This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
Asphaltum, called also Bitumen, Mi-neral Pitch, Jews' Pitch, etc. is a resinous substance rendered brown by the action of fire, natural or artificial. The substances employed in painting under this name are residua of the distillation of various resinous and bituminous matters in preparing their essential oils, and are all black and glossy like common pitch, winch differs from them only in having been less acted upon by fire, and in thence being softer. Asplialtum is principally used in oil-painting; for which purpose it is first dissolved in oil of turpentine, by which it is fitted for glazing and shading. Its fine brown colour and perfect transparency are lures to its free use with many artists, notwithstanding the certain destruction which awaits the work on which it is much employed, owing to its disposition to contract and crack by changes of temperature and the atmosphere; but for which it would be a most beautiful, durable, and eligible pigment. The solution of asphaltum in turpentine, united with drying oil by heat, or the bitumen torrefied and ground in linseed or drying-oil, acquires a firmer texture, but becomes less transparent, and dries with difficulty. If also common asphaltum, as usually prepared with oil of turpentine, be used with some addition of Vandyke brown, umber, or Cappagh brown ground in drying-oil, it will acquire body and solidity which will render it much less disposed to crack, and give it the qualities of native asphaltum: nevertheless, asphaltum is to be regarded in practice rather as a dark varnish than as a solid pigment, and all the faults of a bad varnish are to be guarded against in employing it. This pigment is now prepared in excessive abundance, as a product of the distillation of coal at the gas manufactories.
A specimen of the native bitumen, brought from Persia by Lieutenant Ford, of which we made trial, had a powerful scent of garlic when rubbed. In the fire it softened without flowing, and burnt witli a lambent flame; did not dissolve by heat in oil of turpentine, but ground easily as a pigment in pale drying-oil, affording a fine, deep, transparent brown colour, resembling that of the asphaltum of the shops; dried firmly nearly as soon as the drying-oil alone, and worked admirably both in water and oil. Asphaltum may be used as a permanent brown in water, and the native kind is also superior to the artificial for this purpose.