GLASS. 



By acids. Thofe that contain too much borax and arfonlc, 

 though at firil thfv appear very beautiful, fniiekly tarnifli, 

 and l)t'Com» opaque when cxp'jfod to air. By attending to 

 tiiele properties of different tlu\*-=:, phlogiliic or faline, tlie 

 » artill may know how to adjull the proportions of thcfc to 

 fanj, or powdered flints, for the various kind^ of o-lafs. 



iilAS^i, firofioriion of inri-ei'ieiUs in. Different kinds of 

 glafi recpiire different proportions; nor have thefe been pre- 

 cifely afcertained. We lliall liere, and in fubfeqnent articles, 

 flate leveral of the moil ufual and approved mixtures that 

 have been propofed. \\nien filex is melted with twice its 

 weight, or more, of dry carbonated alkali, either potafh or 

 (oda, the relult is a very foit deliquelcent vitreons mafs, 

 always more or lefs opaque, ftrongly alkaline to the tafte, 

 and which, on expofure to moid air, or more fpeedily in 

 water, totally difl'olves into a clear liquor, which is a foln- 

 tion of filex in alkali. When the fame alkali is equal to the 

 filex in weight, or does not much exceed it, the glafs is now 

 tranfparent, but it is ftill foluble in water. It is not till the 

 alkali is diminiflied to about one-half of the weight of the 

 fdex, that the glafs becomes perfeftly hard and infoluble in 

 any corrofive liquors, (the fluoric acid excepted,) and, in 

 ihort, acquires the character of a perfeil glafs. Tliis propor- 

 tion, therefore, of two parts of land to one of alkali, is 

 ufually the datum on which the dofes of the alkalies atlnally 

 ufed are regulated. Thus, if common wood afhes (of which 

 the alkaline part is reckoned at no more than lo per cent.) 

 are employed, loolbs. of thefe would require no more than 

 about 2olbs. of fand. If the befl Spanilh barilla, contain- 

 ing from 45 to 50 per cent, of carbonate of foda, be ufed, 

 an equal weight of fand may be added; but if purified pearl- 

 afn be taken, it will melt down perfetfly twice its own 

 weight of fand. But glaffes compoted merely of pure alkali 

 and land, require a very ilrong lire for their fufion, and are 

 hard, harfh, and difticult to break : they are therefore never 

 ufed alone. As one half the weight of the fand is reckoned an 

 abundant allowance of alkali, it follows of courfe that when 

 litharge, arfenic, borax, or any other fluxes are employed, the 

 quantity of alkali will be proportionally diniiniilied. The fol- 

 •lowing proportions are extraftcd from Macquer's Chemical 

 Dictionary. If a glafs be required that is denfe, fulible, 

 and not faline, one part and a half of red lead or litharge 

 may be mixed with one part of fand, and fufed together: 

 if equal parts of fand and of calx of lead be employed, a 

 glafs fomewhat lefs denfe and harder will be produced : if a 

 glafs be required of very little denhty, only faline fluxes 

 mufl be employed. A glafs of this kind may be compofed 

 of fix parts of fait of tartar, or of potafh, or of purified 

 foda, mixed with eight parts of fand or of flints; or of four 

 parts of any of the above-mentioned alkalies, mixed v.-ith two 

 parts of nitre or of borax, and eight parts of vitriliable earth. 

 When a cryftal glafs is required, which fliall be of an inter- 

 mediate quality betwixt the metallic and faline glalfes, it 

 may be made from a mixture of one part of the above men- 

 tioned falts, one part of calx of lead, and two parts of fand 

 or other vitrifiable earth. By varying the proportion of 

 thefe ingredients, many different kinds of glalTcs may be 

 produced, each of which may be good, if the quantity of 

 each of the fluxes employed be proportionable to its vitri- 

 fying power. 



