GLASS. 



it to boil four, five, or fix days ; which done, fee whether 

 it have mangancrc enough ; and if it be yet greeniHi, add more 

 manganefe, at difcretion, by little and little at a time, taking 

 care not to overdofe it, becaufe the manganefe inclines it to 

 , a blackifli hue. ThcB let the metal clarify, till it becomes 

 of a cl<'ar and fhining colour ; which done, it is fit to be 

 l)lown, or formed info veflels at pleafure. 



Class, Flhil, as it is called in our country, is of the 

 fame general kind with that which in other places is called 

 cryRal ghifs. It has this name from being originally made 

 with calcined flints, before the ufe of the white fand was 

 underftood ; and retains the name though no flints are now 

 ufcd in the compofition of it. This flint glafs diflers from 

 the other, in having lead for its flnx, and white fand for its 

 body ; whereas the fluxes ufcd for the cryllal ghfs arc falls 

 orarfenic, and the body confills of calcined flints, or white 

 river pebbles, tarfo, or fuch ftones. This glafs, on account 

 of the quantity of litharge, which enters into its compoli- 

 tion, is the ht.avicft, the moil brilliant, the fofteft and muit 

 eafy to work, and alfo the moll expenfive. It is that fine 

 glafs, of which the co;nmon and moil valuable articles of 

 white glafs in domcflic or ornamental ufe are manufactured ; 

 and befides, many optical iu'.lruraents are made of this fub- 

 ftance. To the white fand and lead a proper proportion of 

 jiitre is added, for the purpofes fpecified in a former part 

 of this general article, and alfo a fmall quantity of manga- 

 nefe, and in fome works they ufe a proportionable quantity 

 of arfcnic to aid the fluxing ingredients. The mod perfetl 

 kind of flint glafs may be made by fufing with a very ilrong 

 fire a hundred and twenty pounds of the white fand, fifty 

 pounds of red lead, forty pounds of the bell pearl-aflies, 

 twenty pounds of nitre, and iive ounces of mr.nrrranefe. 



From others we have the following compofition for 

 glafs of this kind, faid to be of the bell quality, I'/'s. 120 

 parts of line clear white fand, 40 of pearl-afhes well puri- 

 iied, 35 of litharge or minium, 13 of nitre, and a fmall 

 quantity of black oxyd of manganefe. 



The following compohLion for a fine cryilal glafs is given 

 by Loyfel ; ico pounds of white fand, 80 to 85 of red 

 oxyd of lead, 35 to 40 of pearl-afli, 2 to 3 of nitre, and one 

 ounce of manganefe. The fpecific gravity of this glafs, and 

 of the common London flint-glafs, is about 3.2. 



Another compofition of flint glafs, which is faid to come 

 nearer to the kind now made, is the following : a hundred 

 and twenty pounds of fand, fifty-four pounds of the bell 

 pearl-alhes, thirty-fix pounds of red lead, twelve pounds of 

 nitre, and fix ounces of manganefe. To either of thefe a 

 pound or two of arfcnic may be added, to increaie the flux 

 of the compofition. A clieapcr compofition of flint glafs 

 may be made with a hundred and twenty pounds of white 

 fand, thirty-five pounds of the bell pearl-afhes, forty-pounds 

 of red-lead, thirteen pounds of nitre, fix pounds of arlenic, 

 and four ounces of manganefe ; or, inllead of tiie arlenic, 

 may be fubllituted fifteen ])ounds of common fait; but this 

 will be more brittle than the other. The cheapell compofi- 

 tion for the worll kind of flint-glafs, confills of a hundred 

 t.ad twenty pound.-; of white fand, thirty pounds of red-lead, 

 tv.-euty pounds of the heil pearl-aflies, ten pounds of nitre, 

 fifteen pounds of common fait, and fix pounds of arfcnic. 

 The bell German cryilal glafs is made of a hundred and 

 twenty pounds of calcined flints, or white fand, feventy 

 pounds of the bell pearl-alhes, ten pounds of falt-petre, half 

 a pound of arfcnic, and five ounces of manganefe. And a 

 clieapcr compofition is formed of a hundred and twenty 

 pound.-: of calcined flints, or white fand, forty-fix pounds of 

 pearl-allies, feven pounds of nitre^ fix pounds ot arfcnic, 

 ar.d five ounces of manganefe. 



