GILDING. 



«/2.. re3 oclire, verdigris, ^eefi vitriol, or alum. Thus the 

 colour of the gilding is heightened by a perfeft diflTipation of 

 fome mercury remaining r.fter the foriner operation. The 

 gilt furface is then covered over with a faline conipofition, 

 confifting of equal quantities of nitre, fal ammoniac, green 

 vitriol, and verdigris, iincly powdered, and mixed up into a 

 pafte with water or urine ; or, this is iifcd inftead of the gilding 

 wax. The piece of metal thus covered is heated till the mix- 

 ture fmokes, and quenched in water cr urine. This cfietl 

 feems to be produced by the acid of nitre, which is difen- 

 gaged by the vitriolic acid of the alum, or other vitriolic 

 felt, during the expofure to heat, afting upon any particles 

 of copper which may happen to lie on the gilded furface. 

 If the colour of the gilding be not fufficieutly heightened 

 by the firll application, a fucceeding one will complete the 

 idefircd effeft. Some artills think they give an additional 

 luftre to their gilt work, by dipping it in a liquor prepared 

 by boiling fome yellow materials, as fulphur, orpiment, or 

 turmeric. The only advantage of this operation is, that 

 a part of the yellow matter remains in fome of the hol- 

 lows of the carved work, in i;!iich the gilding is apt to be 

 more imperfedl, and to which it gives a rich and folid ap- 

 pearance. 



Copper, and the alloys formed by its combinations with 

 zinc, are gilded much in the fame manner as filvcr ; but their 

 affinity for mercury being confiderably lefs than that of fil- 

 *-er, it is not eafy to produce a complete aJhefion of the 

 amalgam of gold to the burnifhcd furface of thcfe metals 

 by the fame means, and with the fame evennefs as in the for- 

 mer cafe. Advantage is here taken of the nitric acid for 

 facilitating the adhelion of the copper and mercury in the 

 following manner. The piece of copper, e. g. a button, is 

 firll cleaned by fteeping it iu acid and fubfequent wafhing, 

 and it i« then burnifhed in a lathe, or by other means : after 

 this, it is dipped in a neutrahzed folution of nitrat of mer- 

 cury, and in a few feconds, on account of the flrong affinity 

 of nitric acid for copper, the mercurial fait is decompofed. 

 The copper takes the place of the mercury, and at tht;fame 

 time the mercury is dcpofited in the metallic Hate, on the 

 furface of the copper, covering it entirely, and ftrongly ad- 

 hering to it. The gold amalgam is now applied, and the reft 

 of the procefs is the fame with that which has been already 

 defcribed. Thus a given quantity of gold may be made to 

 •cover a larger furface than in any other way of gilding on 

 metals ; five grains of gold completely gilding both the up- 

 per and under turfaces of 144 copper buttons, each of them 

 an inch in diameter. (Phil. Mag. ix. 20.) 



Iron cannot be gilt by amalgamation, unlefs it be previ- 

 oufly coated with copper, by dipping it in a folution of blue 

 vitriol, or rubbed with the vitriol itfelf a little moillened. 

 Iron may alfo receive a golden coat from a faturated folution 

 of gold in aqua regia, mixed with fpirit of wine ; becaufe the 

 iron, having a greater affinity for the acid, precipitates the 

 gold from it. 



In thegilding of iron, or rather fteel, bv means of an amal- 

 gam, peculiar difficulties occur. If rceourlebe had to the me- 

 thod of fimple burnifhing down, tjie heat requifite for this pur- 

 pofe will, in many cafes, bring the temper ot ihe fteel too low ; 

 on fuch occafionsihe mode already defcribed of gilding copper 

 is fometiines praclifed : that is, the parts of tlie Heel to be gild- 

 ed are pencilled over with nitrat of mercury, by which they 

 are covered with a fiightly adhering coating of mercury ; 

 th.-n tlie amalgam is applied, and the gilding fmiflicd in the 

 ■U ual way. The objccllonn to this procefs are, firft, that a 

 con iderable heal is required, though inferior to that requifite 

 fur burnilhing down ; and, fecondiv, that even with all pollible 

 f;>re, the giU^ng is apt to be rough and to fcale off. A very 



V(^ xvx. 



confiderable improvemMit on this mtthod is to Knc: ^' figvjr* 

 of the gilding on tiie fteel firft of all with a brufh charged 

 with a ilrong folution of fulphatcd copper, in confequcnce 

 of which a pretty thick plate of this metal is depofited on 

 the fteel to which it may be made to adhere with confiderable 

 firmnefs by means of llie burnidier ; thus the gilding is, in 

 part, performed upon the copper. 



