GILDING. 269 



The gilt surface is then covered over with a 

 saline composition, consisting of nitre, alum, or 

 other vitriolic salt, ground together, and mixed 

 up into a paste with water or urine. The piece 

 of metal thus covered is exposed to a certain de- 

 gree of heat, and then quenched in water. By 

 this method its colour is further improved, and 

 brought nearer to that of gold. This effect seems 

 to be produced by the acid of nitre (which is dis- 

 engaged by the sulphuric acid of the alum, during 

 the exposure to heat) acting upon any particles of 

 copper which may happen to lie upon the gilded 

 surface. 



lastly, some artists think that they give an ad- 

 ditional lustre to their gilt work, by dipping it in 

 a liquor prepared by boiling some yellow materials, 

 as sulphur, orpiment, or turmeric. The only ad- 

 vantage of this operation is, that part of the yel- 

 low matter remains in some of the hollows of the 

 carved work, in which the gilding is apt to be 

 more imperfect, and to which it gives a rich and 

 solid appearance. 



It may here be noticed, that the use of the aqua- 

 fortis or nitrous acid, mentioned in the beginning 

 of the process, is not, as is generally supposed, 

 confined merely tcr cleansing the surface of the 

 metal to be gilt from rust or tarnish $ but it also 

 greatly facilitates the application of the amalgam 

 to the surface of that metal, probably in the fol- 

 lowing manner : It first dissolves part of the mer- 

 cury of the amalgam ; and when this solution is 

 applied to the copper, this latter metal having a 

 stronger disposition to unite with the nitrous acid 

 than the mercury has, precipitates the mercury 

 upon its surface, in the same manner as a polished 

 piece of iron precipitates upon its surface copper 



