410 Chemical Aids to Art. 



and ivory black, with burnt siena, burnt umber, and similar pre- 

 parations, may all be fixed to the prepared wall surface without 

 injury. A great difficulty will be experienced at first in laying 

 on the colours, since no medium but water or lime-water is 

 admissible. By keeping the wall constantly wet with lime- 

 water, or baryta-water, this difficulty may be partially re- 

 moved. All the water used in the process, from the prepara- 

 tion or priming of the walls to the final fixing of the coloured 

 surface, must be pure distilled water. This final fixing of the 

 painting is accomplished as follows : — A weak solution of pure 

 silicate of potash is prepared (several applications of a weak 

 solution are much more effective than two or three applications 

 of a strong liquor) by mixing the ordinary liquid silicate of 

 potash of commerce with thrice its bulk of water. It is of 

 course impossible to lay on the fixing liquor with a brush, 

 which would infallibly cause the removal of much of the 

 colour. A complicated spray-producer, or syringe of peculiar 

 construction, was invented for the purpose, and has been 

 used extensively. But I have found that a very cheap 

 and effective instrument may be made by attaching, with 

 an India-rubber tube, a small pair of hand bellows to 

 the little contrivance familiarly known as La Bouffee, or 

 L'Odorateur. The apparatus should be examined carefully 

 to see that no drop of liquid — nothing but spray — is blown 

 against the painting. The syringing is renewed at intervals of 

 a few days, till — when the painting is dry — a wet. cloth removes 

 no particle of colour. Over-silication will produce a glaze 

 which renders the surface spotty, and unpleasant in appearace. 

 No re-touching, after fixing, is permissible, unless the old 

 colour be first removed by scraping. Any soluble efflorescence 

 which may make its appearance on the wall in the course of a 

 few months, may be removed by a thorough washing of the 

 surface with hot distilled water ; indeed this treatment is in 

 all cases desirable. Another kind of efflorescence also occa- 

 sionally makes its appearance. This latter substance is, un- 

 fortunately, insoluble in water and all usual solvents, and can 

 scarcely be removed even by mechanical means. It consists 

 chiefly of an insoluble silicate, and seems to arise from an in- 

 sufficiency of alkali in the water-glass used. For it is a great 

 mistake to suppose that the excess of potash in the original 

 silicate can be safely removed, as some chemists have recom- 

 mended, by the addition to it of gelatinous silica, or of a 

 diluted solution of silica. Indeed, on the other hand, the 

 introduction of a small quantity of caustic potash, to the diluted 

 medium is often desirable. When this second kind of efflores- 

 cence has once appeared, the unpleasant bloom which it im- 

 parts to the painting may be partially obviated by syringing 



