[32] Lorenz, Pflüger’s Arch., xlvii. pp. 189-95; Journ. Chem. Soc.,

1891, A. p. 477.

[33] Compare Schützenberger, Comptes Rend., cii. pp. 1296-9; Journ.

Chem. Soc., 1886, A. p. 818.

Treatment with acids produces very similar effects. The first products

are soluble peptones. Paal[34] on treating 100 parts of gelatin on the

water-bath with 160 parts water and 40 parts concentrated HCl till the

product was soluble in absolute alcohol, obtained, on purification, a

white hygroscopic mass of peptone salts containing 10-12 per cent. of

hydrochloric acid.[35]

[34] Berichte, xxv. pp. 1202-36; Journ. Chem. Soc., 1892, A. p. 895.

[35] See also Buchner and Curtius, Ber., xix. pp. 850-9; Journ. Chem.

Soc., 1886, A. p. 635.

The products of digestion of gelatin with gastric and pancreatic juice

are peptones which do not differ materially from gelatin in ultimate

composition, and the action is probably mainly hydrolytic.[36]

[36] Chittenden and Solly, Journ. Chem. Soc., 1891, A. p. 849.

The earlier products of putrefaction are very similar. Many bacteria

have the power of liquefying gelatin-jelly. This has been shown by

Brunton and McFadyen[37] to be due not to the direct action of the

bacteria, but to a soluble zymase secreted by them which peptonises the

gelatin. Its action is favoured by an alkaline condition, and destroyed

by a temperature of 100° C.[38] As putrefaction progresses, the solution

becomes very acid from the formation of butyric acid, and later on

ammonia and amido-acids are formed.

[37] R. S. Proc., xlvi. pp. 542-53.

[38] Compare pp. 17, 171; also Ch. Zeit., 1895, p. 1487.

Fahrion,[39] starting with the idea that albuminoids and gelatin were

condensation products of a lactone character (L.I.L.B. p. 185), and

that they might, like lactones, be depolymerised by saponification,

digested these bodies with alcoholic soda till they were dissolved, and

on neutralising the solution with hydrochloric acid, of which the excess

was driven off by repeated evaporation, and removing the sodium chloride

by treatment with alcohol, obtained in each case bodies of acid

reaction, which from their composition he supposed to be identical with

Schützenberger’s proteic acid, C₈H₁₄N₂O₄, which is soluble in water and

alcohol, insoluble in ether and petroleum, uncrystallisable, and forming

uncrystallisable salts. Fahrion suggested that the nitrogenous character

which Eitner attributed to his “dégras-former” (p. 370) was probably due

to contamination by this body; and that its formation might be utilised

in the analysis of leather and other proteid bodies. These products have

since been further investigated by Prof. Paal and Dr. Schilling,[40] who

show that they contain hydrochloric acid, to which their acid reaction

is due, and that they are identical with the peptone salts previously

obtained by Prof. Paal (v. s.) by digestion of proteids with

hydrochloric acid. The free peptones are strongly basic.

[39] Ch. Zeit., 1895, p. 1000.

[40] Ch. Zeit., 1895, p. 1487.

By dry distillation of gelatin a mixture of pyrrol and pyridin bases are

produced. This is commercially obtained by the distillation of bones,

and is known as “bone oil,” or “Dippel’s animal oil.” Pyrrol, C₄H₅N,

resembles phloroglucol in giving a purple-red colour to fir wood

moistened with hydrochloric acid (p. 299).

_Reactions of Gelatin._--Gelatin is precipitated by mercuric chloride,

in this respect resembling peptones, but not by potassium ferrocyanide,

by which it is distinguished from albuminoids, and it differs from

albumin in not being coagulated by heat. Solution of gelatin dissolves

considerable quantities of calcium phosphate; hence this is always

present in bone-glues. Gelatin and some of its decomposition products

are precipitated by metaphosphoric acid.[41] The precipitate contains

about 7 per cent. P₂O₅, but gradually loses it on washing. Various salts

diminish the solubility of gelatin in hot water, and especially those of

the alum type. Chrome alum and basic chrome salts are especially

powerful, rendering it practically insoluble. The addition of about 3

per cent. ammonium or potassium dichromate causes glue or gelatin to

become insoluble by the action of light with the formation of basic

salts of chromium, and has been utilised in photography and as a

waterproof cement. Other colloids besides gelatin are similarly

affected.

[41] Lorenz, Pflüger’s Archiv, xlvii. pp. 189-195.

Gelatin is precipitated by all tannins, even from very dilute solutions;

one containing only 0·2 grm. per liter is rendered distinctly turbid by

gallotannic acid or infusion of gall-nuts; but some other tannins give a

less sensitive reaction. The precipitate is soluble to a considerable

extent in excess of gelatin, so that in using the latter as a test for

traces of tannin care must be taken to add a very small quantity only.

The addition of a little alum renders the reaction more delicate.

Whether the precipitate is a definite chemical compound has been

disputed, as its composition varies according to whether gelatin or

tannin is in excess. Böttinger[42] states that the precipitate produced

by adding gelatin to excess of gallotannic acid contains 10·7 per cent.

of nitrogen, indicating the presence of 66 per cent. of gelatin on the

assumption that gelatin contains 16·5 per cent. N (see p. 57). Digested

with water at 130° C., the precipitate is decomposed, yielding a

solution which precipitates tannin, and probably indicating the

formation of a more acid compound. Gelatin with excess of oak-bark

tannin gives a precipitate containing 9·5 per cent. of nitrogen,

corresponding to 57·5 per cent. of gelatin. Treated with water at 150°

C., this precipitate yielded three products: one soluble in cold water,

another in hot only and one insoluble. On addition of a solution of

formaldehyde (formalin) to one of gelatin no visible action takes place

in the cold, unless the solution of gelatin be very concentrated and

alkaline, but on heating, the gelatin is rendered insoluble owing to the

formation of a compound with the formaldehyde. From the very small

amount of formalin which is required to produce formo-gelatin it is very

doubtful if this is a definite compound.

[42] Liebig’s Ann. der Ch., ccxliv. pp. 227-32.

Weiske[43] states that bone-gelatin, carefully freed from all mineral

matter, is not precipitated by tannin till a trace of a salt (e. g.

sodium chloride) is added. So far as is known, bone-gelatin is identical

with that of skin.

[43] Bied. Centr., 1883, p. 673.

_Chondrin_ is the gelatinous body produced by the digestion of

cartilage with water at 120° C. for three hours. In most of its physical

properties it is identical with gelatin, but differs from the latter in

being precipitated from its solution in water by acetic acid, lead

acetate, alum, and the mineral acids when the latter are not present in

excess. Chondrin also differs from gelatin in producing a substance

capable of easily reducing cupric oxide when it is boiled for some time

with dilute mineral acids. It is extremely probable that chondrin is

merely an impure gelatine.[44]

[44] Cp. Petri, Berichte, xii. p. 267; Mörner, Skand. Archiv f.

Physiol., i. pp. 210-243; and Journ. Chem. Soc., 1889, A. p. 736 and

Zeit. Physiol. Chem., 1895, xx. pp. 357-364; and Journ. Chem. Soc.,

1895, A. i. p. 254. See also Richter, Org. Chem., i. p. 559.

_Coriin._--Rollet[45] has shown that when hide and other forms of

connective tissue are soaked in lime- or baryta-water, the fibres become

split up into finer fibrils, and as the action proceeds, these again

separate into still finer ones, till the ultimate fibrils are so fine as

to be only distinguished under a powerful microscope. At the same time,

the alkaline solution dissolves the substance which cemented the fibres

together, and this may be recovered by neutralising the solution with

acetic acid, when the substance is thrown down as a flocculent

precipitate. This was considered by Rollet to be an albuminoid

substance; but Reimer[46] has shown that it is much more closely allied

to the gelatinous fibres, and, indeed, is probably produced from them by

the action of the alkaline solution. Reimer used limed calf-skin for his

experiments, and subjected it to prolonged cleansing with distilled

water, so that all soluble parts must have been pretty thoroughly

removed beforehand. He then digested it in closed glasses with

lime-water for 7-8 days, and precipitated the clear solution with dilute

acetic acid. He found that the same portion of hide might be used again

and again, without becoming exhausted, which strongly supports the

supposition that the substance is merely a product of a partial

decomposition of the hide-fibre, and indeed that there is no distinct

“cementing substance,” but merely a difference in the hydration or

physical condition of the fibre substance which causes it to split more

readily in certain directions. The dissolved substance, which he called

“coriin,” was purified by repeated solution in lime-water and

reprecipitation by acetic acid. It was readily soluble in alkaline

solutions but not in dilute acids, though in some cases it became so

swollen and finely divided as to appear almost as if dissolved. It was,

however, very soluble in common salt solution of about 10 per cent.,

from which it was precipitated both by the addition of much water and by

saturating the solution with salt. Reimer found that a 10 per cent. salt

solution was equally effective with lime-water in extracting coriin from

the hide, and that it was partially precipitated on the addition of

acid, and completely so on saturating the acidified solution with salt.

Other salts of the alkalies and alkaline earths acted in a similar

manner, so that Reimer was at first deceived when experimenting with

baryta-water, because, being more concentrated than lime-water, the

coriin remained dissolved in the barium salt formed on neutralising with

acid, and it was necessary to dilute before a precipitate could be

obtained. The slightly acid solution of coriin gave no precipitate

either in the cold or on boiling with potassium ferrocyanide, being thus

distinguished from albuminoids. The neutral or alkaline solution showed

no precipitate with iron or mercuric chloride, copper sulphate, or with

neutral lead acetate; but with basic lead acetate, basic iron sulphate,

or an excess of tannin a precipitate was produced. Reimer’s analysis

showed: Carbon, 45·91; hydrogen, 6·57; nitrogen, 17·82; oxygen, 29·60;

and he gives a formula showing its relation to the original fibre, which

does not seem supported by sufficient evidence. In all probability

coriin is merely an impure degradation-product of hide-fibre or gelatin.

[45] Sitz. Wiener Akad., xxxix. p. 305.

[46] Ding. Polyt. Journ., ccv. p. 153.

_Hide Albumin._--The fresh hide contains a portion of actual albumin,

viz. that of the blood-serum and of the lymph, which is not only

contained in the abundant blood-vessels, but saturates the fibrous

connective tissue, of which it forms the nourishment. This albumin is

mostly removed from the skin by the liming and working on the beam,

which is preparatory to tanning. Probably for sole-leather, the albumin

itself would be rather advantageous if left in the hide, as it combines

with tannin, and would assist in giving firmness and weight to the

leather. It is, however, for reasons which will be seen hereafter,

absolutely necessary to get rid of any lime which may be in combination

with it. The blood must also be thoroughly cleansed from the hide before

tanning, as its colouring matter contains iron, which, by combination

with the tannin, produces a bad colour.

The albumins form a class of closely allied bodies of which white of egg

may be taken as a type. They are also related to the casein of milk, to

fibrin, and more distantly to gelatin. A good deal of information on the

class may be found in Watt’s Dict. of Chem., 2nd ed., article

‘Proteids,’ and Beilstein’s article ‘Albuminaten,’ and in Allen’s

‘Commercial Organic Analysis,’ vol. iv.

The most characteristic property of albumins is that of coagulation by

heat. The temperature at which this takes place differs somewhat in

different members of the group, egg and serum albumin coagulating at

72-73° C. Dry albumins become insoluble if heated to 110° C. for some

time. Traces of acid tend slightly to lower, and traces of alkali to

raise the temperature of coagulation. Sodium chloride and some other

neutral salts favour coagulation. Solutions of albumin become opalescent

at a temperature slightly below that at which flakes form.

Albumins are also coagulated by alcohol and by strong mineral acids.

Coagulated albumin is only soluble in strong acids and alkalies by aid

of heat, and strongly resembles keratin (pp. 56, 68).

Solutions of albumin are lævorotatory to polarised light.

“_Acid_” and “_Alkali_” _Albumins_ are formed by the action, in the

cold, of dilute acids (such as acetic, hydrochloric) and alkalies on

albumin solution. They are uncoagulable by heat, and are precipitated by

careful neutralisation, but are soluble in excess of either acid or

alkali, or alkaline carbonates. They are thrown out of solution by

saturation with sodium chloride or magnesium sulphate. It is doubtful

whether albumins combine with either acids or bases, and it is probable

that the “acid” or “alkali” albumins are identical with the parapeptones

formed in the first stage of peptic digestion.

On putrefaction, or on more severe treatment with acids and alkalies,

albumins break down in a way similar to gelatin, and yield almost

identical products (see p. 57); amido-acids of the acetic series, and

tyrosin (para-oxy-α-amido-phenyl-propionic acid) and aspartic

(amido-succinic) acid, being the most important.

Treatment with alcoholic soda (see p. 62) yields peptones similar to

those of gelatin.[47]

[47] Paal, Ch. Zeit., 1895, p. 1487.

Heated for some days with dilute nitric acid (1 : 2) all proteids,

including albumins, gelatin and keratins, yield yellow flocks of

“xantho-proteic acid,” a substance of somewhat indefinite composition,

soluble in ammonia and in fixed caustic alkalies with production of an

orange-red or brownish-red colour.

Millon’s reagent gives an intense red coloration when heated with

albumins, keratins, or gelatin. The reagent is made by dissolving 2·5

grm. of mercury in 20 c.c. of concentrated nitric acid, adding 50 c.c.

of water, allowing to settle and then decanting the clear liquid.

Albumins, previously purified by boiling with alcohol and washing with

ether, when dissolved in concentrated hydrochloric acid (sp. gr. 1·196)

by aid of heat, give a violet-blue coloration, but the reaction is often

somewhat indefinite. Gelatin, chondrin and keratins do not give this

reaction.

Treated with a trace of cupric sulphate and excess of caustic potash

solution, albumins give a violet, and gelatin and peptones a pink

solution (biuret reaction).

