Preliminary Processes. The skin, as re ceived by the tanner, is either green, green salted, dry and dry salted, and in whatever state received, the first process is to trim the hides to remove all useless pieces and adhering fat, but the dry salted skins are first softened. When thoroughly trimmed, so as to pass inspec tion of the foreman, the skins are placed in the soaks. The soaks are large tanks filled with as pure cold water as can be obtained to which salt has been added in order to open the pores of the skin, and the skins are aijowed to remain there for 24 to 48 hours, with periodic agita tion or stirring, due care being taken not to heat the stock. The object of the soak is to soften the skin and to remove all foreign matter such as dirt, salt and blood. Great care must be used to remove all blood and salt, as these materials, if left on the skin, will cause stains which will show dark spots in the finished leather if colored into tan, and dull if made into blacks. Another object of the soak is to add water to the skin to bring it back again as near to the condition in which it was when taken off the animal. The water of the soak must be cold, not above 65° F., so as to prevent all bacterial action and thereby prevent the loss of what tanners call hide substance. The soak ing is controlled by the chemist, whose duty it is to examine the water for dissolved chide substance,* that is, to analyze the water of the soak for ammonia. As mentioned, the hide contains nitrogen. This organic nitrogen is converted either by bacteriological or chemical action into ammonia, due to over warm water or too long in the soaks or both. The skins are then taken from the soaks and placed in a re volving drum and washed thoroughly with clean cold water in order to remove all adhering foreign matter. They then go to the fleshing machine, a machine which consists of spiral blunt knives, set into a revolving cylinder, to remove all superfluous fat on the flesh side of the skin. From the fleshing machine they go to the lime vats for loosening the hair; plump ing the stock and spreading the fibres apart.
Depilation.— In the more modern process of liming, the skins are treated with a mixture of lime and sodium sulphide, a chemical ob tained in the manufacture' of washing soda or by other processes. If it is not desired to save the hair which is valuable, the skins may be dehaired by the use of sulphide of soda alone. Hair can also be loosened by °sweating?), that is, to hang the stock in chambers where the tem perature and humidity can be controlled. This process is simply a partial putrefaction which affects the base of the hair roots, so as to allow them to be easily removed. The sweating process is used, now very seldom, in the manu facture of sole leather, and formerly to remove the wool from sheep skins. The depilation now in use is accomplished by lime or the above former mentioned combination of lime and sul phide. The lime vats are similar to the soaks. The skins are placed in a saturated solution of slacked lime, in which there is always an excess of lime, so that the calcium hydrate, which is slacked lime, is absorbed by the skin, the lime liquor remains at the same strength by the water dissolving some of the excess of lime always present. Lime is used on account of its being a weak alkali and only acts upon the softer parts of the skin, such as the hair bulbs, etc., while it hardly acts upon the harder parts of the skin; yet, by leaving the skin too long in contact with lime, the harder parts are also affected. Bacterial action of the limes must also be taken into consideration for depilating, as new limes are very slow in removing the hair, and the action of the lime is increased as it grows older. The liming takes from two to three days, and sometimes longer, according to the raw materialplaced in the vat and the kind of stock required. Here the chemist is called again to control the time and the action, so that the skins do not become too soft or lose too much substance, thereby making the skins flabby and loose. From the lime vat the skins are
either dehaired by hand or machine. Although machine dehairing is quite satisfactory, the skins' must be worked by hand in order to re-' move the fine hairs which were left 'by the machine. The skins are then placed upon beam, a semi-circular board plated on end, simi lar tb a brtel cut in half in length with the convex side tip,' with the flesh side of 'the skin toward the workman, who cuts off the remain ing flesh, fat and tissue. 'From this operation the skins are washed thoroughly in clean, cold water to remove all adhering lime and as much of dissolved lime as possible. In order• to get all the absorbed lime out of the skins, they are again placed in a vat of water in'which fer mented hen manure, dog dung or pigeon manure is dissolved, and allowed to remain in this solu tion until by chemical reaction all alkali is shown to be removed. The lime is sometimes removed by chemical means in the use of lactic acid, the acid of sour milk, and in the newest processes material used is an extract made from the various organs of the animal such as the spleen, pancreas, etc. This process is known as bating. The fermented animal excrements used produce lactic acid by bacterial action, but also has the advantage of containing bac teria which are very essential in this process of bating. The idea of bating is first to render soluble the insoluble salts of lime, which the skins take up, and then to bring the skin, which swells in the alkaline lime liquor, back to the state desired by the tanner to produce leather according to his needs. The skins are again washed and are ready for the tan, if vegetable leather is required, or for pickling in case chrome tanned stock is to be made.
Tanning Materials.— In vegetable tanned stock the skins are hung on sticks and placed in vats and treated with a dilute water extract of bark, hemlock, quebracho, a South American wood, sumac, myrobalans, mimosa or Australian willow, valonia or oak acorns from Greece and Turkey, and many other materials too numerous to mention or com bination of the different vegetable tannins. The liquor in the vats is strengthened daily with the extracts until the skin is thoroughly tanned or as the tanner would say °struck through?) The vegetable tannages are limited only by the number of plants, barks and other vegetable substances which yield tannins in sufficient quantities for commercial use. In practice only a few tannins offering the advan tages of cheapness, large supply and greater suitability are used. Hemlock and oak bark liquors and extracts separately or in combina tion are used more than any other vegetable tannins. Most sole and belting leather, prac tically all upholstery, harness, bag, strap and case leathers and some shoe and bookbinding leathers are tanned with hemlock or oak or both. Sumac is the principal tanning material for bookbinding leathers, on account of its giving a light-colored leather that is resistant to light and decay. Gall-nuts, myrobalans, divi-divi and oakwood are also used in book binding leathers. Valonia and chestnut oak extract are used for sole leather in connection with oak and hemlock. Quebracho is used in various leathers, often with hemlock and oak. Gambier and palmetto are used in many of the softer leathers for shoes, gloves, etc. Some leathers are tanned with a mixture of three or four different vegetable materials as may be necessary to give the color, firmness and other properties desired. A distinct tan nage of limited use is the oil tannage used for making some glove leathers, particularly chamois and buckskin. Cod oil is the oil com monly used. Its oxidation tans the leather. The leather is then washed, oiled or placed in a revolving drum with a combination of oils and greases which is called °Fat Liquoring* and hung up to dry. The skins are now ready for coloring and finishing which will be ex plained further on under chrome or chemical tanning.