_Extraction of solid materials_, such as_The Principles of Leather Manufacture

difficult, but the following is a convenient method, which has been

recognised as official by the International Association. An ordinary

beaker, of about 200 c.c. capacity, but which may be varied in size

according to the weight of the material which it is necessary to treat,

is placed in a water-bath, as shown in Fig. 65. A thistle-headed funnel,

the stem of which is bent twice at right angles, and of which the head

is covered with a piece of fine silk gauze (such as is used by millers)

to act as a strainer, is placed in the beaker and held in position by a

clamp as shown in the figure. To the free end of its stem a piece of

glass tube, six or eight inches long, is attached by indiarubber tube,

which is provided with a pinchcock to regulate the flow of liquid. Fine

silver-sand, freed from iron and soluble matters by washing first with

hydrochloric acid, and then very thoroughly with water, is now poured

into the beaker, so as to surround the head of the funnel to about half

an inch in depth; and the weighed quantity of tanning material is next

introduced. It is best to cover the material with cold water, and allow

it to stand all night, but in case of haste, water of 30° to 50° may be

used, and the extraction proceeded with after the material is thoroughly

soaked. Percolation is started by sucking the syphon, and allowing the

liquid to drop slowly into a liter flask, the temperature of the

water-bath being maintained by a Bunsen burner, and the beaker being

refilled as it requires it with water at the desired temperature.[155]

At least 500 c.c. must be percolated before the temperature is allowed

to exceed 50°, after which, except in the case of sumach and canaigre,

which should be begun about 30°, and at no time allowed to rise above

50°, the temperature may be raised to boiling point. At least 1¹⁄₂ hour

should be employed in percolating 800 to 900 c.c. and if the material is

not then practically exhausted, the liter flask must be withdrawn, and

an ordinary ungauged flask substituted, into which the percolation is

continued till the material is exhausted. The very dilute liquor in the

second flask is now boiled down till its volume is sufficiently small to

be added to that in the liter flask, a small funnel being placed in its

neck during ebullition, to prevent spirting and the access of air. Under

no conditions must the stronger liquor of the first part of the

percolation be boiled down, as this would involve destruction of tannin.

The solution is now cooled, and made up to the mark as has been before

described. Most ordinary materials may be practically exhausted by the

liter of water if percolation is slow, and the trouble of evaporation

may thus be avoided.

[155] The material should be kept in an even layer, and if necessary

the surface may be stirred at intervals with the thermometer or a

glass rod.

_Total soluble matter._--The solution of which the preparation has been

described, must now be filtered, the size and kind of paper, and exact

method of filtration prescribed by the International Association being

strictly adhered to. All papers and methods of filtration absorb traces

of tanning matters, and but few will give a clear filtrate with such

solutions as those of quebracho and hemlock extracts; and to obtain

uniform results exact uniformity of method is essential.[156] Deviations

from the exact method, in the case of quebracho, easily cause

discrepancies of several units per cent. in the result. The object of

rejecting the first portions of the filtered solution is to prevent, as

far as possible, errors which would arise from the absorption of tannin

by the paper, and to insure a clear filtrate. 50 c.c. of the clear

filtrate is now measured by an accurate pipette, and evaporated to

dryness in a weighed porcelain basin, on a steam-bath, in order to

determine the “total soluble.” This and succeeding operations should be

done in duplicate, even if this has not been the case in making up the

original solution, which is certainly desirable.

[156] Methods of correction for absorption of filter-papers have been

worked out in the Author’s laboratory, and adopted by the last

conference of the International Association. Cp. Collegium, pp.

145-158, 1902, and App. A, p. 477.

Ordinary light porcelain basins, generally of about three inches

diameter, are employed for evaporation, which takes place somewhat more

rapidly if they are flat-bottomed (saucer-shaped). In place of

porcelain, thin glass basins of hemispherical form may be used, and, but

for the cost, platinum would be better than anything else. Aluminium and

nickel basins have been tried, but are slightly attacked by some

liquors, and hence are more liable to vary in weight, though they have

the advantage in rapidity of evaporation. Evaporation takes place most

quickly if the steam-bath can be placed in a draught of air, so as to

rapidly carry away the vapour formed, but the basins must be protected

from dust. Under favourable circumstances, evaporation of 50 c.c. in

porcelain basins occupies one to one and a half hour. An ordinary pan

fitted with a lid of thin copper perforated with holes of two and

three-quarter inches in diameter, makes a useful water-bath; but where

much work is done, it is desirable to have a rectangular bath of thin

sheet copper, taking a single, or at most a double row of basins, and

fitted with the usual appliance for keeping the water at constant level;

or with a supply of steam from a boiler, and an overflow for condensed

water.

As soon as the contents of the basins appear completely dry, they may be

transferred to the drying oven. The most satisfactory form is one in

which the basins are placed in a closed chamber, surrounded by steam at

the atmospheric pressure, and at the same time subjected to a vacuum

maintained by a water-jet air-pump; but as this apparatus is somewhat

costly, it will probably only be provided in laboratories which make a

speciality of such work. Next to the vacuum-oven, an air-oven, heated by

a gas-burner, and with its temperature controlled by a mercurial

regulator to 100-105° C., gives the best results, and it is also the

cheapest; but considerable care and some scientific knowledge are

required to work it satisfactorily. In intelligent hands good results

may be got from the small “breakfast cooker” gas ovens made by Fletcher

of Warrington, which are placed on an iron plate heated by a gas burner,

the supply of gas to which is regulated by a thermostat, or mercurial

gas-regulator, inserted, together with a thermometer, through holes

drilled in the top. The basins must not be placed too near the bottom of

the oven, which must be protected by a perforated metal plate supported

perhaps one inch above it, to prevent radiation and to distribute the

hot air. Any cold air required for ventilation should be admitted below

this plate, and care should be taken to exclude the products of the

burning gas. Contact of the basins with any heated part of the

metal-work should be carefully avoided, and they are best supported on

grid-shelves covered with wire gauze or perforated metal, so as to allow

of free circulation of air. If perforated zinc is used, it must be well

supported, as it is much softened at the temperature used. The least

satisfactory appliance in skilled hands, but probably the most easy to

work by the inexperienced, is the ordinary water- or steam-oven. It is

impossible, in this apparatus, to raise the temperature of the interior

fully to boiling point, and below this gambier, quebracho, and other

solutions containing catechins (p. 298), dry very slowly. On the other

hand, so long as it is kept boiling and supplied with water, the

temperature is necessarily constant, and there is no danger of

overheating, which easily occurs in ovens heated directly by gas. Such

ovens are often fitted with openings at the top for use as a steam-bath.

To get the best results, the basins must be as freely exposed as

possible to the air in the interior of the oven (in no case must basins

be set one inside another, except in the exsiccator for cooling), and

little or no ventilation from the outside is required, as only traces of

moisture remain after evaporation on the steam-bath; so that, after an

hour’s drying, any ventilators may safely be closed. As a good deal of

cooling takes place through the door, it is best to protect it with some

non-conducting material, such as asbestos millboard, which may be

attached with rivets, or even with ordinary paper-fasteners. One to one

and a half hours will be required to dry to constance in the

vacuum-oven; two to three in the air-oven at 105°; and probably about

four hours in the water-oven, except in the case of gambiers, which may

require somewhat longer. Too long heating is disadvantageous, as the

residues begin to oxidise and gain in weight. As soon as it is judged

that the basins will be constant in weight, they are withdrawn from the

drying oven, and at once placed in an exsiccator (a glass vessel with an

accurately fitted lid, which should be slightly greased, in the bottom

part of which is placed either dry calcium chloride or concentrated

sulphuric acid, to absorb the moisture of the air it contains). In this

they are left till thoroughly cold, which if several basins are put in

together, may require half an hour. They are then weighed accurately,

but as rapidly as possible; returned to the drying oven for half an

hour; and replaced in the exsiccator. The exact weight of each basin, as

it comes in turn to be weighed, is now placed on the balance before

removing the basin from the exsiccator, so that it can be seen instantly

if there is loss or gain of weight, before it has time to absorb any

moisture from the air. The weight should not be more than a milligram or

so less than at the first weighing; if weight has been gained, it is

caused by oxidation, and the first weight should be taken as correct if

it is certain that the basin was then perfectly cold; a very slight

amount of warmth easily reducing the apparent weight by several

milligrams. If material loss has occurred, the basin must of course be

returned to the oven, and re-weighed in another half hour; but with

experience, this should rarely be needed.

It is necessary that the balance used should weigh accurately to

milligrams; and it must carry at least 50 grm. on each pan; while it is

more convenient that it should carry 100 or more, it is always possible

with a little ingenuity, to manage within 50 grm.; and if a cheap

balance must be used, the smaller size will probably be more accurate.

Balances of this sort can now be got for two or three pounds, though it

is in all respects better to obtain one of first rate quality, which

should cost about ten pounds. The balances of Verbeek and Peckholdt, of

Dresden, from their simplicity and rapidity of weighing, have given

great satisfaction in technical work in the Yorkshire College. Whatever

economy be exercised in the choice of the balance, it is essential that

the set of weights should be of the greatest accuracy, and especially

that all the weights of one denomination (10 grm., 1 grm., etc.), should

accurately balance each other. Even after all precautions are taken, it

is desirable that those weights which are in duplicate should receive

distinguishing marks (e.g. with a centre-punch), and should always be

placed on the scale in the same order; and, not only on account of

possible inaccuracy, but to save time, it is desirable to reject basins

which are so nearly of an even weight (20, 25, 30 grm.) that when

weighed with the residue (0·3-0·4 grm.), a change of the larger weights

may be required, since it is to be remembered that any error of the

weights employed is concentrated on the small weight of the residue.

After deducting the weight of the empty basins, the weight _in

milligrams_ of the two residues of 50 c.c., which should be practically

alike, are added together, and the sum divided by the weight _in grams_

of tanning material used; which gives the percentage of “total soluble

matter.”

[Illustration: FIG. 66.

Hide-powder Filter.]

_Non-tannins._--It is now necessary to determine the proportion of the

“total soluble” which consists of “non-tanning matters,” that is, of

substances not removed from the solution by treatment with hide-powder.

The so-called “tanning matters” removed, include colouring matters and

some other substances, which though absorbed by hide, are certainly not

tannins in a strictly chemical sense. (See note, p. 480.)

According to the method of the International Association, the apparatus

shown in Fig. 66 is employed for this purpose. The glass bell is

carefully and uniformly stuffed with hide-powder, care being taken that

no channels are left, especially at the sides, through which the liquor

can reach the syphon without traversing the hide-powder. Before filling

the bell, the short leg of the syphon-tube should be loosely plugged

with cotton-wool (of which a little is allowed to project from the end),

in order to prevent the powder from gaining access to the tube. The

powder is retained in its position in the bell by a piece of muslin held

by an indiarubber band, and the bell is then placed in a beaker or

tumbler as shown in the figure; and filtered liquor is gradually added,

as it is absorbed by the powder, till the whole is uniformly wetted. The

liquor which was first filtered through the paper, and rejected for

“total solubles,” may be used for this purpose, and it is not necessary

that it should be absolutely clear. The syphon is now gently sucked, and

the filtrate is allowed to fall, drop by drop, into a gauged cylinder.

The first 30 c.c. which collects is rejected, since it contains traces

of dissolved hide-substance even from the purest hide-powder; and the

next 50 c.c. should give no turbidity if a few drops are mixed either

with clear tannin solution (absence of dissolved hide-substance), or

with the first 30 c.c. (absence of tannin). This 50 c.c. is used for

determination of non-tannins, by evaporation and drying precisely as has

been described in the case of “total soluble.” Some chemists, with very

accurate balances, prefer to evaporate only 25 c.c., which effects a

little saving of time in evaporation; but in any case the whole of the

50 c.c. must be allowed to run through the filter before it is measured,

as the filtrate varies somewhat in solid contents as the filtration

proceeds. The filtration and evaporation should be done in duplicate.

The weight of the residue is calculated into percentage as “soluble

non-tanning matters” precisely as has been described for the “total

soluble”; and when subtracted from the latter, the remainder is the

percentage of “tanning matters.” If the hide-powder now employed by the

English members of the International Association (manufactured by

Messrs. Mehner and Stransky in Freiberg in Sachsen), be employed, no

difficulty will be found in the filtration. This powder is quite

neutral, and contains between 10 and 20 per cent. of cellulose to render

it more absorbent. It does not swell in the filter, and hence should be

stuffed into the bell almost as tightly as possible, about 10 grm. being

required. If the bell is properly filled, the filtration should

altogether take about one hour, but if the liquid runs too fast, it must

be regulated by a pinchcock on the indiarubber tube of the syphon. If

other powders are used, which often contain acid, and swell very much in

the bell, the filling is much more difficult, and while the sides of the

bell must be closely packed, great care is requisite to keep the powder

loose in the centre, or the filter will not run. One point requires

mention with regard to neutral hide-powders. If an extract which has

been rendered soluble by the addition of alkalis or sulphites (p. 388)

be analysed with a perfectly neutral powder, it has been shown by

Paessler and Appelius[157] that a part of the tannin combined with the

alkali will not be absorbed, while with acid powders, the whole will be

estimated.

[157] Wissenschaftliche Beilage des ‘Ledermarkt,’ 1901, p. 107.

[Illustration: FIG. 67.

American Milk-shaker.]

_The “shake-method” adopted by the American Association of Official

Agricultural Chemists_, possesses some advantages, especially in the

analysis of used liquors which, from the acids they contain, are apt to

give somewhat too high results by the filter method (see App. B, page

480). It has the further advantage of being much less dependent than the

filter-method on the quality of the hide-powder employed. It has

therefore been accepted by the International Association as permissive

for all tanning materials, and as compulsory for used liquors (see App.

