G L A

G L A

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gredients has been already explained. Verydifferent proportions will alfo produce a fineglafs of fimilar quality, and in particular thequantity of lead may be much encreafed whichnaturally gives a yellow tinge but which themanganefe counteracts. The following com-pofition for a fine cryftal. glafs is given byLoyfel: ioo pounds of white fand, 80 to 85 ofred oxyd of lead, 35 to 40 of pearlafh, a to 3of nitre; and one ounce of manganefe. Thefpecific gravity of this glafs and of the commonLondon flint glafs is about 3.2.

The oxyd of lead is fo abundant in this glafsthat it may be partially reduced in a very cu-rious manner. If a tube of it be made red-hot and hydrogen gas paffed through, the wholeinner Surface becomes covered with a half-bril-liant black coating owing to the reduced lead,and moifture appears at the further end. Thiswas difcovered by Dr. Prieftley.

Crown-glafs is the name given to the bell fortof window glafs, the compofition of which va-ries confiderably, but it differs effentially fromthe lall in containing no lead nor any metallicoxyd except manganefe and fometimes oxyd ofcobalt in minute dofes, for the foie purpofe ofcorre&ing the natural colour and not as a flux.This kind of glafs therefore is much harder andharfher to the touch than the flint glafs, butwhen well made is a very beautiful and perfectarticle. The compofition is fand, alkali, eitherpotafli or foda, the vegetable afhes that containthe alkali, and generally a fmall portion oflime. A fmall dofe of ari'enic is often added tofacilitate the fufion. Zaffre or the oxyd ofcobalt with ground flint is often ufed to cor-rect th* dingy yellow which the inferior kindof crown-glafs naturally acquires, and by ad-ding the blue, natural to glafs coloured withthis oxyd, to convert the whole into a foft lightgreen. The green hue thus given is veryflight and not difagreeable to the eye, and ishardly perceived unlefs the light pais througha great thicknefs of glafs, as happens when apiece is held up edgeways. The quantity ofzaffre neceflary for this effect is extremelyfmall, one ounce being fufficient for 1000 lbs.fo that it is only Toioo of the whole. It needhardly be added that when the fand, alkali,and lime, are very fine, and only thefe ingre-dients are ufed, no zaffre or corrective of badcolour is required. A great quantity of frag-ments and refufe pieces of glafs is alwayscollected during the working, and thefe areadded again to the next fufion, but as glafs

always lofes alkali by the long continuanceof fire and becomes thereby harfher and lefsfufible, too great a portion of thefe refufe bits(which of courfe undergo twice as long afufion as the raw materials) will very fen-fibly alter the quality of the glafs. As how-ever they confifi of glafs already refined, thepot which contains much of them is muchfooner brought to a working ftate, as it con-tains lefs glafs-gall and impurities. Thefe frag-ments of glafs are reduced to grofs powder bybeing heated red-hot, and immediately plungedinto cold water, which fplits them in everydirection and enables them to be readily brokendown. They fhould not conftitute much morethan a third of the whole compofition. A veryfine glafs of this kind may be made by 200 partsof pretty good foda, 300 of fine fand, 33 of lime,and from 250 to 300 of the ground fragments ofglafs. *

The manufacture of the common windowglafs though made by blowing, is carried onin a confiderably different manner from thatof the common flint-glafs articles, as the ob-jeCl is to produce a large flat very thin plate ofglafs, which is afterwards cut by the glaziers’diamond into the requifite fhape. The ftepsof the procefs could not be underftood by meredefcription, but it may juft be mentioned thatthe workman firft takes a very large mafs ofglafs on the hollow iron rod, and by rolling iton an iron plate and Twinging it backwards andforwards caufes it to lengthen by its own weightinto a cylinder, which is then made hollow byblowing with a force of breath which only thofethat are ufed to the bufinefs can command, andis brought out to the requifite thinnefs. Thehollow cylinder is then opened by holding it tothe fire, which by expanding the air confinedwithin it (the hole of the iron rod being flop-ped) burfts it at the weakeft part, and when{till foft it is ripped up through its whole lengthby iron fhears, opened out into a flat plate andfinilhed by annealing as ufual.

Common green bottle glafs is another kind,which indeed is by no means uniform in itscompofition, but is made almoft entirely offand, lime, and fometimes clay, alkaline afhesof any kind according as cheapnefs or conve-nience diredt, and more efpecially of kelp inthis country, of barilla, varec and the othervarieties of foda in France , and of wood-afhesin many parts of Germany , and the like. To thistoo is fometimes added even the earth remain-

ing from faline alhes, after the alkali and falts1 Encyclo, Method.

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VOL. I.