the expense of the hide-fibre,

and of the fats employed in currying, so that the leather internally

became greyish-green, and really chrome-tanned. Specimens of the early

products of the process, preserved in the museum of the Leather

Industries Department at Leeds, have now all undergone this change, but

are still tough and flexible, showing that the rapid tendering of the

Heinzerling leather, which was one of the causes of its failure, must

have been due to some error in manufacture, and was not inherent in the

process. Interesting, historically, is the fact, that at an early stage

in the life of the patent, a specimen of the leather was submitted to

the late Professor Hummel, in order that he should suggest some means of

overcoming the disagreeable yellow colour of the product. He reduced it

with a bisulphite, and coloured it with an aniline dye, and a piece is

still in the possession of the Yorkshire College, and in perfectly sound

condition. If legal publication of this experiment could have been

proved, it would have invalidated the important Schultz patents under

which most of the chrome-kid of the United States has been manufactured.

As bearing on modern chrome-tanning, the most important reaction in the

process is that of the alum with the bichromate. It has been shown by

Heal and Procter[115] that pelt absorbs practically no chromic acid from

bichromate, unless it has been previously set free by acidification.

When however alum, or sulphate of alumina is added, its sulphuric acid

liberates the chromic acid, leaving a basic alumina salt in solution,

and this fact has been utilised in some modern tanning processes.

[115] Journ. Soc. Chem. Ind., p. 251, 1895.

The first really important advance in practical chrome tanning was made

by Augustus Schultz, in 1884. Schultz was not a tanner, but a chemist,

employed by a New York firm of aniline colour merchants, and his

attention was accidentally drawn to leather by a friend who asked him if

it were possible to produce a leather for covering corset steels, which

would not rust them as ordinary alumed leathers do. The process which he

adopted was probably suggested by a method then recently patented for

the mordanting of wool by chrome oxide, and depended on the power of the

pelt to absorb free chromic acid (as it does all other free acids), and

the subsequent reduction of the latter on the fibre to a basic chrome

salt, which produced the tannage. The reducing substance employed was

the free sulphurous or thiosulphuric acid of an acidified solution of

sodium thiosulphate (hyposulphite), and as it was not certain which of

the two acids was the really active agent, Schultz duplicated his

patent, so as to cover both. Though he made no claim in his patent to

having discovered the best proportions of his ingredients, those which

he specified have proved practically useful after allowing for the

modifications required by different skins, and slightly different

methods of working. His first bath consisted of a solution of 5 per

cent. of bichromate of potash, and 2¹⁄₂ per cent. of concentrated

hydrochloric acid (or 1·25 per cent. of concentrated sulphuric acid),

reckoned on the wet weight of the prepared pelt, and dissolved in

sufficient water for convenient use in the paddle or drum which was to

be used in the process. In this bath the skins were worked till they

took a uniform yellow colour throughout, but without any tanning effect

being produced. They were now freed from superfluous chrome liquor by

draining or “putting out,” and transferred to the second bath, which

consisted of 10 per cent. of “hypo” and 5 per cent. of hydrochloric acid

similarly dissolved. In this, they rapidly took a duck-egg green colour

from the reduction of the chromic acid; and when this was uniform

throughout the skin, the tannage was complete. The exact quantity of

water is not of great importance, and good results can be obtained with

anything varying from 20 to 50 gallons per 100 lb. of pelt (200 to 500

per cent.) if time be allowed for the weaker solution to act. The

quantities of “hypo” and hydrochloric acid given for the second bath are

often somewhat insufficient, and have to be slightly increased to

complete the reduction. The reactions which take place are represented

by the following formulæ, in which the weights of the materials taking

part in the reaction are also given below the symbols. In the first

bath--

Potassium Hydrochloric Potassium Chromic Water.

dichromate. acid. chloride. acid.

K₂Cr₂O₇ + 2HCl = 2KCl + 2CrO₃ + OH₂

294 + 73 = 149 + 200 + 18

As ordinary concentrated hydrochloric acid does not contain more than

about 30 per cent. of actual HCl,[116] about 2·5 parts would be required

to completely decompose 2·94 parts of dichromate, while in Schultz’s

formula 2·5 parts of hydrochloric acid are used to 5 parts of

dichromate. This excess has been found useful in the production of a

good leather, both to prevent accidents from an overdose of hydrochloric

acid, and because of the modifying effect of an excess of neutral salt

on the action of the chromic acid (cp. p. 82).

[116] Acid of S.G. 1·16 (32° Tw.) contains 31·5 per cent. of HCl by

weight or 36·6 grm. per liter, and therefore is practically 10 ×

normal strength. Acid of S.G. 1·2 (40° Tw.) contains 39·1 per cent. or

469 grm. per liter.

The reactions which take place in the second bath are somewhat

complicated. Eitner, in a valuable series of articles on chrome tannage,

which have been appearing in the ‘Gerber’ since January 1900, states

that even better results are obtained by using the hydrochloric acid in

slight excess, as the action of chromic acid (in the presence of the

potassium chloride of the chrome-bath) is not swelling but hardening to

the skin, and the slight swelling action of the hydrochloric acid tends

to counteract this, and also to facilitate the subsequent reduction. The

two views are not contradictory, as the excess of bichromate behaves to

the hide as an alkaline salt, which also produces a slight swelling

effect, and it is quite probable that better results are attained when

the solution is either alkaline or acid, than when the potassium

chromate is exactly decomposed. Eitner recommends the use of four parts

by weight of bichromate, and four parts of the strongest hydrochloric

acid, dissolved in 400 parts of water, for each 100 parts of wet pelt,

which should yield about 40 parts of dry leather. He states that if such

a bath be used, it may be safely and economically exhausted by a second

pack of skins, which is impossible in a bath containing excess of

unacidified bichromate. He gives[117] the following explanation of the

successive changes which take place when acid is gradually added during

the reduction, but points out that in practice the reactions always to

some extent go on simultaneously.

[117] Gerber, p. 297, 1900.

In the first stage, very slight acidification is required, and if the

skins have been chromed with excess of hydrochloric acid, may be

altogether dispensed with. The skins become brownish from the conversion

of the chromic acid into so-called “chromium dioxide” (probably really a

basic chromic chromate, Cr₂CrO₄(OH)₄, which on ignition leaves Cr₃O₆);

no sulphurous acid is liberated, or sulphur deposited, but sodium

tetrathionate is formed in the bath, and the reaction may be represented

as follows:

(1) 3CrO₃ + 6HCl + 6Na₂S₂O₃ = 3Na₂S₄O₆ + 6NaCl + 3OH₂ + Cr₃O₆.

Further addition of hydrochloric acid brightens the colour of the skins,

while the liquid still remains clear, and chromium chloride is formed

instead of chromic chromate, the main reaction being:

(2) 2CrO₃ + 12HCl + 6Na₂S₂O₃ = 3Na₂S₄O₆ + 2CrCl₃ + 6NaCl + 6OH₂.

On still further addition of hydrochloric acid, sulphur is separated

according to the following equation, and is deposited partly in the

skins, and partly in the bath:

(3) 2CrO₃ + 6HCl + 3Na₂S₂O₃ = 3Na₂SO₄ + 3S + 2CrCl₃ + 3OH₂.

After complete reduction, and consumption of the free hydrochloric acid,

further reactions take place at the expense of the excess of

thiosulphate which should be present, resulting in the production of

basic chromic salts, and the further deposition of sulphur, mostly

within the skin, as shown in the following equations:

(4) Cr₂(SO₄)₃ + Na₂S₂O₃ + OH₂ = 2CrOH.SO₄ + SO₂ + S + Na₂SO₄.

(5) 2CrCl₃ + Na₂S₂O₃ + OH₂ = 2CrOH.Cl₂ + SO₂ + S + 2NaCl.

The thiosulphate bath therefore not only reduces, but precipitates

sulphur in the skin, and reduces the chromic salt to a basic state. In

boiling solution, thiosulphate precipitates the whole of the chromium as

chromic oxide, but in the cold, and in presence of free sulphurous acid,

it only reduces to a basic salt. Eitner does not consider the

possibility, which certainly requires investigation, that instead of

basic salts, sulphite-sulphates are formed at least in the first

instance. Such salts of one base and two acids are quite possible, and

it is very probable that in the use of chroming baths containing organic

acids, they have considerable influence on the tannage.

The free sulphur which is liberated is partially deposited on and among

the fibres of the leather, and adds to its softness, and also acts

chemically on the oils used in “fatliquoring,” so that it is probably

one of the main causes of difference between the products of the Schultz

or “two-bath” method, and the “one-bath” processes subsequently to be

described.

It does not fall within the scope of this book to describe in detail the

working methods for the production of the different kinds of chrome

leather, but a few precautions common to all forms of the process may be

named. It is not absolutely important in all cases that goods should be

completely freed from lime before chrome-tannage, but in this case a

sufficiency of acid must be allowed in the first bath to neutralise the

lime introduced. Pretty thorough liming is generally advisable, to plump

and separate the fibres, but as a rule the bating or puering of goods

for chroming should not be excessive,[118] but should be planned not to

remove more than is absolutely necessary of the hide-substance, as the

chrome tannage is in its nature soft and light, and does not lend itself

to artificial fillings, such as the flour and egg-yolk of the calf-kid

process. Skins are sometimes freed from lime by “pickling” (p. 89), and

pickled skins may be chromed without depickling, which will be done by

the dichromate, but in this case the acid contained in the skins must be

considered in the composition of the chroming bath. Skins, indeed, which

are pickled with a sufficiency of acid may be chromed in a neutral

dichromate bath, and this is sometimes a convenient mode of procedure.

