Chemical Aids to Art. 411 



the surface with pale copal varnish, diluted with twice or thrice 

 its own bulk of spirits of turpentine. 



Since Professor J. Fuchs, of Munich, published his impor- 

 tant paper on Water Glass (in 1825), this process, in which it 

 is artistically employed, has developed greatly in the hands of 

 Kaulbach, and other German artists, at Berlin and Munich, 

 and also in this country likewise. Maclise, for instance, has 

 worked most successfully with it in the Royal Gallery of the 

 House of Lords, but it remains to be proved that the works 

 of these artists are safe against all the evils of the process. 

 The silicious bloom has, in several instances, appeared upon 

 the works of those who are thoroughly well versed in all the 

 minute details of the process. It is probable that the method 

 will have to be modified greatly, so as to get rid of its tech- 

 nical difficulties and its chemical defects before it can command 

 the general confidence of artists. In some of the other pro- 

 cesses which we will now describe, there probably exist the 

 germs of real improvements in these particulars. Professor 

 Kuhlmann, of Lille, suggested, some years ago, the combined 

 use of silicate of potash and aluminate of potash for the fixation 

 of colours, as weli as for the hardening of stone. One great 

 objection to this process, in which the colours are mixed with 

 a solution containing both silicate and aluminate of potash, is 

 the excessive alkalinity of the preparation. The union of these 

 two caustic potash compounds yields a solid glassy substance, 

 but this compound is far from being analogous, as has been 

 alleged, to felspar in its constitution, for it contains many times 

 as much alkali as that mineral. Nor is it wholly unchangeable, 

 for it spontaneously undergoes a process of disintegration, 

 although this does not occur generally for some time. A 

 wall decorated by this process never dries, and retains its 

 alkalinity for years, as may be easily shown by placing a 

 piece of moist yellow turmeric paper upon the painted 

 surface ; the alkali will change the yellow of the turmeric paper 

 into brown. 



But if we leave the soluble silicates altogether, we shall 

 find that there are other chemical compounds which can effect 

 the same objects. If a ground for painting be prepared with 

 lime, whitening, and sand, and the colours be mixed with a 

 five per cent, solution of soluble phosphate of lime instead of 

 with water, they will readily adhere, while no soluble salts 

 whatever will be formed in the wall, insoluble bone-phosphate 

 only being produced. Good proportions for the plaster are 

 three parts of burnt lime and two of whitening, both ground 

 together into a fine powder; five parts of pure sand, or of 

 marble grit, are then mixed with this powder, and the whole 

 made into a paste with baryta-water. This material is spread 



