244 



NA TURE 



[January 14, 1892 



author's failure to obtain the best possible result from 

 pure alizarin, although prepared from madder-root. 



Although madder colours are deservedly esteemed for 

 their stability, the author has proved, by a series of 

 experiments with different samples of commercial 

 moist and cake colours, that they are by no means 

 all alike in this respect. Considering their great im- 

 portance for the artist, as being unique in their special 

 qualities of tint and permanency, it appears highly de- 

 sirable that a thorough investigation should settle the 

 question as to the composition of the most stable of them, 

 and thus furnish a guide to the preparation of entirely 

 trustworthy colours. A formula quoted by the author on 

 p. 154 for the preparation of madder carmine seems in- 

 complete, as no mention is made of any alum or alumina, 

 which is an essential constituent of this pigment. 



Passing on, we take notice of a short account of lac dye 

 and cochineal lakes, which, along with the ancient 

 kermes, are furnished by three different species of coccus. 

 Of these only the cochineal carmine is of interest ; but, 

 rich and beautiful as is this colour, the author very justly 

 warns the artist against its use on account of its want of 

 stability. We may take this opportunity of observing 

 that pure carmine, notwithstanding its ready solubility in 

 ammonia, is a true lake, and, in fact, is a definite com- 

 pound of one of the carminic acids with lime and alumina. 



In the chapter on blue pigments, the article on indigo 

 deserves special attention ; and many will be disappointed 

 to find that this highly esteemed colouring-matter, 

 generally supposed to be one of the most permanent, 

 does certainly not deserve this character when used in 

 painting. To this pigment is largely due the faded con- 

 dition of the works of artists who have used it extensively. 

 This proneness to fade is especially noticeable in water- 

 colour paintings where the sulphindigotates have been 

 employed. 



Considering the lamentable deterioration of some of the 

 works of artists who have used asphaltum too freely, it is 

 satisfactory to learn that this substance may be safely 

 employed if prepared as directed by the author. In this 

 connection mention is also made of a brown water-colour 

 said to be an ammoniacal extract of asphaltum ; but we 

 venture to say that this must be some trade mystification, 

 for genuine asphaltum does not yield its colour to 

 ammonia, or indeed to any other alkaline solvent. 



After the detailed description of the pigments, a chemi- 

 cal classification of them is given, which in the main is 

 an arrangement based upon their chemical composition, 

 and which more prominently exhibits the characteristic 

 chemical and physical difference of deportment of the 

 various groups in which the pigments may be classified. 



In chapter xxi. tables are given in which the pigments 

 are ranged according to their stability into three classes, 

 and it may be seen at a glance what degree of per- 

 manency is ascribed to any of them. 



Chapter xxii. is full of most important considerations 

 and useful suggestions with regard to selected and con- 

 structed palettes ; and after discussing the lists of pig- 

 ments used by some well-known artists, the author gives 

 his selected palettes for oil and water colours. 



In chapter xxiii. the chemistry of the various methods 

 of painting in tempera, fresco, stereochromy, oil and 

 water colour, and pastel, are discussed and explained; 

 NO. I 159. VOL. 45] 



and under oil painting the complicated changes which 

 the vehicles and their constituents undergo during the 

 process of painting are recapitulated. 



Chapter xxiv. contains a study of old paintings and 

 drawings, with the special object of giving a brief account 

 of the materials used in the production of these early 

 works, and comments on the lessons to be drawn from 

 individual pictures. 



Chapter xxv. is devoted to a discussion of the best 

 means for conservation of pictures and drawings, with 

 some useful hints on restoration ; and in the concluding 

 chapter xxvi. we have an account of trials of pigments as 

 to their permanency, carried out by the author and other 

 investigators. This is a subject which in recent times has 

 received a good deal of attention, and although the in- 

 vestigations dealing with this matter are by no means 

 concluded, very important results have already been 

 obtained. 



This is especially the case with regard to water-colour 

 paints, which, on account of their thinness of application 

 and absence of a protecting medium, are much more 

 susceptible to fading, and therefore give greater facility 

 for examination than the oil colours. It is now well 

 understood that the action of light in conjunction with 

 moisture and air is the principal cause of the fading 

 of the colours ; and were it practicable to keep water- 

 colour drawings absolutely dry inclosed between glass 

 plates, front and back, their preservation would be much 

 prolonged. With oil paintings the conditions are dif- 

 ferent, and in some respects more favourable, especially 

 when the painting has once reached its proper stage of 

 maturity— that is to say, when the volatile vehicles have 

 entirely passed away, and the linseed oil has completed 

 its course of drying and oxidation. Whilst the process 

 of drying is still going on in the early years of the 

 existence of an oil painting, some amount of moisture 

 is produced within the body of the painting as a direct 

 product of the oxidation of the linseed oil, and con- 

 sequently a picture too freely exposed to sunlight in this 

 stage will be much more liable to suffer and to change 

 than later on, when the resinous and oleaginous vehicles in 

 which the particles of colour are locked up have become 

 firm and quiescent. To fully appreciate this we must 

 bear in mind that even the varnish and resinous vehicles 

 do not so completely exclude air and moisture from 

 without as is commonly supposed. 



There is still much to be learned with regard to the 

 chemical processes involved in the so-called drying of 

 linseed oil, and this subject deserves a thorough re- 

 investigation in the light and with the means of modern 

 chemical research. It is, for instance, a well-known fact 

 that linseed oil under certain conditions becomes itself a 

 most powerful oxidizer, so much so that canvas or paper 

 soaked with it will become destroyed in the course of 

 time, and it seems that this effect is especially marked 

 when oil of turpentine has been used along with it. 

 It is quite conceivable that this activity of linseed oil 

 may be one of the agents at work in the deterioration of 

 oil paintings ; but whatever dangers may arise from this, 

 the use of linseed oil cannot be dispensed with. 

 It is otherwise with oil of turpentine, for which a 

 very much superior substitute might be found in the 

 higher members of the benzene series, which could 



