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G L A

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the oxyds of iron are very numerous. In diegeneral account of glafs-making it was obfervedthat a very fmall portion of iron fully vitrifiedwith a large body of glafs gave different {hadesof green and yellow, and to this the colour ofcommon green bottle glafs feems to be owing.A larger dofe of iron gives a yellow after tho-rough vitrification, and a ftill larger gives abrownifli black, which feems to be only a yellowvery much concentrated, fince this latter colouris again produced by diluting the brownifli-blackwith a greater quantity of uncoloured glafs.The oxyds of iron alfo produce a red uponenamel and porcelain, but as it appears, this isonly owing to imperfect vitrification, that is tofay, it is only red as long as it is merely fufpendedin the glafs flux in a ftate of extreme divifion,for when by raifing the heat a compleat fufionof the oxyd is produced, the colour turns toyellow. This change is ingenioufly preventedby combining the oxyd previoufly with alumine,by mixing the fulphat of iron and alum togetherin folution, and precipitating both together bycarbonat of potafh, as mentioned under thearticle Enamel .

A great many preparations of iron for tnefepurpofes have been given, particularly the fajfronof mars , the ethiops, and the red oxyd precipi-tated from any of the folutions, all of whichwill be mentioned under the article Iron . Itdoes not appear however that there is any cer-tain foundation for preferring one to the other,for even the perfect oxyd by nitric acid, andwhich is itfelf of a dark red-brown, when fullyvitrified produces a yellow or brown accordingto the dofe, as well as any of the fub-oxyds.Brogniart afferts that the prefence of the oxydof lead fingu arly difpofes the perfect oxyd ofiron to lofe its red or rofe colour, for which itis ufed in enamelling, when urged by ftrongfire, but when no lead is prefent the colour isfixed.

Of Copper. All the oxyds and carbonatedoxyds of copper produce a fine green whenthoroughly vitrified with any kind of glafs orflux, and this colour is one of the eafieft toproduce in experiments in the fmall way. Theredoes not feem to be much reafon for preferenceof one preparation over the other. Thofe themoft frequently employed are the carbonatedoxyd produced by adding a carbonated alkali tothe fulphat of copper, and alfo the tes uftum orcopper oxyd'ated and calcined (imply by heatand accefs of air. This metal however may bemade to give a carmine red (or mixed with iron

a full deep red) by adding to glafs containingit a quantity of tartar when in compleat fufionand working off almolt immediately. The oxydof copper muft in this cafe be reduced nearlyto the reguline ftate. A greater continuance ofthe heat reftores the green colour. The oxydof copper is alfo often mixed with manganefeand iron in the compofition of the full-bodiedblack glafs. This oxyd when combined withthrice its weight of alumine runs in a ftrongheat into an opake red enamel.

Of Lead. The oxyds of lead naturally givea yellow tinge to glafs, but only confiderably fowhen in very large quantity. Their ufe as aflux has been repeatedly mentioned, and theywill readily vitrify moft compleatly in a mo-derate heat without addition, or will unite withany other vitrifiable matters and moft power-fully aflift their vitrification. This oxyd is-feldom ufed direftly as a colour, on account ofthe enormous proportion required, which fhouldbe at lead three-fourths of the glafs to give ayellow of any intenfity, and this would give arcompound extremely foft and one that wouldpowerfully corrode the crucibles.

Of Antimony. The perfeft oxyd of this metalgives a full yellow to glafs which is much ufed,.both alone and in compound colours in whichyellow is a neceffary ingredient.

Of Manganefe. The peculiar eircumftancesattending the ufe of this metallic oxyd in glafs-making have already been mentioned. Whennot in contadl with carbonaceous matter theproper colour given by this oxyd to glafs is apurplifh-red, rather muddy when in full body,but ftill very beautiful. It is almoft alwayscombined with nitre when thus employed. Thecolour is totally deftroyed by all the arfenicalfalts as before mentioned, and therefore arfenicin any form fhould be avoided when manganefeis ufed as a colouring matter. It alfo is theprincipal ingredient in moft of the black glades.

Of Cobalt . The’ colour given by the oxyd ofthis metal is a fine deep blue, which is unalter-able in any fire, and fucceeds with any flux.The colouring power is alfo very intenfe. Zaffireis ufually employed for this purpofe. Cobalt is alfo uied for fome of the finer blacks mixedwith manganefe and iron, and with the yellowof antimony and lead it compofes a green.

Of Nickel. Though this oxyd is not aftuallyufed it may here be mentioned that the oxydgives a violet-blue glals with fluxes of potalh andto thofe glades in which it largely enters, butnot with foda or borax. Klaproth has fully