The New International Encyclopædia/Leather

LEATHER. (AS. leþer, OHG. leder, Ger. Leder, leather). The skin of an animal, dressed for use by some process which shall render its texture permanent in character. Untreated fresh skin is easily putrescible; dry skin is hard and horny and almost impenetrable to air. By converging a skin into leather, however, its nature is entirely changed and it is rendered practically imperishable, porous, and flexible. The skin of an animal consists of two layers: the outer or epidermis, which has no blood-vessels and is hard and horny, and the inner true skin or corium, which is made up of gelatinous fibres. This inner layer or true skin is the basis of leather, and the process of leather-making consists of applying to this skin certain substances which shall enter into combination with the gelatin in such a manner as to produce the desired characteristics of durability, penetrability, and flexibility. Three methods of accomplishing this have been practiced from very early times: Tanning, in which the gelatin is combined with tannin or tannic acid, or, by a much later process, with chromium salts. Tawing, in which the gelatin is combined with certain mineral salts. Shamoying, in which the leather is combined with oil or fatty substances.

Historical Development. Probably, the original process of curing skins was that of simply cleaning and drying. Then, the use of smoke, sour milk, various oils, and the brains of the animals themselves was found to improve the texture of the leather. Later it was discovered that certain astringent barks and vegetables effected permanent changes in the texture of skins, and stopped decay. This knowledge was possessed by the ancient Egyptians, for engravings on their tombs depict the process of tanning. In China specimens of leather have been discovered in company with other relics that prove them to be over 3000 years old. The Romans used leather which they tanned with oil, alum, and bark. The earliest explorers of America found the Indians wearing skins prepared with buffalo-dung, oil, and clay. No improvement in the general methods of preparing leather took place from the most primitive times until about 1790, when the use of lime, to loosen the hair, was introduced. By 1825 English tanners were attempting to introduce new methods by which the tanning process could be shortened. One of the pioneers in these experiments was John Burridge, the inventor of the ‘barkometer,’ an instrument for determining the strength of tanning-liquors.

Tanning Industry in the United States. The first tannery in America was built in Virginia, in 1630. A few years later a second one was established in Lynn, Mass. The tanning industry was well represented among the early settlers of Massachusetts, for it is recorded that no fewer than 51 tanners had come over to the new colony before 1650. There was great demand for their labors, for skins accumulated so rapidly that in 1640 it was found necessary to pass a law “that every hide and skin should be dried before it corrupts, and sent where they may be tanned and dressed.” The tanning industry was also encouraged throughout the colonies by many laws forbidding the exportation of untanned leather. Tanneries flourished everywhere, and by 1810 their annual output was $20,000,000. See paragraph Statistics.

Manufacturing Processes. The hides of commerce are brought to the tanneries in four different forms: either they are simply ‘green’ or ‘fresh’ hides, direct from the slaughter-houses, or, in ease they have been shipped from a long distance, they are wet-salted, dry-salted, or simply dried. The preliminary process of preparing the hides for tanning differs somewhat with the condition in which they are received, salted and dried hides requiring much more thorough cleansing and softening than green hides. The process also differs somewhat in preparing sole-leather, harness-leather, and dressed leather. The first step is to soak the skins or hides in water, to soften them, after which every vestige of adherent flesh is scraped from the inside. They are then laid in heaps for a short time, and afterwards hung in a heated room, by which means a slight putrefactive decomposition is started and the hair becomes so loose as to be easily detached. This process of unhairing, called ‘sweating,’ is mostly followed in America for making sole-leathers, while the process of liming or loosening the roots by the milk of lime is used for dressed leather; but in Great Britain milk of lime is used for depilation of all leathers. The process may be hastened by use of sulphuric or other acid. Hides or skins intended for dressing purposes, as for shoes, coaches, harness, or bookbinding, after the hair is taken off by the lime, have to be submitted to a process called bating for the purpose of reducing the swelling or thickening occasioned by the introduction of the lime, and for cleansing the skin from grease and other impurities. This is effected by working the skin in a decoction of pigeon's or dog's dung and warm water. This process does something more than cleanse the leather; it effects a marked change in its texture, reducing it to an extremely flaccid condition. If the old method of tanning is followed, the hides after unhairing are placed in the tan-pits, with layers of oak-bark or other tanning materials between them, and when as many layers of hides and bark are arranged as the pit will hold, water is let in, and the hides are allowed to remain for an indefinite period to be acted on by the tanning material. Various means for shortening this process have been devised, such as forcing the tanning-liquor through the skin by pressure, sewing the skins together into a bag in which the liquor is suspended, and simply substituting for the dry bark which was formerly used liquid infusions of tanning materials, which are gradually increased in concentration as the process advances. The last-named method, though the slowest, is found to produce the best leather, and the process of tanning is still a tedious one, consuming weeks or even months. The general method employed in American tanneries is described by Sadtler as follows:

