456 THE ROLE OF CHEMISTRY IN PAINTINGS. 



the oil ; it is even possible that to these traces may be due the darken- 

 ing of varnishes, the " museum tone " of which we have already 

 spoken. We have recourse, then, to driers of manganese base, wdiich 

 to-day leaves nothing to be desired. 



We have sought to replace white lead — a poison, as are all lead 

 compounds — by zinc white, ZnO, which, while forming compounds 

 with sulphur, does not blacken, for zinc sulphide, ZnS, is white. 

 Unfortunately, zinc white does not spread as well as white lead; at 

 equal weight of pigment it will cause to disappear, even quicker than 

 white lead, a subjacent painting; but this same weight ought, then, 

 to be applied in a greater number of layers, for zinc white does not 

 mix well with oil. This is a great difficulty. Fixed white or sul- 

 phate of barium, SO^Ba, absolutely unalterable, has not given the 

 best results. It was then that '' lethophone " was discovered. 



Lethophone is a mixture of sulphide of zinc and sulphate of 

 barium, both Avhite substances, which are formed when we make 

 solutions of sulphide of barium and sulphate of zinc. BaS-f-SO^Zn^ 

 ZnS-fSO^Ba. 



Unfortunate!}^, though the sul^ohide of zinc and the sulphate of 

 barium taken alone are absolutely unalterable prepared in this man- 

 ner, and without one knowing why, they give a mixture which dark- 

 ens in the light. It is true that the Avhiteness returns after the letho- 

 phone has been placed in darkness. This curious property was none 

 the less an obstacle in its employment. This obstacle no longer exists, 

 for Doctor Ostwald has found the means of avoiding the darkening 

 and guarantees the absolute whiteness of the painting in lethophone 

 should it be exposed three hundred years in full sunlight. This is, 

 it seems, the maximum length of time for which the best of colors 

 are guaranteed. 



All these alterations are serious, but they a^e not definite or with- 

 out remedy, for, strictly speaking, treating the pictures carefully 

 with hydrogen peroxide, they can be whitened. Another serious 

 thing is the cause which produces the cracking and which is due to 

 the diversity of the nature of the layer of paint and its support. 



This support is generally a canvas, which contracts under the effect 

 of dampness and cold, while the layer of paint, on the contrary, 

 expands under the same conditions. When the layer of paint is 

 thin, it maintains a certain elasticity and lends itself to the changes 

 of dimensions, which the variations in temperature and humidity 

 produce ; if it is too thick, it breaks. To quote Doctor Ostwald : 

 " The durability of a picture is inversely proportional to the thickness 

 of the layer of color." 



All the old paintings that have come down to us without cracking 

 were made very thin. This was the style of the early painters, as 



