January 14, 1892] 



NATURE 



!43 



a discussion of its chemical composition, and an outline 

 is given of the most approved methods of preparation. 

 Wherever desirable the chemical test is described by 

 which the purity of the colour may be recognized and 

 any adulteration detected ; but what will be esteemed as 

 most valuable are the observations and suggestions which 

 bear immediately upon the physical and chemical proper- 

 ties concerned in their special applications. Here the 

 author gives us, in most cases, the results of his own 

 experiments, and especially when he deals with the ques- 

 tion of the permanency of the colours. The changes 

 which the colours are liable to, whether under the influence 

 of light, or when mixed with other colours, or under the 

 conditions in which they exist in a painting, are likewise 

 fully gone into and considered. Beginning with the 

 white pigments, the author discusses the flake-white or 

 white lead which occupies a foremost place on the artist's 

 palette on account of its superior whiteness, its excellent 

 working qualities, and the power it possesses of imparting 

 its own strongly siccative character to slow-drying oil 

 colours. He also draws special attention to a valuable pro- 

 perty of white lead which has scarcely been clearly recog- 

 nized, and which depends upon the formation of a kind of 

 lead-soap in the oil colours it is mixed with. This lead-soap 

 imparts a degree of toughness and elasticity to the body 

 of the painting, and thus prevents it from cracking when 

 become dry with age. On the other hand, white lead, 

 like all lead compounds, even when locked up in the 

 medium of the paint, is liable to become brown and dark- 

 coloured when exposed to the action of even minute 

 traces of sulphuretted hydrogen, and this darkening is 

 much favoured by moisture and the absence of light. 

 According to the author, an admixture of baryta white 

 (barium sulphate), in the proportion of one to two, very 

 much lessens this deleterious effect. Zinc white, although 

 free from the last-named defects of white lead, and of 

 great value in water-colour, tempera, and fresco paint- 

 ing, has, nevertheless, only to a limited degree become a 

 substitute for white lead in oil painting on account of its 

 being a bad dryer, its possessing a lesser degree of 

 opacity, and its tendency to crack and scale. 



In the chapter on yellow pigments, the author's remarks 

 on the peculiarities of the different shades of cadmium yel- 

 low deserve especial notice. We may here add, in passing, 

 that any free sulphur in the raw cadmium yellow may be 

 readily removed by carefully heating it in an open vessel. 

 Aureolin, or cobalt yellow, is very favourably spoken of by 

 the author, and so is baryta yellow and Indian yellow, but 

 these have to be applied with caution when used along 

 with certain other pigments to which he refers. 



Very instructive are his observations on Naples yellow, 

 an old favourite of the artist's ; also those on yellow lake 

 or brown pink, which, as he remarks, ought to be rigor- 

 ously excluded from the artist's palette. The chrome 

 yellows are dismissed with a very short notice, for, beau- 

 tiful as they are, they are quite unfitted for use in tempera 

 or water-colour painting, and although showing some 

 degree of permanency when locked up in a resinous 

 vehicle and protected by varnish, they are peculiarly 

 liable to change when mixed with pigments of organic 

 origin. 



Passing on to the red pigments, we find a very full de- 

 scription of vermilion, a colour of great importance, but 

 NO. 1 1 59, VOL. 45] 



unfortunately of a very capricious behaviour in respect of 

 permanency. It is a remarkable fact that vermilion pre- 

 pared from the native mineral cinnabar is found perfectly 

 preserved in old Italian tempera paintings, and even in 

 wall-paintings of Pompeii ; whilst the artificial vermilion 

 now generally in use, whether prepared by the dry or by 

 the moist way, and although of finer tint and chemically 

 pure, is very liable to change. All we know at present 

 on this subject is that this is due not to a chemical 

 alteration, but merely to a physical or molecular change 

 of the red crystalline into the black modification of 

 sulphide of mercury, and that this transformation is 

 especially favoured by strong sunlight. Different samples 

 of artificial vermilion, however, frequently exhibit a 

 different degree of liability to change, and as the more 

 finely ground orange vermilions are generally found to 

 be the less stable, it has been inferred that the fine state 

 of division favours the change. In reference to this we 

 may, however, mention that if such pale vermilion is 

 heated for some hours at a temperature approaching 

 volatilization, it gradually assumes a darker red colour, 

 and becomes crystalline like the sublimed cinnabar ; and 

 yet by this treatment its liability to turn black in sunlight 

 is not lessened, but on the contrary much increased. It 

 is evident that a great deal has still to be learned about 

 this pigment. 



The pigments derived from madder possess particular 

 interest, and are remarkable for their stability as well as 

 for their early introduction in the arts. The author 

 states that such colours were known in Europe as early 

 as the thirteenth century ; but red madder pigments 

 have been found in some quantity at Pompeii, and more 

 recently also amongst ancient colour materials in Egypt. 



Alizarin and its allied colouring principles, which are 

 the essential colour-giving constituents originally derived 

 from madder-root, are now manufactured artificially from 

 anthracene, and most of the madder colours are at present 

 obtained from this source. Nevertheless, it is a note- 

 worthy fact that the choicer kinds of the madder pigments 

 used by artists are still prepared from madder- root, 

 although artificial alizarin, purpurin, and anthrapurpurin 

 in a chemically pure state are accessible to the colour- 

 maker at a cheaper rate. It is not clear whether it is 

 merely habit of the colour-maker, and a difficulty of adapt- 

 ing the use of the pure colouring-matters to the old 

 methods, or whether it is the presence of some still un- 

 recognized constituent of madder-root, which causes the 

 root to be preferred in the production of fine artists' 

 colours. It is, perhaps, of interest to mention here 

 that a difference in the behaviour of the artificial 

 colouring-matter is also observed in the difficulty of 

 applying it on wool or woollen cloth so thoroughly fast as 

 to stand the operation of fulling. On this account the 

 red cloth of the French army is still dyed with madder, 

 and probably this is the true reason why the cultivation 

 of this plant is still to some extent continued. 



The study of the chemistry of the Turkey-red dyeing 

 has fully established the important part which the pre- 

 sence of fatty matter, in some peculiar condition, plays in 

 this remarkable process. That this also holds good with 

 pigments can be shown by trying to produce these 

 colours from artificial alizarins without using such fatty 

 substances ; and this may in some way account for the 



