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GROUP VI

Online Encyclopedia
Originally appearing in Volume V21, Page 600 of the 1911 Encyclopedia Britannica.
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GROUP VI. Silicates.—Terre verte, which is a natural green ochre containing a silicate of iron, potassium and magnesium, and one other silicate, smalt, an artificial glass containing a silicate of cobalt and potassium, constitute this small group. However, some of the ochreous earths contain silicates of iron, manganese and aluminium, as well as hydrates of the two former metals, and so have some claim to be ranked with the silicates. GaouP VII. Chromates.—These salts are rich in oxygen. When in contact with some of the more alterable organic pigments belonging to Group IX. the chromates may lose oxygen, acquiring a somewhat greenish or greyish hue, owing to the formation of the lower or green oxide of chromium. The chromates cannot be trusted as pigments. The yellow chromates, those of barium, strontium, zinc and lead, are represented by the general formula 111"CrO4; chrome red is basic, and is Pb2CrO5. Gaon'. VIII. Various Inorganic Salts.—This group is intended to receive a number of pigments which are solitary, or almost solitary, examples of various classes of salts. There is one cobaltinitrite, aureolin (K3Co(NO2)s, associated with one or more molecules of water), called sometimes cobalt yellow; one antimonate, that of lead, the true Naples yellow; one tungstate, that of chromium, known as tungsten green; a metaphosphate of manganese, which goes under the name of Nurnberg or manganese violet; and several mixed cobalt compounds containing arsenates and phosphates of that metal, and represented by cobalt violet and Thenard's blue. Two sulphates also belong here, namely, baryta white (BaSO4) and lead sulphate (PbSO4) ; also Schweinfurt green, a basic copper arsenite. It is obvious that of the members of so miscellaneous a group of pigments no general characteristics can be predicated. But it may be stated that the two sulphates, the tungstate and the cobalt compounds are practically inert and unalterable, while the copper arsenite and the lead antimonate are sensitive to the action of sulphur and of sulphides. The cobaltinitrite, aureolin, cannot be safely mixed with some of the organic pigments belonging to the next and last group. Gaoue IX. Organic Compounds.—Most of the members of this large and unwieldy group of pigments possess this character in common, proneness to oxidation and consequent deterioration in the presence of light, moisture and air. Such oxidation is accelerated by the action of some highly oxidized pigments belonging to other groups. such as the chromates of Group VII. and aureolin of Group VIII., this action being particularly marked in the case of the yellow lakes, the cochineal lakes and indigo. There are two pigments consisting of copper salts in this group. They are verdigris —both the blue-green and the green varieties being basic copper acetates—and the pigment known in England as emerald-green, which is a basic cupric aceto-arsenite. These copper pigments present the usual sensitiveness to the attack of sulphur which distinguishes compounds of this metal, and cannot therefore be safely mixed with the members of Group III., and more particularly with the cadmium colours. About nine members of Group IX. may be regarded as substantive pigments. These include Indian yellow (mainly magnesium and calcium euxanthates), gamboge, sap green, indigo. Prussian blue, bitumen or asphalt, bistre, sepia, and the bituminous variety of Vandyck brown. The adjective pigments include a great variety of lakes where different kinds of colouring matters of more or less acid character have been thrown upon a base, generally of colourless aluminium hydrate, aluminium phosphate, stannous hydrate, stannic oxide, bartya or lime; sometimes coloured bases containing such metals as copper, chromium, manganese or iron are introduced in small quantities. The colouring matters used are both natural and artificial. Amongst the former may be named Indian lake, from the resinous exudation produced in certain trees by the attacks of Coccus lacca; carmine, crimson and purple lake, from the colouring matter obtained from the cochineal insect, Coccus cacti ; rose-madder and the madder-lakes, from the alizarin and allied bodies derived from the root of the ordinary madder plant Rubia tinctorum; and yellow lakes, from quercitron bark (Quercus tinctoria), and from Persian and Avignon berries (species of Rhamnus or Buckthorn). The lakes derived from alkanet root, archil, Brazil wood, and red sanders wood are of very small interest and value. The same judgment may be pronounced upon the large number of artificial lakes which owe their colours to coal-tar derivatives, with the single exception ofthe important class of pigments obtained from artificial alizarin, and from its congeners and derivatives. Of these, alizarin (q.v.) itself, in its purest state and associated with alumina and a little lime, yields those pigments which possess a pink or rosy hue. When purpurin and its isomers, anthrapurpurin and flavopurpurin, are present, the red hue is more pronounced, and may even tend towards a golden colour, or, when some copper or iron or manganese is introduced, may become decidedly brown. Many of the alizarin crimsons sold as paints are not made from alizarin itself, but from the sulphonic acids of alizarin.. These lakes present a wide range of hues. Another derivative of alizarin, known as 13-nitro-alizarin, yields a rich orange lake, to which such names as pure orange, orange madder and marigold have been applied. Stability.