Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/401

LEATHER 383

barkometer. It is graduated to the standard of pure water; and, when it is placed in a specimen of ooze, the strength of the latter is judged of by the position of the stem above or below the water-mark. But, as bark or other tanning material may contain several soluble substances besides tannin, the barkometer obviously cannot be relied on. Some tanners judge of the strength of ooze by its astringency to the taste. Seguin, who in the end of the 18th century was the first to insist on the advantage of tanning with previously prepared infusions, proposed the use of a solution of gelatin as a test of the presence of the tannin. In trying the quantity of tannin by Seguin's process, 480 grains of the bark in coarse powder should be acted on by half a pint of boiling water. The mixture should be frequently stirred, and suffered to stand twenty-four hours; the fluid should then be strained through a linen cloth, and mixed with an equal quantity of solution of gelatin, made by dissolving glue, jelly, or isinglass, in hot water, in the proportion of a drachm of glue or isinglass, or six table-spoonfuls of jelly, to a pint of water. The precipitate should be collected by passing the mixture of the solution and infusion through folds of blotting-paper, and the paper exposed to the air till its contents are quite dry. Every 100 grains of precipitate contains 40 grains of tannin nearly. As, however, some kinds of tannin produce larger precipitates of gelatin than other kinds, and as the composition of tanno-gelatin varies with the strength both of the solution of gelatin and of tannin, this method is not reliable. Sulphate of cinchonin is said to afford a better test; a solution of this, acidulated with a few drops of sulphuric acid, will, it is said, precipitate tannin completely from the solution. Löwenthal's method, founded on the fact that solutions of tannin in presence of sulphuric acid are readily oxidized by permanganate of potash, is very useful for the comparative determination of the value of different tanning substances. A given weight of tanning material is infused, and the solution is brought up to a definite volume. One half of this measured quantity of tannin solution is mixed with definite quantities of a standard indigo-carmine solution and sulphuric acid, and to this mixture permanganate solution is added from a graduated tube till the colour of the indigo is completely discharged, when both tannin and indigo are oxidised. A parallel experiment is next made with similar measured amounts of indigo and sulphuric acid solutions, but without any tannin infusion. The difference between the quantity of permanganate required to discharge the colour in the two experiments gives the standard for calculating the amount of tannin in the solution to be tested. Another good method of testing the value of tanning material is to digest a piece of dry prepared hide or skin in a known quantity of the infusion, until the whole of the tannin and other matters be separated. The skin is then taken out, slightly washed, dried, and weighed, when the increase of weight is supposed to be the weight of tannin and of the other matters required. An apparatus devised by MM. Muntz and Ramspacker has recently been introduced for facilitating this last test. It consists essentially of a small vessel sufficient to hold a measured quantity of a tanning infusion, the specific gravity of which is carefully ascertained. That vessel is so arranged that strong pressure can be brought to bear on it by means of a screw acting on an india-rubber surface, thereby forcing the liquid through a piece of skin which covers the lower part. The skin absorbs the whole of the tannin during the passage through it of the infusion, and by ascertaining the specific gravity of the escaped liquor the percentage of the tannin material in the infusion can be readily determined. This apparatus has been extensively introduced in practice in England and on the Continent.

Sole Leather or Heavy Leather Tanning. – The hides of oxen are received in the tan-yard in four different conditions. These are – (1) market or slaughter hides, which, coming direct from local abattoirs, are soft, moist, and covered with dirt and blood; (2) wet salted hides; (3) dry salted hides; and (4) sun-dried or "flint" hides, the three last forms being the condition in which the imports of foreign hides are made. The first operation in the tannery is to clean the hides, to free them from salt, and to bring the hard dry hides to the uniformly soft flaccid condition in which all market hides are obtained. The treatment at this stage requires skill and attention to prevent the more soluble constituents of the hide from dissolving out in the washing and soaking processes, and also to secure the complete softening of the entire substance, upon which the successful tanning greatly depends. In the case of market hides cleaning and softening are principally effected by washing and soaking in spent lime-water, while for dry hides and dry salted hides brine is essential. The softening of these materials is helped and rendered thorough by working them for some time in the stocks (fig. 1) after they have been well soaked. After being thus brought as nearly as possible into a uniform condition, all hides are treated alike. The first operation to which they are subjected is depilation, which removes, not only the hair, but also the scarf-skin. This is effected variously in different countries. In England the most common plan is to throw the hide or skin into a strong watery ley of slaked lime, with lime in excess. By this, in a few days, more or less according to the proportion of lime present, the hair is easily detached, the hair-sheath having been dissolved. The hair was formerly taken off by making a sour liquor from fermented vegetable matter, in which the hides lay for several days; they were also smoked in a damp state for the same purpose; but both those methods are now abandoned. They are still sometimes, especially on the Continent, sweated, that is, they are laid in heaps and kept wet and warm, a plan which is still adopted in England for skins. In America the sweating is performed cold; the hides are hung up wet in a damp underground cellar, and are kept moist for ten days or a fortnight. In either of these sweating processes incipient putrefaction takes place sooner or later, when the hair and scarf-skin are easily removed; but the fatty matter remains, and in some cases prevents the hide from taking the tan.

Fig. 1. – Double-acting Stocks.

There have been numerous other methods proposed and patented for unhairing skins, few of which have been received with much favour. Among the agents proposed may be mentioned caustic soda, sulphide of sodium and sulphide of calcium, borax, sugar, and charcoal – substances which it is obvious must act in very different manners. Lime and alkaline solutions not only loosen the hair and scarf-skin, but also "plump" the corium or true skin, that is, they swell it and render it consequently porous and more permeable to the tanning solution. Lime further forms with the fatty matter of the flesh side calcareous soap, thus neutralizing the fat which would otherwise interfere with