C H A P. II
Of filvering.
SILVERING may be pra&ifed on the fame fubftanqes; and by all the fame me-thods, either with leaf or powder we have be-fore pointed out with regard to gilding; varia-
tion being made in a few circumftanees below mentioned. It is neverthelefs but feldom ufed, fiotwithftanding the effect would be very beau-tiful and proper in many cafes; and there is an extreme gdod reafbn for fuch a negledt of it.
This reafon is, its tarnifhing in a very fhort time; and acquiring frequently, befides the general depravity of the whitenefs, fuch foots of various colours, aS reader it very unfightly: and this tarnifh and {peeking is not only the conftant refult of time; but will be often pro-duced inftantly by any extraordinary moifture In the air, or dampnefs, as well as by the fumes dnd effluvia of many bodies which may happen to approach it.
Wherever, therefore, filvering is admitted, a ftrong varnifh ought to be put over it: and. this even is not fumcient wholly to fecure it from this deftrudtive confequence. The var- riifh muft be fbme of the compofitions of maf- tic, fandetac, the gums animi or copal, and White refin; (theparticular treatment of which in the forming Varnifhes will be found in other parts of this work;) for the other fubflances ufed for compounding varnifhes are too yellow. Some put a coat of ifinglafs fize over the lilver : but, befides that the fize itfelf injures the whitenefs in time by turning yellow, it preferves the filver but in a fmall degree.
The methods of making the filver powders is alio the fame as thole of gold, except with regard to one of the German powders, which is correfpondent both in its appearance and ufe, abating the difference of colour, to the aurum mofaicum or mufivum : whence it has been indeed, though improperly, called the argentum mufivum. Tne procefe for this being, therefore, different from any before given, it is proper to infert it fully, as follows.
“ Take of very pure tin one pound. Put it into " a crucible; and fet it on a fire to melt: when it ** begins to run intofufion, add to it an equal “ proportion of bifmuth or tin glafs: and ftir “ the mixture with an iron rod, or the fmall end *c of a tobacco-pipe, till the whole be intirely “ melted, and incorporated. Take thecroci- “ ble then from the fire; and, after the melted “ compofition is become a little cooler, but “ while it is yet in a fluid ftate, pour into it “ a pound of quickfilver gradually; ftirring
it in in the mean time, that me mercury ** may be thoroughly conjoined with theo** ther ingredients. When the whole is thus " commixt, pour the rnafs out of the cruci- “ ble on a ftone; where, as it cools, it will ** take the form of an amalgama or metalline pafte; which will be eafily bruifed into a flaky ** powder; and is then fit for ufe.”
This powder maybe either tempered, in the manner of the fheU gold, with gum water; or rubbed over a ground properly fized, according to any of the methods above directed for gold powders; and it will take a very good polifh from the dog's tooth or burnifhcrs; and hold its colour much better with a flight coat of varnifh over it, than any true filver powder.
The fizes for filvering ought not to be mixed, as in the cafe of gold, with yellow, or bole ar- moniac; but with feme white fubftance, whofe effect may prevent any fmall failures in the covering the ground with the filver from being feen, in the fame manner as the yellow fub- flances do the gold. This may be done with flake white, or white lead, when the fizes formed of oil are ufed: but whiting is the proper matter in the burnifh for filvering ; or where the glover's or parchment fize is ufed. Some recommend tobacco-pipe clay in the place of whiting; and add a little lamp black togive a filver-like greyifhnefs to the ccmpofi- tion.