Dissolved in glacial acetic acid and treated with concentrated sulphuric

acid, albumins and peptones give a violet and feebly fluorescent

solution. A somewhat similar reaction is obtained if sugar solution be

substituted for acetic acid.

A solution of albumin rendered strongly acid with acetic acid is

precipitated by potassium ferrocyanide, salt, sodium sulphate, lead

acetate, mercuric chloride, tannin and picric and tungstic acids.

_Egg-Albumin_ is contained in the whites of eggs in membranes which are

broken up by beating with water and can then be removed by filtration.

When fresh its reaction is slightly alkaline, and it is lævorotatory.

According to Lehmann, white of egg contains 87 per cent. of water, and

13 per cent. of solid matter, the latter being almost entirely composed

of egg-albumin. This latter coagulates and becomes insoluble in water on

heating to 60° C.

_Vitellin_ (the albumin or globulin[48] of the yolk) is insoluble in

water, and is obtained as a white granular residue on extracting undried

egg-yolk with large quantities of ether. It closely resembles myosin,

the chief globulin of muscle, but differs from other globulins in being

soluble in a saturated solution of common salt. A neutral solution of

vitellin in very dilute brine coagulates at 70-75° C.

[48] Globulin is an albumin soluble in dilute salt solutions, but

insoluble in water.

Yolks of eggs, preserved by the addition of salt, borax, or formalin,

are used for dressing skins in the process of “tawing” (see p. 191). For

the analysis of such yolks, see L.I.L.B., p. 159. Their most important

constituent for the leather-dresser is egg-oil of which they contain

about 30 per cent.

_Casein_, the principal proteid of milk, may be mentioned here in

connection with the albumins to which it is closely related, since,

though in no way connected with the animal skin, since it is used to

some extent as a “seasoning” or glaze for leather, for which it is well

adapted, and it is now to a considerable extent a waste product of

butter manufacture. It differs from albumins in being very incompletely

if at all coagulated by boiling, but separates at once in curdy flakes

on the addition of acids (hydrochloric, acetic, butyric), and by the

action of rennet. The curd is easily soluble in small quantities of

dilute alkalies, lime-water, and salts of alkaline reaction, such as

sodium carbonate and borax. If no more than the necessary quantity of

alkali is employed for solution, the compound has an acid reaction to

phenolphthalein, and like the original milk, is curdled by rennet and

dilute acids. Casein may also be dissolved by digestion with diluted

mineral or organic acids.

_Hair, Epidermis and Glands._--These are all derived from the epithelial

layer, and hence, as might be inferred, have much in common in their

chemical constitution. They are all classed by chemists under one name,

“keratin,” or horny tissue, and their ultimate analysis shows that in

elementary composition they closely resemble the albumins. It is

evident, however, that the horny tissues are a class rather than a

single compound.

The keratins are gradually loosened by prolonged soaking in water, and,

by continued boiling in a Papin’s digester at 160° C., evolve

sulphuretted hydrogen, at the same time dissolving to a turbid solution

which does not gelatinise on cooling. Keratin is dissolved by caustic

alkalies; the epidermis and the softer horny tissues are easily

attacked, while hair and horn require strong solutions and the aid of

heat to effect complete solution. The caustic alkaline earths act in the

same manner as dilute alkaline solutions; hence lime easily attacks the

epidermis, and loosens the hair, but does not readily destroy the

latter. Alkaline sulphides, on the other hand, seem to attack the

harder tissues with at least the same facility as the soft ones, the

hair being often completely disintegrated, while the epidermis is still

almost intact; hence their applicability to unhairing by destruction of

the hair. Keratins give the xanthoproteic reaction with nitric acid, and

a red coloration with Millon’s reagent, and also resemble albumin, in

the fact that they are precipitated from their solution in sulphuric

acid by potassium ferrocyanide. By fusion with potash, or prolonged

boiling with dilute sulphuric acid, keratin is decomposed, yielding

leucin, tyrosin, ammonia, etc. The precipitate produced by the addition

of acids to alkaline solution of keratin (hair, horns, etc.), mixed with

oil and barium sulphate, has been employed by Dr. Putz as a filling

material for leather, for which purpose it acts in the same way as the

egg-yolks and flour used in kid-leather manufacture. Eitner attempted to

use it for the same purpose with bark-tanned leather, but without much

success. Putz has also proposed to precipitate the material after first

working its solution into the pores of the leather.

_Elastic Fibres._--The elastic or yellow fibres of the hide are of a

very stable character. They are not completely dissolved even by

prolonged boiling, and acetic acid and hot solutions of caustic alkalies

scarcely attack them. They do not appear to combine with tannin, and are

very little changed in the tanning process. They are present in hide and

skin to the extent of less than one per cent.

_Analytical Methods._--The reactions distinguishing the principal skin

constituents are summarised in the following table:--

+------------------+--------------+---------------+------------------+

| Reagent. | Gelatin. | Albumins. | Keratins. |

+------------------+--------------+---------------+------------------+

|Cold water |Swells only |Soluble |Insoluble. |

| | | | |

|Heated in water |Soluble |Coagulate at |Soluble only at |

| | |72°to 75° C. |temp. over 100° C.|

| | | | |

|Acetic acid and }| | | |

|potassium ferro- }|No precipitate|Precipitate |Precipitate |

|cyanide to aque- }| | | |

|ous solution }| | | |

| | | | |

|Millon’s reagent |No reaction |Red coloration |Red coloration. |

| | | | |

|Hot concentrated }|No coloration |Violet blue |No coloration. |

|hydrochloric acid}| | | |

+------------------+--------------+---------------+------------------+

There is no simple method for the quantitative separation of the

different constituents of skin. It is, therefore, customary to simply

determine the amount of nitrogen which any particular portion of the

material may contain, and, as gelatinous fibre, which constitutes by far

the greater portion of the true skin, contains 17·8 per cent. of

nitrogen, to base the estimation of the amount of skin present upon this

figure (see p. 57).

The most convenient process for the determination of the nitrogen is

that devised by Kjeldahl, which is most easily carried out as follows:--

A known weight of the substance which contains about 0·1 gram of

nitrogen (0·5 gram of skin, or a corresponding quantity of liquor) is

placed in a flask of Jena glass, capable of holding 500-700 c.c.

together with 15 c.c. of concentrated sulphuric acid. The contents of

the flask are then boiled over a small Bunsen flame for 15 minutes, or

more, until all the water has been driven off and the material is quite

disintegrated; and are then allowed to cool below 100°. 10 grams of dry

powdered potassium persulphate is now added, and the boiling continued

till the liquid has become colourless. The operation of boiling should

be conducted in a good draught, or in the open air. Before the substance

has begun to char it is advisable to place a small funnel in the neck of

the flask to prevent, as far as possible, spirting and loss of sulphuric

acid.

[Illustration: FIG. 18.--Kjeldahl Apparatus.]

The colourless liquid is allowed to cool thoroughly, and the flask is

then fitted with a tapped funnel and tube, as shown in Fig. 18. This

tube must not be less than 4 mm. in diameter, and with the end in the

flask cut diagonally to facilitate drops of liquid falling back again

into the flask. It rises obliquely for a height of 12 to 15 inches, is

then bent over as shown in the figure and connected by a rubber tube[49]

to a 100 c.c. pipette, or similarly shaped tube, the other end of which

dips just below the surface of a volume of exactly 50 c.c. of “normal”

hydrochloric acid contained in a second flask. About 50 c.c. of

distilled water is introduced into the flask containing the treated

sample, and after this 100 c.c. of a solution of 50 grams of caustic

soda in 100 c.c. of water is carefully and slowly run into the flask by

means of the tapped funnel with which it is provided. The contents of

the flask are now boiled for about half an hour,[50] the normal acid in

the receiving flask being kept cool by immersing the latter in cold

water. The liquid in this second flask is then titrated with normal

sodium carbonate, using methyl orange as indicator. The difference in

c.c. between 50 c.c., the volume of acid used, and the quantity of

normal sodium carbonate required to neutralise it, when multiplied by

0·014 represents the amount of nitrogen (in grams) in the weight of the

substance used for the determination; or if multiplied by 0·0786 shows

the weight of hide-fibre in the same quantity of material. Some

chemists add copper sulphate, or a drop of mercury before boiling up the

substance with the strong sulphuric acid, but the use of such substances

introduces complications in the process without, in the case of

gelatinous matter, securing more accurate results. It is absolutely

necessary that the acids and alkali used should be free from ammonia,

and a blank experiment should be made using pure sugar which contains no

nitrogen, and a correction applied if necessary for the ammonia they

contain.[51]

[49] The ends of the glass tubes should fit closely together, so as to

expose the rubber as little as possible to the action of ammoniacal

vapour.

[50] “Bumping” is often very troublesome at this stage, and may be

prevented by passing a current of steam from another flask, or

ammonia-free air through a tube with a capillary opening into the

boiling liquid; fragments of pure zinc, of platinum, or broken

tobacco-pipe are much less efficient. It is an additional safeguard

against the escape of ammonia to fix a small absorption-tube

containing fragments of glass to the absorption-flask. The normal acid

is run through this tube into the flask, so as to wet the broken

glass, and is finally rinsed into the absorption flask before

titrating its contents.

[51] Cp. Procter and Turnbull, Jour. Soc. Chem. Ind., 1900, p. 130;

also Nihoul, Composition des Cuirs Belges, p. 14 (Bourse aux Cuirs de

Liège, Sept. 1901), who advocates the use of potassium permanganate in

the oxidation; and Law (Jour. Soc. Ch. Ind., 1902, p. 847).

In place of using 10 grm. of potassium persulphate as described, 10

grm. of ordinary potassium sulphate may be used, and potassium

persulphate added in small quantities towards the end of the operation

till a perfectly colourless solution is obtained.

CHAPTER IX.

_THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE._

The nature of the changes which take place in the conversion of raw hide

into leather, and the causes of swelling and “falling” in the various

stages of the wet-work and tannage are among the most difficult problems

with which we have to deal, and no intelligible explanation can be given

without taking into account facts which are among the most recent

discoveries of physical chemistry; and of which even yet our knowledge

is by no means complete.

We know from our study of the structure of hide, that it consists in its

natural state of gelatinous fibres which are soft and swollen with

water, and easy putrescible. When these are dried, they contract and

adhere to each other, forming a hard and almost homogeneous mass,

resembling in degree, a sheet of glue or gelatine. After the tanning

process, the fibres are changed in character, though not in form; they

no longer absorb water so freely, and in drying they do not adhere

together, but remain detached and capable of independent movement. The

leather is therefore porous, flexible, and opaque on account of the

scattering of light from the surfaces of the fibres, although the

individual fibres are translucent. At the same time, chemical changes

have taken place which render the fibres incapable of ordinary

putrefaction. Our first necessity, therefore, in the conversion of skin

into leather is to dry the fibres without allowing them to adhere. This

is accomplished in the most primitive mode of leather dressing, by

mechanically working fatty substances into the skin as it slowly dries,

so as to coat and isolate the fibres, which are loosened by kneading and

stretching; while at the same time the fat forms a waterproof coating

which prevents them from again absorbing the water which is necessary to

putrefaction. Similar results may be produced by causing chemical

changes in the fibres themselves, which render them insoluble in water,

and consequently non-adhesive; and a sort of leather may even be made by

merely replacing the water between the fibres with strong alcohol, in

which they are insoluble, and which absorbs and withdraws the water from

them, allowing them to shrink and harden, while preventing their

adhesion. The merit of having first clearly seen and expressed these

cardinal principles in leather production belongs to the now venerable

Professor Knapp, who published in 1858 a short paper (_Natur und Wesen

der Gerberei und des Leders_) which is a model of clear explanation and

practical experiment. Knapp, however, deals mainly with the changes in

the condition of the fibre which are necessary to convert it into

leather, and not with their physical causes; and before we can explain

the means by which these changes are brought about, we must be

acquainted with certain facts and theories about solutions which have

become much clearer since he wrote.

The particles (molecules) of all substances are drawn together by

attractive forces somewhat of the same character as the attraction of

gravitation which holds together the solar system, and which is the

cause of weight. It is indeed even possible that these forces are

identical. Like gravitation, these molecular attractions increase

rapidly in intensity as the distance of the attracting bodies

diminishes, so that in solids and liquids, where the molecules are near

together, they are immensely powerful, while in gases and vapours they

are barely perceptible. These attractions are opposed by the motion of

heat, which takes the same part in molecular physics which the energy of

planetary motion does in the solar system. In solids, the attractive

forces hold the molecules rigidly in position, the motion of heat being

limited to short vibrations round a fixed point, the effects of which

are visible in the expansion caused by rising temperature. If the

temperature is increased, most substances become liquid, a condition in

which the particles can roll round each other, but are still held

together by their mutual attractions, as the sun holds the earth from

flying off into space. If the temperature goes on rising, the orbits of

the molecules become greater, the liquid expands, and finally molecules

fly off at a tangent out of reach of the attractions of the mass of

liquid, and are only diverted from their course by colliding with solids

or with other flying molecules, from which they rebound. This

constitutes the state of vapour or gas.

The molecules usually consist of _groups_ of atoms. Thus in the vapour

of water, each molecule contains one atom of oxygen combined with two of

hydrogen, and it is only at immense temperatures that this inner

grouping is broken up. Naturally, the more complicated and heavier the

molecular group, the more easily it is broken up by outside causes into

simpler groupings, and molecules may exist in liquids or solids, which

break up before they reach the gaseous form. Of such substances the

chemist says that they “cannot be volatilised without decomposition.” In

very rare instances does the gaseous molecule consist of a single atom;

even those of the most perfect gases, such as hydrogen, oxygen and

nitrogen consist of pairs which are not broken up at any known

temperature. The pressure of a gas, and its tendency to expand is due

simply to the motion and impact of its flying molecules, and it may be

noted that at the same temperature and pressure equal volumes of all

gases have the same number of molecules, the lighter molecules making up

for their want of weight by their greater velocity. The average velocity

of a molecule of oxygen (O₂) at freezing point is 461 meters per second

or about that of a rifle-bullet. It must not be taken however, that in

any given solid, liquid, or gas, all the molecules at any temperature

move at a uniform velocity, but that each individual molecule may vary

from moment to moment from rest up to a very high speed, while the

temperature of the mass only represents the average. Thus it happens

that in all liquids, and even in solids, a certain proportion of the

molecules at any temperature will have a speed sufficient to enable them

to leave the surface, and take the form of vapour, while a certain

proportion will fall back and be caught and retained. Thus every liquid,

and theoretically every solid, has a “vapour-pressure,” rising with the

temperature, and depending on it only, and at the boiling temperature of

the liquid equal to that of the atmosphere, or about 15 lb. per square

inch, and therefore able to form bubbles in the interior of the liquid.