A), and must therefore be briefly explained. It can be carried out

successfully with somewhat inferior hide-powders to those required for

the filter, but generally gives results 1 or 2 per cent. lower in

tannins than the latter. A special shaking machine must be employed,

capable of thoroughly agitating a mixture of hide-powder and the liquor

to be analysed; and if many analyses have to be done, it is convenient

that it should be driven by power, as otherwise the work becomes

somewhat laborious. A machine called a “milk-shaker,” Fig. 67, employed

in the mixing of summer drinks, is generally used. The quantity of

powder required for the analyses to be made (about 8 grm. of ordinary

air-dried powder for each determination, with say 5 grm. added), is

stirred in a large beaker with 25 times its weight of distilled water,

and allowed to soak for 24 hours, 1·5 per cent. of chrome-alum

previously dissolved in water being added at the beginning of the

operation, and 1·5 per cent. more not less than 6 hours before its end.

The powder is then washed by squeezing through linen, and the washing is

continued till the wash-water no longer gives a precipitate with barium

chloride; and is then well squeezed out in linen, preferably with the

aid of a press. The damp squeezed powder is now roughly weighed, to

determine approximately what quantity it is necessary to take, to give

7·5 grm. of the original dry powder to each estimation (air-dried powder

contains about 15 per cent. of moisture), and a portion is accurately

weighed in a basin, and dried, first on the water-bath, and then in the

drying oven, to determine its moisture by loss. The approximate amount

of powder required for each determination--if possible a round number of

grams--is now weighed into as many bottles of about 300 c.c. capacity as

determinations are to be made, 100 c.c. of the filtered liquors,

prepared as before described, are introduced into each bottle, and the

bottles are then each shaken for 10 minutes (Mr. Alsop states that in

his experience 5 minutes is sufficient). The contents of the bottles are

now filtered through funnels, the stems of which are plugged with pure

cotton-wool, and the liquor is returned till a clear filtrate is

obtained, of which 50 c.c. is evaporated as in the International method.

It is now necessary to accurately correct the residue obtained, for the

amount of water carried in by the wet powder. The loss of weight of the

powder which has been dried, divided by its wet weight, gives the water

contained in each gram of wet powder, and this multiplied by the weight

of wet powder added to the liquor, gives the weight in grams (or volume

in c.c.) of water which has been added to each 100 c.c. of liquor.

Consequently, if the residues found be multiplied by this weight plus

100, and the product divided by 100, the weight will be obtained which

should have been given by 50 c.c. of undiluted but detannised liquor;

and from this the non-tannins are calculated exactly as in the case of

the residues from the filter process. Of course, in practice, a factor

is found, by which it is simply necessary to multiply all the residues,

to correct them to undiluted weight. The process sounds somewhat

complicated, but in reality, where a large number of determinations have

to be made, is quite as quick, if not quicker than the filter method;

which it is quite possible it may ultimately supersede, as much

attention is being devoted to its improvement.

Having determined the tanning, and soluble non-tanning matters of the

materials, it remains to determine the moisture, and the insoluble which

make up the whole. To determine moisture, a quantity, not exceeding two

or three grams of dry solid materials, or half a gram of moist or liquid

extracts, is weighed into a basin, and dried in the same way as has been

described for the residues, only that a considerably longer time will be

required before constancy is attained. The object of employing so small

a quantity of liquid extracts is to abridge this time, and the

consequent oxidation, as much as possible, as the extract soon forms a

hard skin on the exterior, which renders further drying very tedious. It

is advantageous to add a little alcohol to liquid and semi-liquid

extracts, and so dilute them that by inclining the basin they can be

distributed in a thin layer over its sides, while at the same time the

alcohol facilitates the evaporation of the water. The weight of the

dried residue in the basin is the “total solids,” while the loss is the

“water”; and these can be converted into percentages by multiplying by

100 and dividing by the weight of substance originally taken. An

alternate method, which is frequently convenient with extracts, is to

pipette off 50 c.c. (in duplicate) of the dissolved and well-mixed

extract-solution _before filtration_, and dry exactly in the same way as

for “total soluble.” The sum of the two residues in milligrams, divided

by the weight of extract taken for analysis, gives the “total solids”;

subtracting this from 100 leaves the “water,” while the difference

between the “total solids” and the “total soluble” is the percentage of

insoluble matter. Two further points must be noted. If the total solids

are determined by the first method, and the total soluble in the

ordinary way, in an extract which contains no insoluble matter, it

frequently happens that they differ by 0·1 or 0·2 per cent., owing

either to the difficulty of driving off the whole of the water, or to

slight oxidation of the total soluble residue. On the other hand, if the

second method is adopted, a small amount of “insoluble” is invariably

found, even in perfectly soluble extracts, which is due to the

absorption of tannin or colouring matter by the filter paper. On the

correction of this error, see Collegium, 1902, pp. 145-158, and App. A,

p. 477.

As the value of a tanning material often depends very much on the

paleness of its colour, it has become customary to specify in contracts

the intensity of colour of a solution of it containing one-half per

cent. of tanning matter (as measured by the I.A.L.T.C. method of

analysis), in a glass cell of one centimeter thick, by comparison with

standard coloured glasses in the tintometer. On the method of making the

measurement see L.I.L.B., p. 131.

NOTE.--All the apparatus named in this chapter can be obtained of

Messrs. Reynolds and Branson, Commercial Street, Leeds; or of Messrs.

Portway, Jamaica Road, S.E.; and of most other dealers in chemical

apparatus.

CHAPTER XXI.

_GRINDING OF TANNING MATERIALS._

Before the tannin they contain can be extracted, most materials require

to be ground, almost the only exceptions to this rule being divi-divi

and algarobilla, in which the tannin is very loosely contained.

Extracts, whether solid or liquid, merely require to be dissolved in

water or liquor, in which they are, for all practical purposes,

perfectly soluble. With the less soluble extracts it is generally

preferable to dissolve at a temperature of 50° to 60° C. with vigorous

stirring.

The actual method of grinding, and consequently the machinery employed

for the purpose, vary not only with the material to be ground, but with

the method of leaching adopted, as it is essential that the mass of

ground material should be completely permeated by the liquor employed in

leaching; and if it be ground too finely, or subjected to too much

pressure on account of the height to which it is piled in the leaches,

it is apt to form a compact and clay-like mass, the interior of which

remains unextracted.

[Illustration: FIG. 68.--Cone-Mill.]

In the laboratory, where thorough extraction must be completed in a few

hours, the material can hardly be too fine; but on the larger scale a

much coarser product must be used, and leaching requires days, or

sometimes even weeks, and is then seldom successful in removing all the

tannin. It is probable, however, that in the future these mechanical

difficulties of extraction will be overcome; and the material will then

be as finely divided, and as completely extracted on the large scale, as

it is in the laboratory at the present time.

One of the earliest methods of grinding oak-bark, and which is still

used for sumach (p. 271) consists in crushing it under large circular

edge-stones, frequently turned by a horse. This process was very slow

and inefficient for barks, and both it and horizontal millstones

similar to those used for wheat were long ago superseded by iron or

steel mills on the same principle as the ordinary coffee-mill.

These mills, Fig. 68, consist of a “bell” or inner cone, covered with

blades or teeth arranged at a slight angle to the vertical section of

the cone, and which are made finer and increased in number towards its

lower and wider part. This cone rotates within an outer hollow cone or

casing, also provided with blades or teeth which are sloped slightly in

the opposite direction to those of the inner cone, so as to meet them at

an angle, like the cutting-blades of a pair of scissors, and the angles

of the cone are so chosen that the blades approach each other more

closely towards their base. The outer cone is fixed, and is provided

with a hopper like a coffee-mill, while the inner cone is so rotated on

its axis that bark placed in the hopper is screwed down between the two,

and cut finer and finer till it reaches the lower edge, when it drops

out. The blades or teeth are usually cast in one piece with the metal

cones, and sharpened when required by chipping with cold chisels. This

operation should not be conducted in the mill-house, or small chippings

of iron may get mixed with the bark, and cause stains on the leather.

This form of mill, which is run in England at about 30 revolutions per

minute, and at nearly three times this speed in America, works very well

with dry material, but clogs badly if it be appreciably damp. On this

account it is always well to run the mill with a fairly slack belt which

will slip before exerting sufficient pressure to break the machine, as

in such operations as grinding, safety clutches are of but little use.

A type of mill varying somewhat from the above, consists of a pair of

discs or very obtuse cones, the inner one of which runs on a horizontal

axis. The teeth are generally arranged in concentric rings and interlock

with each other. The material to be ground is fed at or near the centre

of the fixed disc, and escapes at the edges. The construction of this

class of mill will be easily understood from Fig. 69. Very small pieces

of iron or steel which get caught between the teeth will often result in

the breaking of the latter, and the formation of iron dust, which is a

serious objection to the employment of this type of mill (to which the

Schmeija “Excelsior,” the Glaeser “Favorita,” and the “Devil

Disintegrator” of the Hardy Patent Pick Co. belong) for grinding barks.

[Illustration: FIG. 69.--“Excelsior” Mill.]

Myrobalans and mimosa-barks have proved especially troublesome to grind,

the former from the hardness of the stones of the fruit, and a tendency

to clog the mill, and the latter from their combined hardness and

toughness. “Disintegrators” of various patterns are now made, which are

capable of grinding both these materials satisfactorily, and but for

their liability to cause fire, and the large proportion of fine dust

which they make, are usually to be preferred to toothed mills. In spite

of their disadvantages, however, they have come very largely into use,

on account of their efficiency in grinding obstinate materials.

Disintegrators work on the principle of knocking or beating the material

to powder, by means of very rapidly revolving beaters, which, in the

smaller machines, are driven at 2500 to 3000 revolutions per minute.

The first disintegrator was made by Carr and consisted of two concentric

cylinders or baskets of steel bars, rotating in opposite directions at a

very high speed. The material was fed between these and was dashed to

pieces by being thrown against the bars and the outer casing.

[Illustration: FIG. 70.--Disintegrator.]

A simpler form was soon introduced by Carter, in which only one axis was

employed, carrying radial beaters which dashed the material against the

serrated outer casing, a portion of the circumference of which was

fitted with gratings, through which the ground material was thrown as

soon as it was sufficiently reduced in size, the fineness of the

grinding being regulated by changing the grates as required. This type

of disintegrator is, with slight variations, made by all the leading

makers of tanners’ machinery; and one form is shown in Fig. 70, and a

similar but smaller machine, opened to show construction, in Fig. 71.

In the more modern machines the sides as well as the circumference of

the casing are frequently corrugated in order to increase the action on

the material.

Mills running at such high rates of speed as 3000 revolutions per minute

will grind most hard substances, such as stone or brick, without

injury, but pieces of iron among the tanning material are apt to cause

damage, and various magnetic devices have been employed for separating

this metal, but with only partial success. In the best mills, therefore,

the beaters and inner casings are constructed so that they can be easily

replaced, and the damage is then rarely serious.

[Illustration: FIG. 71.--Disintegrator opened, showing construction.]

In order to avoid vibration, the discs and beaters of all these

high-speed mills must be balanced with great accuracy. This is best

accomplished by removing the spindle from the mill, and allowing it to

roll on two levelled straight-edges, and then filing or chipping the

beaters on the heavy side until it will remain indifferently in any

position.

A new form of disintegrator has been recently brought out in America by

the Williams’ Patent Crusher and Pulveriser Company, in which a series

of discs are keyed to the main shaft, to the circumference of which a

number of sets of “hammers” are suspended by means of hinge-bolts. Each

of these steel bars, or hammers, has a free arc movement of 120°, and

when the machine is in motion take a position divergent from the centre

on account of the centrifugal force. After striking a blow against any

material fed on to a plate serving as an “anvil,” the hammers recoil,

and, after passing any material which is not shattered by the blow,

again resume their normal position, leaving the next set of hammers to

beat against the unground material. The hinged suspension of the hammers

imparts a degree of flexibility to the mill which is not found in any

other machine of this character, and lessens the risk of serious damage

to the machine by the introduction of pieces of metal along with the

bark. The makers claim that this machine can be repaired more rapidly

and with less expense than any other disintegrator of equal power on the

market. Considerable improvements have recently been made in the details

of its construction. Fig. 72 shows a section of this mill. Of course

only the end hammers of each set can be seen in the figure.

[Illustration: FIG. 72.--Section of Williams’ Crusher.]

When myrobalans or valonia is to be used for leaching, it is generally

better to crush it between toothed or fluted rollers, rather than to

grind it finely, as the cellular structure is just as completely broken

up, and the flakes formed by crushing allow of much freer percolation

than when the material is powdered by the disintegrator, while the

consumption of power is also less. The general construction of the

machine will be easily understood from Fig. 73, and it is only necessary

to point out that the small upper roller acts mainly as a “feed” to the

larger crushing rolls.

In the best mills, the rollers are made up of a series of toothed steel

discs on a square axis, and are on this account easily replaced or

sharpened when they have become broken or worn.

[Illustration: FIG. 73.--Myrobalans Crusher.]

Several mills have been introduced in America in which the bark is sawn

or rasped by toothed discs like circular saws, but these are only

capable of dealing with barks of a brittle nature, and are immediately

choked by tough materials like the bark of the mimosa or oak. A better

form of mill, but one which is, to some extent, subject to the same

disadvantage, is the “shaving-mill,” in which blades are fixed like

plane-irons upon a disc, cones or cylinder, and are rotated at a high

speed against the material which is fed against them by toothed rollers

at such an angle that the shavings are cut diagonally to the grain.

These shaving-mills are largely in use in America for hemlock-bark, with

which they are particularly successful. The principle of the machine is

exactly the same as that of the machines used in cutting oakwood,

quebracho, and the different dye woods. One type of shaving-mill is

illustrated in Fig. 74.

[Illustration: FIG. 74.--Shaving Mill.]

It frequently happens that the material is delivered from the mill in a

very unequal state of division, and it is sometimes necessary to screen

it and thus separate the coarser portion either for use in the leaches

or for re-grinding, while the finer portion is more suitable for

“dusting.” With disintegrators, which deliver the bark with considerable

impetus, the screening can be accomplished by placing a screen

diagonally below the mill, through which the finer parts are projected.

It is, however, essential that this screen should be quite smooth on its

upper surface and very strong, as ordinary wire gauze is immediately cut

through by the impact of the material. What are called “locked wire

screens” in which the wires are supported by being actually twisted

round the transverse bars are very suitable. Where the circumstances

will not permit of screening in this way, cylindrical rotating screens,

or nearly horizontal screens vibrated by an eccentric may be used. The

latter are cheaper to erect and have the advantage that they take up

less room, and by having lengths of wirework or perforated steel of

different coarseness, the material may be separated into more than one

degree of fineness.