“The tan-house into which the cleansed and prepared hides or ‘butts’ now come is provided with rows of pits running in parallel lines, which are to contain the butts during their treatment with tan-liquor. The butts in most cases are first suspended in weak tanning infusions before they go into the first, or ‘handler’ pits. The object of this is to insure the uniform absorption of tannin by the skins, before subjecting them to the rough usage of ‘handling,’ which in the early stages of the process is liable to cause injury to the delicate structure of the skin. During this suspension the skins should be in continuous agitation to cause the tannin to be taken up evenly. Both the suspension and the agitation are accomplished generally by mechanical means. From the suspenders the butts are transferred to the ‘handlers,’ where they are laid flat in the liquor. They are here treated with weak infusion of bark, commencing at about 15° to 20° by the barkometer and are handled twice a day during the first two or three days. This may be done by taking them out, turning them over, and returning them to the same pit, or more generally by running them, fastened together, from one handler pit to another. The treatment of the butts in the handlers generally occupies about six to eight weeks, by which time the coloring matter of the bark and the tannin should have ‘struck’ through about one-third of the substance of the skin. Many of the butts will have become covered, moreover, with a peculiar ‘bloom’ (ellagic acid) insoluble in water. They are now removed to the ‘layers,’ in which they receive the treatment of bark and ‘ooze’ or tan-liquor in progressive stages until the tanning is complete. Here the butts are stratified with ground oak-bark or valonia, which is spread between each butt to the depth of about one inch, and a thicker layer finally on top. The pit is then filled up with ooze, which varies in strength from about 35° barkometer at the beginning to 70° at the end of the treatment. For heavy tannages six to eight layers are required, the duration of each ranging from ten days in the beginning to a month in the later stages. Each time the butts are raised they should be mopped on the grain to remove dirt and loose bloom.”

Many materials besides oak-bark are now used to make tanning infusion, and some of these, being stronger, have hastened the tanning process. Among the most important of these are valonia, the acorn of an evergreen oak found in Asia Minor and Greece, which contains three times as much tannin as the strongest oak-bark; the sumach; the divi-divi and algarovilla, pods of South American trees closely allied to logwood; and the larch, spruce, pine, and hemlock barks. Besides this group there are the tanning materials derived from abnormal growths, caused by the sting of insects or other injuries, as galls (q.v.) and knoppern. The so-called ‘union’ tannage is produced by a combination of oak and hemlock barks.

Undressed leather, after it is tanned, needs simply to be rendered smooth and compact, which is accomplished by scouring and compressing the surface with stones, brushes, the ‘striking-pin’ and rollers, all of which processes are effected by machinery. Dressed leathers must, in addition, be ‘stuffed’ with oils to increase their resistance to water and their flexibility; they must frequently be dyed or stained in black or colors and ‘grained.’ These processes are also performed by machinery. In 1860 a machine was invented for splitting leather to any desired degree of thinness. The practice previously was to shave the leather down, the shavings being wasted. The process of dressing tanned leather known as currying was formerly a separate industry, but is now carried on as a part of the general business of leather manufacture. A favorite oil used by curriers for stuffing leather is the degras, or superfluous oil pressed from shamoyed leather. The demand for this oil is so great that its manufacture has recently become a separate industry.

