Page:Popular Science Monthly Volume 29.djvu/99

Rh on the "Fugacity of Colors." Here is Field's caution on the subject, which deserves attention:

"Others, with some reason, have imagined that when pigments are locked up in varnishes and oils they are safe from all possibility of change; and there would be much more truth in this position if we had an impenetrable varnish—and even then it would not hold with respect to the action of light, however well it might exclude the influences of air and moisture; but, in truth, varnishes and oils themselves yield to changes of temperature, to the action of a humid atmosphere, and to other chemical influences: their protection of color from change is, therefore, far from perfect."

The best way, then, to keep oil-pictures in a state of safety is not to trust much to their power of resisting damp, but to treat them just as if they were notoriously delicate things like water-color drawings, although in reality we know that their constitution is more robust. An oil-picture, it is well to remember, may be attacked by damp from behind. If it is hung on a damp wall, the canvas will absorb damp from the wall, like the mill-board behind a water-color, and this damp will reach the colors through the priming. The proof that canvases absorb damp is that they hang flaccid on their stretching-frames when there is much moisture in the atmosphere. It is some protection to have the back of the canvas protected by a coat of paint applied with varnish, but a still better protection is to have two canvases on the same stretching-frame, the one that bears the work of the painter and another behind it with a coat of paint on both sides. The practice of having two canvases on the same stretcher has been adopted by more than one modern painter for various reasons. One reason is that an accidental blow to the canvas from behind, or an indentation from some angular object, may produce a fracture of the paint in the picture a fracture not immediately visible, perhaps, but likely to show itself later.

It is generally of no use to propose anything that has not been already adopted to some extent in practice, but I may call attention to a plan which is successfully adopted by house-painters to protect wall-papers from damp. Their way (or one of their ways) is first to apply tin-foil to the wall, making it adhere by means of a thick coat of white-lead. This is found to be a good protection for the wallpaper which is pasted on the tin-foil. It would probably, in the same way, be an excellent protection for pictures if the double-canvas system were adopted and the under canvas covered with tin-foil upon