—Some notion of the relative stability of pigments will have been derived from the remarks already made under " Classification." But as permanence is of no less importance than chromatic quality in the case of pigments used in the fine art of painting, to which the present article is mainly devoted, further particulars concerning certain selected pigments may profitably be given here. Beginning with white pigments, these three may be named as useful: white lead, Freeman's white, zinc white. As an oil-colour, white lead of the old type is generally the best to use, but among water-colours its place must be taken by zinc white in the condensed form known as Chinese white. Zinc white, in spite of the qualities which recommend its use in oil, namely, the fact of its being not only unaffected by sulphur, but odourless and non-poisonous, lacks toughness as an oil-paint, and has a tendency to scale. Freeman's white, which consists essentially of lead sulphite, is the best substitute for white lead yet devised. The small percentages of zinc white and baryta white which it contains are not to be regarded as adulterations, for they greatly increase its body, and though of less specific gravity than lead sulphate, actually raise the weight per cubic foot of the dry pigment. Out of a dozen or more familiar yellow paints, a selection may be made of these six: yellow ochre, raw sienna, mars orange, cadmium yellow, aureolin and baryta yellow. Concerning two of these, cadmium yellow and aureolin, the following observations may be set down. Cadmium sulphide, CdS, exists in two forms, which in some measure correspond to the two modifications of mercuric and antimonious sulphides. One of these forms is yellow and the other reddish orange. When sulphuretted hydrogen is sent into a weak, cold, and neutral solution of cadmium salt, the sulphide which separates is pale and yellow—the orange variety is obtained from a strong, hot, and acid solution. The pale variety is more prone to change than the darker one; but as oil colours both forms are sufficiently stable for use, provided they are pure. The value of aureolin as a pigment depends much upon its mode of preparation. A new variety of bright yellow hue was described by Adie and Wood in 1goo, and is represented by the formula K2NaCo(NO2)6, H2O. Of red pigments, six claim special mention. These are vermilion, light red, Venetian red, Indian red, red ochre, and the red lakes derived from madder or alizarin. Vermilion is stable in oils, but as water-colour paint is prone to change, under exposure to strong light, into the black modification of mercuric sulphide. The iron-reds named above, whether natural or artificial, are quite permanent, but so much cannot be said of the various madder-paints. They are of far greater stability under exposure to light than any other red organic pigments, and are absolutely necessary to the artist. It must be noted that those madder and alizarin lakes which contain an element of yellow and brown are less stable than those of a crimson hue. Five green pigments may be recommended, namely, viridian, or the emerald oxide of chromium, the ordinary green oxide, cobalt green, green ultramarine, and terre verte. Except for minor decorative work, where permanence is of secondary moment, one is obliged to exclude from the palette emerald green, green verditer, verdigris, sap-green, and the numerous preparations which owe their colour to mixtures of Prussian blue and chrome yellow, and are sold under the names of green vermilion, chrome green, Brunswick green, and so on. All these pigments usually contain much barium sulphate. Similarly, amongst blue pigments, ultramarine, cobalt blue and coeruleum may be retained, while smalt, indigo and all copper blues should be rejected. Prussian blue, or the mixture of this pigment with a white base which is usually called Antwerp blue, can scarcely be spared, but care should be taken to choose a sample containing no potassium compounds. Coeruleum, which may be described as cobalt stannate presents the peculiarity of appearing a greenish blue in artificial light, not a purplish blue like that of ordinary cobalt blue. Cobalt violet is .a sound pigment, while manganese metaphosphate or Nurnberg violet is said not to be safe in oil. Mars violet, an artificially prepared ferric oxide, is dull in hue but permanent. Passing on to brown pigments, it is matter for regret that there are no permanent colours possessing the artistic capacities of asphalt, madder brown, and the old bituminous Vandyke brown. Cappagh brown, burnt sienna, and raw and burnt umber may be employed safely. Little need be said as to the selection of black pigments, for all are permanent. The soot from burning acetylene, which has recently been introduced, forms a black pigment of remarkable intensity. Uses.—Hitherto pigments have been considered chiefly in relation to the requirements of the painter of pictures. In many merely decorative arts, such as the manufacture of wall-papers and the painting of woodwork and of iron, the pigments available are in one direction, that of cost, more restricted, but, on the other hand, many alterable or weak pigments are commonly employed. In paints intended for the protection of iron-work, the nature of the pigment introduced is a matter of great moment, for red lead, zinc white and white lead are found to exert a strong protective influence, which is not observed in the case of the vast majority of pigments. There are a number of other uses besides those just named for which special pigments, or, more precisely, special paints, are employed. Amongst such preparations may be named luminous paints, anti-fouling paints, metallic paints, damp-proof paints, and asbestos and other fire-proof paints.
End of Article: GROUP VI
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