Chrome or Chemical Tanning.— The vege table method of tanning light leathers is being replaced by chemical tanning or combination of vegetable and chrome now considered chemi cal or chrome tanning. In order to prepare the stock for chrome tannage, it is first pickled after being washed out of the bate; that is, placed in a vat, fitted with a revolving paddle similar to a water wheel, in which is dissolved salt and sulphuric acid mixed in different proportions, according to the ideas of the fore man. When the skins have absorbed enough of the chemicals, they are taken out of the paddle, allowed to drain and then pressed in an hydraulic press to squeeze out the water. They are then split on a machine, which con sists of a fast running horizontal 'belt knife or drawn over a stationary knife being nressed on to the edge of the knife by a roll placed above the knife. The thickness of•the skin is regu lated by the distance between the belt knife or stationary knife and the roll. Light stock such as calf, kip, goat or sheep are not split but shaved on the flesh. The chemicals used in the manufacture of chrome leather are mostly im ported and before we enter the realm of modern chemistry as applied to tanning it will be of interest to know where the raw materials are obtained "Ind what they are. The raw materials are sulphuric acids, glucose, sodium thiosulphate, soda ash and chrome salts. Sul phuric acid or oil of vitriol as generally known is obtained by burning sulphur or brimstone. Glucose or grape sugar is obtained from corn starch. 'Sodium thiosulphate, commonly known as "Hypo,* a salt with which every photographer is familiar, is also obtained from sulphur or brimstone by heating it with caustic soda, known as soda lye. As a by-product sodium sulphide is also obtained, which as already said is used in the dehairing process. Soda .ash or calcined soda is obtained from salt. The chromium salts used in the tanning are made from chrome iron ore, a very hard mineral composed of the oxides of chromium and iron and mined mostly in New Caledonia, although some is mined in Canada, Turkey in Asia and California, but the quantity is so small as to be no factor in the manufacture. The chrome iron ore is mixed with lime and soda or potash, placed in a furnace with free access to the air and roasted at a high temperature. When finished, the mass is treated with water to extract the sodium or potassium chromate formed. Sulphuric acid is then added and the liquor evaporated and allowed to crystalize, and the resulting crystals are sodium or potassium bichromate, the mother substance for the manufacture of the compounds used in the tan nery. This bichromate is used in the tannery, as such, for the so-called two-bath tannage or converted into its salts, to be used in the one bath process. The one-bath and two-bath processes are chemically similar. The two-bath produces the chrome salt desired in the skin, while the one-bath process consists of forming the chrome salt desired and when so made to mill or drum it into the skin.
It is interesting to note that the first tical chemical or chrome tanning was invented in the United States by an American, August Schultz, in 1884. Since that time great advance ment has been made in chrome tanning in America and Europe hut the theory has re mained the same, yet the fact remains that it was discovered in the United States and due to the research of an American. Schultz's patent was for a two-bath, while Professor Knapp of Germany published in 1858a process for' the one-bath process, though he did not recognize its value. In 1893, Martin Dennis, of Newark, N. J., recognized its value and was granted patents on this process. For the two-bath process, the pickled stock is taken and placed m a revolving driun with water in which com mon salt is dissolved, the drum closed and set in motion. Then bichromate, i.e., either sodium bichromate or potassium bichromate, is dis solved in water, a certain amount of sulphuric add added for the purpose of changing the bichromate into another chrome salt which will be absorbed by the skin. Bichromate without the use of an add (and sulphuric is used in preference) will not penetrate the skin, and this mixture is put gradually in the drum. When all bichromate has been added and the skins are ((struck through,)) that is,i the yellovi color of the bichromate has thoroughly pene trated the skin to its fullest extent, a solution of thiosulphate of soda, "Hypo,)) is added. These materials, when in contact with the yellow colored stock, will turn them blue by precipitating an insoluble chrome compound between the fibres of the skin and chrome tanned leather, is the result of this stage. A solution of soda ash or calcined soda is now added to the stock in the revolving drum to neutralize the 'acid present and also to convert any soluble chrome compound present to an insoluble. The stock is now thoroughly
washed, pressed and sent to the machines to be split down to the required thickness and to be shaved, smoothly on the flesh. The next process of tanning is probably the most im portant to the tanner of to-day, namely, the one-bath process. The one-bath process, in which the liqu r used is blue, is the same with the 'exception as above explained, the liquor li is already m de and simply milled into the skin. The tanner tests his .stock from time to time in order to know when it is thoroughly tanned. This test is simply taking a piece of the heaviest part of the skin and ,boiling in water for five minutes. If the stock remains soft and does not curl after the expiration of the time, the stock is considered tanned. After tanning iS finished, the stock is again washed and pressed, and is ready for the fat liqtioring, coloring, drying, resoftening, tacking on boards to' dry, staking and finishing. , The fat-liquoring process is to mill oils, soaps and greases into the stock in order to keep them soft and pliable and at the same time render them semi-water proof. Staking is stretching the skins by mgchine in order to get a smooth 'surface. The coloring of the skins is.. done by the ap plication of wood dyes or coal tar colors. The coloring of the stock is of great import ance. In order to prevent the dye-stuffs from going on unevenly, great care must be exercised. Blacks are made either by the use of lopvood or aniline colors.