Chrome-Tanning. The possibility of tanning by the use of chromium compounds, instead of the older tanning materials, was discovered as early as 1856 by a German chemist, but the first process which attained commercial success was invented in 1884 by Augustus Schultz. The introduction of this process in Philadelphia caused it to become at once a great leather-manufacturing centre. Chrome-tanning consumes only a few hours, as compared with weeks or months required by the older method, and it produces a leather which is extremely soft and pliable, of close texture, and thoroughly resistant to water. At the close of the nineteenth century two-thirds of the glazed kids made in the United States were chrome-tanned, but the process had not been applied successfully to sole-leather. The process consists in treating the skins at first with a weak solution of bichromate of potash, to which sufficient hydrochloric acid is added to liberate the chromic acid. Of course pickled skins may be used without the necessity of adding free acid. After the skins have taken up a bright yellow color, through their entire texture, they are drained and transferred to a bath of hyposulphite of soda, to which some acid is added to liberate sulphurous acid, which reduces the chromic acid to green chromic oxide. The sulphurous acid is at the same time oxidized to sulphuric acid, until the whole of the chromic acid is reduced. The leather so produced is of a pale bluish-green color. The combination of the hide fibre, or corium, with the chromium oxide is apparently more stable than its combination with tannin, and yields less to boiling water. The leather can also be dyed successfully if the dye is applied while the skin is still wet, but so great is its water-repellent character that, once dried, it cannot be wetted sufficiently to dye properly.

Tawing consists in dressing the skins in antiseptic materials, so as to preserve them from decay; but by this operation no chemical change is effected in the gelatin of the skin; hence, scraps and other wastes of tawed leathers can be used in the manufacture of glue. The preliminary process of cleansing and depiling is performed as for skins that are to be tanned. After thorough cleansing, the pelts are steeped in a pit filled with lime and water, being taken out from time to time and drained on sloping benches. When removed finally from the lime-pit, the skins are worked with the knife, to render them more supple, and are then put into the branning mixture. This consists of bran and water in the proportion of two pounds of bran to a gallon of water. From this mixture they are transferred to an alum bath in a wooden tumbler or drum. For every two hundred skins some twelve pounds of alum and two and a half pounds of salt, with twelve gallons of water, are used. After remaining in this mixture about five minutes they undergo what is called pasting. The paste is a mixture of wheaten bran and sometimes flour and the yolk of eggs, which the leather almost completely absorbs. Lastly the skins are dried and examined, and if satisfactory are dipped into pure water, and worked or staked by pulling them backward and forward on what is called a stretching and softening iron and smoothed with a hot smoothing-iron.

Shamoying is effected by treating the skin with oil. After the skins have been thoroughly cleansed with lime, and then by a bran drench to remove the lime, they, while still wet, are oiled with fish, seal, or whale oil to which a slight amount of carbolic acid is sometimes added. The oil works into the skin, displaces all the water, and becomes united with the material, rendering its texture peculiarly soft and spongy. Wash-leather or chamois-leather is so prepared, and for this purpose the flesh halves of split sheepskin are chiefly used.

The skins which form the staple of leather manufacture are those of the ox, cow, calf, buffalo, horse, sheep, lamb, goat, kid, deer, dog, seal, hog, walrus, kangaroo, and alligator. The term pelt is applied to all skins before they are converted into leather. When simply made into leather in the state we find in shoe-soles, it is called rough leather; but if in addition it is submitted to the process of currying, it is called dressed leather. Hides are the skins of large animals, as horses, cows, and oxen. The complete hides when rounded, with the cheeks, shank, etc., cut off, are called butts; the pieces cut off constitute the offal. Skins are all the lighter forms of leather, as sheep, goat, deer, including the skins of fur-bearing animals in which the fur is retained. Kips are the skins of yearlings and animals larger than calves. Alligator leather is chiefly used for small fancy articles. Only the skins of young alligators are used, and of these the backs are thrown away as too horny. Walrus and hippopotamus hides are tanned in considerable numbers for the use of cutlers and other workers in steel goods; buffing-wheels are made of them, often an inch thick, which are of great importance in giving the polish to metals and horn goods. Hog-skins are used for the manufacture of saddles and fancy articles. Dog-skins are used for gloves. The ‘grain leather’ of commerce is leather that has been made from the hides of neat cattle, split so thin by the splitting-machine as to be suitable for the same uses as are goat, calf, and various other skins which it is made to imitate.

Morocco leather, formerly an article of import from the Barbary Coast, is now prepared in the United States from goatskins; sheepskins are also used for imitation. It is always dyed on the outer or grain side with some color, and the leather-dresser in finishing gives a peculiarly ribbed or a roughly granulated surface to it by means of engraved boxwood balls which he works over the surface. Morocco has been largely superseded by glazed kid.

