COLOURS. This very important article includes a variety of matters of peculiar interest to various professions, and re quiring no inconsiderable portion of study. We have only seven natural colours, namely, red, orange, yellow, green, blue, indigo, and violet. See CHROMATICS.
The mathematical use ofcolours is more immediately under our present conside ration. These are either what are called body, or transparent : the former applies to such as have a certain substance, being like very thin paste, and coating the ob ject to which they are applied: these are again divided into oil and water colours. Transparent colours are made either of expressed juices, corrected by inspissa tion, or of the finer particles of earths, gums, &c. highly prepared by levigation, washing, &c.
Oil colours are made by mixing the co louring substances with prepared oils ; that is, such as dry readily, and are at the same time so fine and transparent as not to injure the brilliancy or clearness of the colour. Nut-oil is on this account highly esteemed ; but in a recent publi cation (the seventh number of the Agri cultural Magazine) we are informed, that sun-flower oil possesses qualities of great moment to the painter, and to various other artists. The colouring matter must be minutely mixed with the oil, so that it may work perfectly free and smooth.
Body colours for the limner's use should be prepared of the purest materials, and be triturated in a mortar, and on a slab with water, until such time as the mixture is completely smooth,and leaves no rough ness when rubbed between the thumb and fore-finger: not, however, without making allowance for some particular substances, especially minerals, which, however well they may have been prepared, will occa sion a roughness to the touch. Body co
lours are usually sold in bottles, ready mixed to their proper consistence, and sometimes in cakes, with a small portion of gum Arabic dissolved in the water. Oil colours arc most frequently sold in kegs, and ready ground, but requiring an addition of oil before they can be work ed : these are generally for the use of house painters, &c. : those for the more delicate purposes are usually kept in bladders.
Transparent colours should be so clear, when mixed with abundance of water, as to communicate a strong tint, without in the smallest degree plastering or conceal ing the paper, &c. : hence their designa tion. The best of every kind are made from either vegetable or animal substan ces ; minerals being extremely difficult to prepare, equally so to work with water, and many of them very subject to change. We shall give a concise account of the materials in general use ; observing, that there are an, immense number of com pound colours, sold under various names, that may be made from the following list of simples: