In England, leaches are usually sunk in the ground, and are frequently made of brick and cement, or of large Yorkshire flagstones. Such leaches are somewhat costly but very durable. Square wooden pits, puddled outside with clay, are also used, and last well with cold, or even warm liquors, but will not stand direct steaming, the wood gradually bending, and allowing the clay to leak into the liquor, causing black stains. The large round vats of thick pine, and often holding 10 or 12 tons, which are generally used in the United States, stand boiling much better, and are frequently supported above a tramway or conveyor, into which the spent bark can be discharged through a manhole in the bottom. If this method is adopted, it must be remembered that bark, and indeed most other tanning materials, will not run through a hole like corn, but must be cast into it, so that unless the vat is of great depth, it is simpler and almost as easy to cast over the top. If the manhole is used, a central hole must be made in the false bottom, and this must be surmounted by a copper pipe made in sections of two or three feet, and reaching to the top of the leach. When the pit is to be emptied, the top length is removed, and the tan shovelled down the hole until the second length is reached, and the process repeated. The central pipe serves also for the circulation of the liquor when the pits are boiled, and may be used as the ascending pipe for circulating on the press-leach system.

The question of the influence of temperature on extraction is discussed on p. 344, but except where a pale colour is all important, it is generally profitable to use a moderate degree of heat in extraction. In the opinion of the writer (which is supported by a vast amount of careful experiment) only the nearly exhausted leaches should be heated, not merely to avoid discoloration, but to extract the maximum amount of tannin. In American tanneries the boiling is frequently done by copper coils fixed below the false bottoms of the vats, but such coils are very costly, and, where weak liquors only are to be heated, seem to present no advantage over a well-arranged system of heating by direct steam in which care is taken that dry steam only is used, and that all water condensed in steam pipes, and usually containing iron, is removed by effective steam-traps. If steam is blown into cold liquor through an open pipe, a very disagreeable rattling and vibration is produced, which is not only annoying, but is very injurious to the leaches. This evil may be avoided by the use of “silent boiling jets” on the principle of the steam-jet water-raiser; and, following a suggestion of the writer, these jets may be used at the same time to circulate the water through the tanning material of the nearly exhausted vat, and so wash out the last traces of tan. The simplest way to accomplish this is to lower the boiling jet, directed upwards, and connected with a movable steam-pipe, into the eye of the leach (which is preferably central) so that the heated water flows over its top, and percolates downwards through the material to be washed. Two forms of these boiling and mixing jets made by Messrs. Körting are shown in Figs. 80 and 81.

Batteries of closed copper extractors, worked on the press system, and similar to those used in extracting sugar from beetroot, have frequently been advocated, but are very costly, and have no other advantage over open vats than that the liquor can be forced through the series by pressure, instead of circulating by gravity. No advantage is gained by boiling under pressure, since even boiling in open vats has been shown to destroy tannin, darken the colour of the liquor, and increase the amount of insolubles, and higher temperatures are still more injurious.

Heating the weakest leach in the press-leach system promotes the even circulation of the liquor, since the warm weak liquor is much lighter than the colder and stronger liquors in the forward leaches, and so floats on the top, and presses the stronger liquor uniformly downwards. It also has the advantage that the liquors are cooled before they are strong enough for the yard, while in tanneries where all the leaches are heated, expensive tubular coolers are often employed. As the liquor cools, much of the colouring matters and reds dissolved in the hot liquor separate, and are filtered out by the tanning material, so that much brighter and lighter coloured liquors are obtained.

Sprinkler-Leaches, Fig. 82, were formerly used in many tanneries and extract factories, especially in the United States. They were introduced by Allen and Warren, and yield a liquor which is at first very strong, but which becomes very rapidly weaker as the running is continued. These leaches are similar in principle to the mashing-tub and sparger of the brewer, but the process is not well adapted for tanners’ use, as the material is left too much exposed to the air, which is apt to cause oxidation and loss of tannin. It is also extremely difficult to completely exhaust the material without using an impracticably large volume of water. Sprinkler-leaches are arranged so as to spray the liquor, or water, on to the top of the solid material which is to be extracted at such a rate that it flows out just as rapidly as it flows into the vat. Some idea of the great amount of oxidation and consequent loss of tannin which takes place in this form of extractor may be obtained when it is remembered that this same method is now used for the destruction of sewage matter by spraying it on to beds of coke so that it may be mixed with as much air as possible before it is attacked by the bacteria of the coke-beds (see p. 473), and also to oxidise weak alcohol to acetic acid in the “quick vinegar process.”

