G I L

G L A

( 477 )

according to the tafte of the artift. Inhead ofthe common gilder’s wax a mixture of equalparts of nitre, fal ammoniac, green vitriol, andverdegris moiftened with water, will anfwer thepurpofe.

Copper and the alloys formed by its combi-nation with zinc are g.lded nearly in the fameway as lilver* but as their affinity for mercuryis confiderably lefs than that of filver, it wouldbe difficult to make the amalgam of gold ad-here to the burniflied furface of thefe metalsby the fame means, and with the fame evennefsas takes place in the cafe juft defcribed. Toobviate this inconvenience advantage is very in-genioufly taken of the aftion of nitric acid tofacilitate the adhefion of the copper and mer-cury in the following manner. The piece ofcopper, a button for example, is firft cleaned byfteeping in acid and fubfequent waffiing, andis then burniflied either in a lathe, or by anyother means: after this it is dipped in a neu-tralized folution of nitrat of mercury, and inthe fpace of a few feconds, owing to the ftrongaffinity of nitric acid for copper the mercurialfait is decompofed, the copper takes the placeof the mercury, and at the fame time the mer-cury is depofited in the metallic Hate on thefurface of the copper, covering it entirely, andftrongly adhering to it. The gold amalgam isnow applied, and the reft of the procefs goeson as already defcribed. By this method ofproceeding a given quantity of gold may bemade to cover a larger furface than in anyother way of gilding on metals : five grains ofgold will compleatly gild both the upper andunder furfaces of 144 copper buttons, each ofthem an inch in- diameter . b

There is no metal the gilding of which pre-fents fo many difficulties as iron, or ratherfteel. If the method of fimple burnifhingdown is had recourfe to, the heat requifite forthis purpofe will in many cafes bring the tem-per of the fteel too low : on fuch occafionsthe way already defcribed of gilding copper isfometimes pratftifed; that is, the parts of thefteel to be gilded are pencilled over with nitratof mercury, by which they are covered with a{lightly adhering coating of mercury, then theamalgam is applied, and the gilding finilhed inthe ufual way. The objections to this modeof proceeding are firft that a confiderable heatis required, though inferior to that requifite forburnilhing down, and fecondly, that even withall poffible care the gilding is apt to be roughand fcale off. A very confiderable improve-

»Phil. Mag. i*. p. mo-

ment on this method is to trace the figure ofthe gilding on the fteel firft of all with a brufhcharged with a ftrong folution of fulphated cop-per, in confequence of which a pretty thick plateof this metal is depofited on the fteel to which itmay be made to adhere with confiderable firm-nefs by means of the burniffier : thus the gildingis in fad performed upon the copper.

Anew method of gold gilding upon.fteel haslately been publilhed c poffeffed of many ad-vantages over the others, and which probablyin time may attain a very high degree of per-fection. It depends upon the well-known fadtthat if fulphuric ether and nitro-muriat of goldare mixed together the ether will by degreesfeparate from the acid nearly the whole of thegold, and retain it for fome time in folution innearly a metallic ftate. If ether thus charged withgold is fpread by means of a pen or fine brufh-on the furface of highly polifhed fteel the etherprefently evaporates, leaving the gold behindin dole contact with the fteel, and the ad-hefion is confiderably improved by the fubfe-quent application of the burniffier. The dear-nefs, and efpecially the rapid volatility of etherare objections of fome moment, but may begot over by ufing the beft oil of turpentineinftead of the ether, which has nearly thefame efficacy in decompofing the nitro-muriatof gold, and is both cheaper and not fo veryquickly evaporable.

Cold gilding upon filver is, we believe, atprefent entirely difufed. It was performed inthe following manner. A faturated folution ofgold in nitre-muriatic acid was poured uponfome linen rags, and when they were becomedry they were heaped in a plate and touchedwith a hot eoal. The fire gradually fpreadthrough the mafs and reduced it to a heavyblack afh. A foft cork being moiftened inwater was dipped in this afh, to which a partof it adhered, and was then rubbed on the fur-face of polifhed filver, upon which the minuteparticles- of gold became fixed, and covered itwith an extremely thin coating, which whenburnifhed exhibited the genuine colour and luftreof this precious metal-

GLASS. Verre, Fr. Gins , Germ;

A glafs in the common chemical ufe of theterm is underftood to be any fubfta-nce or mix-ture, earthy, faline, or metallic, brought byigneous fufion to the ftate of a hard, brittle, uni-form mafs, breaking with a conchoidal frac-ture, paffing into fplintery, and with a high'degree of luftre. Tranfparency is alfo a cha-

*• Phil. Mag. xi, p. 144.