[Illustration: FIG. 75.--Bark-Breaker.]

Oak-bark as it is taken off the trees is usually in lengths of perhaps

three feet, and it is necessary to cut or break it into smaller

fragments before it can be ground in most of the machines just

described. This is frequently done by hand by chopping the bark into

pieces about four inches long, and the operation is known as “hatching.”

Machines on the principle of the chaff-cutter, consisting of a fly-wheel

with curved blades radially attached to it, are sometimes used. Instead

of “hatching” it, the bark is frequently broken by passing through

toothed rollers fitting into each other, and often attached to the mill;

the construction of this machine will be readily understood from Fig.

75.

In Belgium, and some other bark-producing districts, the adhering moss

and dead outside bark are usually removed before hatching, but

apparently these impurities are frequently re-mixed with the bark after

the hatching is completed! As such barks often also contain much clay

and dirt, it is generally expedient to pass the hatched bark over a

coarse screen before letting it enter the mill, so as to remove the

greater part of such rubbish, since, if left in the bark, it produces

black and unsatisfactory liquors.

In drawing up policies for fire insurance, it is usual to charge a

higher rate where disintegrators are used to grind the tanning material,

as owing to the amount of dust and the production of sparks by the

striking of the steel parts of the machine on any chance piece of flint

or metal which may get into it, there is a greater liability to fire

than with toothed mills, although with proper precautions the risk is

really small. (Cp. p. 446.)

All disintegrators act like ventilating fans, and suck in air with the

material, blowing it out again with great force at the periphery. This

air is heavily laden with dust from the tanning material which is

extremely irritating to the lungs. The difficulty is to some extent

remedied by an air-channel or flue (generally cast in the casing of the

machine) connecting the discharge with the feed-opening so as to convey

the air back to the disintegrator. The air is thus circulated through

the arrangement, but some is always drawn in from the external

atmosphere and driven out with the ground material, and it is advisable

that the chamber into which it is discharged should be provided with

some means of filtering the air before it escapes. One convenient method

is to have a large flannel bag which is blown out by the air like a

balloon and out of which the dust can be shaken when the machinery has

stopped. Another efficient method is to have one of the walls or the

ceiling of the chamber made of canvas or of sacking; but in any case the

air should be allowed an escape where a little dust will not cause

annoyance.

_Chain-Conveyors._--While, in England, the ground material is usually

carried from the mill to the leaches in barrows or baskets, in America

the use of conveyors is practically universal, and there is no doubt

that they effect a great saving of labour at a comparatively small cost.

The most practical conveyor for tanning materials consists of a trough

through which an endless chain passes, carrying scrapers. The chain

generally used for this purpose is one consisting of square links

fitting into each other and capable of running over toothed wheels.

These chains are made by several firms in America, and in England by the

Ewart Chain Conveyor Co., of Derby, who supply not only plain links but

also those having projections to which buckets, scrapers and a variety

of attachments may be fixed.

[Illustration: FIG. 76.--Chain-Conveyor.]

In many cases the trough is V-shaped with the chain running in the

angle; in others flat-bottomed as in the illustration, or rectangular.

The scrapers may consist either of metal or of wood; and where materials

have to be carried up a steep incline buckets instead of scrapers should

be employed. The arrangement of such a conveyor is illustrated by Fig.

76.

A useful form of conveyor for dry materials consists in a woven cotton

belt running in a smooth trough and with laths riveted across it at

intervals. These laths should project slightly beyond the edges of the

belt so as to prevent wear. Care must be taken with belts of this sort

that the material does not get between the belt and the pulley.

Chain-carriers are often used for conveying the spent tan to the

furnaces from the leaches, and occasionally for carrying skins.

Several other kinds of conveyor are in use in corn-mills, spiral or worm

conveyors which work on the screw principle being very largely used for

carrying corn. They are not very suitable for tanning materials on

account of the coarseness of the latter, by which the friction is

greatly increased; they are however occasionally used. Those built up of

separate blades are specially to be avoided.

An ingenious form of conveyor has been recently introduced from Germany,

and consists of a light trough supported on steel springs and vibrated

longitudinally by means of an eccentric in such a way as to shake the

material from one end of the carrier to the other; the velocity of

motion of the trough being less in the outward than the return stroke,

so that the material is carried with it as it moves forward and slides

over it in its return. It is obvious that the principle may also be

applied to screening or sifting.

CHAPTER XXII.

_THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF EXTRACTS._

_Leaching._--The material, having been reduced to a suitable state of

fineness, is ready for extraction. This requires a considerable amount

of time, as the tannin is contained in cells whose walls are of a

wood-like substance (cellulose and lignine), through which the water

diffuses but slowly. Hence, unless the material be very finely ground, a

long soaking will be necessary before it becomes “spent.” It should be

the aim of the tanner to have his barks, etc. ground so finely that they

may be extracted as rapidly as possible, and yet not be so fine that

they settle to a compact mass in the leaches and so prevent circulation.

Using the present methods of extraction on the large scale it is

necessary to have the material only somewhat coarsely ground or crushed,

so as to render its percolation practicable; but it is quite possible

that in the near future some better mechanical means will be found of

treating the dust and other excessively finely ground matter so as to

bring about a very rapid extraction.

Up to perhaps 150 years ago, no attempt was made to leach the tanning

material, which was simply strewed in layers between the hides, and

moistened with water. Leaching originated in England, and was first

applied merely to complete the exhaustion of the material which had been

already used for layers; but the use of even weak liquors instead of

water in the layers was found so advantageous, that new material was

soon applied to make stronger infusions. The earliest form of leach was

simply a pit with a perforated wooden “eye” or shaft down one corner, in

which a pump could be placed to remove the liquor without being choked

with solid matter. This was considerably improved by the addition of a

perforated “false bottom” to the pit, with which the eye communicated.

The perforations of the latter were found unnecessary, and it now serves

simply for pumping through, or for the manipulation of a plug in a hole

communicating with an underground “trunk” leading into a pump-well. The

false-bottom is best made of laths about 1 inch thick and 2 inches wide,

cut slanting so as to be wider on the upper than the lower surface,

which makes the spaces between them less liable to choke. The laths are

nailed on cross-battens with copper nails, which should be long enough

to clinch, ¹⁄₄-inch to ¹⁄₂-inch spaces being allowed between the laths

according to the fineness of the ground material. The lattice-bottom

should be in at least two sections, so as to allow of its easy removal

for cleaning, and should rest on detached blocks, which are best nailed

to the underside of the battens. A space of 2 inches to 3 inches below

the false bottom will prove sufficient if it is cleared every time the

pit is emptied, but not otherwise. Clearness from obstruction both below

the bottom and between the laths themselves is very important in

securing free running in the “press leach” system about to be described.

A section of the latticed bottom is shown in Fig. 77. The laths are

easily cut by employing a circular saw with a tilted table, and turning

the board at each cut. No advantage is gained by planing them.

[Illustration: FIG. 77.--Section of Leach-Bottom.]

As a strong liquor cannot be made by the use of a single leaching-pit, a

series of pits are now always employed, and it is the leaching,

systematic or otherwise, which determines how much of the total tannin

will be thrown away and lost in the “spent tan.” In the case of properly

extracted materials the “spent tan” will not contain more than one per

cent. of tanning matter, but the degree of extraction which is

profitable is dependent on the tanning material employed and the class

of leather to be produced.

The system of leaches now considered to be the best is based on the

“continuous” process of extraction. Of its different forms, the

“press-leach” is the simplest and in most cases is all that is required.

[Illustration: FIG. 78.--Plan and Section of Battery of Press-Leaches.]

A plan and vertical section of the leaches is shown in Fig. 78. Assuming

that the leaches have been working for some time and that the liquor in

the strongest leach has been run off to the tan-pits, or in the case of

manufacturing extracts to the decolorising tanks or evaporator, the last

vat in the series is now filled with water or spent liquor, which may be

heated by steam if desired, and this water, which completes the

exhaustion of the material in this vat, forces the liquor forward in the

whole series, so that it gets stronger and stronger as it passes from

vat to vat. The very weak liquor remaining in the last vat is now pumped

into a spare pit, or on to the next stronger vat, pressing the liquor

forward as before; the vat is emptied of the spent material and refilled

with new, and now becomes the head leach; and the strongest liquor is

pressed on to it by running water or weak liquor on the weakest vat.

As regards the construction of such a “battery” of leaches, details will

differ according to whether the usual English square sunk pits, or the

American form of circular tub-leaches is employed. In the former case

the vertical spouts connected with the space under the false bottoms are

usually made of wood, like the old fashioned “eye,” and placed at one

side or corner of each pit, and connected with the top of the next pit

by a short trough which may be open above or covered as preferred. Both

eyes and cross troughs must be of ample size, so as not to check the

running of the liquor, and for a set of six or eight leaches, the bottom

of the cross trough should be at least 10 or 12 inches below the actual

top of the leach, which should not be filled with material above that

level. The object of this is to allow of a sufficient fall from the

first to the last leach. Means must be provided for the temporary

closing of the cross-trough between the vats which form the first and

last leach. On a very small scale, this may be done with a plug; sliding

wooden doors are convenient, but difficult to keep tight. A hinged or

sliding door held against an indiarubber facing by a wedge or

toggle-joint would seem a practicable device.

If round tub-leaches are employed, the vertical connection may be

similarly made with a wooden trough, but copper tubes are almost

essential for the cross connections. If a vertical copper eye in the

centre of the leach be provided for boiling, or for emptying the leach

(p. 334), it may be utilised for the upflow by connecting it with the

cross pipe with a thin copper pipe of large diameter, which must be

movable for the purpose of casting the leach. A joint like that of a

stove-pipe will probably prove sufficiently tight, but if necessary may

be made tighter by rolling an indiarubber ring over it.

Six to eight leaches is generally a sufficient number to form a

press-leach “battery.” If more are connected in one series it will

usually be necessary to assist the circulation, either by pumping an

intermediate leach, or by one or more pumps on the Holbrook system, in

which a power-driven pump of simple construction is fitted in the eye of

the leach. It is hardly necessary to note that the liquor must run

_downward_ through the leaches, and _up_ through the vertical pipes, in

order to prevent mixture of the weaker with the stronger liquor.

Several additions and modifications to the system have been made with a

view of obviating the so-called “channel difficulty.” There is always a

fear on the part of some tanners that the liquid in the leaches may push

the material aside and form channels through it, thus preventing proper

extraction of the tanning matter. In the author’s opinion this evil has

been greatly exaggerated, as, unless the liquid be pumped from the

leaches at a very rapid rate while they are in circulation, it is not at

all easy for the formation of such channels to take place. In any case

it can be entirely avoided by turning over the material in the leaches

occasionally, so as to lighten it somewhat and rearrange it a little.

It may also be pointed out that the provision of a proper system for

pressing or circulating leaches does not prevent their being pumped off

as frequently as desired, though this is generally to be avoided, since

when the leach is emptied of liquor, the material tends to settle into a

compact mass, which is not easy to percolate, and which is liable to

shrink from the sides of the pit, thus causing the very trouble which it

is desired to avoid. There are some advantages in taking the first and

strongest liquors off the material in a separate tank, and then

finishing the exhaustion in the press leaches, since many materials

swell, and pack tightly when they are first wetted, but on the whole the

method hardly pays for its added cost.

The press-leach system as above described is well adapted for the

requirements of tanners, as its first cost is very small in addition to

that of the construction of the leaches themselves; it extracts the bark

well, and saves much labour in pumping, and greatly lessens the tendency

of the pumper to miss pits in the series, to save time, when the

master’s eye is not on him. Another advantage which is often important,

is that when the leaches are full, much more than a single liquor can be

run from the head-leach without pumping on; and similarly when they are

run down to their lowest level, much more than a single liquor can be

pumped on to the worst leach before it overflows. As the leaches flow

slowly in comparison to the rate at which liquors can be pumped by a

good steam pump, it is very advantageous to allow the pump to discharge

into a liquor-tank raised to such a height that the liquor can be run

from it into any leach at a suitable rate for the circulation, and it

also enables liquors to be pumped without waiting till room has been

found for them in the leaches. Similar tanks are very useful in running

liquors for the yard, and especially for the suspenders in a

sole-leather yard, enabling circulation to be kept up during the night,

and at other times when the pumps are not running. They may also be used

as filters for the suspender liquors by fitting them with false bottoms

covered with a layer of nearly spent tan. The liquors may be distributed

to the different pits and leaches by means of canvas hose-pipes, or,

what is often more convenient, by overhead troughs, carefully levelled,

and fitted with discharge valves where required. The latter are

conveniently made of lead in a hemispherical form, resting on an

indiarubber washer supported by a light brass casting, or a suitably

turned rebate in a block of wood. (Cp. p. 457 and Fig. 79.) Such valves

if good indiarubber is used, wear well, and are absolutely tight.

[Illustration: FIG. 79.--Valve for Liquor-Troughs.]

In England, leaches are usually sunk in the ground, and are frequently

made of brick and cement, or of large Yorkshire flagstones. Such leaches

are somewhat costly but very durable. Square wooden pits, puddled

outside with clay, are also used, and last well with cold, or even warm

liquors, but will not stand direct steaming, the wood gradually bending,

and allowing the clay to leak into the liquor, causing black stains. The

large round vats of thick pine, and often holding 10 or 12 tons, which

are generally used in the United States, stand boiling much better, and

are frequently supported above a tramway or conveyor, into which the

spent bark can be discharged through a manhole in the bottom. If this

method is adopted, it must be remembered that bark, and indeed most

other tanning materials, will not run through a hole like corn, but must

be cast into it, so that unless the vat is of great depth, it is simpler

and almost as easy to cast over the top. If the manhole is used, a

central hole must be made in the false bottom, and this must be

surmounted by a copper pipe made in sections of two or three feet, and

reaching to the top of the leach. When the pit is to be emptied, the top

length is removed, and the tan shovelled down the hole until the second

length is reached, and the process repeated. The central pipe serves

also for the circulation of the liquor when the pits are boiled, and may

be used as the ascending pipe for circulating on the press-leach system.

