For tanning purposes it is customary to use the bark of various trees, oak bark being the most esteemed in Europe and, in this country, hemlock bark being the most used. These contain from 12 to 15 per cent. of tannin, as a rule, with a moderate amount of brown coloring matter. Pine bark is also frequently used, and the bark of fir, spruce, and larch, while, in Russia especially, much willow bark and birch bark is used for light grades, the so-called Russia leather.
The next most valuable source of tannin is known as sumach, consisting of the finely-ground twigs and leaves of several species of that plant. The American sumach contains more tannin—18 to 25 per cent—than other varieties, but it is less valuable than the Sicilian sumach, which contains less coloring matter, and therefore can be used for tanning light shades of leather. All the materials can be used in the tannery either directly, or in the form of previously prepared extracts. From the Far East come some very important sources of tannin, used for dyeing as well as for leathermaking, in the form of dried extracts of various plants. One of these is Catechu or Cutch, now of value only for its tannin contents, but in former years used as a brown dyestuff as well. A similar product, known as Gambier, is still imported on a large scale from Singapore and other Eastern ports. It contains less tannin than Cutch, but less coloring matter as well. It is used not only for leather but for black silk dyeing with logwood.
The Tanning Process.—Without going too much into detail, the conversion of raw hide into leather by means of tannin is a very lengthy and mechanical process. The hides are first softened by soaking in water, and then are dehaired, usually by steeping in a bath of slaked lime until the hair is loosened and can be scraped off with a blunt knife.
This lime must then be extracted by steeping in an acid bath, preferably containing some organic acid like lactic or acetic acid; some manufacturers, for the sake of cheapness, use dilute sulphuric acid for this purpose, with the invariable result of making the leather brittle and rotten when it is fully dried.
After the acid has been rinsed off, the hides are placed in the tan liquor, made either by dissolving one of the extracts in water, or by mixing the finely-ground bark or sumach with water and placing the hides in the mixture. The tanning process is a very slow one, especially for heavy hides, and it may take several months before the tannin penetrates to the center of the goods. When that time has come, the hides are taken out, brushed off, rinsed with cold water, drained off on horses, and then hung up in a drying shed to slowly dry.
When in the proper condition they are thoroughly rolled by hand or machinery, to break up any adhesions, and to make the leather soft and flexible. Then they are ready to be finished, are dyed to the required shade, rubbed down and polished with wax or varnish, grained by being run through rollers with engraved patterns, and otherwise prepared for the trade.
DYEING AND STAINING OF LEATHER.
General.—It has been mentioned, in previous chapters, that animal fibres of all sorts, such as wool, silk, feathers, etc., seem to possess at the same time both acid and basic properties, and therefore they combine readily with dyestuffs belonging to the Basic and also to the Acid class. This at once distinguishes animal fibres from vegetable fibres such as cotton, linen, and paper, which, being practically neutral in composition, will not combine with either Basic or Acid dyestuffs without the assistance of mordants.
This same rule applies to leather, and we are therefore able to dye leather successfully with either Acid or Basic dyestuffs, using a dye-liquor made acid with, preferably, a volatile organic acid such as acetic or formic acid.
Acids.—For Basic colors acetic acid is generally used, as being cheaper than the other, and quite as good for dissolving the dyes for the dye-bath. For Acid colors it is generally best to use formic acid, for acetic acid in many cases fails to liberate the color-acids from the dyes, and then the colors fail to “bite.”