THE CARE OF PICTURES AND PRINTS. 8 5 



house that is even moderately damp, he can hardly have failed to no- 

 tice that the boards behind framed engravings swell and bulge out, 

 which is the result of an increase in the bulk and area of the boards 

 exactly proportionate to the quantity of water they have absorbed. 

 When there is a sufficient supply of water certain fungoid growths 

 will begin on the surface of the print under the glass, exactly like the 

 growth of plants from the damp earth in a garden or conservatory. 

 If there is iron in the paper here and there (which often happens), 

 there will be spots of oxide of iron, or what we call rust, to give a 

 pleasing variety of color, and if one of them happens to occur on a 

 face, it must of course add greatly to its charm. Wooden backings 

 are safer ; and I have seen a room where the engravings with mill- 

 board behind them were all more or less spoiled by damp, while a 

 large engraving with a thick wooden backing was entirely uninjured. 

 Nevertheless, I would rather not trust to deal boards, as it is well 

 known that deal is very absorbent of moisture. I remember having a 

 heavy block of deal dead-wood removed from the hull of a boat, and 

 when it was sawed through the water oozed freely out of every fiber. 

 Had it been submitted to a powerful pressure, such as that from a hy- 

 draulic press, there can be no doubt that it would have been like 

 squeezing a wetted sponge. 



The necessity for careful precaution about the backing of framed 

 engravings is not simply due to the permeability of walls that let the 

 damp come through them ; it may be also due to mere condensation 

 on the inner surface of the wall even when it is well built and im- 

 permeable. This is best seen on a painted wall, as papers can absorb 

 a great deal of water without letting it be immediately visible. In a 

 very cold winter the external walls of a house become chilled through- 

 out their mass, and when they are painted on the inside a sudden rise 

 in temperature will produce visible condensation from the damp air, 

 because the wall has not yet had time to raise its own temperature to 

 that of the atmosphere. If there are engravings against the wall, they 

 will suffer as much as if the wall itself were damp throughout its sub- 

 stance ; for if the backings are absorbent they will drink in a quantity 

 of moisture from the streaming wall-surface, which they will after- 

 ward slowly give off to the engraving for the encouragement of fungi 

 and rust-spots. If oil-pictures are hung against a wall of this kind, 

 the canvas will absorb moisture (unless certain precautions are taken, 

 of which we may give an account presently), and then the increase in 

 its bulk and area will cause it to hang loosely on the stretching- 

 frames. The only way to combat condensation is by heating the air 

 sufficiently to warm the walls themselves, when, of course, it must 

 cease. Nature herself puts an end to it ultimately in the same way if 

 the mild weather continues, but more slowly, as it takes some time to 

 raise the temperature of a mass of stone by a gentle increase of heat. 

 A thin inner wall, or wainscot separated by a little space from the 