If a little of a liquid is confined in a flask, the flask will become

filled with its vapour, and so long as any of the liquid is present, the

pressure of the vapour will depend only on the temperature and not at

all on the respective quantities of liquid or vapour. Neither will it

be affected by the pressure of other vapours or gases present in the

flask, the total pressure in which will be the sum of the “partial”

pressures of all the gases and vapours present.[52]

[52] Cp. p. 421.

The behaviour of gases and vapours has been described in some detail

because it possesses very close analogy to that of substances in

solution. The molecules of liquids are held together by attractions

which are very powerful over the short distances which separate them,

amounting in most cases to many tons per square centimeter of sectional

area, but the range over which they act is very small. In the interior

of the liquid the attractions on one side of a molecule are of course

exactly balanced by those on the opposite side, so that it is free to

move within the liquid without hindrance, but at the surface a very

small part of the force due to the attractions of the surface-layer is

unbalanced and acts as a sort of elastic skin holding the liquid

together, and is called “surface-tension.” Familiar examples of this are

found in the force which supports a drop on the end of a tube, the

possibility of laying a slightly oily needle on the surface of water

without sinking, and the ability of some flies to walk on water as if it

were covered with a sheet of india-rubber. Many liquids will mix or

dissolve in each other in any proportions, e.g. water and alcohol; the

attraction of the alcohol for the water-molecule being as great or

greater than that of alcohol for alcohol, or water for water. In other

cases, such as water and oil, or water and petroleum spirit, practically

no mixture takes place, their mutual attraction being small; and each

retains a considerable surface-tension at the points of contact, though

less than that of the free surfaces, since each exerts an attraction on

the other. There are also many intermediate cases, such as water with

chloroform, carbolic acid, or ether, in which each solvent dissolves a

portion of the other, but the two solutions do not mix, but form

separate layers. In these cases an equilibrium is attained, in which

there is just as much tendency for either of the liquids to pass into as

out of the other layer. In this there is an extraordinary resemblance to

what has been said of vapour-pressures; and the tendency to pass into

solution is often called solution-pressure; and it may be noted that

when equilibrium has been reached, not only is the solution-pressure,

but the vapour-pressure of each constituent equal in both solutions.

Like vapour-pressures, the solution-pressures usually increase with rise

of temperature, more of each constituent passing into the other, till at

last the composition of the two layers becomes identical, their

surface-tensions disappear, and complete mixture takes place. With

phenol (carbolic acid) and water this takes place at about 70° C.

Most of what has been said of the mutual solution of liquids is also

true of the solution of solids, but the latter may be divided into two

very distinct classes, colloids and crystalloids (which, however, shade

off into each other). The colloid or gluey bodies are mostly miscible in

any proportion with liquids in which they dissolve, and there is no such

thing as a definite point of saturation. There are however some which

form _jellies_ which have great analogy to the partially miscible

liquids; there is a mutual solubility, a portion of the solid dissolving

to a liquid solution, while the remainder of the liquid dissolves in the

solid, increasing its volume, but still retaining the characteristics of

the solid state. As the temperature is raised, this mutual solubility

generally increases, till at a given point the jelly melts, and complete

solution takes place, as in the case of partially miscible liquids.

These phenomena are of prime importance in the theory of tanning, but

their further consideration must be deferred till a few words have been

said about the crystalloids. These are characterised by regular

crystalline form, indicating that the attractive forces of their

molecules are exerted in definite directions, giving them a tendency to

attach themselves together in definite geometrical arrangements. They

dissolve in themselves no part of the solvent, but are dissolved by it

till an equilibrium is reached in which the tendency of further

particles of the solid to pass into the solvent is balanced by that of

those already dissolved to attach themselves to the remaining solid, or

“crystallise out.” Such a solution is “saturated” with respect to the

solid residue, but the word has no meaning unless solid crystals are

present, and where a body has, as sometimes happens, more than one

crystalline form, a solution may be saturated with regard to one of

them, and more or less than saturated with regard to another. In

“supersaturated” solutions, crystallisation is at once started by the

addition of a “seed” crystal of the proper form.

If a crystalloid substance, such, for instance, as copper sulphate, be

placed in a solvent (e. g. water), the dissolved salt will gradually

spread itself through the whole body of the solvent, though in the

complete absence of currents in the liquid, the motion is extremely

slow, and years may be taken for the diffusion to rise through a few

feet. In many cases salts diffuse through aqueous jellies at the same

speed as they would through still water. Colloid substances on the other

hand have little or no power of diffusion and mostly cannot pass through

jellies at all. This is the reason why tannage with mineral salts is so

much more rapid than with vegetable tannins which are of colloidal

character, and diffuse through the gelatinous fibres of the hide with

extreme slowness.

All dissolved crystalloids do not pass through gelatinous membranes with

equal ease, and substances are known, mostly gelatinous precipitates,

which do not permit the diffusion of dissolved salts, though they allow

water to pass freely. Thin layers of such precipitates form what are

called “semipermeable membranes.” The existence of such membranes

affords us the possibility of direct measurement of the tendency to

diffusion, or as it is generally called the “osmotic”[53] pressure of

dissolved bodies. Thus a porous earthenware battery-cell may be immersed

in a solution of copper sulphate, and filled with one of potassium

ferrocyanide. In this way its pores will be filled with a gelatinous

precipitate of copper ferrocyanide, which is pervious to water, but

impervious to most dissolved substances. If now the cell be filled with

a dilute solution of some crystalloid, say sugar, and its top closed by

a perforated cork fitted with a vertical tube, and the cell be plunged

in water, the latter will pass into the cell, and the dilute solution

will rise in the tube to a height of many feet above the water outside.

By substituting a mercury pressure gauge for the vertical tube, exact

measures of the pressure in the cell can be made, which is the osmotic

pressure of the dissolved substance. At first sight it is paradoxical

that the water should flow into the solution, apparently against a heavy

pressure, but the explanation is simple. Mention has already been made

of the enormous internal pressures of liquids produced by the

attractions of their molecules. In the solution a portion of this is

borne by the dissolved substance, and the water flows in from the

outside till an internal mechanical pressure is produced, equal in

amount to the osmotic pressure of the dissolved substance. The

resemblance of the phenomena of solution to those of vapour-pressure has

already been mentioned, and it is found to be even quantitative, since

the measured osmotic pressures are exactly equal in amount to those

which the dissolved body would produce if it were in the state of vapour

at the same temperature and occupying the same volume as the solution.

It acts, in fact, precisely as the “partial pressure” of a vapour. There

are several indirect ways of measuring the osmotic pressure of dissolved

bodies, as for instance, from the lowering of the freezing point, or the

raising of the boiling point of the solution as compared to those of the

pure solvent, all of which confirm the direct measurements, and show

that in a given volume at the same temperature, the same number of

molecules will produce the same osmotic pressure whatever their nature,

or conversely, that at the same osmotic pressure and temperature equal

volumes of any solution must contain the same number of molecules. The

use of these facts in determining molecular weight is obvious.

[53] Solution-pressure and osmotic pressure are really two names for

the same force; the former being employed to signify the tendency of a

solid to dissolve, and the latter the pressure produced by the

dissolved body which tends to prevent further solution. Thus, in a

saturated solution in contact with its solid, the two pressures are

always equal, but exerted in opposite directions.

A curious apparent deviation from this law is however noticed in

solutions of salts, acids, and alkalies, and indeed of electrolytes

generally; thus a dilute solution of sodium chloride produces an osmotic

pressure nearly double that corresponding to the number of molecules of

NaCl present; and in fact behaves as if it were a solution of Na and Cl

existing separately. Such a solution conducts a current of electricity

very readily, while at the same time the chlorine is carried to the

positive, and the sodium to the negative pole, where they separate as

Na₂ and Cl₂ (the Na decomposing the water present and forming NaOH). In

fact, the modern theory of electrolysis asserts that these dissociated

atoms are not separated from each other by electricity, but that they

exist already separated in the solution of the electrolyte, and merely

act as carriers for the electricity, and that the work done by the

latter is not that of breaking up the salt-molecule, but of giving its

dissociated atoms fresh charges of electricity which enable them to

combine as new molecules, and escape from the electrolyte. Complex

salts do not always break up into single atoms, thus calcium sulphate

dissociates into Ca and SO₄, hydrogen sulphate (sulphuric acid) into 2H

and SO₄, and so on. These dissociated atoms and atom-groups are called

“ions,” and may be monovalent, divalent, and so on; the divalent ion

carrying double the electrical quantity or charge of the monovalent.

Without discussing the ultimate nature of electricity itself, the matter

is most easily pictured by assuming that the molecule of the undissolved

salt is made up of an ion with a + charge (“kation,” e.g. Na), and an

ion with a - charge (“anion,” e.g. Cl), by the electrical attraction of

which charges they are held together. In the solution these attractions

are balanced by those of other ions, so that they can wander freely

within the liquid, but in order to take the molecular form of free

elements and escape, say as Na₂ and Cl₂, the pair of kations must go to

the - pole and give up one + charge, and at the same time a pair of

anions must go to the + pole and receive a + charge. Thus the Na and all

other kations separate at the - pole, and the Cl and all other anions at

the + pole.

From what has been said, it will be obvious that free ions can only

exist in solution, and can neither evaporate, nor separate as solids;

but that in the liquid they act much like other dissolved molecules,

exerting their own osmotic pressure independently of each other or of

the dissolved salt, but with the limitation that a solution must always

contain at the same time equal numbers of + and - ions. As a solution is

diluted, more ions are liberated; as it is concentrated, more recombine

to form undissociated salt. This will be made clearer by an example. In

a saturated solution of sodium chloride with solid salt present, we have

dissolved salt at the solution-pressure of the crystallised salt, and Na

and Cl ions at the dissociation-pressure of the saturated salt solution,

and neither affect the others. If we now add hydrochloric acid, it has

no effect directly on the solubility of the _salt_, but as HCl

dissociates largely into H and Cl, it increases the pressure of the Cl

ions, and so compels the salt to recombine till the Cl pressure is

reduced to its normal amount. This increases the concentration of the

undissociated salt-solution, and thus salt is precipitated or

crystallises out till the solution is no longer super-saturated with

respect to the salt-crystals.

Most chemical reactions, and especially those between acids and bases,

are really reactions of the ions. Thus NaOH in dilute solution is mostly

ionised into Na and OH, while HCl is similarly ionised into H and Cl. On

the other hand, water ionises only very slightly. Hence, on mixture, the

H and OH combine and form water, with evolution of heat, while no actual

combination occurs between the Na and Cl, so long as they remain in

_dilute_ solution. For this reason, the heat of neutralisation of all

strong acids and bases is the same, independent of their nature, since

strong acids, bases and salts are almost completely ionised. The

rapidity of action, and consequently what we call the “strength” or

“avidity” of an acid or base depends on the number of its free ions in

solution; very weak acids and bases are very little ionised, though

their salts ionise almost completely in dilute solution. On this depends

the explanation of a fact of great practical importance. Hydrochloric

acid, a strong acid, is almost completely ionised in solution; acetic, a

weak one, very little; while sodium acetate and sodium chloride as salts

are both almost completely ionised. If we add hydrochloric acid to a

solution of sodium acetate, we shall have sodium-ions, acet-ions,

chlorine-ions and hydrogen-ions in the solution. As the pressure of the

acet-ions and the hydrogen-ions will be greater than the

dissociation-pressure of acetic acid, they will combine to form it, till

the pressure is equalised, and we shall have in the solution, free

acetic acid slightly ionised, the sodium- and chlorine-ions of sodium

chloride, and the sodium- and acet-ions of any excess of sodium acetate

left. If the hydrochloric acid were just sufficient to combine with the

whole of the sodium, we should have an equilibrium containing much

(ionised) sodium chloride and little sodium acetate, together with much

free acetic acid, and little hydrochloric. Thus the “strong” acid would

displace the weak one.

Taking another example, we add sodium acetate to a solution of acetic

acid. As the ionisation-pressure of the acetic acid is much less than

that of sodium acetate, and both have a common acet-ion, the ionisation

of the acetic acid will diminish, and more undissociated acetic acid

will form, till by its concentration the two pressures are equalised.

The total quantity of free acetic acid will be unchanged, but a less

proportion of it will be ionised, and it will act like a weaker acid.

This reduction of the activity of a weak acid by the addition of its

neutral salt is often made use of by chemists. Instances in tanning

practice are the use of excess of potassium dichromate with chromic acid

in the chrome tanning process, the effect of neutral salts in

“mellowing” the action of tanning liquors, and the use of salt in

“pickling.”

Let us now try to apply these facts to the physics of tanning, taking

first the simplest cases, where electrolytic dissociation does not take

place. We may consider the wet hide as made up of a mass of fibres of

gelatine-jelly, with interspaces which are filled with water. In fact,

for many purposes of experiment we may substitute for hide, mere sheets

of swollen gelatine, so as to avoid the complications introduced by the

water or solution mechanically retained between the fibres.