To prevent drawing of the grain during tanning, skins not unfrequently

receive a preliminary tannage with alum, or sulphate of alumina, and

these materials, together with salt, may be introduced into the chroming

bath, in which case they will liberate a portion of the chromic acid, as

has been mentioned in connection with the Heinzerling process. Alum,

chrome-alum, and acid salts, such as sodium bisulphate, may be

substituted for the acid in the chrome bath, but organic acids must not

be used, as they would reduce the chromic acid. The quantity of free

chromic acid in the chrome bath is of the most vital importance to

success, as it, and not the dichromate (which may be present in

considerable excess), regulates the amount of chrome taken up by the

skin, and the subsequent degree of tannage. It is very possible to

injure leather by overchroming, rendering it rough, harsh and even

tender. If a bath containing excess of bichromate is to be

re-strengthened, it may be assumed as a rule that all the free chromic

acid has been absorbed by the skins, and while it is merely necessary to

restore the strength of the dichromate to its original amount, the full

quantity of acid must be used which would be required in preparing a new

bath. Where, as in Eitner’s acid chrome bath, the whole of the chromic

acid is liberated, the bath may be exhausted by a second pack of skins.

Many tanners, in order to avoid the complications of remaking a bath,

run away their chrome liquors after once using, containing all the

excess of dichromate which has been used. With proper chemical control,

this should not be necessary, and is objectionable, not only from its

wastefulness, but on account of the very poisonous character of the

unreduced bichromate. Even weak dichromate solutions, especially if

warm, are liable to cause painful and obstinate eruptions on the hands,

but this rarely occurs to tanners, as the poisonous action of the

solution is removed on reduction. It is well, however, to arrange that

men who handle skins in the chrome bath, should subsequently also work

in the reducing bath. Methods of analysis of used chrome liquors are

given, L.I.L.B., pp. 142 _et seq._ Those for the determination of

acidity are not however easily applicable in the presence of alum and

salts of chromic oxide.

[118] Goat-skins for glacé kid need thorough puering to produce a

smooth grain.

The skins, on coming from the chroming bath may be allowed to lie for

some time without serious injury, but should be carefully protected from

the action of light, which reduces the chrome at the expense of the

skin, and renders the subsequent tannage irregular. It is found that

skins, if brought into a weak or neutral reducing bath, are apt to

“bleed” or lose chromic acid, which is reduced wastefully in the bath.

On the other hand a strong “hypo” bath is apt to draw the grain and

contract the skins, owing to the tannage taking place too suddenly. A

somewhat strong “hypo” bath is therefore often employed as a preparatory

“dip,” the skins being simply drawn through it to fix the chrome on the

surface, piled on a “horse” and subsequently reduced in a bath of

ordinary strength. The tendency to bleed is lessened, but at the expense

of the pelt, by the reduction which takes place if the skins are allowed

to lie overnight in the chromed state. Eitner states that skins chromed

in an acid bath (i.e. where the whole of the chromic acid is in a free

state) show little tendency to bleed. After reduction, the skins are

well washed with warm water, and their subsequent treatment is the same

as that of skins tanned by the one-bath process, which is subsequently

described (see p. 211).

Naturally in practical work, the reduction cannot be made to proceed

rigidly in the definite steps described by Eitner on p. 206, but all go

on in different proportions together, though by supplying the acid in

proper quantities, and at proper intervals, they may be made in the main

to follow in the given order. Both on this account, and because neither

the exact amount of chromic acid in the skins, nor the sulphurous acid

lost by escape into the air can be exactly determined, the reduction

cannot be conducted on theoretical principles, but the best conditions

must be empirically determined. Eitner states that 12 parts of

thiosulphate dissolved in 400 parts of water, and 6 parts of (40 per

cent.) hydrochloric acid are sufficient for 4 parts of bichromate per

100 of wet pelt employed in the chrome bath, of which not more than

one-half to two-thirds is absorbed; and that if equal parts of

bichromate and acid are employed in chroming, the acid used in reducing

may be lessened to 5 parts. In this case it must not be forgotten, that

if the partially exhausted chrome-bath is used for a second pack of

skins, which are afterwards finished in a bath of full strength, nearly

the whole quantity of bichromate used in making up one bath will be

absorbed by the skins. The amount of acid consumed in reduction will be

greater, the more rapidly it is added, owing to increased escape of

sulphurous acid. It is better to add the acid, previously diluted with

water, in 8 or 10 successive portions, more rapidly at first, and more

slowly during the latter half of the operation, each portion of acid

being added as soon as no further change of colour appears to be caused

by that already given. These changes are the more rapid the lighter the

goods. The colour darkens at first to olive-brown, then gradually

becomes green, and finally blue, and when this colour is uniform

throughout the thickness of the goods, no further acid need be added.

For goods which have been chromed in an acid bath, Eitner states that no

acid will be needed for the first twenty to thirty minutes. It is

important to have a sufficient excess of thiosulphate in the bath when

reduction is complete, in which case the goods may be left for some

hours or overnight in the bath, to complete “neutralisation,” but Eitner

prefers to use a fresh bath of 1¹⁄₂ parts of thiosulphate in 400 parts

of water for this purpose, the bath being used, after settling, for

making up the reduction bath for the next lot of goods, for which 1¹⁄₂

parts less thiosulphate is used. The goods must be kept in motion during

reduction, either in a drum or a covered paddle.

In a paper on “_Die Natur und Wesen der Gerberei_” published by

Professor Knapp, in 1858, he describes clearly a chrome tanning process

with basic chromic chloride, formed by the addition of sodium carbonate

to a solution of the normal salt, but he expressly states that the

product was not more resistant to water than the ordinary alum tannages.

How he fell into this error is hard to explain, for leathers produced

according to his directions, resist not merely washing in cold but

boiling water. As soon as the Schultz process proved successful, many

attempts were made to evade the patent by the use of other reducing

agents than the “hypo,” and other salts of sulphurous acid which it

covered. Among these, the use of hydrogen sulphide, and acidified

solutions of alkaline sulphides, and especially of polysulphides,[119]

proved capable of practical use, though less convenient than

thiosulphate, but were soon acquired by a combination, the Patent

Tanning Company, together with Schultz’s original patents.

[119] “Liver of sulphur” or solutions, made by boiling sodium sulphide

or soda with excess of sulphur.

Under these circumstances, Martin Dennis, either by fresh discovery, or

otherwise, revived the original process of Knapp, which he patented[120]

almost word for word, and offered a basic chrome tanning liquor for

sale, without further restrictions on its use. This liquor was made by

dissolving precipitated and washed chromic hydrate (easily prepared by

precipitating chrome-alum solution with excess of alkali) in

hydrochloric acid to saturation, and adding washing soda until the

solution was rendered sufficiently basic. Such a solution may be used on

skins prepared in the ordinary way, by diluting with water, and

strengthening as the tannage proceeds, like a vegetable tan-liquor. It

is doubtful if the patent is a valid one, as it was known that the use

of such a solution was not new, and it was only granted in America on

the representation, which has since been found to be mistaken, that

chlorides alone were applicable for tanning, while Knapp had not

restricted his statement to these salts. In reality chlorides and

sulphates seem equally suitable, but to produce similar results the

former must be made more basic than the latter. In any case the patent

cannot cover the general principle of basic tanning, but only the

particular liquor and mode of preparation specified. It was soon

afterwards shown by the writer,[121] that a good chrome tanning liquor

might be prepared by direct reduction of dichromate with sugar in

presence of such a limited quantity of hydrochloric acid as to produce a

basic salt. Suitable proportions are 5 mol. HCl to 1 mol. potassium

dichromate, which produces a salt approximately Cr₂Cl₃(OH)₃. The

solution is easily made by dissolving three parts of dichromate in a

convenient quantity of water, adding six parts by weight of concentrated

hydrochloric acid, and then cane-sugar gradually till a green solution

is obtained, when the whole may be made up to one hundred parts, and

will be approximately of the same strength as a 10 per cent. solution

of chrome-alum. A little heat may be needed to start the reaction, but

too much should be avoided, as considerable heat is evolved by the

oxidation; and as much carbonic anhydride is produced, which causes the

solution to effervesce briskly, the vessel used should be of ample size.

In place of cane-sugar, a good quality of glucose may be used, but some

samples contain some impurity which produces a violet solution which

will not tan satisfactorily. This liquor is in regular use in many

tanneries, producing a good quality of chrome calf, but is somewhat

variable in its effects according to the temperature employed in its

preparation, and it appears to have no real advantage over a simple

solution of chrome-alum, rendered basic by soda and with some addition

of salt. A somewhat similar preparation is Eberle’s “chromalin,”[122] in

which some organic substance, probably crude glycerine, is used to

reduce the bichromate. The organic matters, and especially the organic

acids which result from the oxidation of the sugar or glycerine, are not

without influence on the tanning properties of the liquor. Of course

these solutions may be rendered still more basic by the addition of

sodium carbonate. A good stock-liquor, of approximately the same

strength as that above described, is made by dissolving 10 parts of

chrome-alum in 80 parts of tepid, but not hot water,[123] and adding

with constant stirring a solution of 2¹⁄₂ to 3¹⁄₂ parts of washing soda

in 10 parts of water. The chrome alum dissolves somewhat slowly without

the aid of heat, and the solution is best made either in a small drum

driven by power, or by suspending the crystals in a basket near the

surface of the liquor, so that the saturated solution can descend.

[120] Martin Dennis, U.S.A. Pat. 495028, 1893; and 511411, 1893, 7732,

1893. E. Pat. Gallagher.

[121] Leather Trades Review, Jan. 12, 1897.

