Vegetable Tanning.— The sources of tan nin used in the vegetable tanning processes are chiefly oak bark and hemlock bark. The former is taken from the yellow and red oaks, though all oaks carry a proportion of tannin which may be available in some circumstances. The tannin content of the best oak bark ranges from 9 to 14 per cent. Hemlock bark is in much larger supply, and, therefore, is more largely used. It carries from 7 to 8 per cent of tannin. Chestnut wood is used to a con siderable extent in the South, and an extract of this wood is marketed in other parts of the country showing a tannin content of from 23 to 32 per cent The quebracho wood of South America is one of the largest sources of tan nin, the dry wood yielding 18 to 24 per cept. The extract of quebracho, the form in winch it is generally marketed, carries from 62 to 65 per cent of tannin. Other important sources are: Sicilian sumac, with 25 to 30 per cent tannin; gambler, a dry extract of the plant, with 60 per cent tannin; myrabolans— the dried fruit of a tree of India—with 30 to 38 per cent tannin; Mexican mangrove bark, 30 per cent, and African mangrove bark, 40 per cent tannin; valonia — the acorn cups of an Asia Minor oak-30 to 40 per cent tannin; and palmetto extract, from the roots of the palmetto tree, about 20 per cent tannin. The amount of tanning materials which the United States nonnally imports is shown by the figures for the fiscal year ended 30 June 1916—before the entry of the United States into the war caused a dearth of merchant shipping. In that year the amount of quebracho was 106,864 tons, valued at $1,598,465; of mangrove bark was 21,186 tons, valued at $582,992; and of all other tanning materials and extracts, a value of $668,166. The annual consumption of do mestic tanning materials is about 900,000 tons of hemlock bark, 400,000 tons of oak bark and 500,000 cords of chestnut wood.
Leather tanned with oak bark or its ex tract is of a light fawn color, regarded as the most desirable. Hemlock produces a reddish leather; quebracho a yellowish tint; chestnut, a brown color; and mangrove a still deeper brown. Hemlock •is rarely used alone, being too harsh; it is commonly combined with quebracho, with which it yields a fine, soft and pliable product of superior color. Quebracho and 'palmetto are used together to advantage in both quality and color of the leather. A highly approved process combines hemlock, chestnut and quebracho extracts for a pnmary tanning, and quebracho and gambler in com bination for retanning. This formula is par ticularly suitable for large and heavy hides. All hemlock-tanned leather is improved by retanning with gambier, or gambler and sumac combined. Chestnut tannings are much im proved by retanning with sumac. Thick and heavy hides are usually tanned gn the bark° as it is termed; thinner hides ((in liquor.° In the former process a layer of spent bark is spread upon the bottom of a wood-lined pit, upon this are piled successive layers of hides and fresh bark, the whole is covered with spent bark and the pit is filled with water. After 8 to 10 weeks the hides are removed to a second tank containing less bark, from which, after three to four months, they are again removed to a. third tank containing a yet smaller quantity of bark, where they remain for four to five mouths. If necessary the process is repeated several times. The hides increase in weight from 10 to 12 per cent during this process. The total quantity of bark required
averages six times the weight of the dry hides. Tanning °in liquor° consists in placing th.: hides successively in solutions of tannin of gradually increasing strength. The tannin is thus caused to penetrate the hides completely. The thinner hides must be immersed for 6 or 8 weeks, the thicker for 12 or 24 weeks in the tanning liquor. A more rapid process is ob tained by combining the tanning agents. A liquor compound with quebracho extract and palmetto extract is one of the quickest, and the leather may be pressed and split and the grains retanned after 18 to 20 days. The modern tanner divides his work into three stages: in the first the hides are colored by immersion for 24 hours in a weak liquor, activated hy one part of hemlock to two parts of que bracho; in the second stage the tanning proc ess is carried forward by strenghtening the liquor by fresh additions twice daily until it tests 30° (barkometer); the third stage follows the pressing to remove superfluous liquor and the splitting of the hide into grains, and is a retanning process with a strong gam bier and sumac liquor; for about one hour. The entire process may be completed within eight days.
Mineral This process depends upon the action of chromic acid instead of tan nic acid. Of very recent introduction the chrome process has extensively displaced the much slower tannic acid processes and its prod uct seems equally durable and acceptable in the industries. After the hides are bated and washed they are pickled with sulphuric acid and salt or with aluminum sulphate and salt. They are then put into a solution of salt in a vat with paddles, and the paddles run for half an hour. The chrome liquor, a compound of potassium bichromate and sulphuric or muri atic acid, is then added until the bath reaches 50° B., the paddles being run for one hour. More of the chrome liquor is then added, and the paddles run for three hours longer. The remainder of the chrome is then added and the hides left until the tanning is complete, which will take about six hours. They are left in the liquor for 12 hours longer and then pressed and placed in a second bath of sulphurous acid and finally washed with a solution of borax, when they arc ready for splitting and retan ning with gambier ot palmetto. This is the process commonly employed in making calfskin, goatskin and sheepskin leathers, though they are often tanned with alum. In the latter case, when the skins have been washed and prepared by being submitted to processes closely resembling those already described they are separately soaked in a tepid bath containing alum and common salt dissolved in water; they are then, without being dried, placed in heaps for a few days, after which they are wrung out and dried slowly by exposure to air. The alum bath for 10 skins is usually prepared by dissolving 0.70 kilos of alum and 0.30 kilos of common salt in 22.5 litres of boiling water. Aluminium chloride is produced by the mutual action of the salt and alum, and is absorbed largely by the skins; the excess of salt appears also to aid in the conversion of the skins into leather. The tawed and dried skins are soft ened by being damped and stretched between a curved iron and a movable steel plate, after which they are again dried. Heavy hides are sometimes tawed for the use of the saddler by steeping them in a bath c,ontaining a larger.