Gas and Air in Picture Galleries

A few observations as to gas cannot be excluded. Gas, before and after burning, is bad for pictures. The evil effects of an occasional escape of unburnt gas are less to be dreaded than those caused by the products of gaseous combustion. These products are sulphuric acid, sulphurous acid, carbonic acid, and the moisture which is formed at the same time. Thence results a hot, moist atmosphere laden with these corrosive compounds. The water-vapour condenses into the liquid form and dissolves a part of the acids named above; the drops which trickle down are very injurious to paper, wood, canvas and pigments. In any case, all the products of the combustion of gas should be removed from the room as they are formed. For even when there may be no visible condensation of liquid, the vapours formed are often absorbed as such by paper, canvas, etc., and do in that form their destructive work.

An illustration of this fact is furnished by an analysis of the leather back of an old calf-bound volume. Owing to its absorption of the products of the burning of gas this back had decayed and fallen off, and was found to contain over 6 percent of free sulphuric acid.

With respect to the building itself in which pictures are to be kept, our aim should be to secure as far as possible pure air, an equable and agreeable but moderate temperature, and freedom from dust and dirt. Solidity of construction, a continuous damp-proof course, a certain degree of elevation above the ground-level, and double walls enclosing an air-space, are desirable as conducing to uniformity of temperature, and preventing the condensation of moisture upon the interior of the rooms. Due provision should be made for excluding from the galleries themselves the dust and dirt which may be brought in by visitors. And it cannot be too strongly urged that the supply of fresh air should not, as it were, accompany the visitors, but be brought in from an independent source. The place of in-take of such supply should not be gratings near to, or on the level of the ground, in out-of-the-way and dirty corners, the certain depositories of uncertain rubbish. From such sources air laden with organic and inorganic impurities can alone come. The question of the exclusion of fog and city-smoke may be mentioned here. Some kind of air-filter is useful.

It is astonishing how effectively the solid and liquid particles suspended in a yellow fog may be strained off and intercepted by passing the air through a layer of loosely packed carded cotton enclosed between two sheets of perforated zinc - this air-filter of course requires occasional renewal. Moist white lead, that is, white lead in powder reduced to a paste by admixture with water, will absorb the sulphuretted hydrogen as well as the sulphuric and sulphurous acid present in town air. And if the walls of the galleries are coated with a distemper paint containing white lead, this absorption of impurities goes on continually. For these impurities are more readily absorbed by an unprotected and properly-prepared distemper than by the pigments in the pictures. To secure this result the distemper should be made, not with size, but with starch-water, just sufficiently strong to bind the particles together and to the wall. For further particulars as to these and other arrangements for the conservation of pictures, especially in public galleries, the reader is referred to a paper on the subject in the Portfolio for 1882, pp. 106-108.