The question of the influence of temperature on extraction is discussed

on p. 344, but except where a pale colour is all important, it is

generally profitable to use a moderate degree of heat in extraction. In

the opinion of the writer (which is supported by a vast amount of

careful experiment) only the nearly exhausted leaches should be heated,

not merely to avoid discoloration, but to extract the maximum amount of

tannin. In American tanneries the boiling is frequently done by copper

coils fixed below the false bottoms of the vats, but such coils are very

costly, and, where weak liquors only are to be heated, seem to present

no advantage over a well-arranged system of heating by direct steam in

which care is taken that dry steam only is used, and that all water

condensed in steam pipes, and usually containing iron, is removed by

effective steam-traps. If steam is blown into cold liquor through an

open pipe, a very disagreeable rattling and vibration is produced, which

is not only annoying, but is very injurious to the leaches. This evil

may be avoided by the use of “silent boiling jets” on the principle of

the steam-jet water-raiser; and, following a suggestion of the writer,

these jets may be used at the same time to circulate the water through

the tanning material of the nearly exhausted vat, and so wash out the

last traces of tan. The simplest way to accomplish this is to lower the

boiling jet, directed upwards, and connected with a movable steam-pipe,

into the eye of the leach (which is preferably central) so that the

heated water flows over its top, and percolates downwards through the

material to be washed. Two forms of these boiling and mixing jets made

by Messrs. Körting are shown in Figs. 80 and 81.

[Illustration: FIGS. 80 and 81.--Boiling and Mixing Jets.]

Batteries of closed copper extractors, worked on the press system, and

similar to those used in extracting sugar from beetroot, have frequently

been advocated, but are very costly, and have no other advantage over

open vats than that the liquor can be forced through the series by

pressure, instead of circulating by gravity. No advantage is gained by

boiling under pressure, since even boiling in open vats has been shown

to destroy tannin, darken the colour of the liquor, and increase the

amount of insolubles, and higher temperatures are still more injurious.

Heating the weakest leach in the press-leach system promotes the even

circulation of the liquor, since the warm weak liquor is much lighter

than the colder and stronger liquors in the forward leaches, and so

floats on the top, and presses the stronger liquor uniformly downwards.

It also has the advantage that the liquors are cooled before they are

strong enough for the yard, while in tanneries where all the leaches are

heated, expensive tubular coolers are often employed. As the liquor

cools, much of the colouring matters and reds dissolved in the hot

liquor separate, and are filtered out by the tanning material, so that

much brighter and lighter coloured liquors are obtained.

[Illustration: FIG. 82.--Sprinkler-Leach.]

_Sprinkler-Leaches_, Fig. 82, were formerly used in many tanneries and

extract factories, especially in the United States. They were introduced

by Allen and Warren, and yield a liquor which is at first very strong,

but which becomes very rapidly weaker as the running is continued. These

leaches are similar in principle to the mashing-tub and sparger of the

brewer, but the process is not well adapted for tanners’ use, as the

material is left too much exposed to the air, which is apt to cause

oxidation and loss of tannin. It is also extremely difficult to

completely exhaust the material without using an impracticably large

volume of water. Sprinkler-leaches are arranged so as to spray the

liquor, or water, on to the top of the solid material which is to be

extracted at such a rate that it flows out just as rapidly as it flows

into the vat. Some idea of the great amount of oxidation and consequent

loss of tannin which takes place in this form of extractor may be

obtained when it is remembered that this same method is now used for the

destruction of sewage matter by spraying it on to beds of coke so that

it may be mixed with as much air as possible before it is attacked by

the bacteria of the coke-beds (see p. 473), and also to oxidise weak

alcohol to acetic acid in the “quick vinegar process.”

So far as extraction is concerned, there is no difference in principle

between the methods adopted by the tanner and the extract manufacturer,

though the latter usually works on a larger scale, and not unfrequently,

in order to increase his output, or the gravity of his extract, employs

a higher temperature. This is probably justified by practical

considerations in the manufacture of extracts from very low-grade

materials, such as oakwood, which only contains 2 to 3 per cent. of

tanning matter, or even of chestnut wood which is somewhat stronger, but

it is one of the causes why decoloration of the battery liquor is

generally necessary.

Dried blood is chiefly used as the decolorising agent, but a paste of

blood-albumen has been recently placed on the market, which is said to

be free from several of the disadvantages attending the use of the crude

material.

The liquor to be decolorised is run into a mixing vat fitted with a

steam coil capable of raising the temperature of the liquid to at least

80° C., and usually provided with a simple rotary stirring gear. The

liquor, as run into the mixing vat, must not have a temperature of more

than 48° C. (118° F.) nor a strength of more than about 20° Bkr. (sp.

gr. 1·020).

The blood or albumen dissolved in a little water, is added to the

contents of the vat, which are then well mixed, and the temperature is

raised to 70° C. when the albumen coagulates and carries down much of

the colouring matter. The solution is run into another tank where the

precipitate is allowed to settle, and the clear liquor is then drawn off

for the evaporation. The muddy portion, about 8 inches in depth, is

pumped through filter-presses (which can be cheaply constructed of

wood), the clear liquors going to the evaporators and the press-cakes

being dried for manure.

In addition to blood-albumen, several other substances, such as lead

acetate (sugar of lead), salts of alumina, casein and other albuminous

matters have been employed in the decoloration of extracts, but they are

by no means so efficient as albumen.

Decolorising always causes a loss of tanning matter, some of this being

carried down with the precipitated colouring matter; and is for this

reason to be dispensed with whenever its use is not really necessary. It

may often be avoided by careful extraction at moderate temperatures, and

this is especially to be aimed at in the case of strong tanning

materials, which easily yield battery liquors of much greater strength

than 20° Bkr., and which thus, if they can be sent direct to the

evaporator, save cost in evaporation, which is often an important

consideration.

Another method which is frequently used to brighten the colour of

extracts, is treatment with sulphurous acid. Dilute sulphurous acid

solution may be used for extraction, but a more common method is to pass

sulphur dioxide gas into the liquor before concentration. Sulphurous

acid acts partly as a weak acid, in decomposing compounds of the tannins

and colouring matters with bases, such as lime, iron, copper, but more

actively by reducing oxygen compounds and preventing oxidation.

Bleaching in this way does not actually destroy or remove the colouring

matters, which are apt to reappear on exposure to the air, either in the

liquor, or perhaps more often in the leather tanned with it, so that the

gain is frequently more apparent than real. If present in any

considerable quantities, sulphurous acid may also cause inconvenience by

its swelling action on the pelt, but is mostly expelled in

concentration.

Another process should perhaps also be mentioned here, though not

strictly a means of bleaching. Several tanning materials, and notably

quebracho and hemlock, contain large quantities of “difficultly soluble

tannins,” which render the liquors made from their extracts turbid on

cooling. These tannins form soluble compounds with alkalis and with

alkaline sulphites, in the latter case probably setting free the

sulphurous acid and combining with the base. This has been taken

advantage of in a recent patent[158] in which quebracho and other

extracts are rendered soluble by heating in closed vessels with

bisulphites, sulphites, sulphides, or even caustic alkalis; and many

“soluble quebracho extracts” made on this principle are now on the

market. In this case, even where bisulphites are used, the greater part

of the sulphurous acid, after serving its purpose in preventing

oxidation, escapes in course of manufacture, and the extracts remain

neutral or alkaline. There is no reason that such extracts should not

prove serviceable in tanning, but it has recently been shown by Paessler

that the alkaline tannin is not absorbed by neutral hide-powder, and it

therefore may lead, not only to discrepancies in analysis, but in case

of drum-tannage, where no acid is naturally present, to failure to

utilise the whole of the tannin, though, when added to ordinary liquors,

the acids contained in the latter will set free the tannins.

[158] Lepetit, Dollfus, and Gansser, Eng. Pat. 8582, 1896.

The use of ferrocyanides has been suggested as a means of precipitating

iron and copper present in extracts, and it may also be pointed out,

that with many red-coloured tanning materials, such as hemlock and

quebracho, the addition of small quantities of alum to the tanning

liquor effects considerable improvement in colour, not only by

precipitating a part of the difficultly soluble “reds,” but by

developing the yellow colour of certain colouring matters (quercetin,

myricetin, etc.) which may be present. Such an addition does no harm in

the case of soft leathers, but would probably be injurious in a

sole-leather tannage.

The liquors, whether direct from the leaches or from the decolorising

vats, must next be concentrated by evaporation (Chap. XXVI.), to sirupy

consistency for liquid extracts, or until they will become nearly solid

on cooling, if a solid extract is required. As has already been stated,

the action of heat tends to cause a loss of tannin and a darkening of

colour by decomposition and the formation of insoluble reds. To reduce

this loss to a minimum, the weak liquors are evaporated with as little

access of air and at as low a temperature as possible, and these

conditions are best obtained by the use of steam-heated vacuum pans.

[Illustration: FIG. 83.--Triple-effect Yaryan Evaporator.]

For concentration to gravities not exceeding 1·200, the Yaryan apparatus

made by Mirrlees, Watson and Yaryan, of Glasgow, is that most employed.

The general arrangement of a “triple effect” machine of this make is

shown in Fig. 83, and the internal construction in Fig. 84. Each body

consists of a strong casing into which steam is admitted, and which is

traversed by copper tubes which terminate in a separating chamber at the

further end, which is maintained at a low pressure by an air-pump. The

liquid to be evaporated is admitted into the tubes, and is immediately

converted into spray by the steam generated from it, and swept forward

into the separating chamber, from which it is withdrawn by a pump. The

steam before going to the air-pump (or, in the case of “multiple

effects,” to the next body), is passed through a “catch-all,” to

separate any spray still retained in the steam. Thus the liquor to be

evaporated will pass through the entire apparatus in four or five

minutes, and may be concentrated from a gravity of 1·02 or 1·03 to that

of 1·20 without ever having been heated above 70° C. (160° F.). Unless

fuel is very cheap, which is often the case where the spent tanning

material can be used to raise steam, it is advisable to use a double or

triple effect, in which the steam from the evaporation of the weakest

liquor in the first body is used to heat the second, which is maintained

at a lower vacuum, and so on. In this way the steam is made to do nearly

double or triple duty. As the steam from the extract-liquors contains

acids which corrode iron, it is necessary to have the casing as well as

the tubes made of copper in all bodies in which it is employed. Iron

must, in fact, be carefully avoided in every part of apparatus which

comes in contact with extract-liquor or its vapour. Besides the Yaryan,

there are several other evaporators in which the spray principle is

more or less completely employed. The simplest of these consists in

substituting for the heating coil of an ordinary vacuum-pan a copper

steam-box traversed by vertical tubes open at both top and bottom. This

is immersed in the liquid to be evaporated, which enters at the bottom

of the tubes and is sprayed out at the top. Paul Neubäcker, of Danzig,

constructs a pan on this principle with a very ingenious arrangement for

the destruction of foam, which seems worth attention.

[Illustration: FIG. 84.--Section of Yaryan Evaporator.]

It is unfortunately impossible to carry the evaporation of extracts much

further than sp. g. 1·2 with spray apparatus, as thicker liquors are apt

to clog the tubes, which are then difficult to clean, so that even

liquid extracts are usually finished in vacuum-pans of the ordinary

type, which may also be arranged in multiple effect.

In the case of a solid extract, the evaporation must be carried on until

it is as thick as can be run from the apparatus. To do this

satisfactorily, stirrers must be provided to keep the extract in motion

so long as it is in the pan. The thick, hot, liquid extract is then run

into boxes lined with paper, or other suitable material, where it is

allowed to cool and to solidify.

The pan for the final evaporation of solid extracts should be planned so

as to allow of easy cleaning and ready access to its interior, so that

if accidentally the evaporation is carried so far that the liquid will

not run out, the clearing of the pan may be a comparatively easy matter.

It is also important that the extract-exit should be of large size.

Probably a broad and somewhat shallow pan, heated merely by a steam

jacket, and fitted with rotating stirrers, is the most suitable.

_The Use of Extracts in the Tannery._--One of the great attractions of

extracts is that they save the trouble and cost of leaching, and as the

extract manufacturer makes this his specialty, he can often extract more

tanning matter from a material than the tanner who has no means of

concentrating his weak liquors. The extract manufacturer also can employ

methods of decoloration which would be impracticable to the tanner, and

so enable the latter to obtain better colour than if he employed the raw

material. By the use of extracts a tanner can strengthen weak liquors

without trouble, and with definite quantities of materials; and by

using extracts for this purpose the tanner is enabled to use up the

weaker liquors of his leaches and so employ more water and obtain better

extraction of his solid materials than if he used them alone. In the

case of very weak materials like oakwood, the difficulties of making

liquors of sufficient strength for tanning without evaporation are so

great as to render such materials useless to the tanner for his own

extraction, and their carriage even for short distances may amount to

more than their total value. Even with much richer materials, extraction

effects a saving if the carriage is a long one, as it rarely pays to

import any material containing less than about 25 per cent. of tanning

matter. Even when the strength of the natural material is considerable,

as in the case of quebracho, extraction may be profitable if from its

hardness, or other reasons, the material is difficult for the tanner to

handle. For long voyages, and especially from the tropics, solid

extracts are more suitable than liquid, as the expense of casks is

saved, and the danger of fermentation is lessened. As it is impossible

for the tanner to judge by appearance or consistency of the strength or

value of extracts, they should always be bought and sold on the analysis

of the particular shipment or parcel by a competent chemist. For

directions for sampling see pp. 301, 475.

Extracts simply require to be dissolved in a suitable quantity of water

or weak liquor at an appropriate temperature, to obtain a liquor of any

required strength. Some extracts are completely soluble in cold water or

liquor, but most dissolve better by the aid of heat. 40°-60° C.

(100°-140° F.) is generally sufficient, and probably no advantage can

arise from temperatures over 80° (180° F.). Boiling should be avoided,

as it facilitates the formation of insoluble “reds” with consequent loss

of tanning matter and darkening of colour. The extract should be run

into the vat in a thin stream, and continuously plunged up; where large

quantities of extract are to be dissolved, a mechanical agitator is

advantageous. A “silent boiling jet” (p. 335) may be used, fitted into a

small casing immersed in the liquor and open at both ends, and the

extract run into the current it produces.

