TAN 



TAN 



cipitated is heated to ebullition, the tannogelatin is re- 

 dissolved ; tannic acid also precipitates albumen and 

 starch. 



When dried at 212 tannic acid consists of 



Eighteen equivalents of carbon . . 108 

 Five equivalents of hydrogen . . 5 

 Nine equivalents of oxygen . . 72 



Equivalent . 185 



With Three equivalents of water . 27 



When exposed to a temperature of 240, the water is 

 expelled. 



Tannic acid combines with the alkalis to form salts, 

 which are called tannatfs, and it precipitates most me- 

 tallic oxides from solution. The salts of protoxide of iron 

 suffer no change when a solution of tannic acid is added 

 to them ; but by exposure to the air a deep bluish-black 

 precipitate is formed. Tannate of peroxide of iron, formed 

 by the action of the acid on a persalt of the metal, is the 

 nl writing-ink, and is a black pulverulent precipitate. 



TANNIN, ARTIFICIAL. It has been shown by Mr. 

 Hatchett, that when powdered charcoal has been digested 

 for a considerable time in dilute nitric acid, it is dissolved, 

 and a reddish-coloured liquid is obtained, which by care- 

 ful evaporation yields a brown glossy substance, amount- 

 ing to about 120 parts from every 100 of charcoal em- 

 ployed. 



The properties of this substance are that its taste is 

 astringent and bitter, is soluble in water and in alcohol, 

 and forms with a solution of gelatin an insoluble precipi- 

 tate, consisting, according to Mr. Hatchett, of 36 of tannin 

 mid 04 of gel.it in in 100 parts. Sulphuric acid and hydro- 

 chloric aeid, when added to a solution of artificial tannin, 

 -ion brown-coloured precipitates, which are soluble in 

 hot water ; the alkalis combine with this tannin, and it 

 forms a precipitate of difficult solubility when added to 

 lime, barytes, or strontia water, and also with most metallic 

 solutions. These precipitates are of a brown colour; un- 

 like natural tannin, the artificial resists the action of nitric 

 acid. 



When camphor and various resins, as shell-lac, benzoin, 

 and dragon's blood are digested in sulphuric acid till it 

 becomes black, a variety of artificial tannin is procured; 

 when the blackened acid is poured into water, a black 

 powder is deposited, which, by digestion in alcohol, fur- 

 nishes a brown matter soluble in water, and forming an 

 ; nble compound with gelatin. 



Although in certain respects the above artificial sub- 

 stance agrees with tannic acid, yet the late discoveries as 

 to the real nature of this principle tend to the opinion 

 the natural componnd is essentially different from the 

 artificial. 



TANNIN, PURE, or TANNIC ACID, Medical Pro- 

 firrii<:\ if. This substance in combination with extractive 

 een long known under the name of tannin, and re- 

 coirnixed as the active principle in almost all astringent 

 : ablcs. [ASTRINGENTS.] As many of these are 

 it'ul in restraining excessive discharges, whether 

 bloody or otherwise, it was conjectured that the pure 

 principle would be yet more efficacious than when in a 

 of combination. Accordingly it has been adminis- 

 tered in some passive haemorrhages, chiefly from the 

 ^ and the bronchial tubes. To effect any good it re- 

 quires to be given for several days in small doses. It is 

 with difficulty absorbed into the circulation, being with 

 great reluctance taken up by the lactcals, and producing 

 '.Teat, constipation, from its direct astringent action 

 over the intestinal canal, with which it is brought into 

 contact. Tannic acid has been recommended in cases of 

 incurable organic diseases affecting (he uterus, accom- 

 panied itli wasting discharges. These it may for a time 

 moderate, but the constipation induced ne\er fails ulti- 

 mately to n^irravate the disease and discomfort of the 

 patient. There is little therefore to induce practitioners 

 to employ it. 



TANNING is the process of converting the skins of 

 animals into leather, by effecting a chemical combination 

 between the gelatin of which they principally col 

 and the astringent, \egetable principle called tannin. 

