1

GL A

( 512 )

GL A

thy ft, There appears no agreement in the dif-ferent receipts as to the relative proportions ofmanganefe, only that the latter fliould be inlefs quantity than the cobalt when ufed in thedilute form of zaffre. Tliefe mixtures appearto be materially improved by being twice meltedand poured into water and powdered betweenthe firft and fecond fufion. Among rhe feveralreceipts the following maybe given, namely, toloo parts of a fine glafs, without lead, add ipart of zaffre and T V of a part of manganefe.Another is 240 parts of glafs frit, made withfoda and fand only, 192 of minium, 2 of zaffreand 3- of manganefe melted twice and treated asabove. Or a very fine blue glafs may be madefimply by the glafs No. 5, with a neceffary dofeof oxyd of cobalt or zaffre.

Though blue glaffes cannot be now madewithout cobalt, it is .certain that iron in fememode of combination gives this colour in greatperfection. It appears to be the natural colour-ing matter of the fapphire, lapis lazuli, and fomeother blue minerals, and is frequently producedaccidentally in the fcoriae of iron ores. Theart of colouring paftes and enamels blue withironwas certainly known to the antients but is nowentirely loft. This is proved by the analyfis offome of the antient blue enamels, in which nometallic matter but iron, with a frnall propor-tion of copper, can be detected. Whether thecopper alfilts in the effect cannot be told. Kla-proth analyzed an opake antient Roman enamel,the colour of which was fapphire blue vergingtowards that of fmalt, and found the followingingredients. parts

Silex ----- 163

Oxyd of iron - - 19

Oxyd of copper - - 1

Alumine - - - - 3

Lime ----- p.r

186.5

Some of thefe plates of enamel were colouredequally throughout, others only to a certaindepth on one furface, and the colour was givenfo uniformly that the plate had the appearanceof two plates, one blue and the other colourlefs,adhering to each other.

Of yellow Glaffes. The oxyds of lead, anti-mony, and filver are thofe which are ufed togive a yellow to imitate the varieties of thetopaz, or the yellow diamond. M. Fontanieugives the following proportions: 24 parts of theglafs No. 1, or No. 3, and •£• of a part of glafsof antimony, or for the deeper coloured topaz, f-.

For the imitation of the Brazilian Topaz he ad-vifes 192 parts of the glafs No. 2, or No. 3, 8 ?parts of glafs of antimony, and ^ of a part of thepurple precipitate of gold. An inferior yellowglafs may be made by fufing fimply 2 parts offine fand or filex of any kind, with 7 parts ofminium, but this is very foft. The delicate hueof the yellow diamond is imitated by adding to57 6 parts of the glafs No. 4, 25 parts of lunacornea, or ten parts of glafs of antimony.

A colour varying in fhade from brown todingy fmoke yellow, and thence to a fine trans-parent yellow, Is given to common glafs fimplyby adding to it when in foft fufion, any vege-table carbonaceous matter, part of which rifesto the top and is burnt off, but a part alforemains uniformly diffufed through the glafs,and gives it a fine yellow without impairing itstranfparency. No continuance of the fire willburn out this yellow colour altogether, after ithas once loft its dingy fmoked hue and acquireda clear yellow. Tartar has been commonly ufedfor this purpofe, but almoft any vegetable in-flammable fubftance not fluid, will probably doas well; the foft charcoal of the beech anfwersthe fame purpofe. Manganefe has been em-ployed in the compofition of this glafs, but itappears to be of no ufe. a Sometimes the car-bonaceous matter is added to the glafs fritwhen beginning to melt in the glafs pots, atother times it is mixed with it before firing.A little nitre is found of ufe in clearing thecolour and correcting the fmoakinefs, but toomuch of it will deftroy the colour altogether.This glafs fwells much in the pot, when pre-paring, owing to the efcape of part of thecarbonaceous matter, efpecially when tartar isemployed: but probably with quite dry andfreih burnt charcoal, previoufly heated ftronglyin clofe veffels for about an hour, no fucheffeft would take place.

Of the Artificial Diamond. Though no arthas ever invented any vitreous compofitionwhich can be miftaken for the real cut diamond,by an eye at all praftifed, (unlefs by particularartifices in the fetting, which are eafily dete&ed)yet fome artifts can prepare a very fine brillianthard glafs pafte, which poffeffes great beauty,and a very confiderable water or play of light,which more nearly imitates the diamond thanall the common artificial gems. It will befufficient here to add that M. Fontanieu re-commends his glafs No. 1, for this pur-pofe. Thefe glaffes require a confiderablelength of time of ftrong fufion before they are

* Lewis Commerce of Arts, p. 628.

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