Russia leather is much valued for its aromatic odor, which it derives from the peculiar oil of the birch-bark used in tanning it. The fact that this odor repels moths and other insects renders this leather particularly valuable for binding books; a few books bound in Russia leather being effective safeguards against insect enemies in a library. It is also said to destroy or prevent the vegetable evil called mildew, to which books are so very liable.

Japanned leather, varieties of which are known as patent and enamel leather, which is largely used for fancy work and for shoes, is said to have been made in America as early as 1818, by Seth Boyden, of Newark; but it is only within recent years that the American product has approached in excellence that made in Germany and France. The European method of manufacture is described substantially as follows in the Twelfth United States Census Bulletin on the Leather Industry (No. 195: Manufactures, vol. ix., pt. 3): In the preparation of enameled leather, a foundation coat of lampblack mixed with linseed oil has been laid on the flesh side, since the infancy of the industry in Europe. Successive coats of this mixture are applied, the skin being allowed to dry and the surface ground down with pumice-stone after each coat. Then the skins are blackened again with a fluid black mixed with turpentine, and hung up to dry again. After the skins have been allowed to settle, being laid in a pile for about a month's time, or longer if possible, the leather is tacked on to a frame and receives a brush coat of varnish. A baking follows in an oven of moderate heat. The temperature is gradually raised and the baking continued three days. Exposure to the sun for ten hours completes the process. Recently American manufacturers have been making patent leather from chrome-tanned skins. The product is quite different, as is also the process employed. The leather is softer, more flexible, and takes a less brilliant polish than that made from bark-tanned leather, but it is much less likely to crack and is more suitable for shoes than the brittle and inflexible leather made by the older process.

Cordovan is made from horse-hide and is so called because it was first successfully tanned in Cordova, Spain. Most of the hides of commerce are taken from the wild horses of certain parts of South America. A portion of the skin, oval in shape, taken over the rump, about three feet long and half as wide, is all that is used for leather. Its distinctive quality is that it is nearly water-proof.

Statistics. According to the United States census for 1900, there were in this country, at the close of the century, 40,751 establishments devoted to various branches of leather manufacture. The amount of capital invested is given as $173,977,421 and the annual value of the product $204,038,127. Of this product leather goods to the value of $27,293,019 were exported, while $6,773,024 worth of leather goods were imported. The growth of the industry during the last half of the nineteenth century and its distribution into different branches is shown by the accompanying table.

