390 LEATHER
potassium or chloride of sodium, and sulphate of alumina. These are mixed together in one large stock tank, from which is drawn by means of a system of piping communicating with each pit the quantity required to make the necessary strength of liquor; this at first, as in tanning by bark, is very weak, but is strengthened systematically every few days according to the thickness of the hides being tanned. The quantity of chromic acid used ordinarily amounts to from about 2½ to 5 per cent. of the weight of leather produced. The price of bichrome at present is 5¾ d. per lb, so that 100 lb weight of leather would cost for this agent from 1s. to 3s.
Light skins such as sheep skins and calf skins are tanned in less than a week, ox and buffalo hides in about a fortnight, and walrus hides, over 2 inches thick, in six weeks. After being tanned, the hides, which are at this stage of a yellowish tint like sumach-tanned leather, are dipped in chloride of barium, which converts the soluble chromates on the surface into the insoluble chromate of barium. If any particular shade of colour is desired it is then put on, and in general hides are coloured like ordinary leather. After being coloured the leather is allowed to get nearly dry, when it is immersed in pure paraffin wax and resin dissolved together in certain pro portions. These materials, with chloride of potassium or chloride of sodium and sulphate of alumina, go to give the necessary sub stance, weight, and waterproofing to the leather. The hides are afterwards dried and brushed clean by brushing machinery. Thus finished, the leather differs very little in appearance from ordinary leather.
Dr Heinzerling claims as the meritorious and original features of his process the combined use of chromate compounds and fatty matters. The stuffing with fat or paraffin of chrome leather, ho maintains, in the first place, reduces chromic acid to chrome oxide, and secondly the oxygen thus liberated in the substance of the hide oxidizes the fatty into acid bodies, which, uniting with the chrome oxide, form a third insoluble compound mordanted in the fibre of the leather, rendering it at once supple and waterproof. The leather has been reported on by Mr David Kirkcaldy, London, as considerably stronger than the best bark-tanned leather he was able to procure. After steeping samples of it in cold water six days it has been found that the total quantity of tanning material extracted amounted to from 014 to 135 per cent., while first-class bark-tanned leather similarly treated yielded 679 per cent. By boiling chrome leather in water for half an hour, the loss ranged from 005 to 054 per cent.
The process seems to offer the means of utilizing classes of hides, such as sheep skins, and very heavy hides, as those of the walrus, hippopotamus, &c., in a way which has not hitherto been found practicable by other processes. Sheep skins in chrome- tanning do not require to be pured and freed from their oleaginous constituents, and when finished by this process are no longer porous, but become waterproof. They can be shaved and whitened like calf skins, and may be used for shoe purposes.
The Heinzerling process is at work in various localities throughout Germany. For the United Kingdom and British colonies the patent rights have been acquired by the Eglinton Chemical Company of Glasgow, who, as manufacturers of bichromate of potash, have an indirect interest in the development of the system. Although the method has yet scarcely passed the critical stage of practical experi ment, the products appear to be gaining the favour of men of experience; and, should the system meet the expectation of its originator and promoters, it cannot in the end fail greatly to cheapen many useful classes of leather.
Shamoy or Oil Leather.
The process of preparing leather by impregnating hides and skins with oil is probably the oldest system of leather manufacture. It is that which in earlier times was most largely followed, and among rude and semi-civilized people it is still commonly practised. Not withstanding this, well-shamoyed leather requires the exercise of much care and numerous manipulative processes. Hides and skins of all classes are prepared by shamoying; but sheep, goat, deer, antelope, and small calf skin are those usually treated, an enormous number of flesh splits of sheep being shamoyed for common pur poses. The extensive employment of deer skins in shamoying gives the product the name of buck or doe leather, and from the use of the chamois skin of the Alps is derived the name of the process chamois or shamoy, while from the fact that it maybe easily washed like cloth it is called wash leather. In former times a large num ber of ox hides were shamoyed, but now that is little practised.