So far as extraction is concerned, there is no difference in principle between the methods adopted by the tanner and the extract manufacturer, though the latter usually works on a larger scale, and not unfrequently, in order to increase his output, or the gravity of his extract, employs a higher temperature. This is probably justified by practical considerations in the manufacture of extracts from very low-grade materials, such as oakwood, which only contains 2 to 3 per cent. of tanning matter, or even of chestnut wood which is somewhat stronger, but it is one of the causes why decoloration of the battery liquor is generally necessary.

Dried blood is chiefly used as the decolorising agent, but a paste of blood-albumen has been recently placed on the market, which is said to be free from several of the disadvantages attending the use of the crude material.

The liquor to be decolorised is run into a mixing vat fitted with a steam coil capable of raising the temperature of the liquid to at least 80° C., and usually provided with a simple rotary stirring gear. The liquor, as run into the mixing vat, must not have a temperature of more than 48° C. (118° F.) nor a strength of more than about 20° Bkr. (sp. gr. 1·020).

The blood or albumen dissolved in a little water, is added to the contents of the vat, which are then well mixed, and the temperature is raised to 70° C. when the albumen coagulates and carries down much of the colouring matter. The solution is run into another tank where the precipitate is allowed to settle, and the clear liquor is then drawn off for the evaporation. The muddy portion, about 8 inches in depth, is pumped through filter-presses (which can be cheaply constructed of wood), the clear liquors going to the evaporators and the press-cakes being dried for manure.

In addition to blood-albumen, several other substances, such as lead acetate (sugar of lead), salts of alumina, casein and other albuminous matters have been employed in the decoloration of extracts, but they are by no means so efficient as albumen.

Decolorising always causes a loss of tanning matter, some of this being carried down with the precipitated colouring matter; and is for this reason to be dispensed with whenever its use is not really necessary. It may often be avoided by careful extraction at moderate temperatures, and this is especially to be aimed at in the case of strong tanning materials, which easily yield battery liquors of much greater strength than 20° Bkr., and which thus, if they can be sent direct to the evaporator, save cost in evaporation, which is often an important consideration.

Another method which is frequently used to brighten the colour of extracts, is treatment with sulphurous acid. Dilute sulphurous acid solution may be used for extraction, but a more common method is to pass sulphur dioxide gas into the liquor before concentration. Sulphurous acid acts partly as a weak acid, in decomposing compounds of the tannins and colouring matters with bases, such as lime, iron, copper, but more actively by reducing oxygen compounds and preventing oxidation. Bleaching in this way does not actually destroy or remove the colouring matters, which are apt to reappear on exposure to the air, either in the liquor, or perhaps more often in the leather tanned with it, so that the gain is frequently more apparent than real. If present in any considerable quantities, sulphurous acid may also cause inconvenience by its swelling action on the pelt, but is mostly expelled in concentration.

Another process should perhaps also be mentioned here, though not strictly a means of bleaching. Several tanning materials, and notably quebracho and hemlock, contain large quantities of “difficultly soluble tannins,” which render the liquors made from their extracts turbid on cooling. These tannins form soluble compounds with alkalis and with alkaline sulphites, in the latter case probably setting free the sulphurous acid and combining with the base. This has been taken advantage of in a recent patent[158] in which quebracho and other extracts are rendered soluble by heating in closed vessels with bisulphites, sulphites, sulphides, or even caustic alkalis; and many “soluble quebracho extracts” made on this principle are now on the market. In this case, even where bisulphites are used, the greater part of the sulphurous acid, after serving its purpose in preventing oxidation, escapes in course of manufacture, and the extracts remain neutral or alkaline. There is no reason that such extracts should not prove serviceable in tanning, but it has recently been shown by Paessler that the alkaline tannin is not absorbed by neutral hide-powder, and it therefore may lead, not only to discrepancies in analysis, but in case of drum-tannage, where no acid is naturally present, to failure to utilise the whole of the tannin, though, when added to ordinary liquors, the acids contained in the latter will set free the tannins.

The use of ferrocyanides has been suggested as a means of precipitating iron and copper present in extracts, and it may also be pointed out, that with many red-coloured tanning materials, such as hemlock and quebracho, the addition of small quantities of alum to the tanning liquor effects considerable improvement in colour, not only by precipitating a part of the difficultly soluble “reds,” but by developing the yellow colour of certain colouring matters (quercetin, myricetin, etc.) which may be present. Such an addition does no harm in the case of soft leathers, but would probably be injurious in a sole-leather tannage.

The liquors, whether direct from the leaches or from the decolorising vats, must next be concentrated by evaporation (Chap. XXVI.), to sirupy consistency for liquid extracts, or until they will become nearly solid on cooling, if a solid extract is required. As has already been stated, the action of heat tends to cause a loss of tannin and a darkening of colour by decomposition and the formation of insoluble reds. To reduce this loss to a minimum, the weak liquors are evaporated with as little access of air and at as low a temperature as possible, and these conditions are best obtained by the use of steam-heated vacuum pans.

For concentration to gravities not exceeding 1·200, the Yaryan apparatus made by Mirrlees, Watson and Yaryan, of Glasgow, is that most employed. The general arrangement of a “triple effect” machine of this make is shown in Fig. 83, and the internal construction in Fig. 84. Each body consists of a strong casing into which steam is admitted, and which is traversed by copper tubes which terminate in a separating chamber at the further end, which is maintained at a low pressure by an air-pump. The liquid to be evaporated is admitted into the tubes, and is immediately converted into spray by the steam generated from it, and swept forward into the separating chamber, from which it is withdrawn by a pump. The steam before going to the air-pump (or, in the case of “multiple effects,” to the next body), is passed through a “catch-all,” to separate any spray still retained in the steam. Thus the liquor to be evaporated will pass through the entire apparatus in four or five minutes, and may be concentrated from a gravity of 1·02 or 1·03 to that of 1·20 without ever having been heated above 70° C. (160° F.). Unless fuel is very cheap, which is often the case where the spent tanning material can be used to raise steam, it is advisable to use a double or triple effect, in which the steam from the evaporation of the weakest liquor in the first body is used to heat the second, which is maintained at a lower vacuum, and so on. In this way the steam is made to do nearly double or triple duty. As the steam from the extract-liquors contains acids which corrode iron, it is necessary to have the casing as well as the tubes made of copper in all bodies in which it is employed. Iron must, in fact, be carefully avoided in every part of apparatus which comes in contact with extract-liquor or its vapour. Besides the Yaryan, there are several other evaporators in which the spray principle is more or less completely employed. The simplest of these consists in substituting for the heating coil of an ordinary vacuum-pan a copper steam-box traversed by vertical tubes open at both top and bottom. This is immersed in the liquid to be evaporated, which enters at the bottom of the tubes and is sprayed out at the top. Paul Neubäcker, of Danzig, constructs a pan on this principle with a very ingenious arrangement for the destruction of foam, which seems worth attention.

It is unfortunately impossible to carry the evaporation of extracts much further than sp. g. 1·2 with spray apparatus, as thicker liquors are apt to clog the tubes, which are then difficult to clean, so that even liquid extracts are usually finished in vacuum-pans of the ordinary type, which may also be arranged in multiple effect.

In the case of a solid extract, the evaporation must be carried on until it is as thick as can be run from the apparatus. To do this satisfactorily, stirrers must be provided to keep the extract in motion so long as it is in the pan. The thick, hot, liquid extract is then run into boxes lined with paper, or other suitable material, where it is allowed to cool and to solidify.

The pan for the final evaporation of solid extracts should be planned so as to allow of easy cleaning and ready access to its interior, so that if accidentally the evaporation is carried so far that the liquid will not run out, the clearing of the pan may be a comparatively easy matter. It is also important that the extract-exit should be of large size. Probably a broad and somewhat shallow pan, heated merely by a steam jacket, and fitted with rotating stirrers, is the most suitable.