[122] Compare Eberle’s German patents, 119042, 1898, and 130678, 1899.

The last of these appears to be anticipated, at least as regards the

use of glucose, sugar and starch, by the writer’s publication in 1897

above cited.

[123] Leather Trades Review. Later investigations have shown that the

temperature of the water is unimportant if alkali be added, but

chrome-alum dissociates to some extent in hot water, and comparative

experiments have shown that solutions of the normal salt made with the

aid of heat act on skin as if more acid than those made in the cold.

Eitner[124] has pointed out the important effect that differences of

basicity have on the tanning properties of chrome solutions. Normal

chrome sulphate or chrome-alum colours the leather quickly and equally

throughout, and swells the pelt on account of its practically acid

character, but gives a thin and lightly tanned leather, from which much

of the chrome washes out, unless it is at once “neutralised” in alkaline

solutions. As the chrome solution is made more basic, the tannage

penetrates more slowly, but is heavier and more thorough, the colour is

darker and bluer, and much less of the chromic salt is removed by

washing with water. When the basicity becomes excessive, the solution

becomes unstable, and decomposes on dilution with water into a very

basic salt which is precipitated, and a more acid solution than that

given by a moderately basic salt. The effect of such solutions on the

leather is very unsatisfactory, producing the bad effects both of too

acid and too basic salts. The pelt is apt to be swollen and lightly

coloured by the more acid salt, but at the same time the actual tannage

proceeds very slowly, and in extreme cases it is difficult to tan

through, while the surface becomes over-tanned, and the grain often

tender and even brittle from the incrustation of precipitated basic

salt. Eitner likens the effect of the more acid liquors to the quickly

penetrating and lightly tanning vegetable tans, such as gambier, and

that of the more basic to the heavier tannages, such as valonia; and

within limits, advantage may be taken of these facts in adjusting the

liquors to the character of the leather it is desired to produce. In

sulphate liquors, he considers the salt CrOH.SO₄ as most suited to

general use, and in the case of chrome-alum, this is produced by the use

of 286 parts of soda-crystals, or 106 parts of dry sodium carbonate (1

molecule) to 998 (or practically 1000) parts by weight (1 mol.) of

chrome-alum. (In using washing soda, care must be taken to employ

_clear_ crystals of the salt, and not those which have become white by

loss of water.) In place of soda, Eitner makes a similar basic liquor by

boiling 1000 parts of chrome-alum with 248 parts (1 mol.) of sodium

hyposulphite until the whole of the liberated sulphurous acid is driven

off, and the sulphur deposited. In comparative experiments by the

Author, no difference could be detected between the tanning effects of

the two solutions, and that with soda is both cheaper and more easily

made. If the solution with hyposulphite is not boiled, a more acid

liquor results, in which part of the chromium is combined with

sulphurous acid, forming an unstable compound which may prove useful in

certain cases.

[124] Gerber, 1901, pp. 3 _et seq._

Eitner states that he has made chrome-solutions of various types,

containing organic compounds in combination with the chrome-salt, which

combine with the leather, producing a fuller and softer tannage, but he

gives no details as to their preparation, as they are made commercially

by the “Erste Oesterreichische Soda-Fabrik” at Hruschau. The writer has

found that in some cases by the addition of say 3 parts of sugar, or

still better of glucose, to 10 parts of the chrome-alum in making up the

basic liquor, a much fuller and plumper leather is produced, which dries

perfectly soft, even without staking or fat-liquoring; and it is

probable that many other organic compounds may be found which produce

similar effects. The addition of very small quantities of even neutral

tartrates or lactates, and probably of many other organic salts or

acids, have a remarkable effect in lowering the apparent basicity of the

solution, and it is possible that these may also be usefully employed in

combination with very basic liquors. It is highly probable that the

unsatisfactory tanning liquors produced by direct reduction with some

samples of glucose are due to the presence of small quantities of some

organic acid produced during the oxidation. It has been found that these

solutions may be made to tan by the liberal addition of soda. It is

probable that more satisfactory results in chrome-tanning will be

attained by the direct addition of known organic substances to basic

liquors of definite constitution, than by the somewhat uncertain

products of organic oxidations.

The quantity of salt to be added depends on the qualities desired in the

leather, and upon whether chloride or sulphate liquors are employed;

salt in chloride-liquors increasing the softness of the leather, but in

excess tending to flatness, while in sulphate-liquors it practically

diminishes their basicity by converting the chromium sulphate into the

equivalent chloride, which, as Eitner points out, behaves as a less

basic salt, and hence but little advantage is to be gained from its use.

It is best to begin with a very weak liquor, to avoid drawn grain, and

for the same purpose a preparatory tannage with alumina salts, or an

addition of alum or sulphate of alumina and salt may be made to the

first liquor, as the attraction of the chrome salt for the fibre is

sufficient to produce a chrome tannage, even in presence of excess of

alumina salts. 10 lb. of chrome alum will tan about 100 lb. of wet pelt,

but more must be used for the first parcel; as to avoid loss of time,

the skins may be tanned out in a pretty strong liquor. The bath has a

tendency to become acid by use, and before strengthening, it may be

necessary to add some more soda solution. Very little additional salt is

required, as it is only absorbed by the skins to a small extent,

probably as chromic chloride. As the liquors gradually become charged

with sulphates, it is best to work them out like bark liquors, and not

to go on strengthening the same liquor indefinitely. If old liquors are

used for green goods, it is not necessary to neutralise them with soda

before use, as Eitner has shown that less basic liquors colour more

evenly and with less tendency to produce drawn grain.

Basic chrome liquors, such as have been described, may also be used in

chrome combination tannage. It is generally best to let the light

vegetable tannage precede the chrome, and lightly tanned skins, such as

“Persians” and East India kips, acquire many of the qualities of

chrome-tanned leather by the treatment. The effect is still further

increased by a previous detannisation of the leather with alkaline

solutions (see p. 241). Several firms beside Dennis now supply basic

chrome liquors ready prepared for use.

The time of tannage will of course vary with the thickness of the goods,

and for calf-skins will usually extend over some days, though it can be

much quickened by drumming. The tannage is generally best accomplished

in the paddle, but can be carried out by frequent handling in pits or

tubs, or, where very smooth grain is important, by suspension. When the

goods come out of the final liquor, they may be allowed to lie in pile

for twenty-four hours, or even for some days, with advantage, as the

surplus chrome liquor is pressed out, and the tannage becomes more

complete. They are then washed with plenty of warm water, till it ceases

to be coloured with chrome. They may be kept for an almost unlimited

time in a wet condition, as they do not bleed, and have little tendency

to heat even in pile. They have now reached the stage at which we left

the “two-bath” leather, and the subsequent treatment may be the same in

both cases.

Although by both processes, the chrome-salt fixed in the fibre is of a

decidedly basic character, it still contains enough acid to act

injuriously on the leather in course of time, and to lead to serious

inconveniences in its subsequent treatment. Before proceeding further,

this access of acid must be removed or neutralised, and it is not too

much to say that most of the troubles experienced in the fat-liquoring

arise from neglect or mistake in the washing and neutralisation. The

difficulty in the process arises from the fact that while the acid

should be reduced to a mere trace, it must not be entirely removed,[125]

as chromic oxide itself does not seem capable of tanning, and at any

rate the effect of excess of strong alkalies is at once to render the

leather hard and pelty. Borax is one of the safest neutralising

materials, about 3 per cent. on the wet weight of the pelt being

required, in not more than ¹⁄₂ per cent. solution. Eitner recommends the

use of silicate of soda, which, sold as a solution of S.G. 1·5, is

somewhat stronger and much cheaper than borax. Hyposulphite of soda and

whitening together neutralise more rapidly and completely than either

alone. Other salts of weak acids may also be used, the acids exercising

a regulating influence which prevents neutralisation going too far.

Sodium carbonate or bicarbonate, or ammonia may also be used, but with

these it is difficult to get even “neutralisation,” or to avoid the risk

of carrying the process too far. Even a thorough drumming with a milk of

“whitening” (calcium carbonate) is effective. With the latter there is

no danger of overdoing the process, but in some cases the adhering

whitening and precipitated calcium sulphate are troublesome in later

operations. In any case the neutralising should only be carried so far

that the skins show no acid reaction to litmus paper.

[125] Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223.

It is probable that one of the great causes of difference between

“one-bath” and “two-bath” leathers is the presence of free sulphur in

the latter. This may also be introduced into “one-bath” leather, by

treating it in the wet chromed state, without washing out the chrome

liquor, with excess of a solution of hyposulphite, or of an alkaline

polysulphide, which at the same time will neutralise the skin. The more

acid the chrome liquor, the greater the quantity of sulphur which will

be introduced. The simplest means of distinguishing “two-bath” from

“one-bath” tannages is to test for the presence of sulphur, by wrapping

up a silver coin, with a piece of the leather in paper, and leaving the

parcel for an hour in the water-oven, or some other warm place, when

the presence of sulphur will be shown by the blackening of the coin. Of

course a sulphurised “one-bath” leather will give the same reaction.

The leather must now be dyed and fat-liquored. Which of these two

operations should be first undertaken will depend on circumstances. Most

leathers dye more easily before fat-liquoring, but as many dyes are

soluble in the alkaline fat-liquor, a good deal of colour is often lost.

This may be compensated by dissolving a suitable aniline (acid) colour

in the fat-liquor. “Bluebacking” is generally done before fat-liquoring

by drumming with methyl-violet, or some other aniline colour (with or

without logwood, which gives alone a very dark violet). Any shaving or

splitting required must of course be done before bluebacking.

