To discover if any metal has a thin gold coating or whether its appearance is due to a gilding lacquer, clean a portion of the article which you desire tested with either ether or alcohol and dissolve same in nitric acid free from any trace of chlorine. If the article is gilded, the layer of the gold floats in or upon the solution. When a removal of the varnish coatings cannot be brought about by ether or alcohol, treatment by chloroform is required. To make sure of the presence of gold the following test will be found useful: Completely dissolve the article to be tested in nitric acid, dilute resultant solution with water, filter, preferably with a small filter, wash out, dry and glow. The glowed residue is treated with the aid of heat, with a small portion of aqua regia poured off or filtered, as necessity requires, and the filtrate evaporated until dry, at a moderate heat. If there is gold in the article a slight lustrous separation of some will be seen on the edges of the vessel. Mix the residue from evaporation with from 12-100 to 18-100 cubic inches of water, and divide the resultant solution into three portions, which are to be used in the following tests:
(1) Add a drop of concentrated solution of proto-chloride; if there is any gold in the mixture there is a resultant dark-brown separation.
(2) Add to mixture a drop of solution of ferrous sulphate; the presence of gold gives a brownish or bluish separation.
(3) Add oxygenated water; the pressure of gold will give a blue separation.
(4) To determine whether gold or alloys have been used for gilding, a solution of chloride of copper will be found efficacious. If the gilding be imitation gold, a touch of the solution gives a black mark, the copper separating out through the zinc in the yellow metal - there is no such discoloration if the metal be pure. Another test is a solution of nitrate of silver, or chloride of gold. The first of these will give a gray or black spot and the second a brown spot, if the gilding be imitation gold, but neither have any effect on the genuine metal.