GL A

GLA ( 513 )

brought to the. Hate of the greateft clearnefsand brilliance.

Of Opake Glajfes.

The materials which form the opake gladesare a common vitreous bafis, as for the colouredglafles, and either an exceflive dofe of colouringmetallic oxyd which {hall give fuch a depth andbody of colour as to produce opacity, or afubltance which of itfelf gives an opake white-nefs, to which any colour may be afterwardsadded if required. In general it is only theblack glafles which are made opake by merequantity of the fame colour which, in fmallerproportion, would be tranfparent; and thewhite glafles are made with a glafs renderedopake by fome addition which in any propor-tion impairs the tranfparency; and the blue,green, yellow, and other coloured opake glafleshave the white glafs for a bafis, and are colour-ed in the fame way as the tranfparent glafles.

White Glafs. The fineft white glafs is avitreous bafe made opake by the oxyd of tin,and is then called Enamel , (which fee).

But a very good white may be made at lefsexpence by fubftituting for the oxyd of tin apretty large quantity of bone-afh or phofphat oflime in very fine powder. It appears to beowing chiefly to the extreme infufibility of thisearthy fait that the opacity is produced, fo thatin fad the glafs thus made is a common vi-trefeent compound holding in intimate mixturea quantity of unvitrefcible earth.

Neri’s receipts for white glafs are the follow-ing : mix together 60 parts of white fand, 40of potalh, and 10 of finely powdered bone-afh,and melt for the ufual time of glafs-making.The refult is a glafs which when fully red-hotis tranfparent, but becomes milky and opake asfoon as it cools. It does not appear certainwhether this change is chiefly owing to a de-ception of fight which does not allow the degreeof opacity to be diftinguifhed when red hot, orwhether the glafs really becomes opake onlywhen it cools, and then depofits the bone-afhby the effed of a kind of fuperlaturation by heat.

Another receipt is, 130 parts of fand orCalcined flint, 70 of nitre, 12 of borax, 12 oftartar, 5 of arfenic, and 15 of bone-afh.

An imitation of the opal is made accordingto Fontanieu by mixing 576 parts of the glafsNo. 3, 10 of luna cornea, 2 of magnetic ironore, and 26 of bone-afh.

Black Glafs. It is not eafy to make a veryfull-bodied, perfectly opake, fine black glafs.This is commonly made by manganefe, and:t has been found that one part of this oxyd

» Broclnnt

vol. r.

will give a full body of opake black to about20 of glafs. A finer black ufed chiefly by the e-namellers is made (as appears) by mixing togetherequal parts of manganefe, zaflre, and feales ofiron, and fufing one part of this mixed powderwith 15 or twenty of any kind of glafs.

For the other opake coloured glafles no moreneed be added than that they may be made inthe fame way as the tranfparent coloured glafles,fubftituting the opake white glafs for the com-mon vitreous bafe. Thefe however are notmuch ufed in mafs, or for any of the commonufeful or ornamental purpofes to which thetranfparent glafles are applied. The opakecoloured enamels are compofed on the famegeneral principles, the bafe being the whiteenamel made with the oxyd of tin.

GLASS OF BORAX is borax calcined tillit lofes its water of cryftallization and flowsinto a very thin limpid glafs. A moderate redheat is fufficient for the purpofe. This be-comes flightly opake by expofure to the air, inconfequence of an incipient efflorefcence.

GLASS -GALL, or Sandiver , is the feum ofthe glafs-pots which arifes during the vitrifica-tion of the frit. See Glass .

GLASSES (Metallic), are the o.xyds, orfometimes the fulphuretted oxyds of the differentmetals vitrified by heat. Thofe of lead andantimony are the moft familiar to Chemifts.(S ee the refpeElive metals.)

GLAS SCHORL. See Thumerstein.

GLAUBER’s SALT. See Sulphat ofSoda.

GLAUBER’s SALT, a native. NaturlichesGlauberfalz , Germ.

Its colour is grevifh or yellowifh-white. Itoccurs generally efflorefeent or incrufting, rare-ly ftaladitical, and fometimes in minute acicularor middle-fized prifmatic cryftals. Internallythe cryftals are fliining, wdth a vitreous luftre,but by a fliort expofure to the air they becomedull and efflorefeent. Its fradure when cryf-tallized is conchoidal, when amorphous is earthy.It varies from tranfparent to opake, accordingas it is more or lefs in a ftate of efflorefcence.It is brittle and eafily frangible, has a bitterfaline tafte, and is readily foluble in water.It gives a copious precipitate with barytic falts,but is not altered by carbonated potalh, Whenfully faturated with water of cryftallization itcontains according to Bergman,

Sulphuric acid 27Soda - - - 15Water - - _ 58ICO

. Jamcfon.

3 T