BITUMEN
BITUMEN and its homologues, such as Vandyke Brown, Cassel Brown and Asphaltum, should, under no circumstances, be used by any artistic painter. If you will look up the literature of photography before the days of the daguerreotype, photographs were taken on Bitumen, because it was so sensitive to the light that within a day a negative or positive imprint could be obtained by coating a sheet of silver or glass with a Bitumen solution. Where the light acted on the Bitumen it became Black and insoluble, and where the light did not strike it, it remained Brown and soluble. More damage has been done to artistic painters by the use of Bituminous pigments than by any other.
Dupré and Jacque, of the Barbizon School, are two examples of painters whose work deteriorated through the use of Bitumen. It may be true that a pleasing effect is obtained when Bitumen is employed as a glazing material, but in time the picture darkens, and restoration is impossible—first, because of the solubility of certain parts of the Bitumen that have not been acted upon by light; and second, because any attempt to remove the part that has turned Black destroys the original painting.
Vandyke Brown and Cassel Brown contain Bituminous materials, and Asphaltum is the same thing as Bitumen.