The fat-liquor is an emulsion of soap and oil, which for chrome leather

should be as neutral as possible, if the neutralising has been thorough;

but if any acid be left on the skins, a neutral fat-liquor will be

precipitated as a greasy mass. This can sometimes be remedied by the

addition of a little ammonia or borax, or by re-fat-liquoring with soap

solution only, but if the washing of the skins has been incomplete, and

soluble chrome-salts remain, the mischief is almost irretrievable, as

sticky chrome-soaps are formed, often coloured with the aniline violet,

which adhere to the skins, and which can scarcely be removed by any

solvent which does not injure the leather. As regards the soaps and oils

used, there is considerable latitude: 1¹⁄₂ per cent. of castor-oil soap,

and ³⁄₄ per cent. of castor or olive oil on the wet weight of the pelt

has done good service in my hands, but many manufacturers employ soft

soaps, curd soaps, etc., with castor, olive, cod or neatsfoot oil, and

sometimes sod-oil or degras. Eitner considers olive-oil and olive-oil

potash soap the most suitable, and particularly warns against the use

either of drying oils or of oils containing tallow (such as neatsfoot),

which are not only apt to cause a white efflorescence, but to give the

leather a disagreeable rancid smell. Fish-oils are unsuitable, but

mineral oils are often useful constituents of fat-liquors. Wool-fat also

makes a good fat-liquor, but is unsuitable for goods which are to be

glazed. “Turkey-red oil” (which is sulphated castor) may be used as a

fat-liquor, simply mixed with warm water, without soap, and has been

recommended where delicate colours are to be dyed after fat-liquoring;

but it is said to have an unsatisfactory after-effect, hardening and

tendering the leather. Some soaps made from the saponifiable part of

wool grease, such as “Lanosoap,” also act well in conjunction with

olive, castor, or other oils. Where leather is to be glazed, the amount

of fat-liquoring must be kept very moderate. Fat-liquors should be

thoroughly emulsified, and are generally used warm. They penetrate

better if the leather is partially dried by sleeking out, or pressing,

or cautious “samming,” but the leather must not be completely dried out

before fat-liquoring and dyeing, unless it has been previously treated

with glycerine, glucose, treacle or some deliquescent salt, which will

enable it to be wet back. Chrome leathers are not “waterproof,” as has

often been stated, unless rendered so by treatment with soaps and

greases, and are apparently easily wetted, but the fibre will no longer

absorb water after thorough drying, and consequently will neither dye

nor stuff satisfactorily. In order to enable chrome leather to be kept

in an undyed condition, glycerine or syrup is sometimes mixed with the

fat-liquor, but as the watery portion of this is not generally

completely absorbed, the process is somewhat wasteful. Mr. M. C. Lamb

avoids this difficulty by applying a solution of glycerine to the

grain-side with a sponge after fat-liquoring. In this case the leather

may be dried sufficiently for staking or shaving without risk.

Chrome leather can be dyed by many of the acid aniline colours without a

mordant. Basic colours are only fixed when the leather has been first

prepared with a vegetable tannin, gambier, or a mixture of gambier and

sumach being the most suitable. Considerable care must be employed in

the application of tannins to chrome leather, as they have a tendency to

harden it and diminish its stretch, or even to render it tender, but

traces of tannin in the dye probably facilitate glazing. Before dyeing,

it is advantageous to fix the tannin with tartar emetic, or for browns

and yellows, with titanium potassium oxalate solution, which itself

gives a good yellow-brown with tannin. In place of employing the tannin

and titanium salt in two separate baths, they may be combined, using a

weight of the gambier or tanning extract (oakwood, chestnut, etc.) about

equal to that of the titanium salt, or titanium tanno-oxalate solution

may be used. Chrome leather may be dyed with the various dye-woods,

which are mordanted by the chromium present, but the colours are mostly

dull, that of logwood being nearly black. A good black of a very

permanent character is obtained by dyeing with logwood, and saddening

with a hot solution of titanium oxalate in the drum. A little iron-alum

added to the chrome liquor in tanning will facilitate dyeing the skins

black with logwood and help it to penetrate through the leather, which

is sometimes desired. Several aniline blacks, and notably the

“corvolines” of the Badische Anilin und Soda Fabrik, Casella’s “leather

black C,” and Claus & Rée’s chrome-black give very satisfactory blacks

by brushing or dyeing.

Chrome skins may be glazed in the ordinary way with blood or albumen

mixtures under glass or agate, but require good pressure and repeated

seasonings and glazings, and much care is required in fat-liquoring. The

glazing is often assisted by the previous application of barberry juice

(épine vinette) or of lactic or tartaric acid solution with a trace of

sugar. Much of the difficulty which has been experienced in glazing

chrome leathers is due either to the natural fat of the skin, or to oils

used in fat-liquoring in excessive quantity or of unsuitable character.

CHAPTER XVI.

_PRINCIPLES OF THE VEGETABLE TANNING PROCESSES._

The processes employed in the production of leather with the vegetable

tanning materials vary extremely according to the class of leather which

is being produced, both in the materials selected and in the time

required. In sole-leather tanning, where thick hides are used, and where

diffusion is the only force acting to carry the tannin into the hide,

many months are frequently needed, while with thin skins, and with the

aid of mechanical motion, which circulates the tanning liquid between

the fibres, the process is often complete within a few hours.

Differences in the strength of the liquors according to whether hard or

soft leathers are to be produced, and the mutual action of the acids

naturally present in the liquors, and of the tan, have also a

determining effect upon the quality of the product.

The simplest form of tanning in principle is probably the old-fashioned

method of sole-leather manufacture. For this purpose, the hides are

usually “rounded” or trimmed after liming, unhairing and fleshing, so

that the most valuable part, the “butt,” can be tanned separately from

the “offal.” The butts are usually washed in water to remove a portion

of the lime, considerable care being required at this stage to avoid

carbonation and fixation of chalk by means of free carbonic acid, or

hydric calcium carbonate (temporary hardness) in the water employed, or

by the free carbonic acid of the air. This somewhat primitive process

can at best only remove a small portion of the lime, since so long as

the lime remains in the caustic condition, it is very obstinately held

by the hide-fibre. Advanced tanners now frequently employ weakly acid

baths, in addition to washing, in order to produce more complete

deliming, and this effects a very considerable improvement of colour in

the early liquors. The use of lactic acid (free from iron) or boric

(boracic) acid in solutions of about 4 lb. per 100 gallons, in which the

butts are kept in motion, are among the safest and most satisfactory

ways of removing surface-lime and improving the colour, but even the

stronger mineral acids may be used successfully with caution (see Chap.

XIII.).

Whether acid be used or not, the butts are now usually suspended in deep

pits containing old and nearly exhausted tan liquors. These liquors

contain a certain amount of lactic and acetic acids, derived by

fermentation from the sugary matters of the tanning materials, and also

in some cases, weak acids originally present in the materials

themselves. These acids are most important to successful tannage, and

their effect is twofold; in the first place, they neutralise and remove

any lime which still remains in the butts; and, secondly, they bring the

butt into a slightly acid condition, in which it remains plump and

swollen in the liquors, while the tannin gradually penetrates and tans

the fibre. If, as frequently happens, especially in modern yards where

extracts are very largely used, the natural acid of the liquors is not

sufficient for this purpose, the lime combines with the tanning matters,

and the butts either become discoloured at once, or darken by exposure

and oxidation, when they come to be dried, while the pelt remains flat

and insufficiently swollen. To avoid this trouble, resort is sometimes

had to artificial acidification of the liquors. As a general rule, it

may be stated that it does not answer to mix the stronger mineral acids

directly with the liquors, but lactic and acetic acids may be used, or

even oxalic acid may be added to the suspenders in such quantities as to

precipitate and remove the lime which they contain, setting free the

organic acids with which it had been combined. The use of oxalic acid

should never be pushed further than this, as it has a most powerful

swelling action on the hide; and goods which are too much swollen by

acids tan dark and brittle.

After the hides have remained from ten days to a fortnight in the

suspenders, they are usually laid in pits called “handlers” which are

worked in series of 6, 8, or 10 pits, containing the same number of

packs of goods. The weakest liquor from the youngest pack is run to the

suspenders daily, a new and stronger liquor is run to the pit, which now

becomes the head of the series, into which the oldest and most tanned

pack of butts is moved; and the next takes its place and liquor, and so

on down the series, the youngest pack finally occupying the place which

had previously been taken by the last but one. In this way each pack

receives a change of liquor of regularly graduated strength; and during

the time which it remains in the handlers, passes from a strength of

perhaps 20° Bkr. (sp. gr. 1·020) to one of about 40° Bkr. (sp. gr.

1·040). During this part of the process the butt is completely or nearly

coloured through, and is then ready for the “layers.”

In the forward handlers, dustings of ground bark or other tanning

material are very frequently given, and the layers only differ from

these in having much heavier dusting, stronger liquors, and being

allowed to remain undisturbed for greater lengths of time, ranging from

a week up to a month or even six weeks, as the tannage progresses. The

handler-liquors are principally from the old layers, though they are

frequently made up with weak liquors from the leaches, and strengthened

with extracts or gambier.

Very varied materials are used in the manufacture of sole-leather.