The Use of Extracts in the Tannery.—One of the great attractions of extracts is that they save the trouble and cost of leaching, and as the extract manufacturer makes this his specialty, he can often extract more tanning matter from a material than the tanner who has no means of concentrating his weak liquors. The extract manufacturer also can employ methods of decoloration which would be impracticable to the tanner, and so enable the latter to obtain better colour than if he employed the raw material. By the use of extracts a tanner can strengthen weak liquors without trouble, and with definite quantities of materials; and by using extracts for this purpose the tanner is enabled to use up the weaker liquors of his leaches and so employ more water and obtain better extraction of his solid materials than if he used them alone. In the case of very weak materials like oakwood, the difficulties of making liquors of sufficient strength for tanning without evaporation are so great as to render such materials useless to the tanner for his own extraction, and their carriage even for short distances may amount to more than their total value. Even with much richer materials, extraction effects a saving if the carriage is a long one, as it rarely pays to import any material containing less than about 25 per cent. of tanning matter. Even when the strength of the natural material is considerable, as in the case of quebracho, extraction may be profitable if from its hardness, or other reasons, the material is difficult for the tanner to handle. For long voyages, and especially from the tropics, solid extracts are more suitable than liquid, as the expense of casks is saved, and the danger of fermentation is lessened. As it is impossible for the tanner to judge by appearance or consistency of the strength or value of extracts, they should always be bought and sold on the analysis of the particular shipment or parcel by a competent chemist. For directions for sampling see pp. 301, 475.

Extracts simply require to be dissolved in a suitable quantity of water or weak liquor at an appropriate temperature, to obtain a liquor of any required strength. Some extracts are completely soluble in cold water or liquor, but most dissolve better by the aid of heat. 40°-60° C. (100°-140° F.) is generally sufficient, and probably no advantage can arise from temperatures over 80° (180° F.). Boiling should be avoided, as it facilitates the formation of insoluble “reds” with consequent loss of tanning matter and darkening of colour. The extract should be run into the vat in a thin stream, and continuously plunged up; where large quantities of extract are to be dissolved, a mechanical agitator is advantageous. A “silent boiling jet” (p. 335) may be used, fitted into a small casing immersed in the liquor and open at both ends, and the extract run into the current it produces.

Whether in the manufacture of extracts, or for direct use in the tannery, the temperature at which tanning materials are extracted is of prime importance. It is a common mistake to assume that the largest amount of tannin is extracted by boiling. Mr. A. N. Palmer has pointed out that this is by no means the case, but that each material has an optimum temperature of extraction, at which more tannin is extracted than at any other; and the question has been carefully investigated by J. G. Parker and the author,[159] with results which are given in the following tables. For many purposes the colouring matter which accompanies the tannin is a serious disadvantage, and it is usually most extracted at the higher temperatures; and on this account it is necessary for the tanner who will work his leaches economically to ascertain at what temperature he can extract the largest amount of tannin combined with no more colouring matter than he can permit to enter his leather. Most materials are satisfactorily extracted at 50°-60° C., but as a general rule it is best to begin cold or nearly so, and only raise the temperature as the extraction proceeds. The tables show the percentages of tanning matter, and the amount of colour (as measured by Lovibond’s tintometer), obtained by extracting materials in a Procter’s extractor (p. 306 and L.I.L.B., p. 102) so long as any colour or tannin could be obtained.

Belgian Oak Bark.

Myrobalans.

Smyrna Valonea.

Greek Valonea.

Natal Mimosa.

Sumach.

Quebracho Wood.

Mangrove Bark (Ceriops).

Canaigre Root (three years old).

Effect of Different Temperatures.

Cube Gambier.

Effect of Different Temperatures.

Block Gambier.

Effect of Different Temperatures.

CHAPTER XXIII.
FATS, SOAPS, OILS AND WAXES.

Fats and oils constitute a large class of substances, of animal or vegetable origin, which may be solid, pasty or more or less viscous liquids, but which in the latter case are commonly known as “fixed” or fatty oils, to distinguish them from the volatile, or essential oils, which may be distilled without decomposition, and which are the source of most of the odours of plants, and of quite different chemical constitution. The term “oil” is also applied to various products of mineral origin, and especially to those derived from petroleum, on account of their similarity in appearance and physical properties to the fixed oils, though, chemically, they form a very distinct class. The waxes are another group somewhat closely allied to the fats; and there are certain fixed oils, such as sperm oil, which though very similar in appearance and properties to the fatty oils, are chemically members of the group of waxes.