If we place a sheet of dry gelatine in water, it swells, absorbing

perhaps seven or eight times its weight of water, but does not

appreciably dissolve. A condition of equilibrium is reached when the

attraction of the water-molecules for the gelatine is equal to the sum

of the cohesive attraction of the gelatine for itself and the internal

attraction of the water outside. An increase of the cohesion of the

gelatine would tend to make it contract and expel part of the water, and

this contraction would tend further to increase both the cohesion of the

gelatine, and its attraction on the diminished number of water molecules

it contained, and clearly these causes would act in opposing directions.

The equilibrium is therefore a very unstable one, and slight causes

might be expected to produce great changes in the degree of swelling,

which is indeed the case. If we increase the temperature we diminish the

cohesion of the gelatine, till at a point it becomes less than its

attraction for the water, and the jelly suddenly loses its solid

condition and dissolves.

The absorption of water by colloids (including gelatine) is accompanied

by contraction of volume (compression) of the water absorbed, and by

evolution of heat, and, as has been pointed out by Koerner,[54] it is

opposed (and swelling decreased) by increase of temperature. Solution,

on the other hand, absorbs heat, and is therefore favoured by rise of

temperature.

[54] Beiträge zur wissenschaftlichen Grundlage der Gerberei, Freiberg,

1899.

If we place the swollen jelly in alcohol, it parts with water and

contracts. The gelatine and alcohol are not mutually soluble, the sum of

the attraction of water for alcohol, and the cohesive attraction of the

gelatine is greater than the attraction of the latter for water, and as

the alcohol cannot pass into the gelatine, the water passes out, and the

jelly contracts. The greater the concentration of the alcohol, the more

completely is the jelly dehydrated, and in strong alcohol it may become

quite hard and solid. If we like to express the same facts in language

more familiar to the modern chemist, but perhaps less clear to the

non-chemical reader, we may say that the alcohol exerts an osmotic

pressure outside the gelatine, but little or none inside it, and

therefore the water is squeezed out. It would be equally true to say

that the water passes out of the jelly till its osmotic pressure is

equal in both the jelly and the alcohol. The jelly is a true “solid

solution” of water in gelatine, and in a solution we may regard either

of the two constituents as the solvent. Exact parallels may be found in

the distribution of a third substance between two immiscible solvents

(see p. 76), say alcohol between water and ether.

The osmotic pressure of water into alcohol may be demonstrated in a very

simple way, taking advantage of the fact that a film of jelly is

permeable for water but not for alcohol. If the experiment described on

p. 78 be made by placing alcohol in a cell previously washed out with a

gelatine solution, and the cell be placed in water, the water will pass

into the cell, and the alcoholic solution will rise many feet in the

vertical tube. The insolubility of gelatine in alcohol may be made use

of for its estimation. If three times its volume of absolute alcohol be

added to a solution containing gelatine, the latter will separate as a

solid mass on a stirring rod, or on the sides of the beaker, and may be

washed with further portions of alcohol. The method is useful in the

analysis of gelatine lozenges and “jelly squares,” roller compositions,

hectograph masses, and the like; and for the determination of true

unaltered gelatine in glues, and commercial gelatines (see page 60).

Many other colloids are however also precipitated by alcohol.

If hide be treated with alcohol, as in Knapp’s experiment (p. 74), the

action is precisely the same as has been described with gelatine-jelly.

The water is withdrawn, first from the spaces between the fibres, and

then from the fibres themselves, and the skin dries with the fibres

isolated and non-adherent, and is in fact converted into a sort of

leather, which, however, returns to raw pelt on soaking in water.

The action of solutions of sugars, glycerine, and the like is in

principle similar to that of alcohol, but more complex, since in general

these bodies are soluble not only in the water, but in the gelatine or

hide-fibre, so that their effect cannot be foretold, though usually it

tends towards contraction rather than swelling. In general terms the

equilibrium is a balance of the attraction of the water and the sugar

for the gelatine, against the sum of their mutual attraction in the

solution outside and the resisting cohesive force of the gelatine; and

will depend not only on the nature of the substances, but on temperature

and concentration.

The action of acids, alkalies and salts on gelatinous fibre is yet more

complex, since not only electrolytic dissociation, but most probably

actual chemical combination comes into the question. The chemical

constitution of gelatine is as yet quite uncertain, but it is known that

the molecule contains both amido-groups capable of linking with acids,

and carboxyls which will combine with bases (see p. 58). Hence

hide-fibre absorbs both acids and bases with great avidity, so much so

that the sulphuric acid of a decinormal solution may be completely

removed by hide, leaving only water without a trace of acid recognisable

by litmus. Alkalies are absorbed in a similar way, and in both cases the

gelatine or gelatinous fibre acquires a greatly increased power of

absorbing water, and consequently of swelling. Familiar cases of this

are the swelling of hide by acid, and by lime, and in neither case can

the added substance be removed in any reasonable time by mere washing

with water. Hence to free hides from lime or acids it is necessary to

neutralise the alkali with acids (see p. 153) or the acid with chalk or

alkalies (p. 91). No accurate determination has yet been made of the

amount of acid or alkali with which gelatine or hide-fibre will combine,

since the matter is complicated by the volume of acid or alkaline

solution which is absorbed mechanically, and by the tendency of the

compound to partially decompose on washing with water. Experiments made

by the author lead to the conclusion that 1 grm. of air-dried gelatine

will combine with about 0·025 grm. of actual hydrochloric acid (HCl)

when placed in a very dilute solution of the latter, and this compound

will absorb 40 or 45 grm. of water while still retaining the jelly

state. The maximum swelling, with both acids and alkalies, is obtained

with dilute solutions; and with the stronger acids, the outside solution

must be almost neutral when equilibrium is attained, increasing

quantities of acid diminishing the amount of water absorbed. The same

statement is true of the strong alkalies. Thus in both cases, where

swelling is desired, the object is defeated by the use of too strong

solutions, and the quantity of acid or alkali should be rigidly adjusted

to the weight of pelt, and not to the volume of solution.

As regards a physical explanation of the effect of acid and alkaline

solutions upon gelatine, anything which can yet be said must be regarded

rather as speculation than as actual scientific knowledge. It must also

be admitted that while the view that actual _chemical_ combination takes

place between the gelatine and acids (or alkalies) seems much the most

probable, difficulties arise from the fact that different acids

apparently do not always combine in proportion to their equivalents,

though it is probable that these will prove only apparent anomalies when

more accurate means are known of determining how much acid is really

combined, and how much merely mechanically absorbed.

Leaving out of account for the moment the question of swelling, a few

words must be said about a property of these acid- (and alkali-)

gelatine compounds, a knowledge of which is essential to understanding

the swelling process. If a mass of acid-gelatine be suspended in pure

water, a certain portion of it will be decomposed into neutral gelatine

and free acid, and the latter will diffuse into the water. Thus

acid-gelatine can only exist in presence of a certain amount of free

acid. This dissociation by water is a common property of all salts, and

necessarily follows from what has been said of ionisation; but it is

only where the combining affinity of the constituents is weak, that it

becomes practically perceptible. Water to a _very_ small extent ionises

to H and OH. If we imagine a salt dissolved in it, such as NaCl, which

ionises almost completely to Na and Cl, we see that a certain proportion

of NaOH and HCl must be formed by combination with the water-ions. In

the case named the quantity is absolutely negligible, since both sodium

hydrate and hydrochloric acid are almost completely ionised themselves,

but if either the acid or the base is weak (that is little ionised), the

process of combination must go on till the acid or basic solution is

strong enough to have an ionisation-pressure equal to that of the salt.

As this acid or base is no longer in an ionised condition, it may be

removed from the solution by volatilisation or diffusion. For instance,

if a solution of ferric chloride be confined in a tray of parchment

paper, through which it has little power of diffusion, and this tray

floated upon water which is frequently changed, the dissociated acid

will diffuse through the membrane into the water, and in this way the

whole of it may be ultimately removed, leaving nothing but a colloid

solution of hydrated ferric oxide in the tray. Actions of this sort, in

which the gelatinous fibre of the hide plays the same part as the

parchment-paper membrane, have an important share in many of the

phenomena of tanning.[55] Thus, in the case of hide swollen with acid,

the acid compound with the fibre is somewhat dissociated, and if the

hide be hung in water which is constantly changed, the acid diffuses

into it, and the whole may be ultimately, though slowly removed. A

similar effect is produced in the familiar operation of removing acid

from pelt or chromed leathers by paddling with “whitening” (calcium

carbonate). The latter is insoluble in water, and therefore cannot

penetrate into the hide, but as it instantly combines with any acid

which diffuses out, the acid-gelatine compound is rapidly decomposed,

since it is only permanent in a solution containing enough free acid to

have an ionisation-pressure equal to that of the compound. Similar

statements are true of the alkali-gelatine and lime-gelatine compounds.

[55] Probably chrome, aluminium and iron salts are decomposed in this

way in mineral tanning, and thus fixed in the hide as insoluble basic

salts. Cp. pp. 186, 215.

It will be easier to follow the results of what has been said if we take

a concrete case which has been carefully investigated by the author and

others; that of the action of hydrochloric acid solutions on gelatine.

If a weighed sheet of gelatine be placed in a very dilute solution of

the acid, it swells much more considerably than it does in water, a

maximum swelling being attained with a concentration of the outer

solution of 0·1 to 0·2 grm. of HCl per litre. The swollen jelly has then

a volume of about 45 c.c. per gram of the air-dried gelatine, and a

concentration equal to about 0·75 grm. of HCl per litre of swollen

jelly, or at least about five times that of the outer solution. As the

concentration of the latter is increased, the concentration in the jelly

also increases, but in a much smaller ratio, while the volume of the

jelly diminishes, till, with a concentration of 5 grm. of HCl per litre

in the outer solution, the volume of the jelly is only about 18·5 c.c.,

and its concentration not quite 6 grm. per litre. These facts cannot be

accounted for by any theory of simple solution of the HCl in the jelly,

since the law of such solutions is that the concentration in each

maintains a constant ratio, unless chemical change takes place. It is

possible that they might be explained by adsorption (surface

attraction), but as it is known that gelatine contains both amido-groups

capable of combining with acids, and carboxyl-groups which can combine

with bases, it is much more likely that actual chemical combination

takes place, and that the apparent irregularities in the amount of acid

fixed are due to partial hydrolysis of the compound.[56]

[56] Cp., however, Walker and Appleyard on the ‘Absorption of Acids by

Silk,’ Chem. Soc. Trans. 1896, p. 1334.

The following may be suggested as a working hypothesis. As both water

and hydrochloric acid can pass freely in and out of the jelly, it must

be in osmotic equilibrium with the outer solution in every respect, and

neither the un-ionised hydrochloric acid of the solution, nor the small

amount which may be formed by hydrolysis of the gelatine compound can

have any effect on the swelling. So long as the outer solution is very

dilute, by far the greater part of the acid present is absorbed and

fixed by the gelatine, and almost the whole of the outer acid will be

ionised, as well as a portion of that in combination with the gelatine.

In the latter case, however, the ions will be unable to pass out of the

jelly, and will therefore cause an internal osmotic pressure, and the

gelatine will swell till the Cl-ions are in osmotic equilibrium with

those of the outer solution. At the same time, this internal pressure of

Cl-ions will oppose the entry of the Cl-ions (and therefore also of

their associated H-ions) from the outer solution, and the acid solution

absorbed mechanically will be somewhat less concentrated than that

outside. As the concentration of the outer solution is increased, the

pressure of the outer Cl-ions will repress the ionisation of the

gelatine-chloride, and at the same time its tendency to hydrolyse. Thus

the acid actually combined with the gelatine should somewhat increase,

but the swelling should diminish, as is actually the case.[57] It is

impossible to carry the concentration of the hydrochloric acid much

above 5 grm. per liter without causing solution of the gelatine, but the

addition of common salt to the outer solution should equally increase

the pressure of its Cl-ions, and cause further diminution of swelling,

the Na-ions in this case increasing the outside pressure in the same way

as the hydrogen ions. In fact the addition of salt in sufficient

quantity will reduce the swelling till the gelatine becomes quite solid,

and retains only about its own weight of water, while at the same time

the apparently combined acid largely increases. This cannot be

attributed to any direct dehydrating action of the salt, since

concentrated sodium chloride solutions have no dehydrating, but rather a

swelling effect on gelatine in the absence of acid, and the

concentration of the salt in the outer solution and in the jelly proves

precisely the same within the limits of experimental error. Several

other facts may be noted, tending to support the explanation which has

been given. The tendency to swell gelatine is common to all acids of

appreciable strength, and in all cases where the concentration of the

acid could be increased to a moderate extent without causing solution of

the jelly, the effect of a maximum swelling, diminishing as the

concentration of the acid increased, has been observed. Other salts also

produce similar effects to sodium chloride; thus the swelling caused by

sulphuric acid is repressed by sodium sulphate. Sodium chloride seems to

diminish the swelling caused by all acids, but in presence of large

excess of sodium chloride, most of the acid in combination with the

gelatine will probably be hydrochloric, whatever the acid used to

originally produce the swelling. A curious fact observed by the author,

is that absolute alcohol, which so effectually dehydrates neutral

gelatine, is almost powerless to remove either water or acid from

gelatine swollen by hydrochloric acid. HCl is freely soluble even in

absolute alcohol, but H- and Cl-ions can only exist in it to a very

small extent, so that we may conclude that the acid which causes the

swelling and retains the water of the jelly exists either in actual

combination with the gelatine, or in an ionised condition.

[57] The acid retained by the gelatine, as measured by deducting from

the total contained in the jelly, a quantity equivalent to the volume

of solution absorbed, at first rises rapidly to a maximum, then

slightly diminishes and remains practically constant. On the theory

suggested, it is evident, however, that the absorbed solution must be

more dilute than that outside, and the actual combined acid greater

than that shown by the above calculation. The “combined” acid, as

determined by indicators, shows slight but continuous increase. It is

acid to phenolphthalein, but neutral to methyl orange.