Oak-bark is one of the oldest, and as regards quality one of the most

satisfactory, but it is costly, not only on account of its weakness in

tannin but from the light weight of leather which it gives. Valonia is

one of the favourite materials, giving heavy weight and a solid leather,

in which it deposits a great deal of bloom. Oakwood, chestnut-wood, and

hemlock-bark extracts are now very largely consumed, principally in

strengthening the layer-liquors; the great object being not only to

lessen the cost in material, but to save time, and produce greater

weight and firmness. The layer-liquors in some yards where extract is

used, reach strengths of even 120° to 150° Bkr. (sp. gr. 1·12 to 1·15),

while in pure oak-bark yards it is difficult to get above 30° or 35°

Bkr.; and even these figures are only reached by repeatedly

strengthening the same liquor, in which large quantities of non-tanning

substances accumulate. The opinion of the most intelligent tanners is,

however, that better results are attained by a regular change of liquor,

even if the apparent strength is less.

When the leather has remained a sufficient time in the layers to have

attained all the weight and solidity of which it is capable, it is

washed up in a clear and somewhat weaker liquor or even in warm water,

and taken into the shed to be dried and finished. As this finishing is

almost purely mechanical, and scarcely comes within the scope of the

present volume, a very brief sketch must suffice.

[Illustration: FIG. 39.--Wilson’s Striking Machine.]

The mode of finishing which was formerly, at least, in vogue in

Lancashire and Cheshire may be taken as a type of the best work. (In the

present day, the various methods are so widely known that they have

ceased to be local, and are varied according to the quality and tannage

of the goods.) The butts, which in earlier times were largely

bark-tanned, are taken wet from the pits, and scoured on a rounded beam

or “horse” with stone and brush, till the bloom is completely removed,

and are then lightly oiled on the grain, half dried (“sammed”), laid in

pile to temper, and “struck out” with the “pin,” a two-handled tool of

triangular section shown in Fig. 29. The use of this tool has now been

largely superseded by Wilson’s striking machine Fig. 39, in which knives

or sleekers (or stones and brushes), held in jointed arms, are made to

work on the butt, which is extended over a slowly rotating cylinder. The

object of the pinning is not so much to remove bloom or dirt, which has

been previously effected by the scouring, as to smooth and flatten the

grain. After further drying, a second pinning is generally given, and

the goods are then twice rolled, first with a light weight, and somewhat

moist grain, and then more heavily with the grain nearly dry. This was

formerly accomplished by a sort of box or car, heavily loaded with

weights, supported on a smooth brass roller of about 5 inches diameter

and 9 inches long, and manipulated with a long wooden handle on a floor

of hard wood, or zinc plates. One type of the machines which have now

almost entirely replaced this primitive contrivance is shown in Fig. 40,

but is principally used for offal and common classes of goods. For

better work, traversing rollers, such as Wilson’s ingenious double bed

roller shown in Fig. 41, are to be preferred. After rolling, the goods

are dried pretty rapidly by the aid of moderate heat, and, after

polishing with a brush (hand, or machine, Fig. 42), are ready for sale.

It may be pointed out that although the tools are different, the process

is almost the same as that used for “_vache lissée_” in France and

Belgium, and closely resembles that of currying harness leather except

that the “stuffing” with fats and oil is omitted.

[Illustration: FIG. 40.--Offal Roller.]

In contrast with the rather elaborate method just described, we may

place the American finish of red hemlock sides, which are tanned

throughout with a material which yields no bloom. On these, the

scouring and “striking” is altogether omitted: the goods are completely

dried out from the pits, which is found to fix the dark-coloured liquor,

and result in better colour; they are then damped back, and tempered,

and heavily rolled under a rapidly moving pendulum roller, which

polishes at the same time that it smooths the leather. The saving of

cost by so simple a process is not inconsiderable.

[Illustration: FIG. 41.--Wilson’s Double-bed Butt Roller.]

In the West of England, much heavy leather is still manufactured from

South American hides, which are tanned with a large proportion of

valonia; and which consequently are heavily bloomed. No attempt is made

to remove this bloom, which would too much lessen the weight and

firmness, but the goods, after a light oiling to preserve the colour,

are hung up and partially dried, and are then laid in pile to temper.

The grain side is now wet with soap and water, with which a little oil

is often mixed, and the bloom is “struck in” with the pin or machine; a

somewhat blunt pin being used, or a blunt tool in the striking machine;

which is held at such an angle as to smooth and compress the grain

without taking too much hold on it. After a little further drying, the

striking is generally repeated, the goods are washed over with water,

and rolled “on.” They are now coloured with a mixture of pigment

colour, generally containing a large proportion of whitening, or

sometimes of French chalk coloured with ochres, chrome-yellow and

orange, or whatever may suit the tint preferred by the tanner, or best

imitate the colour of a clean-scoured tannage, and usually mixed with

size and oil, or sometimes with oil and tan liquor. This mixture is well

rubbed in, and smoothed over with a cloth, and then polished by

brushing, when the goods are “rolled off,” rapidly dried, and again

brushed. If the work has been well done, it is not easy to distinguish

from clean scouring, and is much cheaper.

[Illustration: FIG. 42.--Brushing Machine.]

A method intermediate between this and the first described, and which

was formerly much used in London, was to proceed as above, but using

more water and holding the pin in the first striking so as to scour out

as much bloom as possible, and assisting this by the free use of water

and the brush. Instead of using an opaque pigment-colour, the goods were

generally coloured either between striking and the first rolling, or

between the two rollings, with a transparent colour, such as dissolved

annatto, or a mixture of aniline dyes, so as to conceal the traces of

bloom, and to render slight damages to the grain less conspicuous.

The principles of the manufacture have been fully explained in previous

sections, up to the time when the goods are taken into the tanning

liquors. At this stage complicated reactions take place between the lime

in the butts, the free vegetable acids in the liquors, and the tannins;

and on the right adjustment of these three factors much of the success

of the operation, and indeed of the whole manufacture depends. If the

lime is in excess of the acids present, it forms insoluble compounds

with the tannins in the surface of the hide. If these are protected from

the air, they are generally redissolved as they advance into more acid

liquors, but they readily become oxidised into dark-coloured matters,

which can no longer be removed. Their presence in the finished leather

is one of the great causes of darkening in drying. If the hide in the

limy condition has been exposed either to the carbonic acid of the air,

or to free carbonic acid, or acid calcium carbonate dissolved in water

(“temporary” hardness, p. 94), a precipitate of calcium carbonate will

be formed in the surface, which is much more difficult to remove than

free lime, and which is perhaps the most common cause of the stains and

discolorations which are so serious a source of loss to the sole-leather

tanner. These stains may, if not too much oxidised, be removed by

treatment of the tanned leather with weak warm sulphuric acid, but this

remedy brings other evils in its train, and should not be required. The

great remedy is to keep the goods from the time of unhairing till they

go into the liquors, under water in which there is always a trace of

caustic lime, or which at any rate are free from carbonic acid. In

deliming sole-leather with acids, it is best to give the full dose of

acid required, at once, and not gradually, so that it may act most

powerfully on the exterior, and remove any carbonates present, before it

penetrates to and becomes neutralised by the excess of lime in the

interior. This is exactly the reverse of what is advisable with dressing

leather, where the object of the tanner is to remove lime as uniformly

and completely as possible, without excessive acidity of any part. Of

course hides should not, even in the case of sole-leather, be allowed to

go into the liquors while any acid swelling of the surface remains, but

this will soon disappear if the goods are suspended for a time in cold

water after deliming, unless excess of acid has been used (cp. p. 153

_et seq._).

If the proportion of free acid in the suspender liquors is as it ought

to be, it is probably rather advantageous than otherwise for a little

lime to remain in the interior of the hide, as it keeps the pelt in a

plump condition during the first stages of colouring, quickens the

penetration of the tannin, and lessens the tendency to “drawn” or

wrinkled grain, which arises when the goods go into the liquors in a

flat or fallen condition. The causes of drawn grain are often a little

obscure. Of course that case needs no elucidation in which the hides are

submitted to the tanning liquor in a creased or wrinkled condition,

which is simply fixed and made permanent. This may arise, either from

carelessness in handling the goods before taking into the suspenders, or

from the way in which they are slung to the sticks, which often draws

them into long wrinkles, afterwards difficult to remove. Drawn grain in

general, however, arises from the grain-surface becoming tanned and

fixed in area, while the substance of the hide is in a more extended

condition than that which it assumes as tannage proceeds. Hides in a

flat and unswollen condition are thinner, the fibres are slenderer and

looser than when swollen, and consequently the hide has a larger area.

If, after the grain is tanned, the substance of the hide becomes

contracted in the liquor, either by swelling with acids, or by the

direct action of the tannin on the interior fibres, the grain is certain

to be shrivelled, like the skin of a dried apple. A similar effect,

produced in a mechanical way, may always be noted where a hide has been

coloured hanging grain-side out over a pole, so that the surface is

extended at the bend, on which long wrinkles are formed as soon as it is

straightened.

A hide in a slightly alkaline condition colours, and even tans more

quickly than one which is acid. In presence of a trace of lime, and

deficiency of free acid in the suspender liquor, tannages of valonia and

bark give the butt a sort of lemon-yellow colour, which is not in itself

injurious, and which disappears as the hides advance into more acid

liquors, but which is a sign of danger, as showing that no excess of

acid exists in the suspender-liquors. Gambier gives pelt perfectly free

from lime a pale buff colour, but where lime is present, the colour is

always reddish, and much darker, and this coloration does not disappear

so readily as that with valonia, so that if gambier is to be used in the

first liquors, care should be taken to remove all lime from the surface.

The only known tannin which gives no insoluble compound with lime is

that of the babool pod (sometimes called “gambia-pod”), which is

frequently used in India as a bate, and which would probably prove very

useful in colouring liquors (pp. 165, 288).

