242 



NATURE 



[January 14, 1892 



ought it ever to find its way into the body of the painting 

 itself, as is unfortunately the case whenever megilp is 

 used as the painting medium, this being in reality a 

 mixture of mastic varnish with linseed oil. On the 

 other hand, it is important that mastic varnish, when 

 used as the surfacing varnish, should be applied without 

 any admixture of oil varnishes, which, from their 

 rendering the mastic harder, more insoluble, and less 

 friable, make it much more difficult to remove and renew 

 the varnish. 



Of a different character and of far greater importance 

 are the fat or oil varnishes, which are compounds of the 

 harder resins, such as amber and the different kinds of 

 copals, with linseed oil, and diluted with oil of turpentine. 

 They form in reality the vehicle or medium for modern 

 oil painting, and consequently furnish ultimately the 

 matrix in which the particles of the colour are embedded 

 and held together. Next to the stability of the colours, 

 it is then the durability of this varnish medium in com- 

 bination with dry linseed oil, on which the perman- 

 ency of the oil-painting mainly depends. On the assump- 

 tion that the valuable qualities of the hard resins are 

 maintained in the varnishes derived from them, it is 

 understood that artists' varnishes are prepared only from 

 the hardest and most tenacious kinds of resins. Unfor- 

 tunately these are the most unmanageable and the least 

 soluble, and require the highest degree of heat to bring 

 them into fusion for the purpose of effecting the com- 

 bination with the linseed oil and oil of turpentine in the 

 process of making varnish. Even the most powerful 

 solvents, such as acetone, ether, benzene, chloroform, 

 aniline, and phenol, have only a limited solvent action upon 

 amber or the harder semi-fossil kinds of copal ; and their 

 solution can only be effected after they have undergone a 

 profound change by fusion or otherwise. In opposition 

 to what is generally stated, neither amber nor the semi- 

 fossil hard copals are fusible in the ordinary sense of the 

 word ; for they require to be kept at a high temperature, in 

 suitable vessels, for a considerable time before they become 

 gradually liquefied by the action of the heavy oily pro- 

 ducts of their own decomposition. Amber or copal, when 

 thus once melted, are completely changed ; they are now 

 fusible at a low temperature ; they have become readily 

 soluble in ordinary solvents, and miscible with heated 

 linseed oil ; but at the same time the original hardness 

 of the resins is greatly reduced, and the colour has become 

 of a more or less dark tint. 



It remains to be proved whether much is gained by 

 using the very hardest resins, instead of softer and more 

 tractable kinds which yield lighter-coloured varnishes ; 

 and this is a subject which, in the reviewer's opinion, 

 deserves investigation. Varnish-making is still a secret 

 trade ; and the nature of varnishes, more than any other 

 artists' materials, is involved in much obscurity. There 

 are no chemical methods known for ascertaining the 

 nature or proportion of the ingredients used in their 

 preparation, and as time is the most important factor 

 in proving the quaHty of varnishes, direct practical experi- 

 mental tests may be misleading. 



A new process for making varnish, said to be in use on 



the Continent, consists in heating the resins with the 



solvents in autoclaves under high pressure ; and there 



are also processes recommended which seem especially 



NO. I 159, VOL. 45] 



adapted for those who wish to make their own varnishes 

 on a small scale. These latter depend on the peculiar 

 change which the hard copals t undergo when exposed 

 for some time, in a state of very fine powder, to the action 

 of hot air, whereby the resin is rendered more soluble 

 without becoming much discoloured. The author de- 

 scribes such a process,, in which the finely-powdered 

 resin, after having been exposed for some time to the 

 action of air at a temperature of 220° C, is first dissolved 

 in chloroform, then mixed with oil of turpentine, and 

 after the chloroform has been distilled off, the resulting 

 solution is gradually incorporated with the drying linseed 

 oil. Although 220° C. is repeatedly mentioned as the 

 temperature to which the powdered resins have to be 

 exposed, we venture to suggest that this must be a mis- 

 take. Neither amber nor the copals will endure this 

 temperature for any time without melting and becoming 

 of a dark brown colour, or otherwise decomposed. 



On the other hand, the writer of this notice has found 

 that the various kinds of copal, which, by exposure in a 

 finely-powdered condition for some weeks to the action of 

 hot air in a steam closet, have become readily soluble in 

 chloroform or acetone, are nevertheless almost entirely 

 precipitated again on adding oil of turpentine or ben- 

 zene to such a solution, and no amount of digestion, 

 either with or without linseed oil, will redissolve them. 

 Amber thus treated behaves in a similar manner, but 

 the amount rendered soluble in chloroform or acetone is 

 much smaller. It would, then, appear that this matter 

 requires some further elucidation before this new process 

 can be made readily available. 



It has already been mentioned that there are no special 

 tests for ascertaining the quality of a varnish. Spread- 

 ing a thin layer on a sheet of glass, and then observing 

 the character of the film produced on drying, seems all 

 that can be done. It ought to become dry to the touch 

 within eight or ten hours, and not become fissured even 

 when exposed to sunshine during a year ; nor should the 

 surface become dull through the appearance of "bloom," 

 caused by the minute exudation of soHd fatty acids 

 originating from the linseed oil employed in its pre- 

 paration. 



As the drying of the solution of resins in a volatile 

 solvent depends solely on the volatilization of the solvent, 

 this process is accompanied by a shrinkage of the body 

 of the varnish, which , sets up a tendency to breaking up 

 of the surface. Linseed oil, on the other hand, becomes 

 dry or solid in consequence of combining with a large 

 quantity of oxygen, and this is attended by an increase 

 in bulk. It follows that as long as a sufficient propor- 

 tion of oil is in combination with the resin the tendency 

 to crack is compensated ; but if the artist, from habit or 

 other reasons, uses with his colours a medium deficient 

 in oil, he encounters the risk of the body of his paint- 

 ings becoming fissured in the course of time, and readily 

 subject to the destructive action of the atmosphere. 



Part III. treats of pigments, and, as we might expect, this 

 division forms a large and most important part of the work. 

 All the colours which are made use of in painting are here 

 described in detail, and nothing seems to be omitted 

 which may serve to instruct or guide the artist and 

 student. The historical reference with which the descrip- 

 tion of each of the colours is introduced is followed by 



