Prussian Brown.—An artists’ colour known by this name is prepared from Prussian blue, but as it has no superiority over Vandyke brown or umber, and is higher priced, it is not in general use. It consists essentially of carbon and ferric oxide, resembles bistre in tone, and possesses durability and good covering powers. The operation of calcining the Prussian blue should be conducted slowly, and is best performed in a closed vessel, though it may also be done in the open. The pieces of blue should not be larger than a hazel nut. They soon split, scale off, and become red, when the heating should cease. On breaking the cooled particles they will show a patchy coloration varying from yellowish-brown to black. On grinding, the mass assumes the desired brown hue.
Rubens Brown.—Another name for Cassel brown or Cologne earth (q. v.).
Sepia.—This is one of the few pigments derived from the animal kingdom. It is produced by several sea-inhabiting creatures belonging to the class called Cephalopoda, and more particularly by two members of the genus Sepia, known respectively as Sepia officinalis and Sepia loligo. A peculiarity of these cephalopods is that they are provided with what is commonly called an ink bag, in other words a gland or sac filled with a blackish-brown liquid, which possesses intense colouring power. The object of the secretion is the protection of the creature from pursuit by its enemies, a portion of fluid being discharged at will, and so obscuring the surrounding water that escape is facilitated.
For the sake of this pigment the cuttle-fish are sought after by fishermen in the localities frequented by the animals, notably in the warm waters of the Mediterranean. When the creatures are captured, their glands are carefully extracted and sun-dried so as to solidify the contents. In this state ink bags are sent into commerce. The colourman subjects the sacs to boiling in a solution of soda or potash, whereby the colour is dissolved out of the receptacle, and being filtered clear of all fragments of the animal tissue, is next precipitated by the addition of acid, collected on a filter, washed, and dried. It then forms an exceedingly useful pigment, having, according to Prout, the following average composition:—
| Per cent. | |
| Black pigment (melanin) about | 78 |
| Calcium carbonate | 10½ |
| Magnesium carbonate | 7 |
| Alkaline chlorides and sulphates | 2 |
| Organic matter | 1 |
It is remarkably permanent for an organic substance, suffering no alteration on being combined with other pigments, and withstanding the effects of exposure to air and light. Though slightly transparent, and not quite constant in tint, it possesses very great colouring power. Being of extremely fine texture it can be worked up equally well as an oil colour or as a water colour, but it is especially in the latter capacity that it forms an indispensable artists’ colour, and permits the production of a great range of shades and tints.
Ulmin.—The pigments grouped under this name are also of organic origin; but though they possess good colour, mix well, and flow readily from the brush, they lack the durability which is essential to their successful use. The following methods have been employed in their preparation:—(1) Fused caustic potash is digested in alcohol, and the liquor filtered and heated till a brown powder is thrown down, which is filtered and washed with acidulated water; (2) Waste cotton, peat, or brown coal, heated with an alkali; (3) Farinaceous matters carbonised by mineral acids.
Umbers.—These form a large class of natural earths of a brown colour, differing widely in the proportions of their chief constituents, but closely allied to the ochres and siennas in general composition, and owing their colour mainly to the presence of hydrated oxides of iron and manganese, the latter prevailing in the umbers to a greater degree than in the ochres and siennas, which consequently belong to the yellow group (q. v.).
Beds or veins of umber of varying thickness and extent are found in many places, especially in connection with magnesian limestone (dolomite). Apparently they are often derived from decomposition of this rock, perhaps due to the infiltration of carbonated water, which has acted upon the calcium and magnesium carbonates in the dolomite, and left the silica and the iron and manganese as oxides, forming the bulk of the umber. Usually these beds of umber are near the surface, though covered by an overburden of vegetable soil, and the operation of working them may be called quarrying rather than mining, being of a superficial and simple character, often only amounting to small pits.