When sole-leather first goes into liquors, it is generally swollen with

lime to some extent. If the liquors contain, as they usually do,

sufficient free acid (acetic, lactic) in addition to the tannins, these

combine with and neutralise the lime, and the pelt, without absolutely

becoming flat and thin, loses its firmness, and becomes soft and spongy.

This is a favourable condition for the absorption of tannin, but care

should be taken not to allow the pelt to be squeezed or pressed, or

water will be squeezed out, and the pelt will not easily resume its

plumpness. As the tannage proceeds, both the tannin and the acid of the

liquors penetrate deeper into the pelt, the former tending to contract

and the latter to swell the fibres. Thus a given quantity of acid will

cause the greater swelling, the less tannin is present; and therefore in

strong tanning liquors more acid is required. The presence of certain

products of bacterial putrefaction has a great but unexplained effect in

preventing hide from swelling with acids; and in hot weather, much

better swelling is obtained by sterilising and deliming the hides with

one of the coal-tar products mentioned on pp. 30, 162. Boric acid may

also be satisfactorily used for this purpose, but should not be allowed

to get into sole-leather liquors, as it tends to produce a soft and

loose tannage, and from its inorganic and indestructible character, is

apt to accumulate in a yard in which it is used. The same reasons render

unadvisable its introduction into any liquors which are to be returned

to the leaches even in the tannage of dressing leather, though its

presence in the colouring liquors is otherwise very useful in lessening

the astringency of the tannins (“mellowing the liquors”), and making a

fine grain. Its mode of action is by no means clearly explained, but is

in some way connected with its tendency to produce “conjugated acids”

(L.I.L.B., pp. 37, 46).

The so-called “mellowness” of old liquors requires a word of comment. It

is well known to practical tanners, that old liquors are much less

liable to produce drawn grain, and a harsh surface, when used to colour

green goods, than liquors, even equally weak, which have been made from

fresh materials. This is probably due, in part at least, to more than

one cause. Most natural tanning materials contain tanning matters of

varied degrees of astringency and power of attaching themselves to the

leather-fibre. It is obvious that if a tanning liquor is used, the most

astringent and energetic tannins will be first removed from it, leaving

those of a milder character. It is also known that the presence of

neutral alkaline salts of weak acids has considerable influence in

producing mellowness; the addition, for instance, of sodium acetate has

a marked effect. This effect is probably due in the first place to the

action of neutral salts in diminishing the energy of weak acids (see p.

81), and secondly to the fact, that their bases combine to some extent

with the tannins; and that, as was perhaps first pointed out by the

writer, such tannins are, as it were, partially paralysed in their

action on hide (p. 339). Sodium sulphite acts powerfully in this way,

and may perhaps prove of technical value in temporarily diminishing the

astringency of liquors in quick tannage. Borax has a similar effect, but

is too alkaline, and, unless used with extreme caution, spoils the

colour of the liquors by causing oxidation. It is probable that similar

causes explain the mellowness of palmetto extract, which contains large

quantities of alkaline salts, and of some extracts which have been

treated with sulphites, when used undiluted in drum tannage. The

addition of free acid will generally restore these tannins to an active

condition.

As the tannage proceeds and penetrates further into the hide, the

liquors are used stronger, as the outside, once tanned, is to a large

extent protected from their action, and it is only by continuously

increasing the strength of the liquors that more tannin can diffuse into

the interior, since diffusion only takes place from a stronger into a

weaker liquor. The liquor in the interior of the butts is always

exhausted of tannin so long as any part of the hide-fibre remains

untanned, but as the layer of tanned fibre between this and the outside

gets thicker, a greater difference is required to maintain a reasonable

rate of exchange, just as a greater head of liquor is required to

maintain a flow of liquor through an increased number of

percolation-leaches. If the strength of the liquor outside be allowed to

fall off, this graduation of strength from the outside to the inside of

the butt is disturbed, and takes some time to re-establish. As the

liquors become stronger in tannin, they may also become somewhat

stronger in acid, since, as has been stated, the two act to some extent

in opposition to each other. The acid-swollen fibre absorbs the tannin

more slowly than if it were in more neutral condition, but it absorbs it

apparently in larger quantity, and at any rate, makes a firmer, solider,

and less flexible leather.

It has been mentioned that in the latter stages of the process, solid

tanning materials are generally strewed between the butts in the tanning

liquor. It may be pointed out that many materials vary in their tanning

effect, according to whether they are used in solid form or merely in

liquors. It has been shown by Youl and Griffith[126] that such materials

as valonia, oakwood and chestnut extracts, and myrobalans, which contain

both gallotannic and ellagitannic acids, lose strength rapidly when kept

in the form of liquor, the ellagitannic acid becoming decomposed with

separation of insoluble ellagic acid. Now it is just this ellagic acid,

which deposited _in_ or _on_ the leather, gives weight, solidity and

bloom, and the investigation points out not only an important source of

loss in the tanning industry, but also, why valonia, which in

sole-leather tannage is known to give hard and heavy leather, can be

used in large quantities on dressing leathers in Yorkshire, with

gambier, in the form of liquor, giving a soft and mellow leather almost

destitute of bloom. If weight and solidity are required from the use of

such materials, it is obvious that they must be brought into immediate

contact with the leather to be tanned, so that as large a part of the

bloom as possible is deposited in, and not outside the leather. With

many other materials, such as hemlock, quebracho, and mimosa, which

yield no bloom, but “difficultly soluble” tannins (reds or

phlobaphenes), the same rule holds, since in contact with the hides, the

small proportion of these materials which is soluble in the liquors, is

replaced from the materials as rapidly as it is absorbed by the leather,

while, when liquors or extracts only are used, the greater part of these

solidifying and weight-giving constituents remain unutilised in the

spent tanning materials. At the same time, the long “layers” afford an

opportunity for the acetic and lactic fermentations to go on which are

the principal source of the natural acidity of liquors. It must be

understood that what are called layers in England, are not to be

identified with the _Sätze_, but rather with the _Versenke_ of the

German tanner, the former being layers given in much the same manner as

was current in England 150 years ago; in which the leather, with thick

layers of tanning material between it, is laid into the empty pit, which

is afterwards filled up with liquor, often of a comparatively weak

character. In such layers, the acidification, and the solidification of

the leather both go on to a still greater degree; the acid formed,

apparently gradually penetrating to the heart of the leather-fibres, and

producing a solidity, and cheesy texture which can hardly be obtained by

layers of the English kind; which nevertheless have the advantage in

rapidity and cheapness.

[126] Journ. Soc. Chem. Ind., 1901, p. 428.

In drying sole-leather, one of the great objects which must be aimed at

is to remove the dark coloured liquor, with which the goods are

saturated, from the surface, and to prevent further portions of it from

finding their way there from the interior. If a strip of filter-paper be

allowed to rest with one end in a basin containing a little liquor, and

be placed in a draught of air, the exposed end of the paper will rapidly

become dark brown or black, the liquor which evaporates there being

constantly replaced by fresh portions sucked up by capillary attraction

from the basin. A similar action is constantly seen, when filtering

liquors through paper, if the latter be allowed to project above the

edge of the funnel. Precisely the same effect occurs, perhaps increased

by the oxidation of the tannins, on the edges and other parts of a butt

which are most exposed to draughts of air. The use of oiling the grain

is not only, to a certain extent to protect it from oxidation, but also

to check evaporation, and the consequent accumulation there of the

dark-coloured solids contained in the liquor. A very similar result is

attained by wetting the grain-side, and allowing as much of the

evaporation as possible to take place from the flesh.

The process of sole leather tanning has been discussed in considerable

detail on account of its simplicity and importance. It is now time to

point out in what respects the tannage of the lighter leathers differs

from it in principle. Taking the case of ordinary dressing leathers,

such as kips and shaved hides, the first point to remember is that these

goods come into the liquors not merely almost entirely deprived of lime

by bating, but in a very flat and fallen condition from the action of

the bacterial ferments of the bate. As a general rule in this country

the colouring is done in paddles, but where a very smooth grain is

required, the use of suspenders is to be recommended, and in America is

largely adopted. Indeed in the States the entire tannage of much of the

cheaper leather is done in suspension, and the sides are only removed

from the laths to which they have been nailed, when they are required

for splitting. It is obvious, from what has been said of sole leather,

that as the hides are brought into liquors in a very fallen and extended

condition, the grain will be likely to be wrinkled; and indeed this is

sure to be the case unless, by suspension, the hide is more or less kept

in tension till its fibres are fixed by tanning. The free motion in the

paddle favours the formation of a “pebbled” grain, since the hide is

bent now this way, now that, and minute wrinkles and creases are formed

in all directions. For many purposes, and especially if a grain is

afterwards to be raised by “boarding” the curried leather, this graining

in the paddle is not disadvantageous, so long as it is not excessive. In

some other cases it causes much trouble and labour to the currier before

it is removed, and if the English tanner and currier are ever to compete

with the American in smooth grain finishes, it will be necessary for

them to obviate this source of wasted labour. The graining is the less

considerable, and the easier to remove, the weaker and more mellow are

the liquors employed in colouring and the more gradually their strength

is increased.

The production of a soft leather depends on the fibre being tanned in a

fallen and unswelled condition. It is for this reason that bating is in

many cases essential, though where somewhat firmer leathers are

required, mere reduction of the swelling by removal of the lime is

sufficient. For the same reason, no acid-swelling is permissible either

before tanning, or in the liquors, and though liquors for soft leathers

must be rather acid than alkaline, they are incapable of removing any

large quantity of lime, and for the best results, the deliming must be

complete before tanning. As mere bating or puering is mainly designed to

reduce swelling by the action of bacterial products (p. 172), and is not

a very efficient means of removing lime, it is desirable where it is

employed, to supplement it by some more active deliming process. In the

lighter leathers, drenching (p. 166) generally fulfils this purpose and

many of the more intelligent tanners now give bated hides a bath in

boric acid before tanning, which not only removes the last traces of

lime without acid-swelling, but checks the bacterial fermentation, and

prevents its introduction into the liquors. In gambier tannages, a

decidedly better colour is obtained by this treatment (p. 228).

