TANNING. 



which loofeiis the epidermis, and renders the hair and other 

 extraneous matter eafy of feparation from the true (kin. 

 Tiiis is efFefted by extending the hide on a wooden horfe or 

 beam of a convex form, and fcraping it witli a large two- 

 handldd knife, called ■i.fejhing-kr.^fi:, which is bent, to fuit 

 tlie convexity of the beam. 



The hides are then immerfcd in a pit containing water 

 flightlv impregnated with fulphuric acid. This operation, 

 which is called raijing, by diftcnding the pores and fwelling 

 the fibres, prepares the liide for the reception of the tannin, 

 and renders it more fufceptible of its aftion. 



X^Tien the hides are fufficiently raifed, they are removed 

 into a pit, in which they are lain fmooth with a ilratum of 

 oak bark ground to a coarfe powder between each. 



The pit is then filled with the tanning lixivium or ooze, 

 prepared from oak bark and water, and the hides remain 

 a month or fix weeks without being moved. At the end of 

 this time, the tajining prmciple being exhaufted, the ooze 

 and fpent bark are taken out of the pit, and tiie hides put 

 in again, ftratified with frefh bark, and covered with frefli 

 ooze as before. Here they remain about three months, 

 ■when the fame procefs is repeated, at about the fame inter- 

 vals, three feveral times or mor.e, according to the ftrength 

 of the lixivium and the fubftance of the hides. When 

 fuf&ciently tanned, they are taken out of the pit, hung up 

 in a flied to dry gradually, and being comprefied with a 

 fteel inftrument, and beaten fmooth to render them firm and 

 denfe, the operation is complete ; and having been numbered, 

 weighed, and ftamped by the excife officer, to afcertain the 

 amount, and denote the payment of the duty (which will be 

 noticed at the end of this article), they are ready for fale, and 

 are termed butts or backs. Thefe form the thickeft and mofl 

 fubftantial fole leather for very llrong (hoes, and are chiefly 

 intended for exportation. 



Crop hides are thus manufaftured. The horns having 

 been removed, the hides are immerfed in pits containing a 

 mixture of lime and water, where they remain three or four 

 days, being occafionally moved up and down, that each 

 part may be uniformly expofed to the aflion of the lime- 

 water. They are then taken out of the lime-pits, and the 

 hair and other extraneous matter being fcraped off on a 

 wooden beam, as before defcribed, are wafhed in water, to 

 free them from the lime and filth adhering. They are now 

 immerfed in a weak ooze, and by degrees are removed into 

 other pits, containing folutions gradually increafing in ftrength, 

 during which time they are taken up and put down (techni- 

 cally termed handling) at leaft once in every day, that all 

 parts of the hide may be afted upon by the tanning principle 

 equally and uniformly. This is continued for about a month 

 or fix weeks, when they are put into other pits with ftronger 

 ooze and a fmall portion of ground bark ; from whence, as 

 tlie tannin becomes exhaufted, they are removed to other 

 pits in regular fuccelTion, with frefh ooze and frefh bark, for 

 two or three months. 



At the end of this period, the hides are put into larger 

 vats, called layers, in which they are ftratified, or lain fmooth, 

 in a lixivium of greater ftrength, and with a larger quantity 

 of ground bark between each fold. Here they remain about 

 fix weeks, when they are taken up and relaid in the fame 

 manner, with frefli bark and ftrong ooze, for two months. 

 This procefs is repeated, with little variation, once, twice, 

 or thrice, at the difcretion of the manufafturer, till the 

 hides are thoroughly tanned ; when they are taken out of 

 the pits, fufpended on poles to dry, and being comprefied 

 and fmoothed, nearly in the manner before defcribed, arc 

 called crop hides, and form the principal part of the fole 

 leather which is ufed in England. 



Vol. XXXV. 



The procefs ot tanning_/f/«r (cilvcs, feals, &c.) is fomc- 

 what different from hides. They are continued in tlie lime- 

 pits for ten or fifteen days ; they arc then depilated and 

 wafhed in water, after which they arc immerfed in an in- 

 fufion of pigeon's dung, called a graiuer, having the pro- 

 perty of an alkali. Here they remain for a week or ten days, 

 according to the ftatc of the atmofphere and other circum- 

 ftances, during which time they are frequently handled, and 

 fcraped on both fides upon a convex wooden beam. This 

 fcraping, or ivorling, as it is termed, with the aftion of the 

 graincr, helps to difcharge all the lime, oil, and faponaceous 

 matter, and renders the dun foft and pliant, fitted to imbibe 

 the tanning principle. They are now removed into pits con- 

 taining a weak folution of bark, where they undergo nearly 

 the fame procefs of haHdling, &c. d.^ crop hides ; but they 

 are feldom ftratified in layers ; and the time occupied in tan- 

 ning them is ulually from two to four months, according to 

 their nature and fubftance. The (Itins are then dried, and 

 fold to the currier, who dreffes and blacks them for the 

 upper leathers of boots and ftioes, for harnefs, and various 

 other purpofes. 



The light and thin fort of cow-hides and horfe-hides lui- 

 dergo nearly the fame procefs in tanning as calf-(kins, and 

 are applied to Cmilar ufes. 



Thefe procelFes are fuch as are now commonly praAifed, 

 vaiying, however, with the nature and condition of the pe- 

 culiar kind of hides and (Icins — with local habits and cir- 

 cumftances — and with the (kill and experience of the manu- 

 fafturer. The greateft defeft in the common method* ap- 

 pears to exift in the means of extrafting the tannin from the 

 bark. Cold water is chiefly ufed for that purpofe ; but 

 fome perfons conceiving that this does not entirely cxhaull 

 the tanning principle, fubjcdl the bark, as before obferved, 

 to the aftion of boiling water, &c. If, however, as fir Hum- 

 phrey Davy has ftatcd, the extraft as well as the tannin 

 combines with the fkin, the cxtraftion of the tannin by lieat 

 would tend to oxygenate the former, and render it infoluble 

 in the liquid. 



The late ingenious Dr. Macbride. of Dublin invented 

 and publifhedin 1778 a new method of tanning, the leading 

 feature of which was the ufe of lime-wafer, which he con- 

 ceived would extraft the virtues of oak bark more com- 

 pletely than plain water. 



It has, however, been obferved, that both natural and artifi- 

 cial tannin form compounds with the alkalies and the alkaline 

 earths, and thefe compounds are not decompofable by flcin. 

 Lime forms with tannin a compound not foluble in water, 

 and therefore Dr. Macbride's fyftem is founded on erroneous 

 principles, as fo much of the tannin as combined with the 

 lime contained in the water was loft. It was alfo found, by 

 the praftical experience of tanners, that this method was in 

 all refpecls injurious rather than beneficial ; and as it has 

 long been univerfally rejefted, it is not necelTary to enter into 

 the detail. The reader who is defirous of further information 

 on this point, may refer to Phil. Tranf. vol. Ixviii. part i. art.8. 



The application of fome new and cheap fubftitute for 

 oak bark has been long a defideratum in tanning. Cate- 

 chu, the fubftance we have fpokcn of under the article 

 Tannin, has been recommended, and its powerful tanning 

 properties have been fully afcertained by experiment and 

 aftual praftice : but it is not likely that tlie article can be 

 procured in fufficient quantity, or at an adequate price, 

 for the purpofes of manufafture. The bark of elm, wil- 

 low, larch, and other trees, together with vallonia (the 

 acorn of a peculiar fpecies of oak in Turkey), have all been 

 employed in tanning with confiderablc effedl. 



The greateft hope which chemical fcience prcfents, is the 

 M pf^* 