Whether in the manufacture of extracts, or for direct use in the

tannery, the temperature at which tanning materials are extracted is of

prime importance. It is a common mistake to assume that the largest

amount of tannin is extracted by boiling. Mr. A. N. Palmer has pointed

out that this is by no means the case, but that each material has an

_optimum_ temperature of extraction, at which more tannin is extracted

than at any other; and the question has been carefully investigated by

J. G. Parker and the author,[159] with results which are given in the

following tables. For many purposes the colouring matter which

accompanies the tannin is a serious disadvantage, and it is usually most

extracted at the higher temperatures; and on this account it is

necessary for the tanner who will work his leaches economically to

ascertain at what temperature he can extract the largest amount of

tannin combined with no more colouring matter than he can permit to

enter his leather. Most materials are satisfactorily extracted at

50°-60° C., but as a general rule it is best to begin cold or nearly so,

and only raise the temperature as the extraction proceeds. The tables

show the percentages of tanning matter, and the amount of colour (as

measured by Lovibond’s tintometer), obtained by extracting materials in

a Procter’s extractor (p. 306 and L.I.L.B., p. 102) so long as any

colour or tannin could be obtained.

[159] Journ. Soc. Ch. Ind., 1895, 635.

BELGIAN OAK BARK.

-----------+---------+-----------+---------+---------------+---------

| | | Yield. +-------+-------+ Yield.

| | | | Red. |Yellow.|

-----------+---------+-----------+---------+-------+-------+---------

°C. |per cent.| per cent. | | deg. | deg. |

15 | 5·9 | 5·1 | 61·9 | 8·6 | 23·1 | 57·4

15-30 | 6·8 | 5·5 | 70·7 | 9·2 | 26·4 | 64·5

30-40 | 8·0 | 5·5 | 83·5 | 11·6 | 30·4 | 76·1

40-50 | 8·2 | 5·7 | 84·2 | 12·0 | 32·1 | 80·0

50-60 | 8·5 | 5·8 | 87·6 | 12·5 | 36·0 | 84·0

60-70 | 9·1 | 5·9 | 95·5 | 13·1 | 38·1 | 92·7

70-80 | 9·2 | 6·0 | 95·7 | 14·7 | 38·9 | 98·7

80-90 | 9·6 | 6·0 | 100·0 | 14·0 | 36·9 | 93·2

90-100 | 9·6 | 6·1 | 100·0 | 14·0 | 41·2 | 94·6

¹⁄₂ hour | 9·1 | 6·6 | 93·7 | 15·0 | 42·6 | 100·0

-----------+---------+-----------+---------+-------+-------+---------

MYROBALANS.

-----------+---------+---------+---------+--------------+---------

of | Matters.| Non- | of | ¹⁄₂ per cent.| of

| | | Yield. +------+-------+ Yield.

| | | | Red. |Yellow.|

-----------+---------+---------+---------+------+-------+---------

°C. |per cent.|per cent.| | deg. | deg. |

15 | 28·5 | 12·8 | 79·2 | 1·09 | 4·9 | 97·4

15-30 | 30·1 | 13·6 | 83·6 | 1·00 | 4·1 | 82·5

30-40 | 32·3 | 14·3 | 89·8 | 1·03 | 4·1 | 82·7

40-50 | 33·5 | 13·6 | 93·0 | 1·03 | 4·2 | 84·4

50-60 | 34·7 | 14·4 | 96·4 | 1·03 | 4·4 | 87·6

60-70 | 34·8 | 14·4 | 96·6 | 1·03 | 4·5 | 89·3

70-80 | 34·9 | 14·9 | 96·8 | 1·10 | 4·7 | 94·1

80-90 | 35·1 | 15·0 | 97·4 | 1·16 | 4·8 | 96·7

90-100 | 36·0 | 14·9 | 100·0 | 1·12 | 4·9 | 97·0

Boiled | 35·4 | 15·5 | 98·1 | 1·26 | 4·9 | 100·0

-----------+---------+---------+---------+------+-------+---------

SMYRNA VALONEA.

-----------+---------+---------+---------+------------------+---------

of | Matters.| Non- | of | ¹⁄₂ per cent. | of

| | | Yield. +----+-------+-----+

-----------+---------+---------+---------+----+-------+-----+---------

°C. |per cent.|per cent.| |deg.| deg. |deg. |

15 | 25·5 | 19·1 | 70·5 | 2·5| 6·0 | 0·3 | 74·6

15-30 | 29·1 | 18·3 | 74·5 | 2·5| 6·4 | 0·3 | 78·0

30-40 | 33·6 | 18·1 | 86·2 | 2·3| 6·4 | 0·3 | 76·2

40-50 | 35·5 | 18·1 | 86·2 | 2·3| 6·5 | 0·3 | 74·6

50-60 | 39·1 | 16·6 | 100·0 | 2·0| 6·0 | 0·3 | 76·2

60-70 | 38·6 | 17·0 | 99·0 | 2·0| 6·8 | 0·3 | 84·7

70-80 | 38·8 | 17·5 | 99·5 | 2·1| 7·4 | 0·4 | 84·7

80-90 | 36·9 | 17·2 | 95·0 | 2·2| 7·6 | 0·4 | 84·7

90-100 | 36·6 | 17·0 | 94·0 | 2·4| 7·8 | 0·5 | 90·6

Boiled | 35·4 | 17·6 | 90·6 | 3·0| 8·2 | 0·6 | 100·0

-----------+---------+---------+---------+----+-------+-----+---------

GREEK VALONEA.

-----------+---------+---------+---------+------------------+---------

of | Matters.| Non- | of | ¹⁄₂ per cent |of Colour

| | | Yield. +----+-------+-----+of Tannin

-----------+---------+---------+---------+----+-------+-----+---------

°C. |per cent.|per cent.| |deg.| deg. | deg.|

15 | 16·0 | 13·0 | 64·0 | 2·9| 6·3 | 0·3 | 67·3

15-30 | 18·1 | 12·6 | 72·4 | 3·0| 6·6 | 0·3 | 70·0

30-40 | 21·1 | 12·0 | 84·4 | 2·8| 6·5 | 0·3 | 68·0

40-50 | 23·6 | 12·1 | 94·4 | 2·4| 6·6 | 0·3 | 65·9

50-60 | 24·8 | 12·4 | 99·2 | 2·7| 7·0 | 0·4 | 71·6

60-70 | 25·0 | 12·6 | 100·0 | 2·9| 7·3 | 0·5 | 75·8

70-80 | 24·6 | 12·5 | 98·4 | 3·1| 7·9 | 0·6 | 82·3

80-90 | 24·0 | 12·5 | 96·0 | 3·4| 8·1 | 0·6 | 85·8

90-100 | 23·6 | 12·6 | 94·4 | 3·5| 8·8 | 0·7 | 92·0

Boiled | 22·6 | 13·0 | 88·8 | 3·9| 9·4 | 0·8 | 100·0

-----------+---------+---------+---------+----+-------+-----+---------

NATAL MIMOSA.

-----------+---------+---------+---------+--------------+----------

of |Matters. | Non- | of | ¹⁄₂ per cent | of

| | | +----+---------+

| | | |Red.| Yellow. |

-----------+---------+---------+---------+----+---------+----------

°C. |per cent.|per cent.| |deg.| deg. |

15 | 21·2 | 11·6 | 66·2 | 2·6| 4·1 | 51·1

15-30 | 29·0 | 9·8 | 90·6 | 3·0| 4·1 | 54·2

30-40 | 30·1 | 9·8 | 94·0 | 3·0| 4·4 | 56·5

40-50 | 30·2 | 9·8 | 94·4 | 3·1| 5·0 | 61·8

50-60 | 30·4 | 10·4 | 95·0 | 3·9| 6·5 | 79·9

60-70 | 31·5 | 10·6 | 98·4 | 4·2| 6·5 | 81·6

70-80 | 32·0 | 10·8 | 100·0 | 4·2| 7·0 | 85·5

80-90 | 30·8 | 11·2 | 96·2 | 4·9| 7·4 | 93·8

90-100 | 30·1 | 11·8 | 94·0 | 5·3| 7·8 | 100·0

Boiled | 29·4 | 12·0 | 91·8 | 5·7| 7·2 | 98·4

-----------+---------+---------+---------+----+---------+----------

SUMACH.

-----------+---------+---------+---------+--------------+---------

of |Matters. | Non- | of | ¹⁄₂ per cent | of

| | | +----+---------+

| | | |Red.| Yellow. |

-----------+---------+---------+---------+----+---------+---------

°C. |per cent.|per cent.| |deg.| deg. |

15 | 14·2 | 17·8 | 70·0 | 1·6| 5·4 | 63·6

15-30 | 17·6 | 18·1 | 86·7 | 1·4| 4·3 | 51·8

30-40 | 18·5 | 18·1 | 91·1 | 1·3| 4·4 | 51·8

40-50 | 20·1 | 18·5 | 99·0 | 1·4| 4·4 | 52·9

50-60 | 20·3 | 19·1 | 100·0 | 1·5| 4·7 | 56·5

60-70 | 19·0 | 19·4 | 93·6 | 1·7| 5·6 | 66·6

70-80 | 18·0 | 19·9 | 89·1 | 1·9| 6·2 | 72·8

80-90 | 16·9 | 21·1 | 83·2 | 2·3| 6·8 | 82·7

90-100 | 16·6 | 22·3 | 81·7 | 2·6| 7·0 | 87·7

Boiled | 15·2 | 24·0 | 74·8 | 3·3| 7·7 | 100·0

-----------+---------+---------+---------+----+---------+---------

QUEBRACHO WOOD.

-----------+---------+---------+---------+--------------+---------

of |Matters. | Non- |of Tannin|¹⁄₂ per cent | of

| | | +----+---------+

| | | |Red.| Yellow. |

-----------+---------+---------+---------+----+---------+---------

°C. |per cent.|per cent.| |deg.| deg. |

15 | 7·6 | 2·2 | 35·0 | 8·9| 14·1 | 71·3

15-30 | 10·1 | 2·4 | 46·5 | 6·4| 10·7 | 68·7

30-40 | 11·8 | 2·4 | 54·4 | 5·9| 9·6 | 65·2

40-50 | 15·1 | 2·4 | 69·5 | 5·3| 8·4 | 60·0

50-60 | 16·5 | 2·4 | 76·0 | 5·4| 8·5 | 60·4

60-70 | 17·4 | 2·4 | 80·0 | 5·6| 8·2 | 59·9

70-80 | 19·1 | 2·7 | 88·0 | 6·4| 8·6 | 67·4

80-90 | 21·7 | 3·0 | 100·0 | 6·4| 9·4 | 74·3

90-100 | 19·5 | 3·0 | 89·8 | 6·6| 9·8 | 100·0

-----------+---------+---------+---------+----+---------+---------

MANGROVE BARK (_Ceriops_).

-----------+---------+---------+---------+--------------+---------

of | Matters.| Non- |of Tannin|¹⁄₂ per cent. | of

| | | +----+---------+

| | | |Red.| Yellow. |

-----------+---------+---------+---------+----+---------+---------

°C. |per cent.|per cent.| |deg.| deg. |

15 | 13·0 | 10·4 | 61·6 |14·2| 20·8 | 64·7

15-30 | 16·1 | 10·4 | 76·3 |16·1| 21·7 | 69·8

30-40 | 17·4 | 12·5 | 82·4 |15·8| 23·0 | 71·7

40-50 | 18·5 | 11·4 | 87·7 |16·5| 33·5 | 73·8

50-60 | 20·3 | 10·3 | 96·2 |16·0| 23·4 | 72·8

60-70 | 20·0 | 11·4 | 94·7 |17·5| 31·2 | 90·0

70-80 | 20·4 | 11·2 | 96·7 |16·5| 28·3 | 82·8

80-90 | 21·1 | 10·8 | 100·0 |15·4| 24·6 | 73·8

90-100 | 20·2 | 11·4 | 95·7 |23·0| 34·1 | 100·0

-----------+---------+---------+---------+----+---------+---------

CANAIGRE ROOT (three years old).

_Effect of Different Temperatures._

-----------+---------+---------+---------+-------------------+-------

| by Hide.| Matters.| Maximum +----+-------+------+ Colour

-----------+---------+---------+---------+----+-------+------+-------

°C. |per cent.|per cent.| |deg.| deg. | deg. |

15 | 21·1 | 13·0 | 78·7 | 1·6| 4·1 | 5·9 | 41·5

15-30 | 26·2 | 12·5 | 85·6 | 1·6| 3·8 | 4·4 | 38·0

30-40 | 28·1 | 12·5 | 91·8 | 1·4| 3·7 | 5·1 | 35·9

40-50 | 30·5 | 13·1 | 99·6 | 2·1| 4·2 | 6·3 | 44·3

50-60 | 30·6 | 13·6 | 100·0 | 2·4| 4·8 | 7·2 | 50·7

60-70 | 27·2 | 14·1 | 88·8 | 2·5| 5·0 | 7·5 | 52·7

70-80 | 26·4 | 14·6 | 86·2 | 2·8| 6·1 | 8·9 | 62·6

80-90 | 23·2 | 14·8 | 75·8 | 3·1| 6·9 | 10·0 | 70·4

90-100 | 22·8 | 14·8 | 74·5 | 4·3| 7·4 | 11·7 | 82·4

¹⁄₂ hour | 19·2 | 12·3 | 62·7 | 5·6| 8·6 | 14·2 | 100·0

-----------+---------+---------+---------+----+-------+------+-------

CUBE GAMBIER.