In most cases the production of bloom is not desired in dressing leather

tannage, and is prevented by relying chiefly on liquors, and avoiding

the use of bloom-giving solid materials, which include most pyrogallol

tannins. Dressing leather tannages can frequently be advantageously

hastened by drumming: which by continuously bending the leather in all

directions, constantly widens and contracts the spaces between the

different fibres, and, as it were, pumps the liquor through the skin.

The softness of dressing leathers is increased, and the hardening action

of acids present in the liquors is prevented by the addition of salt, or

of some sulphates (sodium, magnesium, ammonium) which exercise a sort of

pickling action on the fibre, and prevent its swelling, but at the same

time tend to light weight and a somewhat empty tannage. It by no means

follows that a hide or skin which is thoroughly coloured through, is

really fully tanned; as, though the fibres may be actually tanned or

coated on the surface, time is required for the tannins to penetrate

them to the centre. This incompleteness of saturation is often found in

drum tannages. Such leathers are generally tough, and gain weight and

softness in currying. In order to “carry grease” well, that is, to

absorb a large quantity without appearing greasy, it is essential that

the fibre-bundles should be thoroughly split up or differentiated; and

the degree to which this is attained largely depends on the extent of

liming. There is also considerable difference in different tannages, as

to the amount of grease which they will carry.

It is now not uncommon to combine a degree of alum or chrome tannage

with vegetable tannage in the finer dressing leathers. For further

information on this the reader must be referred to the next chapter.

The finest sorts of leather such as goat, calf, sheep and seal for

bookbinding, upholstery and the like, are mostly tanned with sumach;

paddles and drums being largely used to quicken the operation. Leather

tanned with sumach has been proved by the researches of the committee of

the Society of Arts on the decay of bookbinding leathers[127] to be the

most durable leather for this purpose, some other tanning materials of

the pyrogallol class coming near it in this respect, while all catechol

tannages are found peculiarly liable to destruction by the action of

sunlight, dry heat, gas fumes, and traces of sulphuric acid from other

sources, although in many cases more durable than the pyrogallol

tannages when exposed to mechanical wear and moisture, as is the case

with shoe-leather (p. 298). East India sheep- and goat-skins (so-called

“Persians”) are tanned with the catechol tannin of turwar or cassia

bark.

[127] Soc. of Arts Journ., 1901, p. 14.

[Illustration: FIG. 42_a_.--Interior of Light Leather Tannery.]

The finer leathers of which we are now speaking are almost invariably

prepared for tanning by puering with dog-dung, and drenching with bran,

as colour and softness are the special characteristics aimed at. A

somewhat interesting style of tannage is occasionally used for

sheep-skins (roans), and calf-skins, in which the skin is sewn into a

bag, flesh side out, with only a small aperture left for filling at one

of the shanks. It is then turned grain-side out, and filled with strong

sumach liquor, and a little leaf sumach, and floated in a bath of warm

sumach liquor. After a short immersion, the skins are piled on a stage,

so that the liquor is pressed through them by their weight; and when

partially empty, they are refilled and the process repeated. The time of

tannage is very short, not exceeding about twenty-four hours, and the

leather produced is very soft.

CHAPTER XVII.

_COMBINATION OF VEGETABLE AND MINERAL TANNAGE._

In very early times leathers were produced, which were partly tanned

with alum, and partly with vegetable materials. One of the earliest of

these was probably the Swedish or Danish glove-leather. The principle

has long been applied to the production of certain very tough and

flexible leathers known as “green leather,” and used for “picker-bands”

for looms, laces for belting, “combing-leathers” and some other purposes

where softness and toughness are of principal importance. About

twenty-five years since, it was applied in America by Mr. Kent to the

manufacture of an imitation of glazed kid, which he named Dongola

leather; and since that time, the method in various modifications, has

taken a considerable place in the manufacture of the finer leathers for

shoe purposes, especially in the United States.

Alum-tanned leathers, as has been already stated, are remarkable for

softness and toughness, and the mineral (crystalloid) tannages have the

power of penetrating and isolating the individual fibrils of the skin in

a much greater degree than the vegetable tannins, and hence are less

dependent than the latter on a previous isolation produced by liming. On

the other hand, they give much less plumpness and solidity, and more

liability to stretch, and are less resistant to the action of water; and

are, as a general rule (to which some chrome-tannages are an exception),

incapable of producing a soft leather without mechanical softening

(staking) after the tannage is completed. Purely mineral tannages have

always a woolly fibrous structure, and never the firm and compact flesh

which is required in leathers which are to be “waxed,” or finished on

the flesh side to a smooth surface, and as they communicate more or less

of these peculiarities to combination-tannages, the latter are mostly

used, either for grain-finish, or for uses where a soft and velvety

flesh-side is required, as in the case of “ooze-” or “velvet-” calf. On

the other hand, the partial use of vegetable tannage communicates to

them a degree of plumpness, fulness and resistance to water which is not

possible to alum-tannages pure and simple, and a softness which is not

easily obtained in vegetable tannage without the use of large quantities

of fats or oils. A preliminary mineral tannage also greatly increases

the rapidity of the penetration of the vegetable tans, by isolating the

fibres, and rendering them less gelatinous. Once a leather is

_thoroughly_ tanned by vegetable materials, it is little affected by

subsequent treatment with alumina, or even with chrome; and on the other

hand, though chrome and alumina leathers are still capable of absorbing

considerable quantities of vegetable tannins, they always retain, in a

degree, the qualities which the mineral tannage has communicated to

them. The resulting leathers are thus not only modified by the different

proportion of vegetable and mineral tannages which have been given, and

by the properties of the particular vegetable tannage used; but by the

order in which the several treatments have been given, and always

retain, to a considerable extent, the characteristics of that which has

been first applied. We have thus in our hands a powerful means of

modifying the character of our leather to suit the special requirements

which it is to fulfil.

So long as tanners were restricted on the one hand, to the ordinary

methods of stuffing tanned leathers with oils and fats, and on the other

to the use of egg-yolk, which had long been common in alum-tannages,

combination-tannage remained of but secondary importance, and it was the

application of the method of “fat-liquoring” by James Kent to his

Dongola leather, which gave them the place they now possess, by

providing a cheap substitute for egg-yolk, and enabling the tanner to

obtain softness and resistance to water, without producing the greasy

feel which is common to curried leathers. The process of fat-liquoring

has already been mentioned in connection with chrome leathers, to which

it was subsequently applied, and we shall return to it, after having

given some further details of the methods of tannage.

In the first place we must consider briefly the mutual action of the

mineral and vegetable tannages on each other. It has been pointed out

by Eitner, and also mentioned (p. 339) in connection with the

decolorisation of extracts, that the addition of say ¹⁄₂ per cent. of

alum, or aluminium sulphate to tanning liquors, lightened their colour,

not only by giving a degree of acidity to the solution, but by

precipitating a portion of the less soluble and more darkly coloured

tannins. Chrome-alum, and basic chrome salts produce a similar effect,

though from their marked colour, the lightening of the solution is not

so easily observed. It is therefore advisable if these salts are to be

used in actual mixture with the vegetable tans, to allow the solution

time to subside, or to filter off the dark-coloured precipitated

matters. Larger quantities than ¹⁄₂ per cent. of the alum do not appear

materially to increase the effect just described.

A second effect produced by these mineral salts on vegetable tannins, is

in many cases to develop mordant colouring matters which are present;

and thus, since most of these colouring matters are yellow, to produce a

yellower leather than would be obtained with the vegetable material

alone. This effect is very marked in the cases of sumach, gambier and

quebracho. The compounds which these colouring matters form with chrome

are mostly of a darker shade than those with alumina, tending to olive,

and therefore chrome-combination leathers are generally dull in colour.

Potassium dichromate, especially if acidified, generally oxidises and

precipitates tannins, and darkens their colours, so that it is not

practical to follow a vegetable tannage by the two-bath chrome process;

and though the reverse order may be pursued, the single-bath chrome

process, and that following and not preceding the vegetable tannage,

generally gives the best results. If lightly tanned leathers, such for

instance as the imported East Indian tannages, with babool or turwar

barks, be treated with a basic chrome tanning liquor, such as is

described on p. 215, so large a proportion of chrome will be absorbed,

that the leather will possess most of the characteristics of a genuine

chrome leather.

Combination-tannages for glove-leathers, such as the Danish and Swedish

leathers already alluded to, are generally first tawed with alum and

salt, with or without addition of flour and egg-yolk, and are then

coloured, and more or less tanned with vegetable materials. That

employed on the original Danish leather was willow bark (of _Salix

arenaria_). In France, where this willow is not found, the bark of the

commoner _Salix caprea_ was substituted; and as it is much weaker in

tannin, additions of oak-bark or sumach to supply the deficiency, and of

madder to give a redder colour were made to it. The dyeing of these

leathers is frequently combined with the tannage, dyewoods or dyewood

liquors being mixed with the tanning liquors. In the manufacture of

glazed French kid, indeed, the process is so arranged, by brushing on

dye-liquors mixed with tannins, as merely to tan the grain-surface,

which is necessary to enable it to be glazed by friction, leaving the

substance of the leather of purely alum tannage.

