G L A

GL A

fufed bafalt, long after it had loft the liquidityof fufion. The circumftance of no materialchange occurring in the weight of glafs by thisconverfion into the fibrous ftate, thews incon-teftibly that it cannot be owing either to anything gained during the procefs, nor to anymaterial lofs of the alkali, and this is alfo ren-dered manifeft by its return to the vitreousHate and vitreous qualities when again melted.This too may again be porcellanized in the fameway, and again be melted into glafs, and fo onalternately.

Glafs has been often found cryftallized quiteat the bottom of the pots, and in places whereit has cooled undifturbed for a length of time.

■ This is particularly the cafe in that mafs ofrefufe and fpilled glafs, with afhes, melted dropsfrom the clay of the furnaces and pots, &c.which falls down behind the pots in the fur-nace. x This is generally raked out, but partof it remains, and when the fire is let to go out,and the furnace is become ufelefs, thefe cry-ftallizations are ufually found here. They arefound in all kinds of glafs, but lefs often in thefine flint and the very faline glafles, than in thecoarfer green and crown glafles.

* * #

Of Coloured Glasses.

The metallic oxyds when mixed with any ofthe glafles, diflove in them with eafe at a melt-ing heat, and always change the colour moreor lefs, fometimes producing very beautifulcompounds, which when well prepared, havea luftre and richnefs of colour ftrongly refem-bling that of the natural gems, though in aninferior degree. The bufinefs of making thefecoloured glafles or artificial gems, is carried onto a very great extent in the manufacture of avariety of ornaments, and though much of tiremanagement remains a fecret in the hands ofthe artifts, a good deal of valuable matter hasbeen made public by the labours of Neri, aKunckel, b Margraaf, c Fontanieu, d and manyother practical chemifts.

It is very eafy for any perfon at all ufed tochemical experiments to repeat moft of thefein the fmall way, with fufficient fuccefs tofatisfy himfelf of the leading facts, though forthe purpofe of manufacture more care in thechoice and preparation of the materials, and agreater length of time in the melting part is re-quired than moft: experimenters choofe to bellow.

It may be premifed too that the art of makingcoloured Enamels is eflentially the fame as thatof coloured glafles, the chief difference being

* Dartigucs J. Fhyf, tom. 59. * Art. de la Verrerie.

that in the former cafe the ground or vitreousfubftance that receives the colour is an opakeenamel glafs, and in the latter a clear tranf-parent glafs. The way of preparing the enamelground is defcribed under that article. Thecolouring power of the metallic oxyds is alfoin many inltances much affected by the degreeof heat to which they are expofed, and to theother ingredients with which they are mixed,and hence arife a good many precautions andniceties of management, many of which are onlyknown to the practical artift. There feems goodreafon to fuppofe that much of this differencedepends on the degree of oxygenation in whichthe colouring oxyd is left alter the action offire, or of the other ingredients. The pheno-mena that occur in the ufe of manganefe havealready been defcribed, and fomething fimilarhappens in ufing the oxyds of iron and filver.Sometimes a metallic oxyd may be fo nearlyreduced to the reguline ftate, as not to be per-fectly foluble in the glafs, but only fufpendedin it. This happens occafionaliy with the oxydof copper, which when perfectly oxydated givesa fine blueifh-green, but when nearly in themetallic ftate produces a brown-red and notperfectly tranfparent glafs. There is alfo amutual aftion of the oxyds upon each other,fo that the glafles in which oxyd of lead enters,will not receive a red colour by iron, an efteCtwhich is not produced with the merely alkalineglafles. Much remains to be done on this verycurious and entertaining fubjeCt of the colour-ing power of metallic oxyds, and it is only by awell-conduCted fcientific feries of experimentsthat the perplexing intricacy of many of thereceipts of the articles on this fubjeCt can bereduced to certain rules.

In making coloured glafles to refemble artifi-cial gems, the glafs which is to receive thecolour (which is often called cryjlal) ought tounite the qualities of great purity, luftre, andhardnefs, together with a fufficient fufibilityto melt at a moderate heat thofe oxyds thatare in danger of being decompofed by a hightemperature. A vaft variety of receipts have beengiven for fuch a glafs, and it appears that feveralforts are actually in ufe according to the priceand object of the manufactured article. Theglafles that poflefs the greateft luftre and are atthe fame time eafily fulible, are unqueftionablythofe in which the oxyd of lead enters verylargely, and it appears that many of them arelittle elfe than oxyd of lead vitrified with amuch ftnaller proportion of filex than is ufed

* Mem, Acad. Berlin, 1780. i £ucycl. Method.