	Year	Number of  establishments	Capital	Wage-earners		Cost of materials used	Value of products
				Average number	Total wages
Boots and shoes, factory product	1900	1,600	$101,795,233	142,922	$59,175,883	$169,604,054	$261,028,580
Leather, tanned, curried and finished	1900	1,306	173,977,421	52,109	22,591,091	155,000,004	204,038,127
Saddlery and harness	1900	12,934	43,354,136	24,123	10,725,647	33,127,926	62,630,902
Leather goods, pocketbooks, trunks, and valises	1900	772	13,505,819	14,990	6,679,767	13,485,761	26,905,814
Boots and shoes, custom work and repairing	1900	23,560	9,262,134	9,698	4,128,361	8,288,664	26,550,678
Boot and shoe, cut stock	1900	342	7,003,080	6,155	2,230,691	17,800,282	23,242,892
Belting and hose, leather	1900	105	7,410,219	1,667	913,937	7,500,413	10,623,177
Boot and shoe uppers	1900	132	273,796	256	125,627	401,680	700,225
Total	1900	40,751	$356,581,838	251,920	$105,571,004	$405,208,784	$615,720,395
Boots and shoes, factory product	1890	2,082	$95,282,311	133,690	$60,667,145	$118,785,831	$220,649,358
Leather, tanned, curried and finished	1890	1,749	97,653,898	42,095	21,090,176	122,221,982	17l,063,337
Saddlery and harness	1890	7,931	35,346,620	22,672	10,908,918	24,674,225	52,970,801
Leather goods, pocketbooks, trunks, and valises	1890	613	11,148,694	10,074	4,448,796	8,785,822	18,814,885
Boots and shoes, custom work and repairing	1890	20,803	14,230,081	16,981	7,422,377	10,403,383	34,856,651
Boot and shoe, cut stock	1890	344	5,401,834	4,992	1,891,031	13,744,655	17,903,846
Belting and hose, leather	1890	93	4,973,420	1,342	780,615	6,132,704	8,663,634
Boot and shoe uppers	1890	317	1,216,026	1,353	609,324	1,902,926	3,343,002
Leather, dressed skins	1890	38	434,800	297	159,813	724,739	1,072,755
Total	1890	33,970	$265,687,684	233,496	$107,978,195	$307,376,267	$529,308,269
Boots and shoes, factory product	1880	1,959	$42,994,028	111,152	$43,001,438	$102,442,442	$166,050,354
Leather, tanned, curried and finished	1880	5,426	67,117,674	34,887	14,062,456	145,320,852	184,865,633
Saddlery and harness	1880	7,999	16,508,019	21,446	7,997,762	19,968,716	38,081,643
Leather goods, pocketbooks, trunks, and valises	1880	378	3,961,256	6,998	2,737,726	5,951,039	11,068,749
Boots and shoes, custom work and repairing	1880	16,013	11,364,273	22,667	7,993,706	12,524,133	30,870,127
Boot and shoe, cut stock	1880	172	1,210,300	2,885	735,482	5,939,249	7,531,635
Belting and hose, leather	1880	97	2,749,299	1,229	607,287	5,021,203	6,531,249
Boot and shoe uppers	1880	81	209,264	437	170,425	448,104	790,842
Leather, dressed skins	1880	202	6,266,237	5,395	2,441,372	11,063,265	15,399,311
Total	1880	32,327	$152,380,350	207,096	$79,747,644	$308,679,003	$461,189,543
Leather, tanned, curried and finished	1870	7,459	$59,784,362	34,345	$14,108,201	$116,469,899	$154,377,625
Saddlery and harness	1870	7,607	13,935,961	23,557	7,046,207	16,068,310	32,709,981
Leather goods, pocketbooks, trunks, and valises	1870	295	2,638,389	4,329	2,171,416	3,889,695	9,091,543
Boots and shoes	1870	23,428	48,994,366	135,889	51,972,712	93,582,528	181,644,090
Belting and hose, leather	1870	91	2,118,577	808	454,187	3,231,204	4,558,043
Leather, dressed skins	1870	110	1,340,450	898	397,574	2,099,735	2,859,972
Total	1870	38,990	$128,812,105	199,826	$76,150,297	$235,341,371	$385,241,254
Leather, tanned, curried and finished	1860	5,175	$38,908,170	26,145	$8,144,278	$49,534,318	$75,318,475
Saddlery and harness	1860	3,621	6,478,184	12,285	4,150,365	6,606,415	14,169,037
Leather goods, pocketbooks, trunks, and valises	1860	214	1,244,000	3,160	901,741	1,990,573	4,163,956
Boots and shoes	1860	12,486	23,357,627	123,026	30,938,080	42,728,174	91,889,298
Boot and shoe, cut stock	1860	1	25,000	15	8,184	31,400	149,740
Belting and hose, leather	1860	46	588,000	354	134,952	915,271	1,481,750
Leather, dressed skins	1860	13	117,450	101	31,230	278,341	380,272
Total	1860	21,556	$70,718,431	165,086	$44,308,830	$102,084,492	$187,552,528
Leather, tanned, curried and finished	1850	26,664	$22,582,795	25,379	$6,492,130	$26,038,743	$42,932,528
Saddlery and harness	1850	3,515	3,969,379	12,958	3,154,008	4,427,006	9,935,474
Leather goods, pocketbooks, trunks, and valises	1850	165	522,610	2,142	543,840	1,056,835	2,213,363
Boots and shoes	1850	11,305	12,924,919	105,254	21,622,608	23,848,374	53,967,408
Belting and hose, leather	1850	8	40,800	39	15,208	111,785	160,500
Leather, dressed skins	1850	22	192,000	216	49,548	391,138	525,370
Total	1850	41,679	$40,232,503	145,988	$31,877,342	$55,873,881	$109,734,643

Bibliography. Consult Sadtler, Handbook of Industrial Organic Chemistry (Philadelphia, 1895); Davis, The Manufacture of Leather (Philadelphia, 1897); Standage, Leatherworker's Manual (London, 1900); Stevens, Leather Manufacture (Saint Louis, 1891); and Watt, The Art of Leather Manufacture (London, 1897).