On the other hand, in the “green leathers” (so-called from their

greenish-yellow colour, and largely made in the West Riding of

Yorkshire), the hides usually receive a light gambier tannage, extending

over a week or so in weak gambier liquors in handlers, and are then

“cured” by handling in hot and strong solution of salt and alum, in

which they are finally left all night, and then dried rapidly without

washing out the alum, much of which consequently crystallises on the

surface. This is slicked off, and the leather damped back, and heavily

stuffed with sod-oil. If, however, the combination-tannage is properly

carried out, it will stand liberal washing without losing the necessary

alum, and of course a tougher and more satisfactory, though somewhat

lighter weighing leather results. It is in many cases a better plan to

combine the two tannages in one bath, mixing the alum and salt with the

gambier, and handling or paddling the goods in the mixture. This is the

plan usually adopted for Dongola leather, in the United States. For

skins which are to be glazed, it is important that the _surface_ should

be tanned with the vegetable material, and the goods are therefore

worked into gambier liquors, to which the salt and alum are only added

after the tannage has made some little progress; while for dull Dongola,

intended rather to imitate calf-kid, it is best for the alum and salt

tannage to begin first. For goat-skins for glazed Dongola kid, about 4

lb. of block gambier, ¹⁄₂ lb. of alum, and ¹⁄₄ lb. of salt are used per

dozen, and the tannage occupies in all about twenty-four hours.

After the skins are tanned, they are thoroughly washed out with tepid

water, to remove loose alum and gambier, and are then ready for

fat-liquoring. As in the case of chrome leather, it is of great

importance that this washing should be done thoroughly, as any remaining

alum which diffuses into the fat-liquor, will cause it to curdle. If the

washing is thorough, the more neutral the fat-liquor and the better; but

a somewhat alkaline soap-solution is less liable to curdle. The original

fat-liquor used by Mr. Kent was the alkaline liquor which had been used

in washing the surplus oil from chamois leather (see p. 380), but now

soap- and oil-solutions are generally made specially for the purpose.

Most of the remarks in the chapter on chrome tannages are applicable in

this case, but probably fat-liquoring is somewhat easier than in the

case of chrome. Mixtures of either soft soap or curd soap with cod, sod,

and olive oil are frequently employed. Sesame oil also seems well

adapted for the purpose. The better these are emulsified, and the more

satisfactory is the result; a cylinder of zinc or copper fitted with a

plunger, something like that of a “Lightning Egg Beater,” but covered

with perforated zinc, or wire gauze, does very good service as an

emulsifier on a small scale.

Another method is to melt the soap with just sufficient water to make it

pasty, and to incorporate the oil thoroughly with the mass, which is

afterwards dissolved in hot water. Oils are the most easily emulsified

when they are somewhat acid. For this reason rancid olive oils are often

used for the Turkey-red process, but a similar effect can be obtained by

adding a small quantity of candlemaker’s oleic acid to the oil before

mixing. The addition of sulphated castor oil (Turkey-red oil) also helps

emulsification, and is in itself a very good softening agent. One of the

commonest mistakes in fat-liquoring is the employment of too strong an

emulsion--even so small a quantity as ¹⁄₂ per cent. of soap, and half

that quantity of oil, reckoned on the wet weight of the well-drained

leather, will produce a very notable softening effect. Of course, for

dull finishes, much larger proportions may be used. Not only

combination-tannages, but those entirely vegetable, can be fat-liquored

with excellent effect, and the process is now largely used for coloured

calf, and other leathers, which are required soft and nourished, but

without any appearance of greasiness. Leathers absorb the fat-liquor

most readily if put into it in a sammed or partially dried condition,

but even if quite wet, they soon take up the whole of the oil and soap

on drumming, leaving only a little clear water in the drum. Goods may

be blacked while still wet with fat-liquor, but should generally (except

in the case of chrome leathers) be dried out before dyeing, as this

fixes the oil and soap in the fibres.

Many coloured leathers are now made by a process which may be considered

a combination of the Dongola process itself with the ordinary process of

vegetable tanning, the goods being coloured and partially tanned as if

for a vegetable tannage, and then finished in Dongola liquors with alum,

salt and gambier. Very good leathers are made in this way in the United

States, with a tannage begun in suspension in hemlock bark liquors.

Imitations of Dongola leather are made by treating East India sheep or

goat with alum liquors, and afterwards fat-liquoring (if necessary)[128]

and finishing like genuine Dongola leather. The treatment is most

effective, if a portion of the original tan be removed by washing with

warm water, with a little borax, ammonia, or even soda, and the goods

then alumed with a “neutralised” or basic alum solution such as that

described on p. 187. Goods treated with a basic chrome-liquor, like that

used for the one-bath chrome process, p. 212, are almost converted into

chrome-tanned goods, and will even stand some degree of boiling. The use

of a liquor made like the Martin-Dennis liquor, by dissolving chromic

oxide in hydrochloric acid, was the subject of an American patent[129]

which in this country is owned by Wichellow and Tebbutt, but which

expires in 1903.

[128] East India sheep and goat are generally so heavily oiled with

sesame oil (up to 30 per cent. of their weight), that it is desirable

in many cases rather to diminish than increase the oil, which may be

done by washing with soap solutions, preferably before aluming.

[129] Eng. Pat. Jensen 13126, 1889.

Chrome-combinations may also be made by retanning goods tanned by either

of the chrome processes with vegetable materials, of which gambier seems

the most suitable. The use, even of very weak liquors of sumach and most

other tanning materials, deprives chrome leather of its stretch, and if

carried to excess, readily makes it hard and tender.

CHAPTER XVIII.

_VEGETABLE TANNING MATERIALS._

As has been stated in the previous chapter, our knowledge of the

chemistry of tannins is not sufficiently advanced to render possible any

strictly chemical classification, while an additional complication

arises from the fact that very different tannins may coexist in the

wood, bark, fruit, galls, etc. of the same plant. It therefore seems

best to follow the example of Prof. Bernardin in his ‘Classification de

350 matières tannantes,’[130] and arrange the plants under the orders of

the natural system of botany, as has already been done by von

Höhnel[131] and A. de Lof.[132] In the following pages, only those

materials which from their high percentage of tannin, or from some other

cause, are of commercial interest or value, are included, as the tannins

are so widely distributed in the vegetable kingdom, that any exhaustive

list would be quite out of the question.

[130] Gand, 1880.

[131] ‘Die Gerberinden,’ Berlin, 1880.

[132] ‘Matières tannantes,’ Halle aux Cuirs, Paris, 1890. See also

‘Agricultural Ledger,’ 1902, No. 1 (Government Printing Office,

Calcutta, 6_d._), by Mr. D. Hooper, which contains much valuable

information.

Tannins are not confined to any particular part of the plant, though

they are usually most abundant in barks and fruits. Insect-galls are

often very rich in tannin, usually gallotannic acid; while in several

cases woods are of commercial importance from their cheapness, though

the percentage of tannin they contain is not generally high. The

function of tannins in the vegetable economy is not well understood. In

some cases they are probably a waste product of plant-life, and may help

to ward off attacks of insects. They usually exist as cell-contents, and

as vegetable cells have frequently thick and impermeable walls, and the

diffusive power of tannins is low, much time is required for extraction,

unless the cells have been previously crushed or broken.

It would be beyond the scope of this text-book to describe in detail the

structures of the tannin-yielding parts of plants; but barks are of such

general importance, that some particulars seem desirable.

The detailed structure of bark varies greatly in different trees, though

its general principles remain unaltered. One of the best short accounts

of these is given by Prof. H. Marshall Ward on page 199 of his little

book on ‘Timber and some of its Diseases,’[133] and further information

may be found in Van Tieghem’s ‘Traité de Botanique’ and other works on

structural botany.

[133] Macmillan & Co.

With regard to detailed structure of various tanning barks, ‘Die

Gerberinden’ of von Höhnel[134] is one of the best authorities.

[134] ‘Die Gerberinden,’ Berlin, 1880.

The inner surface bark of a young tree, or twig, consists of a layer of

soft and living cells resting on the outer surface of the wood, and

called the _cambium_. These cells multiply by division (cp. p. 12) and

produce from their inner surface the successive annual layers of wood,

and on their outer a fibrous tissue called the bast (_phloem_),

consisting of lengthened cells, and tubes with perforated divisions

(sieve-tubes) which convey sap, and mostly run in the direction of the

branch, but are crossed transversely by cells in a line with the

medullary rays of the wood. All these cells when first produced in the

cambium-layer have thin and soft cellulose walls, but the inner layer

forming the wood becomes lignified, or hardened, by deposits of lignine

on the interior of the cell-walls, while their contents of living

protoplasm disappear. The outer layer forming bast remains much softer

and more fibrous, and retains its vitality longer. The outer surface of

the young branch is covered by a thin layer of flat cork-like cells

forming the epidermis, developed from the growing tissue of the bud,

beneath which is a layer of growing cells frequently called the

_cork-cambium_. This produces, on its inner side, a layer of soft,

juicy, thin-walled cells (_parenchym_), which are living and capable of

growth, and contain protoplasm and often chlorophyll, to which the green

colour of young twigs is due. This layer at first rests on the bast. On

the outer side, the cork-cambium produces corky layers beneath the

epidermis. The section of an oak-twig is shown in Fig. 43.

[Illustration: FIG. 43.--Section of Oak Twig, drawn by Prof. Bastin:

_c_, corky layer; _t_, tannin-cells; _St_, stone-cells; _Ca_, cambium;

_Mr_, medullary ray; _P_, pith.]