G L A

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rafter of mod glaffes, for when opake, it iscrenerally owing to an excefs of fome matter,which, in a fmaller quantity, would only give atinge of more or lefs denfity.

Befides the general acceptation of the termglafs, it is alfo applied more commonly to thatmoft beautiful manufacture of filiceous earth,vitrified by various falts and metallic oxyds,which contributes fo much to the purpofes offcience, to ornament, luxury, and to domefticcomfort.

We thall firft fhortly give the refults of avariety of experiments on Vitrification in general,and afterwards defcribe the manufafture ofglafs.

Vitrification.

An immenfe repofitory of valuable fafts onvitrification is to be found in the voluminousexperiments of Mr. Achard, of Berlin , 1 fromwhich, together with the later experimentsof Kirwan, b Klaproth , 0 Morveau/ and otherchemifts, the following fafts may be deduced.It may be premifed that in all thefe experimentsthe mixtures were firft reduced to fine powder,well mixed, put into a crucible, and expofed tonearly as ftrong a heat as the power of furnaceswill give. Thofe of Achard were ufually in-troduced into the porcelain furnaces of the royalporcelain manufaftory of Berlin ; of Klaproth ,the fame •, of Kirwan and Morveau, in a windfurnace of very powerful draught, the intenfityof which was in moft cafes marked by Wedge-wood’s pyrometer. The effefts produced bythe aftion of the materials on the crucibleitfelf will be prefently noticed.

§ I. Of the Vitrification of Earths alone.

All the earths are of themfelves infufible byany heat except that of a blaft of oxygen gas,as afcertained by Lavoifier. Hence infufibilityin culinary fire is properly given as a charadterof earthy bodies. But various mixtures ofearths alone may be vitrified with more or lefseafe.

Of the binary combinations of earths noneare vitrifiable except thofe in which one of theingredients is lime or barytes (and probablyftrontian), and in equal proportion with theother, and at a very intenfe heat.

Thefe earths therefore agree with the alkaliesin their vitrifying power as well as in other alka-line properties, and like the alkalies this poweris more aftive in diflolving filex than alumineor magnefia.

Silex and lime, in equal parts (according toKirwan) melted at 15°° °f Wedgewood, into a* Mem. Acad. Berlin , for 1779, 1780, 1781. k Mir

mafs between porcelain and enamel, white,glazed at the furface, femitranfparent at theedges, and feebly giving fire with fteel.

Silex and barytes, in equal parts, (by Kirwan)gave at 148* Wedgewood, a hard mafs notmelted, but (by Morveau) melted into a greenglafs exceffively hard and cellular. Two partsof barytes and one of filex gave (by Kirwan) ayellowifh green porous enamel at 150°.

Silex with alumine, or with magnefia fingly,and in any proportion, gave only a flightlycohering crumbly mafs fcarcely altered by thefire except in the parts touching the crucible.

Alumine and magnefia heated with the alka-line earths in any proportions gave only flightlyagglutinated maffes, but would not melt into aglafs or even an enamel.

The alkaline earths mixed together fufedmore readily than any of the above mixtures,forming very perfeft glaffes, but which almoft-immediately begin to attack the fides of thecrucible, and melt down with it in a ftrongfire into a vitreous flag.

With regard to combinations of three or moreearths it would be endlefs to enumerate all thepoflible or aftual experiments. Kirwan deducesfrom thofe which he made the following generalrefults, which indeed might partly be expeftedfrom the known properties of the feveralearths: namely,

No compound of lime, magnefia, and alu-mine, in which the magnefia predominates willmelt at a lefs heat than 160°: and to producecompleat vitrifaftion, the lime (which is pro-perly the flux in this inftance) muft be equal tothe two others. Where either the magnefia oralumine predominate the mafs forms a porce-lain, or imperfeft glafs.

In compounds of lime, magnefia, and filex,if the lime exceeds, perfeft glaffes will beformed, if the filex exceeds, few glaffes will beformed, and never, if the magnefia exceeds.

In compounds of alumine, magnefia, andfilex, the refults are fo far curious, that a porce-lain, and even a perfeft glafs, may be producedin the proportions of 3 filex, 2 magnefia, and 1alumine. The filex therefore here feems toaft as a powerful flux, for when either of thetwo latter exceed, fcarcely any approach tofufion can be obtained.

It follows, therefore, that the more fufibleearths, of thofe here examined, when in mix-ture, are lime, and efpecially barytes, and nextto them filex, and that alumine and magnefiaare remarkably difficult of fufion in mixture,iralogy. * Effays, yoj. 4 - * Jour, Folyiech. p. 3.