As it is obvious that there is no chemical distinction between the fats and fatty oils, except that of melting-point, it will be convenient to treat them together; especially as what is a solid fat in one climate may be an oil in another. Palm and cocoa-nut oils are cases in point, as the first is buttery, and the second a hard fat in this country, though they are both liquid in tropical climates.

For more detailed information on the chemistry of fats and oils, the reader must be referred to the ‘Leather Industries Laboratory Book,’ sect. xviii., or to the larger manuals devoted specially to the subject by Lewkowitsch, Jean, and others, or the very excellent section on oils in Allen’s ‘Commercial Organic Analysis,’ vol. ii.; but a few general facts must be recapitulated.

The true fats contain carbon, hydrogen and oxygen, but no nitrogen. They are all compounds of glycerin with organic acids which are generally termed “fatty acids,” and which resemble in many of their characteristics the fats themselves. Glycerin is a very weak base, of the nature of an alcohol, and consequently, when a fat is heated with a solution of one of the caustic alkalis, the fatty acid combines with the latter, and the glycerin is set free. The salts thus formed are denominated “soaps.” The reaction with stearin (glycerin stearate), the principal constituent of hard animal fats, is shown in the following equation.

If a soap is treated with an acid stronger than its own, the latter is set free, while the new acid combines with the base. The following equation, for instance, shows the action of hydrochloric acid on the stearic soap.

If any soap be dissolved in hot water, and sufficient hydrochloric or sulphuric acid added to render the solution acid, the latter will turn first milky, and (if it be kept warm) the fatty acid will finally rise in an oily layer to the surface, which in many cases will harden, as it cools, to a solid mass. The amount of fatty acid in a soap may be roughly determined by weighing 25 grm., dissolving in 50 c.c. of boiling water, and adding excess of acid, and allowing the reaction to take place in a graduated cylinder, or a flask with a graduated neck, in a vessel of boiling water. When the fatty acid has risen to the top, its volume may be noted, and each c.c. may be roughly reckoned as 0·9 grm. (For more detailed methods cp. L.I.L.B., Sect. XVII.).

Soaps are insoluble in strong caustic alkaline solutions, and therefore saponification (as the decomposition of fats by alkalis is called), does not readily take place in them, and for this reason the soap-boiler generally dilutes his caustic soda solutions to a strength not exceeding 18° Tw. (sp. gr. 1·090) in gravity, and separates the soap at the end of the operation, by the addition of brine, in which it is insoluble. An easier method, and one which is often useful for the preparation of small quantities of special soaps for fat liquors and the like, is as follows.[160] 10 lb. of a good caustic soda, free from common salt, is dissolved in 4 gallons of water, and 75 lb. of oil or fat is warmed to about 25° C. or just sufficiently to render it liquid, and the soda solution is added in a thin stream, with constant stirring, which must be continued until the mass becomes too pasty. It is now set aside in a warm place for at least twenty-four hours, during which saponification gradually takes place. For leather purposes, a neutral soap, with a slight excess of fat, is generally advantageous, so that the fat may be increased to 80 lb.; or, in place of this, the operation will be facilitated by the addition of 5 lb. of commercial oleic acid. If soft soap is desired, 14 lb. of caustic potash may be used in place of the 10 lb. of caustic soda. The hardness or softness of soaps varies to some extent with the fat used, but potash soaps are always much softer than the corresponding soda soaps. It is obvious that with soaps made in this way, all the glycerin remains mixed with the soap. If, on testing, the soap does not prove to be free from caustic, it may be re-melted, which will generally complete the reaction. Before attempting to work with large quantities, a laboratory experiment is desirable, using 10 grm. of soda in 40 c.c. of water, and 75 to 80 grm. of oil or fat. The neutrality or freedom of the soap from caustic alkali may be tested by touching a freshly cut surface with an alcoholic solution of phenolphthalein, which the least trace of caustic soda or potash will render pink.