It has been advised through excessive precaution to put also a fireproof coating of some kind as a preservative against fire; but this cannot be put on the front of the picture and would be consequently of little use in case of fire. A better plan would be to nail a metallic canvas on a light frame, attached by hinges behind the picture (so as to be easily opened at will). This metallic canvas, being thus held about half an inch off from the painting canvas, would allow the air to circulate and would preserve the picture from shocks, such as contact with flame, of which there is much more danger from behind than in front; for it is noticeable that in museums, at picture dealers' and in the houses of amateurs, servants who have the greatest respect for pictures when hung pay no attention to them when standing with their faces to the wall: they drag ladders about, and move round with lighted candles through piles of heaped up frames, without taking the smallest precaution, on the contrary, they ought to take the greatest care.,