The pigments comprising this group include asphaltum, bistre, bitumen, Caledonian brown, Cassel or Cologne earth, mineral or metallic brown, sienna, raw and burnt; umber, raw and burnt, sepia and Vandyke brown. Asphaltum is of far greater interest to the varnish maker than it is to the color grinder, the latter's chief use for it being for the artist's color list. For this purpose only the genuine Egyptian or Syrian asphaltum, as the product from the Dead Sea is known to commerce, should be selected as other grades vary too much in their nature, containing too great a percentage of hydrocarbons.
To prepare asphaltum for artists use in the safest manner is to take the grade referred to and subject the crushed material to a slow heat in an iron kettle, gradually raising the temperature to 480 deg. F. until it becomes like a cinder, then, when cooled, crushing it to a powder, which is soaked over night in enough spirits of turpentine to cover it and then mixed with and ground in borate of manganese boiled linseed oil of good body until impalpably fine. In this way the color is not treacherous as are some of the asphaltums prepared as an oil color for artists with the addition of wax, shellac or Venice turpentine. When asphaltum is wanted for water color painting or for overgraining in distemper, it is treated by heat as above to drive off the hydrocarbons, when the powder may be stirred in a solution of strong ammonia, thus giving up its coloring matter to a great extent. The solution is then precipitated by adding acetic acid, the top liquor decanted and the precipitate washed to remove both alkali and acid as much as possible. This done a little gum arabic or dextrine and glycerine is added and the mixture partly dried to acquire the right degree of consistency. Aside from this the color grinder has not any use for as-phaltum, excepting in the form of varnish that comes to him in ready prepared style.
Bistre is used as a water color only and very rarely at that. It is the collected soot of beechwood, and when well washed with hot water until no more soluble matter is extracted, the soot is dried and ready for use after the gum water and glycerine are added. It is sold as moist color in cakes, has a rich brownish-yellow tint and is used as a wash for water color paintings, but is not very permanent.
Bitumen is simply another name for asphaltum, and what we said about the latter applies here. Bitumen of Judea is simply the asphaltum procured from the Dead Sea, where it is said to be cast up by the water.
Caledonian brown is hardly known here and may be simply classed as a native brown oxide of iron, consisting of quite a high percentage of manganese oxide in combination with iron oxide. In the raw state it almost resembles a light shade of metallic brown, but when calcined or burnt it looks like an inferior Vandyke or Cologne brown.
Cappagh brown, which is referred to at length in some of the text books from the other side of the Atlantic, has very little interest to the color grinder on this side, there being no demand for colors of that description, and we cannot really see why an artist should have a desire to have colors of the character of this type on his palette, no more so than he should care for Prince's metallic or other mineral browns. However, an analysis of Cappagh brown by A. H. Church, in his "Chemistry of Paint and Painting," would indicate that this pigment only differs from Turkey or Cyprus umber in so far that it shows a much greater percentage of manganese dioxide and less ferric oxide, while the total percentage of the two oxides is about the average of the totals found in umber, when the difference in hygroscopic and combined moisture is taken into consideration. Cassel or Cologne browns (or earth) are pigments of natural origin. They vary in composition and are imported into this country under the name of Vandyke brown and will be considered under that caption.