Solutions of caustic alkalies are in most respects analogous in their

swelling action to those of strong acids. A portion of the alkali is in

some way fixed by the gelatine, while another portion is simply absorbed

as solution. A maximum swelling effect is also noticed with dilute

solutions, which is diminished as the concentration increases. Swelling

by alkalies is not diminished by chlorides so far as has been observed,

and especially it may be noted that the swelling produced by caustic

soda is not diminished by sodium chloride. On the theory which has been

suggested there is no reason why alkaline swelling should be reduced by

chlorides, since the swelling agent has no Cl-ion, but it is somewhat

singular that the sodium salt, having a common Na-ion should produce no

repression of the swelling by caustic soda. In the present state of our

knowledge no definite explanation can be given, but it is quite possible

that the swelling in this case is not produced by the sodium-ion but by

some more complex one, or even by the hydroxyl-ion, like most of the

characteristic reactions of alkalies. Apparently the gelatin-alkali

compound is still strongly alkaline, affecting phenolphthalein indicator

like uncombined alkali--an effect which is known to be due to the

presence of free HO-ions.

The effect of acids and alkalies has been studied by Procter and others

on actual pelt as well as on gelatine, and has been found to be

qualitatively, if not quantitatively quite similar to that on gelatine,

though from the acid retained mechanically in the interfibrous spaces,

exact quantitative determination is more difficult. The amount of

swelling produced is not proportional to the strength of the acid, some

weak and little ionised acids such as lactic producing larger swelling

than stronger acids such as hydrochloric and sulphuric, of which the

ionic pressure in the external solution is greater. Dilute solutions

generally produce greater swelling than more concentrated, so that where

swelling is required without destructive effect on the fibre, dilute

solutions of such weak acids are to be preferred, and the presence of

neutral salts is to be avoided. On the other hand, where it is desired

to remove lime, or to bring the pelt into an acid condition without

swelling, the addition of neutral salts, and especially of chlorides is

advantageous. A very important application of this principle is the

“pickling” of sheep-skins, and especially of sheep-grains, in order to

preserve them for export. The principle of this operation is that the

skins are first swollen slightly with sulphuric acid, and the swelling

is then reduced by salt, either added, or used in a subsequent bath. In

practice, salt is now generally also added to the first bath to moderate

the swelling. A suitable strength for the “rising solution” is about 80

grm. common salt, and 7·5 grm. sulphuric acid per litre. 100 c.c. of

this solution will therefore require about 15 c.c. of N/1 alkali to

neutralise it, and it should be tested after each lot of skins, and

maintained at the same strength by suitable additions of acid. The acid

absorbed by the skins is mainly hydrochloric, sodium sulphate

accumulating in the bath. The salt is not absorbed by the skins in the

same way as the acid, but will be continually diluted by the water they

bring in, and occasional additions of salt must therefore be made, the

density being maintained at about 65° Bkr. (1·065 sp. gr.) After

paddling or being stirred in this bath for about ¹⁄₂ or ³⁄₄ hour the

skins are transferred to saturated brine, and stirred in it till fully

fallen in thickness, the density of the liquid being maintained by

excess of salt. They may be allowed to remain some hours in the

saturated brine with advantage.

Within moderate limits, the strength of the rising liquor is not of

great importance, since the skins will only absorb a certain amount of

acid (increasing with the concentration of salt). In the second or

falling liquor the large excess of salt forces all the acid present into

the skins, none diffusing into the bath. Skins may be effectively

pickled with very much smaller quantities of acid than those prescribed

above, or ordinarily used, and are much easier to tan satisfactorily;

but it is said that they are more liable to suffer from mildew. Pickling

may also be done by placing the skins in a concentrated brine-bath, and

adding a calculated quantity of acid, not exceeding 0·1 grm.-molecule of

sulphuric acid per kilo. of dry hide substance, but the method is not

economical in practice from the dilution of the bath produced by the

water brought in by the skins and the necessity of constant large

additions of salt.

Pickled skins must not be brought in contact with water, which by

diluting the brine they contain, allows the excess of acid to act upon

and destroy the fibre. Even drops of water, accidentally sprinkled on

the skins produce this effect, and it is said that it spreads to parts

which have not been wet. For similar reasons, it is necessary in

tanning pickled skins, at least to begin the process in liquors to which

salt has been added, the quantity required being dependent on the amount

of acid used in pickling the skins, and where this is reduced to a

minimum, it is even possible to tan without further addition of salt

than that contained in the skins.[58] The pickling process converts the

skins into a species of white leather, and skins tanned in salted

liquors after pickling, or by addition of both acid and salt to sumach

liquors give good colour, and tough leather with a much diminished

consumption of sumach. The permanency of such leather is somewhat

doubtful, but the writer was unable to detect free sulphuric acid in a

sample which he examined, and it may be that when no acid is added to

the later liquors, that derived from the pickling is expelled by the

tannin; but this is very doubtful.

[58] Instead of using salted liquors, the skins maybe “depickled” by a

bath of whitening and water, borax, or some other mildly alkaline

solution before tanning.

The facts which have been discussed in the preceding pages offer a

sufficient explanation of the causes which operate in those deliming

processes which depend on the simple neutralisation of the alkaline

matters present in the hide, and of the swelling by means of acid which

forms a step in the manufacture of many sorts of sole-leather, but they

by no means fully elucidate the causes of the much more complete

depletion of the pelt brought about by the bacterial products of bates

and puers. It has been pointed out (p. 82) that gelatine and hide-fibre

in a neutral condition are swollen by water, but that the equilibrium so

reached is an unstable one, easily influenced by slight causes. Among

these, as has been pointed out by Koerner,[59] the surface-tension

between the water and the swollen fibre holds a place; and

surface-tensions of this sort are greatly influenced by many substances

of the class to which bacterial ferments belong. Many salts also alter

the water-absorption of gelatinous fibres, sometimes causing swelling,

and sometimes contraction, according to temperature, concentration, and

the nature of the salt. Though most salts do not seem to be absorbed by

hide-fibre, it is possible, as suggested by Koerner (_loc. cit._), that

in some instances the base may combine with the acid-groups, and the

acid of the salt with the basic groups of the gelatine-molecule, while

other cases are known in which salts are actually dissociated, and their

acid fixed by the affinities of the hide-fibre. An interesting case of

this sort was recently proved by Paessler and Appelius,[60] who showed

that sulphuric acid was absorbed from a solution of hydric sodic

sulphate, and the neutral sulphate left in the solution. Similar

reactions undoubtedly occur with some salts of strong acids and weak

bases, but this point must be more fully discussed in connection with

the theory of mineral tannages.

[59] Beiträge zur wissenschaftlichen Grundlage der Gerberei,

Jahresberichte der deutschen Gerberschule zu Freiberg, 1898-9 and

1899-1900.

[60] Wissenschaftliche Beilage des Ledermarkt, 1901, ii. p. 106.

CHAPTER X.

_WATER AS USED IN THE TANNERY._

Of all the materials employed in tanning, none is of more indispensable

importance than water, and its quality has undoubtedly great influence

on tanning, though it is constantly blamed for faults and troubles which

are really due to the mistakes of the tanner.

Water is chiefly used in tanneries for soaking and washing hides and

skins, for making the limes, the bates, and the tanning liquors, for

steam boilers, and in dyeing. For all these purposes it should be as

free as possible from impurities, but since water is the most universal

solvent in Nature, it is never found pure, but always contains mineral

matter derived from the rocks and soil through which it has flowed, as

well as organic impurities from decaying animal and vegetable matter.

Associated with the latter are usually living organisms of putrefaction

(_bacteria_) which may affect the quality of the water for tanning even

more seriously than the mineral impurities. The purest natural waters

are those which have flowed only over hard sandstones and volcanic

rocks. Water sufficiently pure for laboratory use can only be obtained

by distillation. The steam-water from heating pipes usually contains

large quantities of dissolved iron, and often also volatile organic

matters from the oil, etc., which finds its way into the boiler. It may

sometimes be made fit for use by boiling (which precipitates the ferrous

carbonate present), and subsequent settling or filtration. The use of

steam-water containing iron is a frequent source of stains and

discolorations in the tannery which more than counterbalances the

advantage of its softness.

The “hardness” of natural waters is mostly due to the salts of lime and

magnesia which they contain, which precipitate soap in the form of

insoluble stearates and oleates, which are useless for washing. It is

commonly estimated by determining the amount of a standard alcoholic

soap solution which must be added in order to produce a permanent froth

on shaking. Theoretically about 12 parts of soap (sodium stearate or

oleate) are destroyed by 1 part of calcium carbonate or an equivalent

quantity of other lime salts, with formation of insoluble lime soaps

(calcium stearate or oleate). Really, the reaction is much more

complicated, owing to the dissociation of the soap into free alkali and

acid-salts on solution in water. Teed[61] estimates that ¹⁄₃ to ¹⁄₂ more

is required than the theoretical quantity, and more in hot water than

cold. This uncertainty is partially overcome by testing the soap

solution against a known solution of calcium chloride. The presence of

magnesia also complicates the test and leads to discrepant results.

[61] Journ. Soc. Chem. Ind., 1889, p. 256. Cp. also Allen, ibid. 1888,

p. 795.

The methods of determining hardness originated by Hehner (see L.I.L.B.,

p. 19) are simpler and more accurate than the soap-test, and are to be

preferred, except for direct determination of the suitability of a water

for scouring with soap. “Degrees” of hardness in England are calculated

as parts of CaCO₃ per 100,000, or sometimes grains per gallon (70,000

grains).

Hardness is of two kinds, “temporary” and “permanent”; the former being

removed by boiling, while the latter is not so removed.

Temporary hardness consists of the carbonates of alkaline earths held in

solution by an excess of carbonic acid. Lime combines with 1 molecule of

carbon dioxide to form the ordinary normal carbonate (chalk), which is

practically insoluble in water. When, however, excess of carbonic acid

is present, hydric calcic carbonate (bicarbonate) which is fairly

soluble is produced. This is easily demonstrated by passing carbon

dioxide into somewhat diluted lime-water, which at first becomes turbid

from precipitated chalk, but soon clears by formation of soluble hydric

carbonate. If the solution be now boiled, the hydric carbonate is

decomposed, and the excess of carbonic acid is driven off as CO₂, and

the chalk again precipitated. The reactions are represented by the

following equations:--

Ca(OH)₂ + CO₂ = CaCO₃ + OH₂. (1)

{ CaCO₃

CaCO₃ + CO₂ + OH₂ = { (2)

{ H₂CO₃

Magnesia forms soluble double carbonates in a similar manner, but on

continued boiling gradually loses the whole of its carbonic acid, and is

precipitated as magnesium hydrate, Mg(OH)₂.

One of the most important reactions in connection with temporary

hardness is that caused by the addition of calcium hydrate (slaked

lime), which forms the basis of Clark’s softening process. When an

equivalent amount of lime is added to a solution of hydric calcic

carbonate, it displaces the water of the “half-bound” carbonic acid,

forming a second molecule of calcium carbonate, which is precipitated

together with that originally present, as is represented in the

following equation:--

CaCO₃ }

} + Ca(OH)₂ = 2CaCO₃ + 2OH₂. (3)

H₂CO₃ }

Hydric magnesium carbonate is also precipitated by lime, but the

reaction is somewhat different, the magnesia being removed as hydrate as

follows:--

{ MgCO₃

{ + 2Ca(OH)₂ = 2CaCO₃ + 2OH₂ + Mg(OH)₂. (4)

{ H₂CO₃

It will be noted that 2 equivalents of lime are required to precipitate

1 of magnesia. Two molecules of sodium hydrate (NaOH) or potassium

hydrate (KOH) may be substituted for 1 of Ca(OH)₂ with similar results,

and in some cases it is practically advantageous to use the former, as

the sodium carbonate formed in precipitating the temporary hardness

reacts again on the permanent, throwing down the lime and magnesia as

carbonates. (See p. 101.)

[Illustration: FIG. 19.--Plan of Archbutt and Deeley’s Apparatus.]

The use of lime for softening temporary hard waters was originally

proposed by Thomas Henry, F.R.S., of Manchester, but was first applied

as a practical process by Clark, who, after adding the requisite

quantity of lime to the water in a mixing vat, allowed it to stand in a

large tank to clear by subsidence, the precipitated carbonate of lime

taking from 6 to 12 hours to settle. The process in its original form is

a perfectly satisfactory one, except for the capacious settling tanks

which are required, which in some cases are inconvenient and expensive.

Messrs. Archbutt and Deeley[62] have patented a modification of the

Clark process, by which the time of subsidence is much shortened, and

according to which the precipitated carbonate of lime of previous

operations is allowed to remain in the tank, and the fresh charge of

water and lime is mixed up with it by means of steam-injectors, which

blow in a current of air through perforated pipes at the bottom of the

tank, and at the same time very slightly warm the water. The action goes

on much more rapidly at a slightly raised temperature than in the cold;

and rather curiously, the stirred up precipitate, instead of increasing

the time of clearing, settles rapidly and carries down with it that

formed in the new operation. It is particularly suitable for treating

waters containing magnesia, from which a compound of lime and magnesia

is apt to be precipitated in a colloid form which chokes filter-cloths

and will not readily settle. After softening, the water is usually

“carbonated” by passing the gases produced by burning coke into the

floating exit-pipe through which it falls, in order to retain any

remaining traces of carbonates of lime and magnesia in a soluble form,

and prevent their subsequent precipitation in the pipes. The apparatus

is made by Messrs. Mather and Platt, of Manchester, and its arrangement

is shown in Figs. 19 and 20.

[62] Journ. Soc. Chem. Ind., 1891, p. 511.

[Illustration: FIG. 20.]

Several modifications of the Clark process have been introduced, in

which the precipitation is carried on continuously instead of

intermittently. The most important of these is the Porter-Clark, in

which one portion of the water to be softened flows through an agitator

containing excess of lime, with which it forms saturated lime-water,

which is passed slowly up a cylinder where it deposits the excess of

suspended lime. The clear lime-water so produced is mixed with a fresh

portion of the water to be softened in a second cylinder also provided

with an agitator, the proportion of the two liquids being regulated by

cocks. The carbonate of lime is at once precipitated, and is removed by

passage through a filter press. This process is in successful operation

on a considerable scale at Messrs. Hodgsons’ tannery at Beverley.