Punishing Processes.—Those leathers which are not shined and which always have a dull appearance such as used in work shoes may be sponged with oils, .measured and sorted into grades to be sold. A good many leathers are boarded In 'order to give them a distinc tive pattern. The grain side is folded on itself and rolled under pressure with a cork surface board attached to the arm. This makes a square pattern on the surface when done up and down and from side to side of the skin. Leather for dress shoes on which a bright or semi-bright lustre is desired must be seasoned and glazed. Seasoning consists in sponging the surface of the leather with a liquid dress ing consisting of albumen or some similar preparation which will coagulate with the heat of friction. After seasoning and drying leather is glazed. The glazing process brings up the polish in about the same way that a brush does on shoes. The skins are glazed or rubbed over with a glass cylinder under great pressure. This glass cylinder is placed horizon tally in a pocket at the end of a long wooden arm, which is brought down over the skin by a connecting rod from the fly-wheel of the machine. The skin is laid over a slanting board and held there by the operator who shifts its position so that all parts of the skin come beneath the glazing cylinder which moves up and down in a straight line, continually bear ing on the skin with a heavy pressure. Some skins on which a dull polish is wanted are brushed over the surface with a revolving roller brush. Leathers on which a high polish is desired are sponged with finish, dried and glazed several times in succession. If an es pecially smooth surface is desired the leather is embossed after the first glazing. This is done by placing the skin beneath the hot steel plates of a heavy hydraulic press. The last operation is sponging a light coating of oil over the skin.
Shoe upper leather is sold by the square foot and is measured by power driven measuring machines. Each skin is passed through the machine and a dial on the machine indicates the measurement to one-fourth of a square foot. Patent leather is chrome tanned, dried, fat liquored and dyed black as nreviously described up to the finishing process. Some of the grease given in fat-liquoring and some of the oil applied to the grain are not combined with the fibres, and unless removed will cause the varnish to slip over the leather so that finish ing becomes impossible. Degreasing is best done by soaking the leather in naphtha, and then pressing out the naphtha and drying the leather. When ready for finishing the skin is spread out and tacked on a frame. and the varnish, which is mainly oxidized linseed oil, is applied with a brush similar to a paint brush. The skins on these frames are then placed in ovens so that the varnish may be baked on. Generally several coats of varnish are baked on the skin.
Bibliography,--'Bibliograph Modern Amer ican Tanning' (2 vols., Chicago) ; Allen, Fred erick J., 'The Shoe Industry' (Boston, Mass.); Burt, E. W., 'Shoe Craft—Its Organization' (Boston) ; Fleming, Louis A., Tan ning' (Hillsdale, Mich.) ; Hill, Herbert and Yeoman, Henry, 'Manual of Boot and Shoe Manufacturers' (London, Eng.) ; Pfister and Vogel, 'Leather Specimen Book' (Milwaukee, Wis.) ; Pratt Institute of Tanning, 'Lecture Courses' (Brooklyn, N. Y.) ; and files of the Boot and Shoe Recorder (Boston, Mass.).