Gl.^s.s, inj!ruments for nmnufiiauring. Thefe are fubfer- 

 vient to two different purpofes; viz the levigation and mix- 

 ture of the ingredients, and the fufion or vitrification of 

 them. To the former chfs belong horfe or hand-mills, 

 mortars and peftles, flat ftones and mullars, and fearces or 

 Ceves. l"ho other fort of utcnfils are furnaces, witU tlic 



proper iron work, pots for containing the compofition when 

 put into the fire, and iron inflruments for (hifling the matter 

 out of one into the other, in cafe of accidents; and f<>r 

 taking out finall portions, in order to Judge of tlie progrefi 

 of the vitrification, and the qualities of the glafs, &:c. See 

 the following articles. See alfo Glass-Ao///^, Flun.^ce, 

 and Gi,A.s.s-/o/j-. 



Gi.A.ss, fufion of. When tlie ingredients are fekcted and 

 duly proportioned, they arc firil calcined for a longer or 

 fhorter time, jjefore they are put into the glals-pots. Thij 

 operation is called " fritting,'' and is performed either lit 

 fmall furnaces adjoining to the proper glafs-finnace, and 

 heated by the fame fuel after its chief force has been fpciit 

 upon the glafs-pots, or elfe in fmall furnaces or ovens con- 

 llrudled for this purpofe. The ufes of fritting arc, to expcf 

 all moifturc from the ingredients, by wliich tiie glals-pot.i 

 would be endangered ; to difcharge j>art of the carbonic 

 acid from the alkalies and chalk and thus to moderate th,r 

 fwelling in the glafs-pots, and efpecially to caufc an adhe- 

 fion, or commencement of chemical union, between th'; 

 alkali and filex, and metallic oxyds. This operation (hould 

 be performed gradually, and carried to the point of femi- 

 vitrification, in which tlie materials flrongly adiiere, aTid 

 begin to become pally, but are flill opaque and not homo- 

 geneous. This operation ferves alfo to dellroy any carbo- 

 naceous matter. When the ingredients are fufficiently 

 fritted, they are t'u-own with clean iron fhovels through tlie 

 fide-opening of^lhc furnace into the glafs-pots, the fire 

 having been prcvioufly raifed to its greatefl intenfity, lo 

 prevent the furnace from being chilled and to fave time. 

 The pots are charged by two or three fucccffive portions^ 

 the preceding one being thoroughly melted down before 

 another portion is thrown in. When filled, the fide -opening 

 is clofed up with wet cl.iy, excepting a fmall liole for ex- 

 amining the work, which clofure is pulled down when the 

 glafs is well refined and al)out to be worked off. As foon 

 as the frit begins to feel the adiion of the fire in the glafs- 

 pots, which is immediately raifed to its greatefl pitch, it 

 finks down into a foft paily flate, increafing in tena- 

 city till the fufion is complete. However, it is flill 

 opaque, from the rifing of a white porous fcum, known 

 by the name of " fandiver,' or " glafs-gall." This fub- 

 ilancc appears to be a confufed mafs, confifling of all thofe 

 falts contained in common alk.ilies, which readily melt at 

 fomewhat lefs than a glafs-mclting heat, and are either 

 naturally foluble in a confiderable degree, or have little, if 

 any, afiinity for filex, and not uniting in the compofitioH of 

 giafs, but being lighter, rife to the top. Another hetero- 

 geneous fubflance, called " fandiver," is fometimes found at 

 tile bottom of the pots. This is quite different tVom the 

 other, and feems to confifl of a vitrified mafs of arfenic and 

 other impurities. But the fcum, or proper " glafs-gall," 

 is almofl entirely faline. When laded out and cooled, it 

 forms a crumbly mafs, fometimes white, at other times 

 brown and fouled, and flrongly fahne, but not uniform in 

 its compofiiion, being fometimes merely fait, often very bit- 

 ter, probably as common fait or fulpiiat of potalh predomi- 

 nates. It is fo volatile in a ftrong fire, that it is conflantly 

 difperfing frcmi llie furface of the glafs in a deufe vapour, 

 which is firil thick and black, afterwards whiter, and which 

 corrodes the top of ths crucible in its paffage. With long 

 continued fufion it would entirely cfcape in this flate, if it 

 were not fcummcd off with long ladles, and fold to metal 

 refiners as a powerful flux. Abundance of this glafs-gall 

 is attended with one of the greatefl inconveniences to tlie 

 maker of glafs, as it requires a confiderable continuance of 



ftrong 



