strongly heating Aluminum Hydrate it is converted into the oxide. Owing to its physical characteristics it is the ideal base on which to precipitate many organic dye-stuffs to form transparent lake pigments. See description of lake pigments, page 74. AMERICAN VERMILION Imitation Vermilion, Vermilionette, Persian Red, A color of varying composition. consisting usually of a base of Red Lead, tinted with Eosine dye, to yield a brilliant substitute for the genuine quicksilver vermilions. The greater number of American Vermilions are not very permanent to light; however, there are some in which the fugitive Eosine dye, has been displaced by more permanent toners, yielding pigments very permanent (to light). Owing to the base of red lead, it is opaque and it is to be classed with all the lead colors; to be used only with limitation. Discolors in presence of sulphurous gases. Heat destroys the color. Can be recognized from pure mercuric vermilions by their partial solubility in akalies, or dilute acids, or upon treatment with alcohol, by parting with most, or all, of their dye coloring matter, the genuine quicksilver vermilions resisting these reagents. Sodium Sulphide converts the lead constituent to black lead sulphide. Red Lead and Chrome Red with barytes or lead sulphate are the most common carriers used with toning colors, to produce very rich hues of American Vermilion. Chrome Red, the most basic lead chromate PbCrO4. PbO-is sometimes sold as American Vermilion, but owing to its high specific gravity it separates quickly when ground in oil and is only used in fresco with a certain amount of success. Chrome Red has also the limited fastness of all lead colors. In oil, American Vermilion, is a good dryer and in the heavy paint trade is especially recommended for its rust protective qualities, when applied on metals. ANILINE COLORS (See Coal-Tar Colors) ANTWERP BLUE Bleu mineral, Bleu d' Anversa, Azul de Amberes, Prussian Blue, weakened in strength, with Alumina. Antwerp Blue, a product of iron and zinc cyanogen compounds, yielding a color paler than Prussian Blue, has more recently been replaced with above mentioned Prussian Blue, reduced in color strength with Alumina. The properties of this color are almost identical with those of Prussian Blue (which see). ASBESTINE Hydrated Metasilicate-3MgO.2SiO2.2H2O. Used principally in the heavy paint trade as a neutral carrier, or extender in ready mixed paints, to prevent settling of pigments heavy in gravity. Finds no practical use as a pigment for fine arts. Asbestine differs from Asbestos, a calcium magnesium silicate of varying composition, by its short fibre, the asbestos having a long fibre. To the native Magnesium Silicates belong also Talc, Talcum, Soapstone or Steatite, French Chalk (a hydrated Magnesium Silicate; 3MgO.4SiO2.H2O) and also another variety of this compound, Meerschaum. These compounds are not very important in the fine arts, being of very soft body, transparent and do not yield brilliant lake colors, when used as bases. Talc finds use for polishing marble and gypsum articles and also in the manufacture of face powders, etc. ASPHALTUM Bitumen, Mineral Pitch, Antwerp Brown, Mum- A natural mixture of solid hydrocarbons, very often containing also sulphur and nitrogen compounds, generally considered to be a product of the decomposition of animal and vegetable matter. Under this group we have a variety of pitchy, tarry substances ranging in color from brown to black, found principally native in the great pitch lake of Trinidad. The best quality, used for artists' paints, is obtained from the Dead Sea. This Syrian Asphalt is of rich brown color, exceedingly hard with bright and lustrous surface fracture, melts at 135° C. with a characteristic bituminous odor; is slightly soluble in Alcohol or Ether, more so in Benzol and dissolves entirely in Turpentine or Linseed Oil. Strong acids, such as Nitric Acid or lyes, do not affect this pigment, which property makes it particularly useful in the manufacture of etching grounds. Asphalt as a color pigment is used only as an oil color. By virtue of its solubility in the vehicle of oil, yielding a very transparent paint, it resembles the transparent lake colors. The Asphaltums decompose in direct sunlight. In presence of moisture, combined action of the oxygen of the air and the actinic rays of light, favor the combination of the oxygen and hydrogen, setting the carbon free, thus producing a dull black color. This is especially noticeable in admixtures of Asphaltum and white pigments; the resultant warm tints on exposure to light are changed to dull grays. When used for under-painting, the Asphaltums 'bleed' or diffuse through the overlying layers of paint, darkening them, this being due to solubility in the oils, of the asphaltums. Asphaltums are very poor dryers, even when used in conjunction with the best siccatifs. They enter largely into the manufacture of varnishes, partly as coloring principle and partly as a resinous matter. As an oil color, the Asphaltums are very treacherous when applied in thick layers. On the surface they appear dry, but even after years, exposed under strong summer heat, they soften and "slide" on the surface of the painting. The Asphaltums, especially when applied in heavy layers, tend to crack on the surface. Carefully selected Asphaltums when properly treated to extract the number of liquid, semi-liquid and solid colorless hydrocarbons, which have no color-giving value as a pigment, are not given to cracking or moving on the canvas like the unpurified material and if used with precaution will yield fairly good results; however, not to be recommended where permanency is desired. The modern coal-tar pitch browns, offered as substitutes for the Asphaltums are far more treacherous on account of their easy fusibility and are more liable to stain contiguous pigments by reason of their solubility in oils or varnishes. The safest substitute for Asphaltum is found among the Alizarin Lake Colors, or mixtures of Raw Sienna, Bt. Umber and Carbon or Ivory Black. Mummy is a bituminous product associated with animal remains and derives its rather gruesome name from its source. The bituminous matter used in the embalming of the Egyptian mummies is perhaps the active coloring principle. Through the lapse of time the bituminous matter of the mummy, having been subjected to a considerable degree of heat, has lost some of its volatile hydrocarbons and ranks as a pigment superior to the raw and inferior to the refined Asphaltums. This color is gradually becoming extinct in the fine arts, as it does not possess any quality not embodied in the refined Asphaltums. It was usual to grind up the entire mummy, thus the resulting powder had more solidity, was less fusible than the native Asphalt and less transparent than the refined Asphaltum. Mummy was used as an oil paint as early as the close of the sixteenth century. AUREOLIN Cobalt Yellow, Jaune Indian Aureolin, Aureolina, Kobaltgelb A yellow crystalline compound salt of the metal Cobalt. Double Nitrite of Cobalt and Potassium-Co2 (NO2) 6. 6KNO 2. If precipitated in chemically pure condition, as a fine powder, the pigment is of pure yellow color, transparent, very slightly soluble in water. On account of its complex chemical composition if not properly washed of soluble salts, it is not absolutely dependable. It is very fast to air and light, and is unaffected by sulphuretted hydrogen, H2S, although blackened by sodium sulphide or caustic soda. Like many other mineral pigments Aureolin when in admixtures with certain organic pigments, such as the lakes from cochineal, indigo, etc., causes decomposition of these; the organic colors becoming altered in hue as also the Aureolin itself becoming of a brownish hue. Various other yellow pigments of different chemical composition appear on the market under the name of Aureolin. The genuine Cobalt Yellow is only adaptable to Oil and Water Color techniques and if chemically pure is considered permanent. Cobalt Yellow is converted into black cobalt oxide by gently heating in a bunsen flame. Like all cobalt compounds a blue glass bead is formed, when heated with borax on a platinum wire. This pigment was introduced about 1861. |