Skins for shamoying are in the preliminary stages treated almost as for ordinary tanning; but, beyond unhairing, the surface of the grain is shaved off in all except the small thin skins. They are afterwards treated with fresh lime solution, and repeatedly washed to bring the pelt to somewhat open and porous condition, drenched with bran to remove all lime, and rinsed in an acid liquor. The skins are next staked out and taken to the fulling machine or stocks, where, after being rubbed over individually with fish oil, they are hammered for about two hours to force the oil into the substance of the skin. They are then stretched, hung up for some time, again oiled, and fulled; and these operations may be repeated from six to twelve times according to the thickness of the skins treated. After thorough impregnation the skins are dried, then heaped up in a heated room, where a process of oxidation is quickly set up. So soon as the skins assume a yellow colour and give off a peculiar odour, not at all like fish oil, the process is complete and the fermentation is stopped. It is now found that about one-half of the oil is oxidized within the skin and combined with the tissue to form leather, while the remainder is present only in the condition of mechanical impregnation. This uncombined oil is washed out with a warm potash solution, and the fat so recovered, known as degras, forms a valuable material for the dressing of common leather by curriers.
Parchment, Vellum, and Shagreen.
These substances, properly speaking, do not come under the heading of leather at all, seeing they are neither tanned nor tawed, but simply are dressed skins dried and prepared for their peculiar uses. Parchment is made from calf, goat, sheep, ass, and swine skins by the unhairing and dressing processes through which all skins pass preparatory to tanning. When they are thoroughly scudded and fleshed, the skins are stretched tightly in every direction over a frame, and in that condition shaved and equalized on both sides with the currier's knife. After drying, the skins are ready for use as drum leather and for the other ordinary applications of parchment. The common kinds of vellum are made from sheepskin splits, of which two may be obtained from a single skin. To prepare these for use, the splits are, after stretching and drying, repeatedly rubbed over with powdered chalk and powdered stone to raise the fine even velvety surface peculiar to vellum. Common shagreen consists of the skins of various species of sharks and rays prepared in a similar manner to parchment; and Persian shagreen is a kind of tawed parchment with an artificial grain embossed in it, by pressing into the substance while in a damp condition the small round seeds of a species of Chenopodium. Shagreen is frequently dyed in bright colours, and used for ornamenting the surfaces of small articles, and the handles of daggers, swords, &c.
Commerce and Statistics.
It is quite impossible to form any adequate estimate of the extent and value of such a trade as that in leather. The raw materials are obtained, in almost equal abundance, throughout the civilized world, and the manufacturing operations are, to a greater or less degree, carried on in every centre of industry. Thus local wants are largely supplied by local producers, and much of this trade is never recorded in any statistical returns. There can be no doubt that leather takes rank among the foremost half dozen of human industries. Both in Europe and America there is a large international trade in the raw materials and manufactured products, while from the East Indies and the British colonies the largest supplies of various untanned and tanned hides and skins are now exported. Taking cattle hides alone, the import trade of the great manufacturing centres has been thus stated for 1879: –
Hamburg 646,330 lioston and New York (U.S.).. 3,153,026 United Kingdom 1,102,405 Marseilles C.W,<)(;7 Havre 853,741 Antwerp 958,41-2 Total for Europe and America 7,426,881
These came principally from the River Plate, Rio Grande, and Brazil, and are exclusive of 250,000 horse hides imported into Europe, and the whole of the enormous exports of the East Indies. The total number of hides, raw and tanned, exported from India in the year 1877-78 was 9,300,955, and the average for the four preceding years exceeded 7,250,000. A great proportion of these are tanned kips, shipped from Madras almost exclusively to the United Kingdom, – buffalo and the heavier flint dry hides going more largely to the United States. The imports of East Indian kips into the United Kingdom during 1880 amounted to 6,135,978, and in 1881 the number fell to 4,580,303. Of tanned goat and sheep skins, again, the annual export from Madras alone reaches nearly 10,000,000 skins, the numbers for 1880 having been 9,799,900 skins. The total number of sheep and goat skins, raw and rough tanned, imported into the United States in 1880 was 11,731,885, of which 6,332,635 were raw soft stock, 3,353,750 were raw hard East Indian skins, principally from Bengal, and 2,055,500 were tanned East Indian skins.
The following table shows the sources and number of hides imported into the United Kingdom during 1880: –
River Plate, salted hides . 310,745 ,, dry . 31,057 Rio Grande, suited ,, . 178,897 Cape . 36,538 dry . 209,176 Australia 196,140 Brazil 96,240 West Indies 10,584 West Coast of America 15,107 Cape kips and gnus 13,097 North America 5,983 East India kips and buffalo ... 6,707,142 Horse li ides 98,200
The following are the Board of Trade returns of articles connected with tanning imported into the United Kingdom, and exported, during the five years ending 1880: –