A new method of gold gilding upon fteel has lately been 

 publiflied (fee Phil Mar. xi. p. 144), podeffing many ad- 

 vantages over the others, and capable of ultimately attaining 

 a very high degree of perfection. This method depends upon 

 the well-known faft, that if fulphuric etlier and nitro-miiriat 

 of gold are mixed together, the ether will, by degrees, fepa- 

 rate from the acid nearly the whole of the gold, and retain 

 it for fome time in folution in nearly a metallic ftate. If 

 ether, thus charged with gold, is fpread, by meant of a pen or 

 fine brufli, on the furface of highly poliffied fteel, the ether 

 prefently evaporates, leaving the gold behind in clofc contaft 

 with the fteel, and the adhelion is confiderably improved by 

 the fubfequent application of the burniflicr. The dearnefs, and 

 efpecially the rapid volatility of ether, are objeiElions of fome 

 moment, but may be got over by ufing tiiebeft oil of turpen- 

 tine inftead of the ether, which has nearly the fame efficacy 

 in decompofing the nitro-muriat of gold, and is both cheaper, 

 and not fo very quickly evaporable. 



On the fubject of gilding by amalgamatia^i. Dr. Lewis hai 

 the following remarks : " There are two principal inconveni- 

 encies in this bufinefs ; one, that the workmen are expofed to 

 the fumes of the mercury, and generally, fooner or later, have 

 their health greatly impaired by them ; tl>e other, the lofs of 

 the mercury ; for though part of it is faid to be detained in 

 the cavities made in the cliimnies for that purpofe, yet the 

 greateftpart of it is loft. From fome trials I lia%'e made, 

 it appeared that both thefe inconveniencies, particularly the 

 firft and nioft confiderable one, might be in a good meafure 

 avoided, by means of afurnr.ce of a due conftruftion." 



If tlie communication of a furnace with its chimney, in- 

 ftead of being over the fire, is made under the grate, the 

 alh-pit door, or other apertures beneath the grate, clofed, 

 and tlie mouth of the furnace left open, the current of air, 

 which otherwife would have entered beneath, enters now at 

 the top, and paffing down tl-.roHgh the grate to the chimney, 

 carries with it completely both the vapour of the fuel, and 

 the fumes of fuch matters as are placed upon it. The back 

 part of the furnace ftiould be raifcd a little higher above the 

 fire than the fore-part, and an iron plate laid over it, that 

 the air may enter onlv at the front, where the workman 

 ftands, who will be thus efieclually fecured from the fumes, 

 and from being incommoded by the heat, and at the fame time 

 have full liberty of introducing, infpedting, and removing 

 the work. 



If fuch a furnace is made of ftrong forged (not milled) 

 iron plate, it will be fufficiently durable. The upper end of 

 the chimney may reach above a foot and a half higher than 

 the level of the fire ; over this is to be placed a larger tube, 

 leaving an interval of an inch, or more, all round between it 

 and tlie chimney, and reaching to the height of ten or twelve 

 feet ; the higher the better. The external air, paffing uj» 

 between the chimney and the outer pipe, prevents the latter 

 fiom being much heated, fo that the mercurial fumes will 

 condenfe againft its iides into running quickfilver, wliich fall- 

 ing down to the boLtom, is there catched in a hollow rim, 

 formed by turniisg inwards a portion of the lower part, and 

 conveyed by a pipe at one fide into a projier receiver. 



Another method is mentioned by authors of gilding upon 



metals, and aHb upon earthen-ware and glafs ; which is, to 



f ufc gold with regulus of antimony, to pulverize tliis maft, 



JiL k and 



