TUR

TUR

C 451 )

and proportion of refin, empyreumatic oil andacid which it contains, according to the age ofthe tree, the foil on which it grows, the partfelefted, the management of the heat, &c.

The extradition of tar from pine-trees is veryantient, being defcribed by Theophraltus, Diof-corides, and other old authors, and the methodof proceeding was extremely limple. Verylarge ftacks were made of billets of pine andcovered with turf to prevent the volatile partsfrom being diffipated. They were then kindledand lufFered to burn with a low fmotheredflame, during which the tar melted out by theheat, flowed to the bottom of the flack, and ranout by a fmall channel cut for the purpofe. Avery large proportion of the tar actually madein Norway , and the other Baltic countries, isprepared in this rude manner. The flacks arebuilt upon the Hope of a hill, covered withmofs and turf, kindled, and the tar that oozesout is collefted and put into barrels.

But a more economical and fcientific methodis praftifed in France and Switzerland , which isto heat the wood in large brick ovens built forthe purpofe, whereby the wood is charred muchmore equally, and the tar is of a more uniform,and probably a better quality. The followingis the method of proceeding in the Valais : thepines are previoufly felled in the preceding yearthat the wood maybe dry enough when wanted,and the outer bark and twigs being ftripped off",the reft of the tree is cut up into billets of tole-rably equal fize. The oven is built of {toneor brick in the form of an egg (landing on itsfmall end. The floor is made either of a Angle ftone fcooped out into a hollow, or of feveraljoined very accurately. On one fide of it, aboutfive inches above the lowed part, is a hole inwhich a large gun -barrel is thruft, which fervesto convey oft' the liquid tar as it collects. Alarge iron grate is laid at the bottom of theoven. The largeft of thele ovens are aboutten feet high, and live or fix feet in the largeftdiameter. To charge the oven, bundles ofthefe billets are thrown in, and the wood fpreadas evenly as poffible, filling the interftices withthe chips, till it nearly reaches the top. Thewhole is then covered with a layer of chips,and the top of the furnace is clofed with flatftones heaped one upon the other, graduallylefiening the opening, and forming a kind ofvaulted chimney, the mouth of which is fouror five inches acrofs. The dry chips at topof the furnace are then fet on fire, and the heatfpreads downwards till the whole of the chargeis judged to be fufficiently kindled'. The chim-

ney is then entirely clofed with a large ftone,and wet earth is heaped on the ftones at top,and thrown on wherever the fmoke burfts outtoo ftrongly. The melting now begins, the tarfalls down to the bottom, fills the hollow of thefloor (which laft detains any bits of wood andother impurities) and runs off through die gun-barrel into calks placed to receive it. Some(kill is required in managing the fire, whichmull fometimes be refreihed by letting in afmall draught of air through fmall holes left forthe purpofe in the (ides of the kiln. When theprocefs is over, the wood is found compieatly -charred, and is taken out, and the oven afterbeing cleared out is again filled. It is foundthat the red wood and knots, which are thericheft in refin, will yield about a fourth oftheir weight of tar, but the general average ofproduft is about 10 or 12 per cent, of theweight of the whole charge. After each pro- :.cefs a quantity of lamp-black is collected beneaththe ftones that form the vault of the temporarychimney.

This latter fubftance is alfo another productfrom the pine, and in the large way is pro-cured by a different procefs, as defcribed underthe article Carbon .

A fubftance fomewhat refembling tar, calledBrai-gras, and much ufed in the French portsfor careening (hips, is made in the followingway. The oven above defcribed is chargedwith alternate layers of chips of green wood,'and billets of dry, and all the refufe matter ofturpentine, the draw through which it is (train-ed, and the like. Over the whole is laid aftratum of brai-fec, or rofin, and the gun-barrelpipe is (topped up, and not tapped till the wholeof the wood is reduced to charcoal. The vaultof the oven is alfo covered more carefully afterthe charge is fufliciently kindled, and the wholeprocefs is carried on more (lowly. The heatof the fire melts the rofin at top, which mixeswith the refinous fap, and the whole col leftsinto a dark refinous liquid at the bottom. Whenfufficiently cooled it is drawn off and barrelled.The brai-gras is of an intermediate confidence,between tar and rofin.

Pitch. The fubftance called Pitch in thiscountry, is Amply tar, infpiflated to the requifitedegree by boiling.

It does not appear, however, that the French have this precife preparation, as the fubftancecalled Poix is made either by melting togetherdue proportions of rofin with tar, or elfe by-filling a kind of oven with various refufe mat-ters from the turpentine, fuch as the ftraw3 L 2