G L A

GLA

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glafs after mixture are ufually firft calcined fora longer or fhorter time by a feparate operationbefore they are transferred to the glafs-pots.This operation is called fritting , and is per-formed either in fmall furnaces clofe adjoiningto the proper glafs furnace and heated by thefame fuel after the chief force has been fpentupon the glafs-pots, or elfe in fmall furnacesor ovens conllructed for the exprefs purpofe.The ufes of fritting are, to drive oft' all moif-ture from the materials which might endangerthe glafs-pots, to expell part of the carbonicacid from the alkalies and chalk, and thus tomoderate the fwelling up in the glafs-pots, andefpecially to caufe an adhefion or commence-ment of chemical union between the alkali andftlex, and the metallic oxyds. For if the rawmaterials were immediately expofed to the in-tenfe heat of the glafs-pots, the alkali wouldflow thin like water, and the grofler particlesof the fand and the heavier oxyds would fallto the bottom, leaving the alkali above nearlydifengaged, and therefore liable ftrongly to addon the crucibles, and alfo a large portion wouldbe rapidly volatilized to mere wafte. Theglafs materials therefore would thus have anexcefs of fand from the lofs of the alkali, anda portion would remain at the bottom unvit-rified.

Fritting fhould be gradual, and carried tothe point of femi-vitrification, in which thematerials ftrongly adhere and begin to becomepafty, but are {till opake and not yet homoge-neous. It has the further ufe of deftroying anycarbonaceous matter.

When the materials are fufficiently frittedthey are thrown into the glafs-pots with cleaniron {hovels through the fide opening. Thefire is previoufly raifed to its greateft intenfityto prevent the whole furnace from being chilledand to fave time. As the fritted materials aremuch more bulky than when they fall into athin-flowing glafs, the pots receive their fullcharge by two or three fucceflive portions, thelaft-added being always thoroughly melteddown before a frefh charge is thrown, in.When full, the fide opening is clofed up withwet clay, except a fmall hole for examining thework, which clofure is pulled down when theglafs is well refined and about to be workedoff.

As foon as the frit begins to feel the actionof the fire in the glafs-pots which is imme-diately raifed to its greateft pitch, it finksdown into a foft pafty ftate, which graduallyencreafcs in tenacity till a perfect fufion is

effected. It is ftill however opake at firft, owingto the rifing of a quantity of a white porousfcum, the nature of which has engaged muchattention, and is known by the name of fan-diver, or gla/s-gall. This fubftance appears tobe a confuted mnfs, confifting of all thofe faltscontained in common alkalies which readilymelt at fomewhat lefs than a glafs-making heat,and are either naturally confiderably volatile,or have little if any affinity for filex, and donot unite in the compofition of glafs, but beingfuperficially lighter, rife to the top. There isanother heterogenous fubftance alfo called fan-diver, which fometimes is found at the bottomof the pots, and is taken out when the wholeis worked off. The nature of this is very dif-ferent from the other, and confifts apparentlyof a vitrified mafs of arfenic and earthy impu-rities. But the fcum or proper glafs-gall is al-moft entirely faline. When laded out andcooled it forms a white crumbly mafs, fome-times quite white, and at other times brownand fouled, and ftrongly faline, but not veryuniform in its compofition, being fometimesmerely fait, often very bitter, probably as com-mon fait or fulphat of potafh predominate. Glafs-gall is very volatile in a ftrong fire, fo thatit is conftantly difperfing from off the furfaceof the glafs in a denfe vapour, at firft thick andblack, afterwards whiter, which very power-fully corrodes the top of the crucible in itspaflage. If the fufion were continued longenough the whole would be difperfed merely inthis way, but it is generally fcummad off withiron ladles, and fold to metal-refiners as a pow-erful flux. As part of the alkali itfelf certainlyis diilipated by the continuance of fire, partlybefore it can unite with the filex, partly fromthe glafs itfelf, much of the corrofion of thepots muft be owing to this circumftance, andprobably it muft alfo unite in part with theglafs-gall, which renders it flightly deliquefcent.An abundance of this glafs-gall is one of thegreateft inconveniences that the glafs-maker canmeet with, for it requires a confiderable timeof very ftrong heat before the whole can bediffipated, or if the glafs be wrought before itis thoroughly purged of this material it is fullof bubbles, unfound, and has a cloudy gela-tinous appearance. Glafs made with potafh ismore likely to, fuffer from glafs-gall than the-foda glafles, for the potafh-glafles are harderand do not run fo thin as the others, and theglafs-gall arifing from them does not fo eafilydiflipate in the.fire.

During this part of the procefs fmall famples