G L A

G L A

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afcertaincd this in his analyfis of the Chryfo-prafe, c which, frpm its giving a blue to potafnglafs, was thought to contain cobalt, but this isnot the cafe, the colouring matter being nickel.Thus i part of the rough chryfoprafe, and 2 partsof carbonat cf potafli fufed into a violet-blueglafs; and 80 parts of filex, 6 o of carbonat ofpotafn, and 3 of oxyd of nickel from the chry-foprafe, alfo gave a violet-blue glafs. The famerelult was given by ufing an oxyd of nickel froma known ore of that metal. Equal parts of thechryfoprafe and of carbonat of foda gave atourmaline-brown glafs nearly opake. Equalparts of chryfoprafe and calcined borax gave abrown tranfparent glafs: and 60 parts of filex, asmuch borax, and 3 parts of oxyd of nickel, alfogave a clear light brown glafs. Silex, phofphoricacid, and oxyd of nickel in the fame proportionsgave a honey-yellow glafs but not quite clear.

Of Tungsten . Though this is not ufed itmay be mentioned that with fluxes of phofpho-ric acid it gives a blue glafs, but not whenborax or alkalies are ufed.

Of Chrome. This metal which is the naturalcolouring matter of the ruby and emerald, wouldbe a mod valuable ingredient for the artificialgems if it could be procured with tolerableeafe. It has been found to give a fine red andalfo a moll beautiful and exquifite green toglaffes, but its great fcarcity has prevented itsfrequent ufe. (See Chrome.)

Having thus generally defcribed the colouringproperties of the feveral metallic oxyds, fomeof the afifual recipes for the different colouredglaffes may be mentioned. Thefe we fhallchiefly give from M. Fontanieu, and from Neriand Kunckel, to which may be added the adfualcompofition of a few of the beautiful antiquecoloured glaffes, as found by the analyfis ofKlaproth . rt With regard to the recipes how-ever, it may be added that there appears fuchenormous difference in the relative proportionsof the metallic oxyds to the fluxes ufed, aseither to give a fufpicion of extreme inaccuracyor to fhew that it mull in mofl cafes be deter-mined by individual experience.

Of the Ruby red, Purple, Violet , tsfe. by Gold.Little can be added here to what has been faidabove under Gold. The particulars of the pro-eefs have always been carefully kept fecret bythe fuccefsful artifts, and from the frequentfailures there can be no doubt that it is a verydifficult procefs to manage. According toKunckel and others who have fucceeded, it ap-pears that the colouring power of the purple

, c Etuys, vol,i.

precipitate of Cajfnts, or the mixed oxyds of goldand tin, is fo great that one part will give a fullrich body of colour to from 600 to loco ormore of glafs. The glafs of antimony too feemsan important addition. This ruby glafs comesout of the fire colourlefs, but affumes its beau-tiful hue as it cools. It has been thought thatthe colour is alfo further brought out by ex-pofure to fmoak.

Of other Reds, Purples, and Violets. Some ofthefe are compofed of a colourlefs glafs balls,fuch as one of the five before mentioned, withmanganefe either alone or with the purple pre-cipitate of gold, or more commonly with theoxyd of cobalt. The colour given by man-ganefe being of a violet red, the cobalt will giveit a more decided purple by adding its naturalblue. It is impoffible to pick out from thevarious recipes any other proportions than thatthe oxyd of manganefe will colour very fullyabout 100 times its weight of glafs when ufedalone, and when employed with cobalt, 200parts of glafs will be highly coloured with 1 ofmanganefe, and (for a purple) about j to | ofzaffre.

Neri gives the following receipt for a glafs toimitate the garnet, namely, 2 oz. of rock crys-tal, 6 oz. of minium, 16 grains of manganefe,and 2 grs. of zaffre. Kunckel gives for a violetred glafs a common lead-glafs bafis withof manganefe, mixed with nitre. Fontanieugives for the imitation of the amethyfl, 24 oz.of his glafs, No. 5 (as already defcribed), i oz.of manganefe, and 4 grains of purple precipi-tate of gold, together with if ounce of nitre.But the quantity of colouring matter here is foenormous that the vitreous bafis fhould probablybe 24 pounds inftead of ounces.

A fine red has been mentioned to be procuredfrom the oxyd of copper (with or without oxydof iron) mixed with the due proportion of glafsand with carbonaceous matter ftirred in. Fora full deep red the oxyd of iron ftiould be threeor four times as much as the copper, and inproportion as the latter predominates the colourapproaches to carmine. The glafs at firft fhouldappear when hot only of a faint greenifh-yel-low, and, when in full fufion, fome tartar isftirred in, which inftantly reddens the wholeand caufes it to fwell prodigioufly, after whichit again fubfides into a clear red glafs, whichfhould be worked off without delay. Probablycharcoal would anfvver as well as tartar.

The antique red glafs analyzed by Klaproth mult probably have been made in the fame way,a Phil. Jouru. vol. 8.