The chief difficulty in painting on the back of the glass is to calculate the effect each touch will have when viewed in front. On so viewing his touches the artist will often be surprised at the discordancy they present, though he may have calculated their relative effect very carefully. There is no expedient that will materially lessen this difficulty. All that can be done is to register, so to speak, the value of the transparent, and the solidity of the opaque couches of paint, by placing a sheet of white paper behind the former, and a sheet of black paper behind the latter.
The colors that are more or less transparent must be applied at the outset, but they will only appear as such and of their proper tint and hue when opaque paint is spread over them. Of course, the transparent colors must not be reserved for final glazing, the whole process of ordinary painting being reversed, the last strokes of the latter having to be the first strokes of painting on the back of a mirror.
As the face of the paint must be as smooth as the polished surface to which it clings, texture, for the representation of the surfaces of objects, can only be obtained by means less direct, for the most part, than those available for other applications of oil-painting. When using transparent colors, and texture be required, they must be applied in a broken manner, and when using opaque colors for the same purpose they should be spread thinly, then scraped, and other tints or hues passed over them so as to show between the interstices of the scraping, according to the requirements.
When the work is otherwise complete a solid coat of white should be spread over the whole, and when this is dry a thick coat of Brunswick black. The first will prevent the second from showing through, as it might to the great detriment of the coloring. Brunswick black is used as the overcoat, because it effectually protects the painting proper from injury by the subsequent silvering.