_Effect of Different Temperatures._

-----------+---------+---------+---------+-------------------+-------

| by Hide.| Matters.| Maximum +----+-------+------+ Colour

-----------+---------+---------+---------+----+-------+------+-------

°C. |per cent.|per cent.| |deg.| deg. | |

15 | 46·8 | 21·8 | 78·0 | 2·5| 7·8 | 10·3 | 57·2

15-30 | 48·8 | 21·0 | 81·3 | 1·7| 8·0 | 9·7 | 54·9

30-40 | 50·2 | 22·0 | 83·7 | 1·7| 8·6 | 10·3 | 57·2

40-50 | 51·9 | 23·0 | 86·5 | 1·7| 8·8 | 10·5 | 58·3

50-60 | 51·1 | 20·3 | 91·9 | 1·7| 8·9 | 10·6 | 58·8

60-70 | 55·6 | 20·3 | 92·7 | 1·9| 9·4 | 11·3 | 62·7

70-80 | 55·7 | 20·3 | 92·8 | 2·2| 10·1 | 12·3 | 68·3

80-90 | 55·8 | 21·2 | 93·1 | 2·3| 10·6 | 12·9 | 71·6

90-100 | 56·1 | 22·0 | 93·3 | 2·8| 11·6 | 14·4 | 80·0

¹⁄₂ hour | 60·0 | 20·0 | 100·0 | 3·2| 14·8 | 18·0 | 100·0

-----------+---------+---------+---------+----+-------+------+-------

BLOCK GAMBIER.

_Effect of Different Temperatures._

-----------+---------+---------+---------+-------------------+-------

| by Hide.| Matters.| Maximum +----+-------+------+ Colour

-----------+---------+---------+---------+----+-------+------+-------

°C. |per cent.|per cent.| |deg.| deg. | |

15 | 30·1 | 27·4 | 50·1 | 2·6| 8·1 | 10·7 | 33·5

15-30 | 34·8 | 26·2 | 69·6 | 2·4| 8·0 | 10·4 | 34·0

30-40 | 40·8 | 27·2 | 81·6 | 2·0| 9·0 | 11·0 | 55·0

40-50 | 44·8 | 27·6 | 89·6 | 2·4| 9·8 | 12·2 | 61·0

50-60 | 46·8 | 27·8 | 93·6 | 2·4| 10·1 | 12·5 | 62·5

60-70 | 47·3 | 27·6 | 94·6 | 2·5| 10·6 | 13·2 | 66·0

70-80 | 47·4 | 27·6 | 94·7 | 2·8| 10·9 | 13·7 | 63·5

80-90 | 47·6 | 27·3 | 95·2 | 3·2| 11·6 | 14·8 | 74·0

90-100 | 48·2 | 27·1 | 96·4 | 3·8| 12·8 | 16·6 | 83·0

¹⁄₂ hour | 50·2 | 26·4 | 100·0 | 5·0| 15·0 | 20·0 | 100·0

-----------+---------+---------+---------+----+-------+------+-------

CHAPTER XXIII.

_FATS, SOAPS, OILS AND WAXES._

Fats and oils constitute a large class of substances, of animal or

vegetable origin, which may be solid, pasty or more or less viscous

liquids, but which in the latter case are commonly known as “fixed” or

fatty oils, to distinguish them from the volatile, or essential oils,

which may be distilled without decomposition, and which are the source

of most of the odours of plants, and of quite different chemical

constitution. The term “oil” is also applied to various products of

mineral origin, and especially to those derived from petroleum, on

account of their similarity in appearance and physical properties to the

fixed oils, though, chemically, they form a very distinct class. The

waxes are another group somewhat closely allied to the fats; and there

are certain fixed oils, such as sperm oil, which though very similar in

appearance and properties to the fatty oils, are chemically members of

the group of waxes.

As it is obvious that there is no chemical distinction between the fats

and fatty oils, except that of melting-point, it will be convenient to

treat them together; especially as what is a solid fat in one climate

may be an oil in another. Palm and cocoa-nut oils are cases in point, as

the first is buttery, and the second a hard fat in this country, though

they are both liquid in tropical climates.

For more detailed information on the chemistry of fats and oils, the

reader must be referred to the ‘Leather Industries Laboratory Book,’

sect. xviii., or to the larger manuals devoted specially to the subject

by Lewkowitsch, Jean, and others, or the very excellent section on oils

in Allen’s ‘Commercial Organic Analysis,’ vol. ii.; but a few general

facts must be recapitulated.

The true fats contain carbon, hydrogen and oxygen, but no nitrogen. They

are all compounds of glycerin with organic acids which are generally

termed “fatty acids,” and which resemble in many of their

characteristics the fats themselves. Glycerin is a very weak base, of

the nature of an alcohol, and consequently, when a fat is heated with a

solution of one of the caustic alkalis, the fatty acid combines with the

latter, and the glycerin is set free. The salts thus formed are

denominated “soaps.” The reaction with stearin (glycerin stearate), the

principal constituent of hard animal fats, is shown in the following

equation.

Stearin Sodium Sodium Glycerin

hydrate stearate

(C₁₇H₃₅CO.O)₃C₃H₅ + 3NaOH = 3C₁₇H₃₅CO.ONa + C₃H₅(OH)₃.

If a soap is treated with an acid stronger than its own, the latter is

set free, while the new acid combines with the base. The following

equation, for instance, shows the action of hydrochloric acid on the

stearic soap.

Sodium Hydro- Stearic acid Sodium

Stearate chloric chloride

acid

C₁₇H₃₅CO.ONa + HCl = C₁₇H₃₅CO.OH + NaCl.

If any soap be dissolved in hot water, and sufficient hydrochloric or

sulphuric acid added to render the solution acid, the latter will turn

first milky, and (if it be kept warm) the fatty acid will finally rise

in an oily layer to the surface, which in many cases will harden, as it

cools, to a solid mass. The amount of fatty acid in a soap may be

roughly determined by weighing 25 grm., dissolving in 50 c.c. of boiling

water, and adding excess of acid, and allowing the reaction to take

place in a graduated cylinder, or a flask with a graduated neck, in a

vessel of boiling water. When the fatty acid has risen to the top, its

volume may be noted, and each c.c. may be roughly reckoned as 0·9 grm.

(For more detailed methods cp. L.I.L.B., Sect. XVII.).

Soaps are insoluble in strong caustic alkaline solutions, and therefore

saponification (as the decomposition of fats by alkalis is called), does

not readily take place in them, and for this reason the soap-boiler

generally dilutes his caustic soda solutions to a strength not exceeding

18° Tw. (sp. gr. 1·090) in gravity, and separates the soap at the end

of the operation, by the addition of brine, in which it is insoluble. An

easier method, and one which is often useful for the preparation of

small quantities of special soaps for fat liquors and the like, is as

follows.[160] 10 lb. of a _good_ caustic soda, free from common salt, is

dissolved in 4 gallons of water, and 75 lb. of oil or fat is warmed to

about 25° C. or just sufficiently to render it liquid, and the soda

solution is added in a thin stream, with constant stirring, which must

be continued until the mass becomes too pasty. It is now set aside in a

warm place for at least twenty-four hours, during which saponification

gradually takes place. For leather purposes, a neutral soap, with a

slight excess of fat, is generally advantageous, so that the fat may be

increased to 80 lb.; or, in place of this, the operation will be

facilitated by the addition of 5 lb. of commercial oleic acid. If soft

soap is desired, 14 lb. of caustic potash may be used in place of the 10

lb. of caustic soda. The hardness or softness of soaps varies to some

extent with the fat used, but potash soaps are always much softer than

the corresponding soda soaps. It is obvious that with soaps made in this

way, all the glycerin remains mixed with the soap. If, on testing, the

soap does not prove to be free from caustic, it may be re-melted, which

will generally complete the reaction. Before attempting to work with

large quantities, a laboratory experiment is desirable, using 10 grm. of

soda in 40 c.c. of water, and 75 to 80 grm. of oil or fat. The

neutrality or freedom of the soap from caustic alkali may be tested by

touching a freshly cut surface with an alcoholic solution of

phenolphthalein, which the least trace of caustic soda or potash will

render pink.

[160] Carpenter, ‘Soap, Candles and Lubricants,’ p. 144.

If solutions of soaps are mixed with those of salts of the heavy metals

or of the alkaline earths, a mutual decomposition takes place, the acid

of the salt combining with the alkali of the soap; and the fatty acid

with the metallic base, to form a metallic soap. Most of these soaps are

sticky masses, insoluble in water, but not unfrequently soluble in

turpentine or petroleum spirit, if previously thoroughly dried, so that

some of them have been applied to the production of varnish. Alumina

soaps are occasionally used to thicken mineral oils, or render them more

viscous. The general reaction of the stearin soap with calcium sulphate

is shown in the following equation, though in practice it is sometimes

more complex:

Stearin soap Calcium Sodium Calcium

sulphate sulphate stearate

2C₁₇H₃₅CO.ONa + CaSO₄ = Na₂SO₄ + (C₁₇H₃₅CO.O)₂Ca

This is the reaction which causes the curdling of soap by hard water,

page 93.

True fats cannot be distilled alone without decomposition. When

distilled in a current of steam, some undecomposed fat passes over, but

the greater part is broken up into free fatty acid and glycerin; and

hydrocarbons practically identical with mineral oils are also formed.

Fats and oils are insoluble in water, and in most cases only sparingly

soluble in alcohol, but freely soluble in ether, petroleum spirit,

benzene, and most other hydrocarbons, as well as in chloroform, carbon

tetrachloride, and carbon disulphide. Petroleum spirit, often called

benzine, is largely used for their extraction, and for de-greasing

leather, and removing grease from clothes. In the laboratory, carbon

disulphide, or carbon tetrachloride is to be preferred. Castor oil is an

exception to the rule, owing to the large proportion of oxygen which it

contains, being readily soluble in alcohol, and very sparingly in

petroleum-spirit; and other oils, when oxidised, usually become more

soluble in alcohol, and less so in hydrocarbons.

Oils vary much in their tendency to “dry,” or become converted into

solid or sticky resin-like substances. This tendency is greatest in some

of the seed oils, and least in olive oil, and the oily part of animal

fats (tallow oil, neatsfoot oil). Sperm oil, a “liquid wax,” is also

very free from this tendency, but all fish oils possess it in a greater

or less degree. It is not due to evaporation, but to the absorption of

oxygen by the fatty acid. The tendency to oxygen-absorption, and

consequently to drying (and, in the case of leather-oils, to “spueing”),

is measured analytically by the “iodine-value,” the absorption of iodine

being proportional to that of oxygen, while it is much more easily

measured.

There are no simple tests by which the purity of oils can be determined,

though in a few cases the presence of particular oils can be detected.

The mixing and adulteration of oils is now a science, and those who

practise it are well acquainted with the customary tests, and take care

to adjust their mixtures so as to meet them. Taste and smell however,

with practice, often furnish useful indications.

Natural oils and fats are invariably mixtures of the glycerides of

several fatty acids, and their qualities depend simply on the character

of these glycerides and the proportions in which they are mixed. The

fatty acids form several groups, differing in their degree of

“saturation,”[161] or, inversely, in their power of taking up oxygen, on

which their tendency to drying depends. The members of any one of these

groups resemble each other strongly, differing principally in melting

points, density, and other physical characteristics.

[161] A “saturated” compound is one, the constituents of which are

present in such proportions that all the combining affinities of each

are satisfied by the others. Iodine value, see L.I.L.B., p. 176, and

Jour. Soc. Ch. Ind., 1902, p. 454.

_Saturated Fatty Acids._--Stearic acid, C₁₈H₃₅O.OH, and palmitic acid,

C₁₆H₃₁O.OH, are the most important. At ordinary temperatures they are

hard, white, crystalline bodies, and melt at 69° and 62° C.

respectively. They do not, under ordinary circumstances, absorb any

oxygen, nor iodine, and are very little liable to chemical change.

Together with oleic acid, they are the principal acids of tallow and

other animal fats, while palmitic acid and some lower members of the

same group are more common in vegetable oils. Free stearic acid is an

important constituent of the “distilled stearines” used in currying;

while “oleostearine” consists mainly of the neutral fats or glycerides

of stearic and palmitic acids.

_Liquid Fatty Acids, Non-drying._--Of these, oleic acid is much the most

common and important; its glyceride, olein, forming the liquid part of

animal fats, and being the principal constituent of vegetable non-drying

oils. Olive oil consists almost entirely of olein, with a little

palmitin. The formula of oleic acid is C₁₈H₃₃O.OH, thus differing from

stearic acid in having two less atoms of hydrogen. The “bonds” or

affinities corresponding to these two atoms are linked together, but can

separate, and attach two atoms of iodine, bromine, or chlorine, or one

of oxygen. The iodine-value of pure olein is 83·9 (that is, 100 grm.

absorb 83·9 grm. iodine); and that of olive oil about 83. Any oil with a

higher “iodine-value” than olein must contain drying oils, though a

lower value does not necessarily indicate their absence, if palmitin or

other saturated acids are also present.

_Unsaturated Liquid Fatty Acids._--Of these there are several groups,

differing in their degree of saturation, and also probably in their

structure. Their glycerides, together with olein, and sometimes

palmitin, are the constituents of the seed oils, the drying tendency of

which depends on their proportion of unsaturated acids, and the

particular group to which they belong. The fish oils contain a peculiar

group of unsaturated acids, together with olein, and usually stearin and

palmitin, like the other animal fats. Linolenic acid, C₁₈H₂₉O.OH, one of

the acids of linseed oil, has six hydrogen atoms less than stearic acid,

and therefore three double linkings, and will take up six atoms of

iodine. Its theoretical iodine-value is 274, while linseed oil itself

often has an iodine-value exceeding 180. The iodine-value of cod-liver

oil is sometimes nearly as high. Both oils therefore contain other acids

less unsaturated than linolenic.

The “spueing” of leather is due to the absorption of oxygen and

consequent resinification of the oils, and therefore all drying oils,

however pure, are capable of producing it, though some are more liable

to do so than others (cp. pp. 363, 365, 366, 368, 390).

Linolenic acid, and probably other allied acids, become converted by

absorption of oxygen into solid varnish-like substances, which are

important to the tanner, as furnishing the principal constituents of

japans for leather. The unsaturated acids of fish oils seldom give hard

varnishes, though menhaden oil (page 367) is sometimes used as paint-oil

for outside work.

Most fats are liable to become rancid by exposure to the air, acquiring

a disagreeable taste and smell, and an acid reaction from the liberation

of the fatty acids. The changes which take place are somewhat complex.