Several other forms of filter have also been employed with success, and

also methods in which the treated water traverses tanks with sloping

partitions on which the carbonate of lime is deposited. The latter plan

was originally patented in France by Gaillet-Huet, and has been

introduced into England by Stanhope.

So far as is yet known, from the tanner’s point of view, it is hardly

necessary to make any distinction between lime and magnesia, either or

both of which may be considered simply as “hardness.” A hard water

probably softens dried hides more slowly than a purer water, though it

is possible that the observed difference in the time required may be due

in many cases to the lower temperature of wells from which hard water is

generally derived. In the actual “limes” the hardness of the water can

have no appreciable influence, though if sodium sulphide be used alone

for unhairing, a certain waste occurs from temporary hardness which may

render it advisable to add a little lime. It is in washing the hides

free from lime that the influence of hard water is first distinctly

felt. If limy goods, after unhairing, are placed in a water with much

temporary hardness, the same action occurs as in Clark’s water-softening

process, and chalk is deposited in the surface of the hides, making them

harsh and apt to “frize” or roughen the grain in “scudding.” The common,

but not wholly satisfactory expedient is to add a little lime, or

better, a few pailfuls of lime liquor to the water before putting in the

hides. The best plan is to use a properly softened water. Permanent

hardness is not injurious in this way.

Unfortunately it is not the grain alone which is injured by the use of

hard water for washing the hides, but on coming into the liquors the

precipitated bases combine with the acids and tannins, forming compounds

which oxidise and darken when exposed to the air, and which are the

commonest causes of stains and markings on all descriptions of leather.

Even when goods are drenched or bated before tanning the injury is not

prevented, since the weak organic acids which are capable of removing

the lime (as such) from the hide have little effect on the precipitated

carbonate, which can only be dissolved by the use of stronger acids. It

must be noted that the same injurious effect on limed goods is produced

by free carbonic acid, which may be present even in soft waters.

When temporarily hard waters are employed for leaching tanning

materials, the carbonic acid is displaced by the tannins, which form

compounds similar to those just mentioned, which are incapable of

tanning, and darken and discolour when exposed to the air. Though the

amount of lime present in a liter of even the hardest water is very

small, yet in the aggregate of thousands of gallons used weekly in a

good-sized yard it amounts to something very considerable, and as the

molecular weight of tannins is very high, the quantity destroyed is many

times that of the lime present. This loss can be prevented (_a_) by the

addition of sufficient mineral acid to convert the temporary into

permanent hardness, (_b_) by the use of oxalic acid, which precipitates

the whole of the lime as oxalate, or, (_c_) best of all, by softening

the water by suitable treatment before use. Each part of temporary

hardness reckoned as CaCO₃ (L.I.L.B., p. 19), requires 1·26 parts of

crystallised oxalic acid or 0·98 parts of H₂SO₄, or say one part of

ordinary oil of vitriol of sp. gr. 1·840 per 100,000 parts of water.

As the lime and magnesia of temporarily-hard water is thrown down by

boiling, it is deposited in steam boilers as a soft precipitate, much of

which can be blown out by suitable sludging; but if oils or fats obtain

access to the boiler, a soft, bulky, adherent deposit is formed, keeping

the water from the plates, which may become red hot, and lead to

collapse or explosion. This effect is not produced by mineral oils,

which, on the contrary, tend to prevent adherence of scale to the

plates, and as suitable mineral oils are not only cheaper, but much less

injurious to the working parts of steam engines than animal or

vegetable oils or tallow, they should always be used in preference for

cylinder purposes.

Water which is temporarily hard owing to calcium and magnesium

carbonates, is unsuitable for _dyeing_, as the carbonates react with

basic dyes, precipitating the colour-base, and so rendering a part of

the dye useless. Further, as this precipitate is deposited on the skins

it causes uneven dyeing and gives rise to spots and streaks. In dyeing

with basic dyes, therefore, it is advisable to add sufficient acetic

acid to the water before use to exactly neutralise the carbonates

present. Of course this treatment is quite unnecessary when acid dyes

are employed, as acid is usually added with the dye, and with dyewoods

the presence of a little calcium salt is advantageous.

As each “degree” of total hardness represents a soap-destroying power of

at least 2 oz. of soap per 100 gallons of water, allowance must be made

in making up “fat-liquors” with soap and oil for the loss of soap due to

its precipitation by the mineral matter in the water. The sticky

lime-soaps are apt to adhere to the leather and interfere with glazing;

so that it is much better to employ a soft water.

_Permanent hardness_ of water is generally caused by sulphates of lime

and magnesia, and more rarely by chlorides and nitrates. As none of

these can be precipitated by lime, permanent hardness cannot be removed

by Clark’s process, nor can it produce the injurious effect on limed

hides which have been attributed to temporary hardness. Neither can the

lime and magnesia present combine with the tannins if used for leaching,

since they are already fixed by stronger acids, and at most can only act

injuriously by slightly lessening the solubility of the tannins. Even

this effect cannot be regarded as proved, though it deserves further

investigation.[63] Permanent hardness is therefore of little moment as

regards the ordinary uses of the tannery, though it has considerable

influence in some of the processes of dyeing, and acts very injuriously

where soap is used for scouring, as in the washing of sheep-skins for

wool mats, since each part of lime reckoned as carbonate destroys at

least twelve parts of pure soap (sodium stearate or oleate), producing

a sticky and insoluble lime-soap which adheres to the fibre. In

sole-leather tanning, permanent hardness is sometimes advantageous,

especially if it be due to calcium and magnesium sulphates, and Vignon

recommended that sulphuric acid should be added to the water before use

in quantity sufficient to exactly neutralise the carbonates which cause

temporary hardness, as magnesium and calcium sulphates are not

injurious, but tend to plump the hides. It must be remembered, however,

that the carbonic acid liberated may still have prejudicial effects on

limed hides.

[63] Recent investigations by Nihoul (‘Influence de la nature de l’eau

sur l’extraction des matières tannantes,’ Bulletin de la Bourse aux

Cuirs de Liège, Sept. 1901) on the tanning waters of Belgium seem to

show that permanent hardness is more injurious in the extraction of

tannin than has generally been supposed.

Permanent hardness is most objectionable in waters employed for

boiler-feeding, and calcium sulphate is especially so, as it becomes

nearly insoluble in water at 150° C. or 55 lb. steam-pressure, and is

deposited on the plates as a hard crystalline scale which has to be

chipped off with a hammer. Where many boilers have to be worked with a

hard water, it is much the most satisfactory to soften the water with

caustic soda, or with lime and soda together before it comes into the

boiler, but in cases where the plant required would be too costly,

boiler-compositions are sometimes used with good effect, though

considerable caution is advisable, since some of them affect the plates

injuriously. The active constituent of many boiler-compositions is

soda-ash or sodium carbonate, which acts by double decomposition with

the calcium sulphate, forming sodium sulphate, and precipitating calcium

carbonate as a sediment which is easily washed out. Most tanning

materials, and even spent tan liquors, will prevent or lessen

incrustation if mixed with the feed water, but sometimes corrode the

plates if used too freely. This danger is lessened if they are used in

conjunction with soda. Heavy mineral oils, either introduced in small

quantity with the feed water, or painted on the sides of the boiler when

cleaned, are useful in preventing the formation of a coherent scale.

The removal of permanent hardness from water is easily effected in most

of the forms of apparatus employed for the softening of water by lime,

by using a calculated quantity of sodium carbonate in addition. The

reaction is represented in the case of calcium sulphate by the following

equation--

CaSO₄ + Na₂CO₃ = CaCO₃ + Na₂SO₄.

The conversion of magnesium sulphate into carbonate may be similarly

effected, but as the latter is somewhat soluble, an additional

equivalent of lime must be used to precipitate it as hydrate. Magnesium

salts, from their solubility, do not cause scale on boilers (though the

chloride is apt to produce corrosion), but they are equally destructive

of soap with the calcium salts. Caustic soda will remove temporary

hardness, and after becoming converted into carbonate will further react

on any permanent hardness present; and its use is therefore sometimes

convenient in small softening plants, but it is not more effective, and

considerably more costly than a suitable mixture of lime and sodium

carbonate. Even with these, Archbutt states that the cost of softening

permanent hardness is about ten times as great as that of removing

temporary hardness with lime only.[64]

[64] Proceedings of Inst. of Mech. Engineers, 1898, pp. 404-54, in

which much valuable information on water-softening is given.

As regards the influence of other impurities, our knowledge is far from

complete, but the following are the most important matters likely to be

present.

_Mud_ under any circumstances is objectionable. It frequently contains

organic slime and organisms which encourage the putrefaction of hides

placed in it to wash or soften. It also almost invariably contains iron

as one of its constituents, and hence stains leather and gives dark

coloured liquors. It is not easily removed by filtration, as large

filter-beds are expensive and difficult to keep in order, and much space

is required to clear water by subsidence. Some mechanical filter which

can be easily cleaned, and used under pressure, offers the best chance

of success. The Pulsometer Company make one consisting of sponge tightly

packed below a perforated piston. To cleanse the filter a stream of

water is passed the reverse way, and the piston raised and worked up and

down, either by hand or power, so as to loosen and knead the sponge.

Filter-presses, in which cloths, or in some cases sand, are used as the

filtering medium, are also well adapted for the purpose. If a water be

softened by Clark’s or other process the precipitated chalk carries down

the mud with it, together with most of the organisms.

_Iron_ is always an objectionable impurity in the tannery, though it is

less injurious to the quality than the appearance of the leather

produced, and indeed German sole-leather tanners frequently put old

iron in the handlers to darken the colour of the leather, and

apparently, if not really, to quicken the tannage. It must not be

present in waters used for dyeing. Iron oxide is frequently present as a

mud merely, and in this case can be removed by filtration. It is rarely

in solution in any other form than that of acid carbonate, since

sulphate or chloride could not exist in presence of bicarbonate of lime.

In this form, iron is precipitated at once by boiling or on the addition

of lime, like the temporary hardness due to other bases, in the form of

ferric hydrate, and more slowly by oxidation on exposure to the air. The

mud produced by softening waters which contain iron must be completely

removed by filtration, or subsidence, before the water is used for

leaching, or the iron will redissolve in the acids of the liquors. Iron

is not perceptibly injurious in the limes, but in the bates and

wash-pits sometimes causes stains, which are scarcely visible till

blackened by the tanning liquors. In presence of sulphur (from sulphide

of sodium or the decomposition of sulphates by the sulphur-bacteria

nearly always present in bates and soaks), the stains become bluish or

greenish black, and a black deposit is frequently produced on the sides

of the pit, in which the threads of sulphur-bacteria (_Thiothrix_) can

often be recognised by the microscope. As ferric salts not only combine

with the tannins, but are themselves tanning agents (see p. 198), they

are rapidly absorbed by leather, and iron is always present in leather

ash. (For detection and estimation see L.I.L.B., p. 218.)

_Alumina_, except as clay, is rarely present in waters, and probably

harmless in any water likely to be used in tanning.

_Soda_ is sometimes present in considerable amount, as sulphate,

chloride, or carbonate. The sulphate is probably inoperative. The

chloride, if present in material quantities, prevents plumping, and may

be the cause of thin and soft leather, and in large amounts will greatly

impede the proper exhaustion of many tanning materials. Sodium carbonate

is sometimes present in considerable quantities, as in some of the

waters of the Leeds district. It may coexist with temporary hardness,

and produces similar injurious effects. Waters in which it is present

cannot have any real permanent hardness. It may be neutralised by the

very cautious addition of an acid; or by admixture of a permanent-hard

water. It tends to increased plumping in the limes, but neutralises the

free acids of the tan-liquors which are necessary in sole-leather

tanning.

_Copper_, _lead_, and other metallic bases are not likely to be present

in any waters used for tanning in quantities sufficient to be injurious.

_Sulphuric acid_ rarely occurs free in water, and then only in such

traces as would be harmless for tanning, though possibly injurious to

steam boilers. As sulphates it is most common. Alkaline sulphates are

not known to have any deleterious action. The sulphates of lime and

magnesia are the principal cause of permanent hardness, q.v. Iron

sulphate is sometimes found in colliery waters.

_Nitrates and nitrites_ in water are usually the result of “previous”

sewage contamination, and are only important as an indication of the

possible presence of the putrefactive ferments, and are of little moment

in waters only used for manufacturing purposes, while they seem to be

even useful in promoting the “working” of bran drenches, by supplying

the nitrogen required by the ferment.

_Chlorine_ is seldom or never present in water in the free state, but

only in the form of chlorides, most frequently of sodium chloride

(common salt), the effect of which has been referred to above, and also

at p. 88. The action of other chlorides is probably similar as regards

the swelling of hide. _Magnesium chloride_ is very objectionable as a

constituent of boiler-waters, as it liberates hydrochloric acid at high

temperatures, and corrodes the plates at the surface of the water. This

injury can be prevented by addition of soda.

_Carbonic acid_ has been referred to under temporary hardness. Its

presence in the free state is a matter of some importance to the tanner

(see p. 99).

_Silicic acid_ in a soluble form is present in some waters in

considerable quantity. Such waters are said to harden leather, but of

this the writer has no personal experience.

Few accurate researches have been made on the effect of the impurities

of water on tanning,[65] and though, from what has already been said, it

will be seen that they are not without effect it is probable that in

many cases the water is blamed for troubles which are simply the result

of mismanagement, and credited with virtues which are really due to

careful and skilful manufacture.

[65] See Nihoul, ‘Influence de l’eau sur l’extraction des matières

tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901.

The hardness of water, and the dissolved carbonic acid which it

contains, are, together with its temperature, the principal factors

which determine whether a hide will plump or fall in it. Almost the only

accurate investigation of this point has been made by W. Eitner.[66] He

placed pieces of hide, unhaired by sweating, and quite flat and fallen,

in water for four days at a temperature of 46° F. (8° C.), with the

following results:--

1. In distilled water Scarcely at all

plumped.

