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.” Professional leather dyers, for the sake of economy, often use a little sulphuric acid in the dye-bath, a practice which is one of the chief causes of the short life of modern leathers. With other animal fibres, such as wool and silk, the Acid colors take quite as readily as the Basic; but with leather, there is some little difference, according to the way in which the leather has been prepared.