The fatty acid of castor oil is of peculiar constitution, being an oleic

acid in which one of the hydrogen-atoms is replaced by a “hydroxyl” or

OH group. The solubility of castor oil in alcohol has already been

alluded to. It does not dry, and is an excellent oil for lubricating

heavy machinery. It is sometimes adulterated with “blown” oils, which

are made from non-drying, or slightly drying seed oils, like cotton-seed

or rape, by blowing air through them in a warmed condition. Under this

treatment they increase greatly in viscosity and density and in their

solubility in alcohol, but do not acquire the other valuable properties

of genuine castor oil.

The “foots” or sediments which oils deposit on standing, sometimes

consist of animal or vegetable fibres, or mucilage combined with water,

but often are simply the harder fats, stearin, palmitin, etc., which

crystallise from the oil on cooling. In this case they are re-dissolved

on warming the oil. Such oils, which like neatsfoot and tallow oils

become turbid in cold weather, are styled “tender.”

NON-DRYING FATS AND OILS.

_Tallow_ (Fr. _Suif_; Ger. _Talg_) is the fat of various mammalia,

principally of the ox and sheep, but occasionally also of the goat. The

mixed fat obtained from all parts of the carcass is known as “rendered

tallow,” while that obtained from the region of the kidneys (suet) is

harder. A substance commonly referred to as “pressed tallow” or

“oleo-stearine” is obtained by pressing ordinary tallow, in cloths, in

the hydraulic press. The more liquid portion which is expressed is

tallow-oil, the finer qualities of which are used in making margarine.

Oleo-stearine must not be confounded with the “distilled stearine,”

obtained from Yorkshire grease by distillation and pressure (page 359),

nor with candlemakers’ “stearine,” which is a mixture of free stearic

and palmitic acids.

Pure tallow is white and tasteless, but much of that sold is yellowish

and of a disagreeable, slightly rancid flavour. Mutton tallow is usually

harder and whiter than that of beef. Goat tallow has a characteristic

odour, as have the recovered stearines and other waste greases from

glue-works. Buck tallow, which is particularly hard, has now been

largely replaced by oleo-stearine.

Beef tallow melts at about 40° C.; mutton tallow at 45°.

In chemical composition, tallow consists chiefly of a mixture of the

tri-glycerides of palmitic, stearic and oleic acids; its hardness

diminishing with the increase of the last.

Tallow should, when melted, be perfectly clear, turbidity indicating the

presence of water or other foreign matters, due either to carelessness

in the manufacture or, possibly, adulteration. Traces of phosphate of

lime, or fragments of animal tissue, may be present as accidental

impurities; lime, on the other hand, is sometimes added to thicken the

tallow and enable it to retain more water; starch, china clay, whiting,

heavy spar, etc., are also occasionally employed. Tallow has been not

infrequently adulterated with the distilled fatty acids from wool

grease. When this is the case, crystals of cholesterol (see L.I.L.B., p.

181) may be detected by examination of the unsaponifiable matter of the

mixture under a microscope. It would also give the tallow an unusually

high “acid-value.”

Methods for the proximate analysis of tallow are given in the

‘Laboratory Book,’ pp. 189 _et seq._

The fats produced by the boiling of fleshings for glue, and by the

pressing of sheep-skins, are of the nature of soft tallows. If the

fleshings are delimed with acid, and boiled fresh, the grease is

generally of good colour, and with little unpleasant odour, but contains

traces of free fatty acids derived from the decomposition of the

lime-soaps. If the fleshings have been dried and the lime carbonated,

the grease will generally be brown, and more or less rancid; but the

lime-soaps are not decomposed, unless the “scutch” or refuse be treated

with acid, when a further yield of grease is obtained. The grease from

sheep-skins is generally somewhat brown, and often smells of the

volatile acids and other constituents of the tan-liquors, especially if

larch bark has been used. These greases are usually much improved in

appearance and odour, if well washed by boiling or steaming on water, or

by blowing a mixture of air and steam through them, or sometimes even by

mere heating to a sufficient temperature to evaporate the water and

drive off the volatile matters. By allowing the grease to cool slowly,

so as to favour crystallisation, till it is of a soupy consistency, and

then pumping through a filter press with woollen cloths, the more liquid

is separated easily from a more solid portion, and both may in many

cases be used in leather manufacture, the tallow for currying, and the

oil in place of neatsfoot oil.

_Horse-fat_, and especially that from the fatty portions of the neck

(Ger. _Kammfett_), as well as various other animal greases, are used in

the manufacture of leather. They differ from tallow chiefly in that they

have a lower melting-point, and contain more olein in proportion to the

stearin and palmitin than true tallow, and are consequently somewhat

softer. Though often almost white, these greases are sometimes darkened

in colour by the products of putrefying animal matter, but this does

not, as a rule, interfere with the oil being used for leather dressing.

They are usually so cheap that they are but little adulterated; means of

determining their purity are, however, given in L.I.L.B., p. 191.

_Neatsfoot oil_ is a yellowish, nearly odourless oil, of bland taste,

which is largely employed in the dressing of calf-kid. It has a similar

composition to tallow oil and the other oils obtained by subjecting the

soft animal fats to great pressure at a low temperature. It is often

adulterated with bone oil, lard oil and cotton-seed oil, and

occasionally with mineral oil and recovered wool-grease.

As neatsfoot oil is somewhat costly, curriers may with advantage often

use ordinary animal greases (horse-fat, etc.) after they have had the

harder tallow extracted by cooling and pressure, the product thus

obtained being, chemically, the same as neatsfoot oil, and in every

respect as suitable, while it is much less liable to adulteration.

The true neatsfoot oil is prepared by boiling the feet of cattle, and

sometimes of sheep and horses, with water, and skimming off and

clarifying the oil which is thus obtained.

The physical and chemical characteristics of this oil are described in

L.I.L.B., p. 192.

_Wool-Fat_ (Fr. _Suint_, _oesype_; Ger. _Wollschweissfett_) is a grease

of high specific gravity, exsuded from the sebaceous glands of the

sheep, together with organic salts of potassium. It is obtained by

extracting wool with solvents; or by washing with alkaline solutions,

from which it is recovered by precipitation with acid, and subsequent

hot-pressing of the “magma,” or, more recently, by evaporating the

scouring liquor to small bulk, and centrifugating. Wool-fat is

characterised by its low percentage of glycerides, the fatty acids which

it contains being mainly combined with higher alcohols (bodies of

alcoholic structure, but of a waxlike consistency), and chemically it is

rather a wax than a true fat. Among the alcohols which it contains is

included a marked percentage of cholesterol and isocholesterol. It is

difficultly saponifiable, requiring to be heated to 105-110° C. with

alcoholic potash under pressure; and even then about 44 per cent. of

alcohols remain, which are incapable of further saponification. Care

must therefore be taken not to assume that unsaponifiable matter in

greases which may contain wool-fat is necessarily mineral oil. For

details of analysis see L.I.L.B., p. 194.

Pure wool-fat is nearly white, of salve-like consistency and very slight

smell, with a density of 0·973 at 15° C. Crude wool-fat is yellow or

brown, with an unpleasant and very persistent characteristic smell. Both

the pure and the crude wool-fat have an extraordinary power of

emulsifying with water, which makes them very valuable as substitutes

for dégras in stuffing greases. Lanoline (and several other preparations

under different names) are mixtures of purified wool-fat and water, of

which lanoline contains about 22 per cent.

“Yorkshire grease” differs from crude wool-fat, in being recovered from

the waters employed in scouring woollen cloths, as well as wool, and

hence contains the free fatty acids of soaps used in scouring, as well

as the “oleines,” etc., used in oiling the cloth, and although it often

contains much wool-fat, it is occasionally destitute of this substance.

_Holden Fat_ consists of ordinary wool-grease mixed with fish oil, and

is used either as a substitute for, or in admixture with dégras (q. v.).

_Distilled Wool Grease_ is produced by distilling crude Yorkshire grease

with steam. Most of the glycerides are broken up, but many of the free

fatty acids, alcohols and waxes distil over unchanged, though a

considerable part is decomposed into volatile hydrocarbons strongly

resembling mineral oils. The distillate is separated by cooling and

pressure into a liquid “oleine” and a solid “stearine.” The latter forms

a very valuable stuffing-grease which, in England, largely takes the

place of the “oleo-stearine” used in the United States--with which,

however, it must not be confounded.

_Distilled Stearine_, prepared as above described, is a pale

yellow-to-brown fat, which varies in hardness and in its melting point

according to the conditions of its preparation. It has a characteristic

odour which is very persistent, and it consists largely of free stearic

and palmitic acids; most of the liquid hydrocarbons formed by

distillation being removed with the “oleine.”

_Olive Oil_ (Fr. _Huile d’olive_; Ger. _Olivenoel_, _Baumoel_) finds

extensive use in leather dressing, and especially in the manufacture of

“fat-liquors” (pp. 217, 240). It is extracted from the fruit of the

olive tree by pressure, and of late years from the residues by

extraction with carbon disulphide. Although it chemically resembles

tallow and lard oils very strongly, its adulteration with these

substances may usually be detected, at any rate roughly, by the taste

and odour of the oil. It is principally characterised, from a chemical

point of view, by containing the glyceride of palmitic but not that of

stearic acid, and by having a much larger proportion of olein to solid

glycerides than most of the non-drying animal oils. At low temperatures,

olive oil solidifies to a product which can be separated by pressure

into a solid tallow-like fat, and a fluid oil consisting essentially of

tri-olein.

Olive oil is the type of a non-drying vegetable oil, but though it does

not thicken materially on exposure, it becomes rancid somewhat rapidly,

and is thus rendered unsuitable for lubrication. Unless the acidity is

excessive it does not appear to spoil the oil for leather manufacture,

and for some purposes is actually an advantage as aiding emulsification.

Free acids in oils may be removed by shaking with sodium carbonate

solution.

Olive oil always contains some free acid; which is of importance in the

preparation of fat-liquors, as it facilitates the production of an

emulsion. This quality may be increased by the addition, when necessary,

of a little oleic acid.

Olive oil is frequently adulterated with other vegetable oils. Probably

the most useful criterion is the iodine-value, which is raised by the

addition of any seed oil. Examination in the refractometer also affords

useful indications. Cotton-seed, sesame and arachis (earth-nut) oils are

the most frequent adulterants of the better qualities, and in many cases

may be recognised by special tests.

_Castor Oil_ (Fr. _Huile de ricin_; Ger. _Ricinusoel_) is the oil

expressed from the seeds of _Ricinus communis_, and is a transparent,

colourless or pale yellowish liquid, having a faint odour and a

disagreeable taste. At a low temperature it thickens and deposits

slightly, and at -18° C. it solidifies to a pale yellow mass.

Castor oil is distinguished from all other natural fixed oils by its

high density (0·960 to 0·964) and viscosity, and by its solubility in

alcohol and its insolubility in petroleum ether. Genuine castor oil is

completely soluble in an equal volume of absolute alcohol, or in four

times its volume of “rectified spirit” at the ordinary temperature. It

is practically insoluble in petroleum ether, but can dissolve an equal

measure of that liquid.

For the purpose of the leather manufacturer, the ordinary hot-pressed

oil, such as is used for lubricating machinery, is quite as good as the

more costly cold-pressed oil which is used for medicinal purposes. It is

generally imported in tins holding about 40 lb. of oil. Castor oil, and

castor-oil soap made as described on p. 352, are very good for

fat-liquors, seeming to interfere with dyeing and glazing less than most

other oils. Boots oiled with castor oil may be blacked at once, and will

take a good polish.

The only oils which are usually mixed with castor oil are “blown” or

oxidised seed oils, or resin oil. Any other oils would so seriously

lower the specific gravity as to render their use impracticable. For the

detection and estimation of these the ‘Laboratory Book’ should be

consulted, or if fuller details are required the reader is referred to

Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s

‘Commercial Organic Analysis,’ vol. ii.

Sulphonated castor oil or Turkey-red oil is now largely used for

“fat-liquoring,” for which it was probably first employed by the author,

about 1890. This material--which must be carefully distinguished from

the olive oil preparation which is also used for dyeing cotton a

Turkey-red colour--is made by treating castor oil with one-quarter of

its weight of strong sulphuric acid (specific gravity 1·8), adding the

latter in very small quantities at a time, and taking care that the

temperature of the mixture at no time exceeds 35° C. The mixture is then

allowed to stand for twenty-four hours, with occasional stirring, and is

washed with its own volume of water, allowed to stand until the water

has all separated, and the oil is then syphoned off. If desired, the oil

may be further washed once or twice with a solution of strong brine, but

this is of doubtful advantage, and should in no case be excessive. The

washed oil is finally neutralised by the cautious addition of

one-hundredth of its volume of strong ammonia solution (sp. gr. 0·880).

If properly prepared, Turkey-red oil (sulphonated castor oil) will, when

_largely_ diluted with water, bear the addition of ammonia to alkaline

reaction without showing any turbidity even on standing several hours.

If a turbidity is produced, it indicates that the castor oil used was

impure and contained some oil rich in stearin.

The alcohol test described on p. 360 may also be applied, as the oily

layer will be entirely soluble if castor oil alone was used in the

preparation of the red oil.

Turkey-red oil usually contains about 50 per cent. of fatty acids

(Allen).

_Linseed Oil_ (Fr. _Huile de lin_; Ger. _Leinoel_) is used by leather

manufacturers in the preparation of the japan for making “patent

leather,” and to some extent also in currying, for oiling off levants

and moroccos, though for these purposes it has been largely superseded

by mineral oils. It is obtained from the seeds of the flax plant, _Linum

usitatissimum_, chiefly grown in Russia and India. The Russian oil is

usually mixed with the oil from hemp to the extent of about 20 per

cent., while that from India, being grown as a mixed crop with mustard

and rape, is never perfectly pure. The Baltic oil is considered best for

japans, and is improved by storing for a considerable time in tanks in a

warm place.

When obtained by cold pressure of the seeds, linseed oil is of a bright

yellow colour; if a higher temperature be used in the extraction the oil

is more or less brown, and tastes much more acrid. On exposure to air,

linseed oil turns easily rancid, absorbs oxygen, and if spread out in a

sufficiently thin film it dries to a neutral substance (linoxyn), which

is insoluble in ether. This property is the one on which the chief value

of linseed and other “drying oils” depends.