A glafs, much harder than any prepared in tlic commoB 

 way, may be made by means of borax in the following 

 method : take four ounces of borax, and an ounce of fine faridj 

 reduce both to a fubtile powder, and melt them together 

 in a large clofe crucible fet in a wind-furnace, keeping up 

 a Ilrong fire for half an hour ; then take out the cruciblv, 

 and when cold break it, and there will be found at the 

 bottom a pure hard glafs, capable of cutting common glafs 

 like a diamond. This experiment, duly varied, may lead 

 to feveral uleful impi-ovcments in the arts of glafs, enamels, and 

 faftitions gems, and fliews an expeditious method of making 

 glafs, without any fixed alkali, which has been generally 

 thought an efl'ential ingredient in glafs ; and it is not yet 

 known whether calcined cryilal, or other fubilanccs, bring 

 added to this fait inllead of fand, it might not make a glaU 

 approaching to the nature of a diamond. Shaw's Lectures, 

 p. 426. 



Gl.^ss, Croivn, is the bed fort of window-glafs, and differs 

 from the flint-gUifs in containing no lead, nor any metallic 

 oxyd, except manganefe, and fonietimts oxyd of cobalt, in 

 minute dofes, not as a flux, but for correcting the natural 

 colour. This glafs is much harder and liarflier to the touch 

 than the flint-glafs ; but when well made it is a ver)- beautiful" 

 article. It is compounded of fand, alkali, either potalh 

 or foda, the vegetable a flies that contain the alkali, and 

 generally a fmall portion of lime. A fmall dofe of arfcnic 

 is often added to facilitate the fufion. ZafTre, or the oxyd 

 of cobalt, with ground flint, is often ufed to corredl the- 

 dingy yellow of the inferior fort of crown-glafs, and by 

 adding the blue, natural to glafs coloured with this oxyd, 

 to convert the whole into a foft light green. One ounce of 

 zafPre is fufficient for looolbs. But when the fand, 

 alkali, and lime are very fine, and no other ingredients arc 

 ufed, no zafl're nor corrertive of bad colour is required. A 

 very fine glals of this kind may be made by 2co parts of 

 pretty good foda, 300 of fine fand, 33 of lime, and from 

 250 to 300 of the ground fragments of glafs. We had. 

 formerly in London two kinds or crown glafs, dillinguillied 

 by the places where they were wrought ; ^'/^. i. Ratcrt/T 

 cro-zun ghfs, which is the beft and clearelt, and was firtl made 

 at the Bear-garden, on the Bank-fide. Southwark, but fince 

 at Ratcliff : of this there are twenty-four tables to the cafe, 

 the tables being of a circular form, about three feet fix inches. 

 in diameter. 



2. Lamlilh croivn glafs, which is of a darker colour tlian 

 the former, and more mchning to green. The following 

 compofition has been recommended for the bell window 

 or crov>n glafs, vi%. white fand, fixty pounds ; of purified 

 pearl-allies, thirty pounds ; of lalt-petre, fifteen pounds ; 

 of borax, one pound ; and of arfenic, half a pound. If tl^c 

 glafs fliould prove yellow, manganefe mull be added. A 

 cheaper compofition for window glafs confills of fixty pounds, 

 of wliite fand, twenty-five pounds of impurified pearl-alhes,. 

 ten pounds of common fait, five pounds of nitre, two pounds 

 of arfenic, and one ounce and a half of manganefe. The 

 common, or green window glafs, is compofod ot fixty pounds 

 of white fand, thirty pounds of unpurified pearl-alhes, ten 

 pounds of common fait, two pounds of arfenic, and two. 

 ounces of manganefe. But a cheaper compofition for this. 

 purpofe, confills of a hundred and twenty pounds of the 

 cheapell white fand, thirty pounds of unpurified pearl-aflies, 

 fixty pounds of wood-aflics well burnt and fitted, twenty- 

 pounds of common fait, and five pounds of arlenic. 



The manufacture of the common window glafs, though 

 made by blowing, is conduced differently from that of the 

 flint glafs articles ; as it is the objed to produce a large, flat, 

 very thin plate of glafs, which is afterwardi cut by the 



glazier"^ 



