LEA

LEA

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being felled in fpring (when the fap has rifen)the bark is ftripped off and piled in large Hacks,protected from the wet by a Ihed, but open atthe fides to ^ admit a free circulation of airthrough it. The bark before ufing is groundinto coarfe powder, and is thrown into pits withwater, by which an infufion of the tan andother, foluble parts is made, which is calledtechnically ooze. The hides previoufly preparedin one or other of the ways above mentionedare then put firft into fmall pits with a veryweak ooze, where they are allowed to maceratefor fome weeks with frequent llirring or hand-ling as it is called. The ftrength of the diffe-rent oozes is increafed gradually, after whichthe half-tanned hides (if of the thick kind in-tended for fole-leather, and which require verycomplete tanning) are put into larger pits withalternate_ layers of ground bark, in fubftance,till the pit is filled, over which a heading ofbark is alfo laid, and the interftices filled upwith a weak ooze to the brim. The hides thusare expofed to the full action of an ooze nearlyfaturated with tan, and fupplied with more ofthis principle from the bark in fubftance, inproportion as the Ikin abforbs that portionalready diffolved, till the tanning is judged tobe complete. This for the heavieft kind of lea-ther requires never lefs than fifteen months.Skin is known to be fully tanned by cutting afmall piece off the edge of the hide, and obferv-ing the change of colour. As far as the tanhas fully penetrated, the colour is of a nutmeg-brown, but the reft is white; and thereforebefore the procefs is complete the upper andunder fides are brown, and a white line orftreak is feen in the middle.

Laftly, when fully tanned the hide is takenout to drain, and ftretched upon a convexpiece of wood called a horfe, on which it isthoroughly fmoothed, and beaten with a heavyfteel pin, or fometimes paffed between ironcylinders, to make it more folid, and at thefame time fuppie, after which it is taken to thedrying-houfe , a covered building .with aperturesfor the free admiffion of air, where it remainstill perfectly dry.

The common calves-fkins require for thewhole procefs of converfion into leather in thisway from two to four months, the thick fole-leather hides from fifteen to eighteen or twentymonths, and a boar’s ftiield can hardly befinilhed in lefs than two years. Leather gainsin weight and improves in quality the longerit is fullered to remain in. the ooze (within cer-

tain limits), and, as it is fold by weight, this isalfo fometimes an object to the tanners, thoughcounterbalanced to a great degree by the lengthof time that muft elapfe before his capital isreturned.

The art is indebted to M. Seguin, a tannerof extenfive bufinefs in France , for the firftaccurate explanation of the rationale of theprocefs of tanning. According to the ancientidea of this procefs the effecb of the infufion ofaftringent vegetables was fuppofed to be littleelfe than mechanical, and that it a fled in con-Jlringing or condenfwg the fibres of the deadIkin, as it corrugates the fkin of the palatewhen tailed, and hence rendered it nearly im-pervious to moiflure and unfufceptible of pu-trefaction. This explanation however did notaccord with the actual increafe of weight whichthe Ikin acquires by tanning, and whichamounts on an average to an increafe of fromone-third to one-fourth of the weight of tiieIkin when dry. M. Seguin, reafoning fromthe circumftance that Ikin before tanning iscompletely refolved by water into a liquid jelly,but is infoluble after tanning, was led to theAmple experiment of adding a folution of fkin(or glue) to an infufion of oak-bark, and foundan immediate precipitate of a thick tough ex-tenfible dun-white matter, ftrongly fmelling oftan, and infoluble in water at any heat, and.which when dry becomes of a dark-brown, co-lour and brittle.

This precipitate is an intimate combinationof gelatin with that part of the vegetable infu-fion which gives the tanning property, and be-ing altogether a peculiar fubftance is denomi-nated tannin or tan, whofe properties will bemore fully defcribed under that article. Thisprecipitate therefore hardly differs from tannedleather in any thing but in wanting the fibrousorganized texture, and what other principlesthe Ikin may have abforbed from the bark-infufion during the maceration of feveralmonths, which a fudden precipitation wouldnot effe£l. Tanning therefore confifts chieflyin a flow and moft intimate combination of ve-getable tan, with the fibre of the Ikin, whichcontinues till the latter is faturated through itswhole thicknefs.'

But oak-bark contains other foluble matter,which certainly alfo enters the texture of thefldn along with the tan, and moft intimatelycombines with it, for Ikin, when it has under-gone the previous preparation already men-tioned, appears to be able to abforb, and when