Linseed oil is chiefly adulterated with other seed oils, cottonseed

being the most often used for this purpose, though menhaden and various

other fish oils are occasionally employed. As the density of raw linseed

oil varies between 0·932 and 0·936 at 15° C., the addition of other seed

oils or of mineral oil would cause an appreciable lowering of this

figure, whilst rosin or rosin oil would raise it. A judicious admixture

of both mineral and rosin oils would give a product of normal density.

Fish oils can be detected by their characteristic smell, especially on

warming.

Various methods have been proposed for judging the quality of linseed

oil, but none of them are perfectly satisfactory. The best oil is that

which dries the most perfectly; but the rapidity of the drying, and the

consistency of the dried product, are most important factors which must

also be taken into account. The iodine-valve, which is a measure of the

drying power, should not fall much below 180.

A satisfactory practical test, recommended by Allen,[162] consists in

mixing the oil with three times its weight of genuine white lead, and

covering a perfectly clean glass surface with the paint. An exactly

similar experiment is made simultaneously with a standard sample of

linseed oil, and the rates of drying and the characters of the coating

of paint compared.

[162] Commercial Organic Analysis, ii. p. 122.

J. Muter has simplified this test by merely flooding a plate of glass

with the oil and then exposing it to a temperature of 38° C. (100° F.)

in a good current of air. The time required for drying, to such an

extent that the coating will not come off when lightly touched, is

noted, and compared with standard samples of oil. By applying the finger

at intervals to different parts of the film surface the progress of the

drying can be readily observed.[163]

[163] Kathreiner states that this method is a useful test for fish and

liver oils, those which dry most rapidly being specially liable to

“spue.”

_Boiled Oils._--Its capacity for thus drying is much enhanced by

heating, with addition of “driers,” to a temperature of 130° C. and

upwards, while passing a current of air through the oil and then

increasing the temperature until the oil begins to effervesce (“boil”).

Large quantities of linseed oil are now treated in this way for use in

the arts. The driers used are metallic salts, principally those of lead

and manganese, which apparently act as oxygen-carriers. Litharge was

formerly most commonly used, but its place has been taken to a

considerable extent by acetate, borate and resinate of manganese. From 1

to 2 per cent. of either litharge or manganese borate may be used,

though less quantities produce a marked effect. Apparently litharge

gives the most rapid drying, and manganese a much paler colour.[164]

Linseed oil is usually darkened by boiling, and increases both in actual

weight and in specific gravity and viscosity. The chemical reactions

which take place in boiling are not well understood, but it is in the

main a process of oxidation and polymerisation, perhaps accompanied by

the formation of anhydrides of the fatty acids, and a portion of the

drier remains dissolved in the boiled oil. These driers may be detected

by boiling an ounce or so of the oil with dilute hydrochloric acid,

allowing the mixture to separate into two layers and then syphoning off

the lower into another vessel, and testing for metals (lead, manganese,

zinc) or acids (boric, oxalic, etc.).

[164] Cp. F. H. Thorpe, Abst. Jour. Soc. Chem. Ind., 1890, 628, from

Technology, Quart., iii. pp. 9-16.

Black japan for patent leathers is made by boiling linseed oil, without

blowing air through it, for at least seven or eight hours, with Prussian

blue, or with oxides of iron. The japan is brownish rather than blue in

colour, and it is probable that the Prussian blue serves merely as a

source of iron oxide, which acts both as a colouring matter and a drier.

Other driers, such as litharge, are sometimes added, and for coloured

enamels other pigments are substituted for the Prussian blue.

_Cotton-seed Oil_ (Fr. _Huile de coton_; Ger. _Cottonoel or

Baumwollensamenoel_) is now expressed in enormous quantities in the

United States, on the continent of Europe and in Great Britain. The

crude oil contains a very characteristic colouring matter which, though

naturally ruby red, is sometimes so intense as to make the oil appear to

be nearly black. This colouring matter causes the oil to produce stains,

and is therefore removed by a process of refining, and a product of a

straw- or golden-yellow colour is thus obtained. The refining is usually

effected by shaking the crude oil with a cold 5 per cent. solution of

caustic soda, using about ten times as much oil as soda solution.

Cotton-seed oil is, on account of its price, seldom or never

adulterated, but is itself frequently employed as an adulterant of olive

and neatsfoot oils. It is a semi-drying oil, and unsuitable for most

purposes in leather manufacture. For a description of its characteristic

properties, both chemical and physical, the reader is referred to

Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s ‘Commercial Organic

Analysis,’ vol. ii.

_Sesamé Oil_ (Fr. _Huile de sésamé_; Ger. _Sesamoel_; Teel oil, Gingeli

oil) is another seed oil, usually of paler colour than cotton-seed oil,

but resembling it in having scarcely any odour, and possessing a bland

and agreeable, though not very characteristic taste. It is often used as

an adulterant of olive oil.

Sesamé oil is a non-drying oil, which does not easily turn rancid. When

present in other oils, it may be detected by agitating 10 c.c. of the

sample with 5 c.c. of concentrated hydrochloric acid in which 0·1 grm.

of white sugar has previously been dissolved. After shaking together for

at least ten minutes, the oil and acid are allowed to separate, when,

if sesamé oil be present, the acid layer will have a marked rose colour,

the intensity of which increases with the amount of sesamé oil in the

sample (Baudouin’s test).

Sesamé oil is largely used in India for oiling tanned sheep- and

goat-skins (“Persians”), and has the characteristic property of being

assimilable in large quantities by leather without the latter appearing

oily. East India tanned skins often contain 25 and even 30 per cent. The

oil is applied to them in the wet condition before they are dried. It is

easily detected in the oils extracted from these skins by Baudouin’s

test. The oil seems well adapted for many purposes in leather

manufacture.

_Cod Oil_ (Fr. _Huile de morue_; Ger. _Leberthran_) is by far the most

important oil used by leather manufacturers, and is obtained from the

liver of the common cod-fish (_Gadus Morrhua_) and several other members

of the genus _Gadus_. The chief seats of the cod fishery are the coasts

and banks of Newfoundland, Nova Scotia, the Gulf of St. Lawrence, the

coasts of Norway, Denmark and Germany, the Dogger Bank in the North Sea,

and the shores of Alaska in the Pacific Ocean.

The oil was formerly obtained by keeping the livers of the fish in large

wooden vats, stirring constantly until so much decomposition has taken

place that the cells containing the oil burst, and the oil thus released

rises to the surface and is skimmed off with wooden ladles. The crude

oil is allowed to deposit any suspended matters by sedimentation in a

tank, and is then poured into casks ready for sale. The “brown oil” so

often used by tanners is obtained by boiling the solid matter left after

extracting the oil as above in iron tanks until all the water has

evaporated; the oil thus liberated is then strained off, clarified and

put into barrels.

The purer qualities of cod-liver oil are now obtained by boiling the

livers with water and skimming off the oil which rises to the surface.

Three grades are on the market at the present time: medicinal, or

ordinary bright; an inferior “light brown”; and “dark-brown,” or

“tanners’ oil.” It is probable that these steam-extracted oils are much

more liable to “spue” than those extracted by the old method at a higher

temperature, since Eitner[165] has shown that seal oils extracted at a

low temperature spue badly, but lose the tendency if heated for some

time to 250-300° C.

[165] Gerber, 1880, p. 244.

Genuine cod oil, as suitable for use in leather manufacture, is always

more or less brown in colour, of specific gravity about 0·928, and

refractive index 1·482. At present prices it can only be adulterated

with other fish oils, rosin, or mineral oil, or with water, gelatine or

mucilage. Of these, rosin oil and petroleum are the most frequently

employed in sophistication.

An inferior variety of oil, known as “coast cod,” made from the livers

of various fish, such as ling, haddock and hake, is also sold, but, as

it is frequently mixed with oils from other fish refuse, it has a very

poor reputation.

Cod oil, together with most of the other oils obtained from fish livers,

has the property of producing an intense reddish-violet colour when a

drop of strong sulphuric acid is dropped upon ten or fifteen drops of

the oil contained in a white porcelain tray or saucer. The reaction

succeeds still better, if, instead of the oil itself, its solution in

chloroform, carbon disulphide or tetrachloride is employed. This test,

although very useful for the detection of liver oils when they are

present in oils of a totally different character, such as rape or olive

oils, does not in any way indicate whether a sample of fish oil is pure

or otherwise. A very similar reaction is given by cholesterol which is

present in wool-fat.

_Shark-liver Oil_ (Fr. _Huile de requin_; Ger. _Haifischthran_) is

obtained from the liver of the “basking shark,” or “ice-shark,” chiefly

caught off the coast of Norway; but the livers of the dog-fish and

several allied fish also are sometimes substituted.

Shark oil has been employed in tanneries as a substitute for cod-liver

oil, but, according to Lewkowitsch, and to Allen, it is no longer

employed in England. From its pale colour it is probably principally

used to improve the appearance of darker oils. According to Eitner,[166]

its use causes leather to “spue” badly if not previously heated.

[166] Gerber, 1886, p. 266.

Shark oil is characterised by the very notable proportion of

unsaponifiable matter which it contains, which is of the same character

as that of sperm oil, and not easily removed from its soap solution by

petroleum ether. It gives a strong violet-blue coloration with

concentrated sulphuric acid, the reaction being even more marked than

with cod-liver oil itself, and of a bluer violet.

_Whale Oil_ (Fr. _Huile de baleine_; Ger. _Wallfischthran_) is extracted

from the blubber of various species of whale, and often contains traces

of spermaceti, the substance which characterises the oil from the sperm

whale. This yields on saponification higher alcohols, which are found in

the unsaponifiable matter; but in ordinary whale oil the total

unsaponifiable matter seldom exceeds 1¹⁄₂ to 2 per cent. Whale oil is

largely used on the Continent for “chamoising” (q.v.), and is

consequently a constituent of dégras. It is much less oxidisable than

cod.

_Seal oil_ (Fr. _Huile de phoque_; Ger. _Robbenthran_) is obtained from

the common rough-coated seal, abundant in the Arctic regions. It bears a

strong resemblance to both whale and fish oils, and cannot be detected

in mixtures of these. The Swedish “Dreikronenthran” (Three Crown Oil) is

a mixture of seal and fish oils. As genuine seal oil only contains about

¹⁄₂ per cent. of unsaponifiable matter, its adulteration by mineral or

resin oils may be detected by a determination of the matter extracted by

petroleum ether after saponification of the oil (see L.I.L.B., p. 178).

There is no simple test by which the purity or otherwise of a sample of

oil can be determined, as the dealers know all the best tests which the

users could try, and fake up their oils accordingly. For instance, if

petroleum is to be added surreptitiously to a cod oil, the decrease in

specific gravity of the oil caused by this addition would be corrected

by the addition of a suitable quantity of soap or rosin oil, which would

scarcely affect the colour, taste or odour of the sample. The only

satisfactory method of detecting adulteration is to submit the oil to a

complete chemical examination, and for this purpose L.I.L.B., pp. 156

_et seq._, or the larger text-books already named may be suitably

consulted.

_Menhaden Oil_ (Porgie oil, Straits oil) is largely used in certain

districts as an adulterant or substitute for cod oil. It is obtained

from the _Alosa Brevoordia_ or _menhaden_, a member of the herring

family, about a foot long. The fish is caught on the Atlantic coast of

America, and is so plentiful that it is very doubtful whether cod oil

can ever compete with it successfully in price. The fish are boiled in

steam kettles, the oil squeezed by hydraulic presses, clarified, and

bleached by exposing to the sun in shallow glass-covered tanks. An

inferior grade is known as “Bank oil.” Menhaden oil is chiefly

characterised by its very high “specific temperature reaction”

(L.I.L.B., p. 169) which is about 306. It is not a good leather-oil,

being very liable to “spue.”

Many other varieties of oil extracted from the bodies, and not from the

livers only of fishes, are classed as _fish oils_. Menhaden oil is the

principal of these; but Japanese oil, sardine and herring oils, and

those obtained from the refuse of other fish are scarcely less

important, though as they are derived from such different sources it is

not possible to quote any definite characteristics by which they may be

identified when mixed with more valuable oils. They are usually very

liable to “spue.”

_Fish Tallow_, which, according to Eitner, is a good and cheap

substitute for dégras, is the solid grease obtained from different kinds

of fish oil by subjecting them to a low temperature and separating the

matter which is thus precipitated, or (as in China and Japan) the solid

fat which is extracted at the same time as the oil from the body of the

fish. Formerly fish tallow was only obtained from and with Japanese

train oil, but it is now obtained from whale blubber. This latter yields

a very pure form of the tallow, which does not need any rectification;

but the Japanese variety, which is obtained from fish of the herring

family, contains a sort of fish glue, which greatly deteriorates the

quality of the product. By careful purification, however, this glutinous

matter may be removed, and the refined product has none of the

leather-staining properties so characteristic of the crude tallow. The

refined tallow is sold in square flat cakes, melts at 42° and is not

quite so stiff as ox tallow.

_Dégras_ and _Sod Oil_ are products of chamois-leather dressing (p. 378)

which are used in currying. Skins are treated with marine animal oils,

and submitted to oxidation, and the surplus and partially altered oil is

recovered. In the French method, whale and seal oils as well as liver

oils are used, and the oxidation is slow and gradual, and the residual

oil, being liquid, is recovered by pressure, and constitutes _moellon_,

of which the first pressing (_première torse_) is the best. This is

never sold for currying in its original purity; but, mixed with further

quantities of fish oils, tallows, and sometimes wool-fat, it constitutes

the ordinary dégras of commerce. The additions, though they lower the

value, are not to be considered as simple adulterations, since the

_moellon_ alone would be less suitable for the purpose. After removal of

as much oil as is possible by dipping in hot water and pressing, a

further quantity is recovered by washing with solutions of potash or

soda, from which it is separated by addition of acid, and constitutes a

lower quality of degras. The _moellon_ is of such value as a currying

material, that factories are run in which chamoising is carried on

solely for its production, the skins being oiled and oxidised

repeatedly, till reduced to rags.

_Extraction of solid materials_, such as barks, or valonia, is more