If the plale is already silvered a separate study should be made and the outlines of this study traced on the back of the silvering. This being done, the portion of the silvering that the painting must occupy is etched away by a scalpel or other sharp blade, taking care not to scratch the glass. The etching may be effected and outlines obtained without much trouble if the silvering has been done by the mercurial process. But it is not so with the modern silvering, this being covered with a coat of hard varnish-paint that is almost impossible to remove without leaving ragged edges. Only for a large plate to be viewed at a distance should it be attempted, and then, so great is the labor involved, it would generally prove more economical to exchange the plate for a new one unsilvered.
All painting on the back of mirrors has, however, inevitable defects, which are apt to prove somewhat antipathetic to artists. Its difficulties, while augmenting the cost to the purchaser, preclude commensurate results. For the reason that the painter cannot see the progress of his work with the usual facility, the coloring can hardly be very harmonious. To mix each first touch of paint to the required hue or tint and lay it on at once in the right place is not easy to an experienced artist, but the difficulty is enormously increased when the work has to be turned to ascertain how the last touch behaves relatively to all those which preceded it. The coloring must also be comparatively dead, owing to the opaque ground. The painting can hardly appear other than flat and monotonously smooth, or with little spirit of handling, or touch, descriptive of texture and expression of light. And, although by this method there are no reflections from edges of the painting when the mirror is viewed at an angle, yet the painting is obscured by reflections from the surface of the glass before it, as well as lowered in brilliancy by the thickness and any greenness of the glass.
