THE PROPERTIES OF PIGMENTS 89 



cakes), supplement the fine powder-colours for the same purpose, 

 and are also used for diagram - making and other artistic 

 purposes. 



The artists' oil-colours in tubes supplement both, but are 

 useful in a much less degree, for the above purposes ; they are 

 indispensable for the legs and bills of birds, soft parts generally 

 of all animals, colouring of models of fishes, reptiles, and in- 

 vertebrates, skulls, etc., for index collections, small or delicate 

 dried ferns or grasses, and small masses of rockwork. All 

 except the common powder-colours are absolutely permanent, 

 i.e. unchangeable either by the action of direct or transmitted 

 light, impure atmosphere (resisting one charged with sulphur- 

 etted hydrogen, etc.), or by mixture one with the other, or 

 with flake white. The only colours which require partial 

 safeguarding are the madders, which must not be mixed with 

 flake white but with zinc white to be absolutely permanent, 

 nor must they be mixed with any of the ochres, nor, indeed, 

 with any colour which, like these, contains iron, nor with any 

 colours containing lead.'' 



The properties of pigments have been studied for many 

 years in the management of the Leicester Museum, and, before 

 the writer went into this subject thoroughly, the changes of 

 colour were remarkable under the influence of direct light, and 

 of town atmosphere sometimes charged with deleterious gases. 

 For instance, the changes in the colours of certain elements of 

 the skulls exhibited in the index collection were noteworthy. 

 The pre-maxilla, coloured formerly in a fairly permanent blue 

 mixed with chrome yellozv (a lead pigment), although at first a 

 fine green colour, blackened rapidly. The maxilla, coloured with 

 chrome yellow, also blackened to an ugly dirty yellow. The 

 frontal, a pale lavender made by mixing permanent blue with 



1 For further and interesting details of the properties of colours, consult Field's 

 Chromato^aphy (last edition) : Winsor and Newton. 