2. „ water saturated with CO₂ Well plumped.

3. „ „ with lime bicarbonate, 20° German} Tolerably plump.

scale of hardness }

4. „ „ „ magnesia bicarbonate, 20° do. „ „

5. „ „ „ lime sulphate 20° do. Well plumped.

6. „ „ „ magnesia sulphate, 20° do Best plumped.

7. „ „ „ magnesium chloride, 20° do. Not at all plumped.

8. „ „ „ common salt, 20° do. „ „

(1 German degree of hardness corresponds to 1 of CaO in 100,000.)

[66] Gerber, iii. (1877) p. 183.

The peculiarities which were shown by the hide pieces on removal from

the water were maintained throughout the tanning, which was conducted in

imitation of the German method, the hide being swollen and coloured

through in weak birch-bark liquors, made with distilled water and

acidified in each case with equal quantities of lactic acid, and finally

laid away, till tanned, in a mixture of oak bark and valonia. No. 6,

from magnesium sulphate, was the best; then No. 2; No. 3 was less good,

but all the pieces from 1 to 6 were firm, close and of good substance

and texture, No. 1 having swelled well in the sour liquor. On the other

hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout,

and were looser, thinner and of finer fibre. From this experiment it is

clear that while sulphates and carbonates exert a favourable influence

on plumping, chlorides do the reverse, as they themselves not only do

not plump, but they place the hides in an unfavourable condition for the

plumping action of acids in the liquors. These experiments are quite

borne out by the writer’s experience in practice. The water at the

Lowlights Tannery, which in dry weather was mostly obtained from beds of

what was originally sea-sand, and which consequently contained a very

abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000),

required special and very careful management to make thick leather,

notwithstanding the fact that it contained a considerable quantity of

calcium and magnesium sulphates. These facts also indicate the

importance of the thorough removal of salt from hides intended for

sole-leather. Plumping is not a desirable thing in leather intended for

dressing purposes, and it is possible that the use of a small percentage

of salt in the liquors or wash waters might in some cases enable bating

to be dispensed with. Like a bate, salt would dissolve a small

proportion of hide substance (see p. 65). There is no practicable means

of removing chlorides from water, but Eitner suggests the addition of a

small quantity of sulphuric acid to water containing much temporary

hardness (bicarbonates), in order to convert it into permanent hardness

(sulphates), which, as stated above, plumps better. The amount required

may be calculated from an acidimetric determination of temporary

hardness (see L.I.L.B., p. 19). A simple but not very accurate guide, is

to add enough acid to purple, but not to redden litmus paper even after

moving the latter about in the water for some minutes. In practice the

acid must of course be very thoroughly mixed with the water by stirring

and plunging. It must be borne in mind that Eitner’s experiment was on

sweated hides, and that with limed hide, which is kept plump by the

dissolved lime retained in the hide, different results as regards

carbonic acid and bicarbonates would be obtained. Both these would

convert the lime in the hide into chalk, which is insoluble and inert,

and the hide would fall, at any rate when the lime was completely

carbonated, while hides would remain plumpest in waters most free from

substances capable of neutralising lime. From this we may conclude, what

may be _a priori_ expected, that the purer the water, the plumper limed

hides remain in it. In soft but peaty waters, hides fall rapidly, from

the neutralisation of the lime by the weak organic acids of the peat.

Such waters are dangerous for domestic use from their solvent action on

lead, but this danger can be entirely removed by storing the water in

limestone reservoirs, or allowing it to flow slowly through a limestone

culvert before use. In some towns in the north of England a small

quantity of lime is added so as to neutralise the water as it leaves the

reservoir and before it enters the mains.

Wherever the conditions of putrefaction or decaying organic matter are

present, as in a bate, hides fall rapidly, and in extreme cases even the

presence of the stronger acids will not maintain plumpness. Eitner

mentions the case of a stream at Vissoko in Bosnia, which was in special

repute among the tanners from its power of pulling down hides rapidly,

and which took its rise in a common on which the pigs of the town were

pastured. The causes of this action are no doubt due to the products of

putrefaction, but are somewhat obscure. Bacteria present in water are a

frequent source of injury in the soaks, and probably in other stages of

the tanning process.

Rain water and the water of streams in mountain districts of hard

igneous rock are generally nearly free from mineral constituents. This

is the case with the Glasgow water from Loch Katrine, and the Thirlmere

water which supplies Manchester. Such water, if cold enough, and free

from mud and organic impurity, is the best for almost every purpose in

the tannery. Most river-water contains material quantities of mineral

matter, though it is usually softer than that of springs or wells.

For further details as to the chemical examination of water, and the

methods of determining the amounts of its different constituents, see

L.I.L.B., pp. 18 _et seq._

CHAPTER XI.

_SOAKING AND SOFTENING OF HIDES AND SKINS._

As has been explained in the last chapter, hides and skins come into the

hands of the tanner either uncured (“green”), as they are taken off the

animal, preserved with salt or some other antiseptic, dried, or

“drysalted” in which both methods are combined. His object in each case

is to remove blood and dirt, and to restore the hide to its soft and

natural condition; but the treatment required varies much with the state

of the hides.

_Fresh hides_ merely require cleansing from blood and dirt. This is

necessary because the blood causes bad colour, and both blood, lymph and

adhering dung are sources of putrefaction, which ultimately attacks the

grain and fibrous structure of the hide. Hence washed hides keep better

than unwashed. Cold water is most desirable, as checking putrefaction.

If the water is much over 10° C., or if it is charged with organic

matter and ferment-germs; or if, as is too generally the case, the hides

are in a partially putrid state when received, the time of soaking must

be reduced as much as possible, and it may be necessary to sterilise the

water with carbolic acid or creolin (pp. 26, 28). In such cases the use

of a wash-wheel, or tumbler, is very desirable, rapidly cleansing the

hides and removing adhering dung, which interferes with the liming, and

is a serious cause of damaged grain. The American pattern of wash-wheel

shown in Fig. 21 is very suitable for the purpose. In no case is it

desirable to allow green hides to lie for more than a few hours in

water; and unwise treatment at this time is the cause of many troubles,

which are only detected at later stages, and which are very difficult to

trace to their source. “Weak grain,” in which the hyaline layer (p. 50)

is destroyed, and which tans a whitish colour; “pricking,” or

perforation of the grain with small pinholes, which may go on to

“pitting” with larger holes, and a general weakening of fibre, with

softening and needless loss of weight, are among these results. An

instructive instance may be quoted. A large tanner found that his

curried leather was affected with small spots and rings of darker

colour, which rendered it quite unfit for staining, and which reappeared

even when the leather was buffed. When finished as black grain, these

spots had a tendency to “spue,” or rise as little pimples of resinous

matter. Before the leather was stuffed no defect was noticeable to the

eye, but either then, or on stripping the grease by a solvent, they

could be seen under the microscope as lighter patches of open and porous

grain which absorbed more than their share of fat. During the tanning

process they could hardly be detected, but in the first colouring they

appeared for a few hours as blackish specks almost exactly like those

caused by particles of iron or iron-rust. By careful observation they

were traced back to the limes; specimens of the limed hide were

submitted to Director Eitner, who identified the defect as “_Stippen_,”

caused by a species of bacteria, which cannot subsist in limes, and

which therefore must have been in the soaks. These, which had been

somewhat neglected from pressure of work, were cleaned out and

sterilised with creolin solution, and the mischief ceased. It is worth

noting that the tanner dated the beginning of the trouble from the

soaking of some “Spanish” horse-hides, which may have introduced the

infection. Several very similar cases have come under the writer’s

notice.

It is not absolutely necessary to soak fresh hides or skins at all

before liming, and where the water is scarce or unfavourable, or the

skins tainted or “slipping” hair, it is best to pass straight into a

weak lime. In this case the limes must be worked in shifts (see p. 131)

and the whole of the oldest liquor run away and the hides rapidly

changed into a fresh lime, or the limes will become so charged with

organic matter and bacteria that the hides will cease to plump, and may

even putrefy.

_Salted hides and skins_ require more soaking and more thorough washing

than fresh ones, as it is not only necessary to remove the salt, but to

soften and plump the fibre which has been dehydrated and contracted by

salting. If goods with salt in them are taken into limes, they will not

plump properly,[67] and creases and wrinkles (drawn grain) are formed

which no after-treatment will remove. This is especially important in

sole leather. In deciding on a method, we must bear in mind that salt is

easily soluble, and diffuses rapidly into water or weaker solutions, and

that weak salt solutions tend to prevent the plumping of the fibre,

while those of about 10 per cent. have considerable power of dissolving

the cementing matter of the fibres (p. 65) and so lessening weight and

firmness. It may also be noted that though salt is not a true

disinfectant (p. 22), salted hides are much less prone to putrefaction

than fresh ones, and therefore a longer soaking may be safely given.

[67] Experiments mentioned on p. 89 throw some doubt on the power of

salt to prevent plumping in the limes, though the opinion in the text

is generally held by tanners.

These conditions point to the desirability of free exposure to water,

attained by suspending, handling frequently, or tumbling, and repeated

changes to remove the salt. The degree of removal of salt is easily

determined by the estimation of Cl in the last wash-water (L.I.L.B., p.

18). American tanners universally soak wet-salted hides three or four

days with as many changes of water, and frequently finish by a few

minutes in a wash-wheel. Any washing tumbler may be used; but the cheap

and simple construction of the American wash-wheel will be easily

understood from Fig. 21. The sides are open, so that hides can be put in

or removed between the spokes. The rim of the wheel is generally

perforated, for the escape of water which is supplied by a pipe passing

through the axis; and the wheel is often driven by a chain or rope round

its circumference. No severe mechanical treatment, such as “stocking,”

is necessary or desirable for green or salted hides.

_Dry and dry-salted hides_ require much longer soaking than wet-salted,

the amount naturally depending on the thickness of the hide and the

character of drying. Even thin skins when strongly dried require

considerable time to soften and swell the fibres, although they soon

become wet-through and flexible. Many different methods of soaking have

been employed. Sometimes hides are suspended in running water; sometimes

laid in soaks which may be either renewed, or allowed to putrefy;

sometimes in water to which salt, borax or carbolic acid has been

added, to prevent putrefaction; and more recently weak solutions of

caustic soda, sulphide of sodium or sulphurous acid have been used with

much success.

[Illustration: FIG. 21.--American Wash-wheel.]

The first of these methods, were it desirable, is rarely possible in

these days of River Pollution Acts; of the others, it is difficult to

say which is better, since the treatment desirable varies with the

hardness of the hide and the temperature at which it has been dried. The

great object is to thoroughly soften the hide without allowing

putrefaction to injure it. As dried hides are often damaged already from

this cause, either before drying, or from becoming moist and heated on

shipboard, it is frequently no easy matter to accomplish this. The fresh

hide, as has been seen, contains considerable portions of albumin, and

if the hide is dried at a high temperature, this may become wholly or

partially coagulated and insoluble. The gelatinous fibre and the coriin

(if indeed the latter exists ready formed in the fresh hide) do not

coagulate by heat, but also become less readily soluble. Gelatin dried

at 130° C. can only be redissolved by acids, or water at 120° C.

Eitner[68] experimented with pieces of green calf-skin of equal

thickness, which were dried at different temperatures, with results

given in the following table:--

-------+-----------+---------+---------+-------------+--------------

Sample.|Temperature|Remarks. |Time of | Remarks. | Coriin

|of Drying. | |Softening| |Dissolved by

| | |in Water.| |Salt Solution.

-------+-----------+---------+---------+-------------+--------------

I. | 15° C. |In vacuo |24 hours |{Without } |1·68 per cent.

| | |{mechanical} |

II. | 22° C. |In sun |2 days |{ work } |1·62 „

| | | | |

III. | 35° C. |In drying|5 „ |Twice worked |0·15 „

| | closet | | |

| | | | |

| | | {Refused to soften} |

IV. | 60° C. | „ | {sufficiently for } |traces

| | | {tanning } |

-------+-----------+---------+---------+-------------+--------------

[68] Gerber, 1880, p. 112.

Hence it is evident that, for hides dried at low temperatures, short

soaking in fresh and cold water is sufficient, and, except in warm

weather, there would be little danger of putrefaction. With harder

drying, longer time is required, and more vigorous measures may be

necessary. A well-known tanner recommended a brine of 30°-35° barkometer

(sp. gr. 1·035, or about 5 per cent. of NaCl). This has a double action,

not only preserving from putrefaction, but dissolving a portion of the

hide-substance in the form of coriin, which is undoubtedly a loss to the

tanner, though it is questionable if there is any process which will

soften overdried hides without loss of weight; since even prolonged

soaking in cold water at a temperature which is too low to allow of

putrefaction taking place will dissolve a serious amount of

hide-substance. Chlorides, however, do not seem well adapted for the

purpose in view, from their weak antiseptic power and tendency to

prevent swelling. To prevent this Jackson Schulz advised the use of

water at 80° F. for soaking during the winter months. Water containing a

small quantity (0·1 per cent.) of carbolic acid has been recommended for

the purpose, and will prevent putrefaction, while it has no solvent

power on the hide, but, on the contrary, tends to coagulate and render

insoluble albuminous matters. Borax has been proposed for the same

purpose, and, in 1 per cent. solution, certainly prevents putrefaction,

and has considerable softening power, but is far too costly. Other

methods of chemical softening are described on p. 115.

For some descriptions of hides, and notably for India kips, putrid soaks

were formerly much employed, the putrefactive action softening and

rendering soluble the hardened tissue. In India the native tanners

soften their hides in very few hours by plunging them in putrid pools,

into which every description of tannery refuse is allowed to run.

