When paint is applied to bright metals like tin or zinc, it is very apt to peel off. This difficulty is greatly lessened if the metal be hot when the paint is applied, but in many cases this cannot be done. In such cases the surface of the metal should be corroded, for which purpose a solution of sulphate of copper, acidulated with nitric acid answers well. The metal should be washed with the solution, allowed to stand a couple of hours, and then washed with clean water and dried.
The black coloring matter is the soot obtained by holding a clean copper or sheet metal plate over the flame of an oil or petroleum lamp (a glowing tool serves the purpose very well). As soon as a sufficient deposit is produced it is collected on a piece of glass, care being taken not to mix any foreign substance with it. A few drops of essence of lavender are then poured on the soot and the mixture pounded with a spatula. This done, just sufficient copal varnish is added to give the composition a proper thickness, so as to prevent it spreading when applied. The varnish thus prepared is put on by means of a very fine brush. To secure brilliancy the dial is dried at a slow heat, by passing it lightly over a spirit flame, the reverse side of the dial being, of course, the only part exposed to the flame. This composition must be made in quantities large enough for present use only, as it dries very rapidly and cannot be utilized afterwards. To secure good results this process requires some experience, which can only be obtained by careful experiments. The painting especially requires a certain aptitude and lightness of hand, which may, however, soon be attained by strict attention.
This process, which gives very excellent results, is evidently applicable to a great variety of purposes.