 [BARK, vol. iii.. p. 4.">f; ; LEATHER, vol. xiii., p. :)7!) ; 

 and the preceding chemical articles on TANNIN.] The 

 object of the tanning process is to produce such a che- 

 mical change in skins as may render them, as observed by 



Dr. Ure, unalterable by the external agents which tend to 

 decompose them in their natural state ; and, in connection 

 with the subsequent operations of dressing, or currying, to 

 bring them into a state of pliability and impermeability to 

 water which may adapt them for the many useful pur- 

 poses to which leather is applied. Similar effects are 

 produced by forcing oil or grease into the pores of the 

 skin, or by preparation with alum ; processes which mav 

 be briefly noticed in connection with the more immediate 

 subject of this article. 



The preparation of skins by tanning or other analogous 

 processes has been practised from the earliest times ; and, 

 although it has engaged the attention of several scientific 

 men, and has been the subject of many curious experi- 

 ments, it has received less modification from recent im- 

 provements in chemical science than many other manu- 

 facturing processes. Several plans which have been sug- 

 gested with a view to expediting the process, which, on 

 the old system, is a very tedious one, have been found to 

 deteriorate the quality of the leather, and have therefore 

 been wholly or partially abandoned ; and others, which 

 appear to be more successful, are as yet adopted by a few 

 manufacturers only. One of the probable causes of this 

 comparatively slow progress of improvement in the leather 

 manufacture is suggested in an interesting article on 

 ' Tanning,' in the seventh edition of the ' Encyclopaedia 

 Britannica,' the author of which observes that, owing to 

 the slow turning of money in consequence of the length 

 of time occupied in tanning the heavier kinds of skins or 

 hides, the tanner ' must have capital enough to pay for 

 twelve months' hides, bark, &c., labour, and contingent 

 expenses, besides keeping a stock of leather ; and, when 

 his capital has been turned at the end of twelve or more 

 months, it must pay him, in one single profit, the in'erest, 

 &c. of twelve months.' ' This,' he proceeds to say, ' has 

 confined the trade to a few wealthy individuals, who look 

 upon tanning as an investment for capital rather than as a 

 business which might be improved by science ; and, being 

 in comfortable circumstances, they are not driven to per- 

 sonal exertion and close application, which would be 

 required of less wealthy tradesmen.' ' It is,' he adds, 

 'from these circumstances, that tanning has been more 

 stationary than any other manufacture, and that the few 

 improvements which have been made in it have not been 

 made by tanners.' 



The larger and heavier skins operated upon by the tan- 

 ner, as those of bulls, buffaloes, oxen, and cows, are tech- 

 nically distinguished as hides; while the name skins is 

 applied to those of smaller animals, as calves, sheep, and 

 goats. The process necessary to convert hides into the 

 thick hard leather used for the soles of boots and shoes, 

 and for similar purposes, will be first noticed. The hides 

 are brought to the tanner cither in a fresh state, when 

 from animals recently slaughtered, or, when imported 

 from other countries, dried or salted, and sometimes both, 

 for the sake of preserving them from decomposition. In 

 the former case the horns are removed, and the hide is 

 scraped to cleanse it from any small portions of flesh or 

 fatty matter which may adhere to the cutis ; but in the 

 latter it is necessary to soften the hides, and bring them 

 as nearly as possible to the fresh state, by steeping them 

 in water, and repeated rubbing or beating. After this 

 the hair is removed ; sometimes by steeping the hides for 

 several days in a solution of lime and water, which has 

 the effect of loosening the hair and epidermis, or outer 

 skin ; and sometimes by suspending them in a close 

 chamber called a smoke-house, heated a little above the, 

 ordinary temperature of the atmosphere by means of a 

 smouldering fire ; in which case the epidermis becomes 

 loosened by incipient putrefaction. In either case, when 

 the hair and epidermis, or cuticle, are sufficiently loosened, 

 they are removed by scraping with a curved knife, the 

 hide being laid upon a convex bench, or ' beam.' The, 

 hides are prepared for the actual tanning, or immersion in 

 a solution of bark, by steeping them for a few days in a 

 pit containing a sour solution of rye or barley flour, or in 

 a very weak menstruum consisting of one part of sulphuric 

 acid mixed with from five hundred to a thousand parts of 

 water. By this process, which is called ' raising,' the pores 

 of the hides are distended and rendered more susceptible 

 of the action of the tan, and the substance of the skin is 

 apparently increased ; but, as the process does not add to 

 the gelatin of the skin, a hide which is much thickened by 



