Gilding
"Act and result of gilding. The gilding of watch-movements is done by electrolysis. The composition of gilding-baths is varied according to the colour required, yellow, red, green, etc. The object to be gilded is suspended from the cathode of an electrolytic bath. The gold salts present in the bath are deposited on the object. Parts are pickled and lightly copper-plated before gilding.
Generally speaking, the process of gilding without using an electric current, i.e. by means of baths of varied composition, used hot or cold, or special powders. Formerly, the gilding of watch-movements and, later, hands was done by covering them with a gold amalgam, then lightly heating them to cause the mercury to disappear." (Berner)
Amalgam. An alloy of mercury and metal. A term used in gilding.
The mix for gilding was a prepared amalgam of mercury and gold. The ratio was always consistent. If the material was a silver alloy - the gold took better, if it was a copper alloy one had to prep the surface beforehand with mercury and then the amalgam was applied. The heat was usually from a coal fire. Mercury was used throughout the process - so from creating the amalgam to the gilding one was exposed to hazardous mercury fumes. Mercury is always off-gassing. There were a few variations on the finish. Usually one sought a bright finish, but you could achieve a soft yellow as well by using a pickle pot or paste of alum, potassium nitrate, and salt, which was then heated and removed. Sometimes cleaned with cream of tartar after.