Putrefactive processes, however, are always dangerous, as the action,

through changes of temperature, or variation in the previous state of

the liquor, is apt to be irregular, and either to attack one portion of

the hide before another, or to proceed faster than was expected. Hides

are also frequently more or less damaged by putrefaction and heating

during the process of cure, and these damages are accentuated in a

putrid soak. Hence hides in the soaks require constant and careful

watching, and the goods must be withdrawn as soon as they are thoroughly

softened, for the putrefaction is constantly destroying as well as

softening the hides. It is possible that putrefactive softening is less

injurious to kips, and such goods as are intended for upper-leather,

than to those for sole purposes, as it is generally considered necessary

in the former case that a good deal of the albumen and interfibrillary

matter be removed, and that the fibre be well divided into its

constituent fibrils for the sake of softness and pliability; and thus

the putrid soak, if acting rightly, accomplishes part of the work which

would afterwards have to be done by the lime and the bate, as the actual

fibre of the hide seems less readily putrescible than the softer

cementing substance.

Putrefaction is caused, as we have seen, by a great variety of living

organisms, each of which has its own special products and modes of

action. It is quite possible that, if we knew what precise form of

putrefaction was most advantageous, we might by appropriate conditions

be able to encourage it, to the exclusion of others, and obtain better

results than at present. Putrid soaks (in the old sense) are, however,

disused in the present day by all enlightened tanners, as it is

recognised that the risks outbalance the advantages, and when drysalted

hides are worked, the soluble salts of the cure accumulate to an

injurious extent. The modern method, where no chemicals are used, is to

give one fresh water at least to each pack of hides or skins. Even in

this case considerable putrefaction takes place where the soaking

occupies 7 to 14 days, as is the case with kips and hides, and it is

probable that the use of chemical and antiseptic methods of soaking will

ultimately be generally adopted, both on technical and sanitary grounds.

The use of dilute acids for softening has much to recommend it, their

power of causing the fibre to swell and absorb water being quite equal

to that of the alkalies, while few, if any, putrefactive bacteria can

thrive in an acid liquid. Very dilute sulphuric acid has been used with

success to dissolve the alkaline “plaster” of East India kips (p. 39).

It has considerable disinfectant power (p. 23), but its action on the

hide-fibre is undesirably strong.

Sulphurous acid is much more suitable. Its use for this purpose was

patented by Maynard, along with a number of other possible uses, but the

patent has now lapsed, and he does not seem to have succeeded in

introducing it into practice. Experiments at the Yorkshire College, and

also at a tannery on a manufacturing scale, have shown that the method

is capable of excellent results. The hides are soaked for 24-48 hours in

a solution of sulphurous acid containing about 2 per cent. of SO₂ (for

manufacture, compare p. 24; for testing, L.I.L.B., pp. 16, 37), and are

then transferred to water, where they swell freely to their full

thickness. They may be either limed at once, or first neutralised with

dilute caustic soda, ammonia, or sulphide of sodium, which, for dressing

leather, is perhaps desirable. No putrefaction takes place, even if they

are retained for a considerable time in water, and the acid has little

or no solvent effect on the hide-fibre, the strength of which is well

preserved. The liming, however, must either be conducted with the aid of

sodium sulphide or in old limes, since the sterile condition of the

hides renders liming in fresh lime very slow (cp. p. 137). For

experimental purposes a ¹⁄₂ per cent. solution of Boakes’

“metabisulphite of soda” may be used, to which ¹⁄₄ per cent. of

concentrated sulphuric acid previously diluted with water is gradually

added during the soaking, the hides being first withdrawn. For permanent

work it will be found much cheaper to manufacture the acid on the spot

by burning sulphur.

The use of solutions of caustic soda (1 part per 1000), or of sodium

sulphide (1¹⁄₂-3 parts per 1000) as suggested by Eitner, seems at

present likely to supersede all other methods of softening from their

simplicity and safety. Twenty-four to forty-eight hours in either of

these solutions, which may if necessary be followed by a short soak in

plain water, seem sufficient to soften either kips or hides. Experiments

at the Yorkshire College have shown that solutions of this strength have

little or no solvent action on the hide-fibre, but promote its swelling

in water so effectively that no mechanical softening is needed (though a

slight drumming is advantageous), while putrefaction is almost entirely

prevented, so that the solution may be repeatedly used if kept up to its

original strength, which is easily determined with standard acid and

phenolphthalein (see L.I.L.B., p. 17). Neither caustic soda nor sodium

sulphide have any injurious effect on liming, though it may prove

somewhat slower than with the older methods, where the epidermis was

partially destroyed by the action of putrid ferments. The dilute

solutions used are not only less injurious to the hide than those of

greater strength, but they are also more effective in softening. Eitner

(Gerber, 1899, p. 584) states that when using a solution of caustic soda

of 1 part in 1000 strength, the time required to soften some hides was

only two days, as against three days for a sodium sulphide liquor, and

four days for pure water, and that with the soda solution only about 0·6

per cent of the hide-substance of the skin was dissolved out, whilst

when sodium sulphide was used it was 0·7 per cent., and with pure water

alone no less than 1·9 per cent. was lost by solution.

The use of moderately warm water (40° C.) in a drum is quite successful

in rapidly softening sound hides after they have previously been soaked

for some days in cold water; but if they are tainted in the cure, it is

very apt to intensify the mischief. Hides which have partially putrefied

internally, or which have been exposed to a hot sun while the interior

is still moist, are very apt to appear sound while dry, but to blister

or go to pieces from the destruction of the fibres as soon as they are

limed, and this in spite of even the most careful treatment. For tainted

hides, caustic soda is probably preferable to sodium sulphide.

Many chemicals have been patented for softening hides. Sulphide of

arsenic is said to be in use, and if dissolved in caustic soda solution

would differ little in its effect from ordinary sulphide of sodium.

Saltpetre has also been employed, but its effect, if any, was probably

merely antiseptic. Ordinary sodium carbonate has been used, but is less

effective than caustic soda. Gas liquor and mixtures of this with tar

and water were patented by Barron, and probably the first would soften

by virtue of its ammonia and sulphides, while tar contains carbolic

acid. Probably the most absurd mixture of all was patented by Berry,

which consisted of ¹⁄₂ bucket of slaked lime, ¹⁄₂ bucket of wood-ashes,

12 lbs. of potash, 5 lbs. of oil of vitriol, and 4 lbs. of spirit of

salt!

[Illustration: FIG. 22.--Faller Stocks.]

Beside merely soaking the hides, it is sometimes necessary to work them

mechanically, to promote their softening; this was formerly accomplished

by “breaking over” the hides on the beam with a blunt knife. This

process is still in use for skins of many sorts, but for the heavier

classes of leather is now usually superseded or supplemented by the use

of “stocks,” or drums. The former consist of a wooden or metallic box,

of peculiar shape, wherein work two very heavy hammers, raised

alternately by pins or cams on a wheel, and let fall upon the hides,

which they force up against the curved end of the box with a sort of

kneading action. The ordinary form of this machine is shown in Fig. 22.

A more modern form, which seems to possess some advantages, is the

American “double-shover,” or “hide-mill,” seen in Fig. 23. “Crank

stocks,” similar in form to the faller stocks, but driven by cranks,

are sometimes used for softening, but are better adapted to lighter

uses.

[Illustration: FIG. 23.--American Hide-Mill.]

The number of hides which can be stocked at once naturally varies with

the size of both hides and stocks, but should be such that the hides

work regularly and steadily over and over. The whole number should not

be put in at once, but should be added one after another, as they get

into regular work. The duration of stocking is 10-30 min., according to

the condition and character of the hides. Hides should not be stocked

until they are so far softened that they can be doubled sharply, without

breaking or straining the fibre. After stocking, they must be soaked

again for a short time, and then be brought into an old lime. A small

quantity of sodium sulphide added to the soaks or in the stocks has been

recommended as of great value in softening obstinate hides, and probably

with justice, from its well-known softening action upon cellular and

horny tissues.

Tumbler drums of various forms may also be used with good effect for

softening purposes, especially for skins, and are much less detrimental

than stocking, both as regards the weight and quality of the goods.

For sole leather, and even for kips, the use of stocks has in recent

years been entirely discarded by many of the more advanced tanners. If

mechanical work is required at all, the drum is preferred, and is

sometimes employed after a few days’ liming, the goods being first

merely softened in fresh water. The use of caustic soda, sodium

sulphide, or sulphurous acid renders mechanical softening almost

unnecessary.

[Illustration: FIG. 24.--Drum for Washing or Tanning.]

The drums employed are in principle like a barrel-churn, and are large

cylindrical wooden chambers 6 to 12 feet in diameter, and fitted inside

either with shelves like the floats of a water-wheel, or with rounded

pegs on which the hides fall. The American wash-wheel figured on p. 111

is a machine of this kind, and one of a more elaborate description is

shown in Fig. 24. Drums are not only used for softening, but for

tanning, dyeing, and many other purposes in leather manufacture. It is

advantageous to be able to reverse the direction of their rotation to

prevent the rolling up of the hides.

CHAPTER XII.

_DEPILATION._

After the softening and cleansing of the hide or skin is completed, and

before proceeding to tan it, it is usually necessary to remove the hair

or wool. The earliest method of accomplishing this was by means of

incipient putrefaction, which attacks in the first instance the soft

mucous matter of the epidermis, and thus loosens the hair without

materially injuring the true skin. This loosening of the hair often

takes place accidentally in hides which have been kept too long without

salting, and is known as “slipping,” and is apt to be accompanied by

some degree of injury to the grain. The old method of loosening the hair

by putrefaction, or, as it is generally called, “sweating,” was to lay

the hides in piles, usually in some warm and damp place. Occasionally a

slight preliminary salting was given to prevent too much putrefaction of

the hide. The action in this case, however, was very irregular, and it

has been quite abandoned in all civilised countries.

[Illustration: FIG. 25.--Sweat-Pit.]

The method which is now used is to hang the hides in a closed chamber,

generally called a “sweat-pit,” Fig. 25, but usually constructed above

the ground-level and protected from sudden changes of temperature by

double walls, or by mounds of earth. The hides are hung in the

sweat-pit, in small chambers each capable of holding 50 or 100 hides.

The temperature is kept at about 15° to 20° C., the air being warmed, if

necessary, by the admission of steam below a perforated floor, or cooled

by a shower of water from sprinklers, so arranged as not to play

directly on the skins, and is thus always kept saturated with moisture.

Little if any ventilation is allowed, and a large quantity of ammonia is

given off from the decomposition of the organic matter, and no doubt

contributes to the solution of the epidermis and the loosening of the

hair, as the writer has found that ammoniacal vapours alone very

speedily produce this effect.

After 4-6 days of this treatment, the hair is sufficiently loosened to

be removed by working the skin over the beam with a blunt knife, or by

means of the stocks or hide-mill (see p. 116). Great care and

watchfulness are required to avoid injury to the grain by putrefaction.

The hide is in a slimy and completely flaccid and “fallen” condition,

and some trouble is occasioned by the hair being worked into the flesh

by the hide-mill, to obviate which, a slight liming is frequently given

after the sweating. Hides which have been unhaired in this way require

to be swollen by acid in the liquors in order to produce a satisfactory

sole-leather, as the sweating process does not swell or split up the

fibres.

In some European tanneries a similar process, but at a higher

temperature, is employed, and it is also largely used for sheep-skins

under the name of “staling,” but in this case is sometimes conducted in

a very rude and primitive manner, and frequently with the result of

considerable injury to the pelt.

The great objection to the sweating process, however carefully

conducted, is the liability of putrefaction to attack the skin itself,

causing “weak grain.” Its most advantageous use is for sole leather, as,

although the solution of the hide-substance may not be very much less

than in the case of liming, the dissolved matter remains in the hide

instead of being washed out, and being fixed by the tannin, contributes

to the solidity of the leather.

In England, lime is the agent almost universally employed for unhairing,

though every tanner admits its deficiencies and disadvantages. It is

hard, however, to recommend a substitute which is free from the same or

greater evils, and lime has one or two valuable qualities which will

make it very difficult to supersede. One of these is that, though it

inevitably causes loss of substance and weight, it is also impossible,

with any reasonable care, totally to destroy a pack of hides by its use;

which is by no means the case with some of its rivals. Another advantage

is that, owing to the very limited solubility of lime in water, it is of

comparatively small consequence whether much or little is used; and even

if the hides are left in a few days longer than necessary, the mischief,

though certain, is only to be detected by careful and accurate

observation. With all other methods, exact time and quantity are of

primary importance, and it is not easy to get ordinary workmen to pay

the necessary attention to such details. Again, the qualities of lime,

its virtues and failings, have been matter of experience for hundreds of

years, and so far as such experience can teach, we know exactly how to

deal with it. A new method, on the other hand, brings new and

unlooked-for difficulties, and often requires changes in other parts of

the process, as well as in the mere unhairing, to make it successful. As

our knowledge of the chemical and physical changes involved becomes

greater, we may look to overcoming these obstacles more readily.

The universal source of lime is chalk or limestone, which consists of

calcium carbonate, and from which the carbon dioxide is driven off by

burning in a kiln. Many limestones, however, are far from being pure

calcium carbonate, but contain large proportions of magnesia, iron and

alumina, the latter perhaps originally deposited in the form of clay

with the sediment from which the stone was formed. Such clay limestones

when burnt yield natural cements, like oolite and other “hydraulic”

limes, which are capable of setting even under water. The presence of

magnesia and clay is injurious not only by diminishing the amount of

lime present, but by making the lime much more difficult to slake; and

iron oxide, though quite insoluble, may become mechanically fixed in the

grain of the hide, and may be the cause of subsequent stains. The

burning of lime in the kiln is probably not quite so simple an operation

as the equations of the text-books would suggest. By mere heating, the

carbonate can, it is true, be decomposed, but to do this completely a

good white heat is required, which is rarely attained in practical

burning, and it is probable that at least a part of the carbon dioxide

present is reduced to carbon monoxide by the combustible fuel-gases, and

so separated from the lime, for which it has no affinity. Carbon

monoxide is the cause of the intensely poisonous character of limekiln

gases, the pure dioxide being irrespirable, but not strictly poisonous.