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GROUP VI

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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 See also:green ochre containing a silicate of See also:iron, See also:potassium and See also:magnesium, and one other silicate, smalt, an artificial See also:glass containing a silicate of See also:cobalt and potassium, constitute this small group. However, some of the ochreous earths contain silicates of iron, See also:manganese and See also: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 See also:rich in See also:oxygen. When in contact with some of the more alterable organic See also:pigments belonging to Group IX. the chromates may lose oxygen, acquiring a somewhat greenish or greyish See also:hue, owing to the formation of the See also:lower or green See also:oxide of See also:chromium. The chromates cannot be trusted as pigments. The yellow chromates, those of See also:barium, See also:strontium, See also:zinc and See also:lead, are represented by the See also:general See also:formula 111"CrO4; chrome red is basic, and is Pb2CrO5. See also: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 See also:water), called sometimes cobalt yellow; one antimonate, that of lead, the true See also:Naples yellow; one tungstate, that of chromium, known as See also:tungsten green; a metaphosphate of manganese, which goes under the name of Nurnberg or manganese See also:violet; and several mixed cobalt compounds containing arsenates and See also:phosphates of that See also:metal, and represented by cobalt violet and See also:Thenard's See also:blue.

Two sulphates also belong here, namely, baryta See also:

white (BaSO4) and lead sulphate (PbSO4) ; also See also:Schweinfurt green, a basic See also:copper arsenite. It is obvious that of the members of so See also: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 See also:action of See also: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 See also:character in See also:common, proneness to oxidation and consequent deterioration in the presence of See also:light, moisture and See also:air. Such oxidation is accelerated by the action of some highly oxidized pigments belonging to other See also:groups. such as the chromates of Group VII. and aureolin of Group VIII., this action being particularly marked in the See also:case of the yellow lakes, the See also:cochineal lakes and See also:indigo. There are two pigments consisting of copper salts in this group. They are See also:verdigris —both the blue-green and the green varieties being basic copper acetates—and the pigment known in See also:England as See also:emerald-green, which is a basic cupric aceto-arsenite. These copper pigments See also: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 See also:cadmium See also:colours. About nine members of Group IX. may be regarded as substantive pigments. These include See also:Indian yellow (mainly magnesium and See also:calcium euxanthates), See also:gamboge, See also:sap green, indigo.

Prussian blue, See also:

bitumen or See also:asphalt, See also:bistre, See also:sepia, and the bituminous variety of Vandyck See also:brown. The See also:adjective pigments include a See also:great variety of lakes where different kinds of colouring matters of more or less See also:acid character have been thrown upon a See also:base, generally of colourless aluminium See also:hydrate, aluminium phosphate, stannous hydrate, stannic oxide, bartya or See also: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 See also:lake, from the resinous exudation produced in certain trees by the attacks of Coccus lacca; See also:carmine, See also:crimson and See also:purple lake, from the colouring See also:matter obtained from the cochineal See also:insect, Coccus cacti ; See also:rose-See also:madder and the madder-lakes, from the See also:alizarin and allied bodies derived from the See also:root of the See also:ordinary madder plant Rubia tinctorum; and yellow lakes, from See also:quercitron bark (Quercus tinctoria), and from See also:Persian and See also:Avignon berries (See also:species of Rhamnus or See also:Buckthorn). The lakes derived from See also:alkanet root, See also:archil, See also:Brazil See also:wood, and red See also:sanders wood are of very small See also:interest and value. The same See also:judgment may be pronounced upon the large number of artificial lakes which owe their colours to See also:coal-See also: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 See also:state and associated with alumina and a little lime, yields those pigments which possess a See also: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 See also:golden See also: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 See also:orange lake, to which such names as pure orange, orange madder and See also:marigold have been applied. Stability.—Some notion of the relative stability of pigments will have been derived from the remarks already made under " See also:Classification." But as permanence is of no less importance than See also:chromatic quality in the case of pigments used in the See also:fine See also:art of See also:painting, to which the present See also: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, See also:

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 See also:place must be taken by zinc white in the condensed See also:form known as See also: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 See also: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 See also:body, and though of less specific gravity than lead sulphate, actually raise the See also:weight per cubic See also:foot of the dry pigment. Out of a dozen or more See also:familiar yellow paints, a selection may be made of these six: yellow ochre, raw sienna, See also: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 See also:hydrogen is sent into a weak, See also:cold, and neutral See also:solution of cadmium See also:salt, the sulphide which separates is See also:pale and yellow—the orange variety is obtained from a strong, hot, and acid solution. The pale variety is more prone to See also:change than the darker one; but as oil colours both forms are sufficiently See also: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 See also:

bright yellow hue was described by Adie and Wood in 1goo, and is represented by the formula K2NaCo(NO2)6, See also:H2O. Of red pigments, six claim See also:special mention. These are See also:vermilion, light red, Venetian red, Indian red, red ochre, and the red lakes derived from madder or alizarin. Vermilion is stable in See also:oils, but as water-colour paint is prone to change, under exposure to strong light, into the See also: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 See also: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 See also:ultramarine, and terre verte. Except for See also:minor decorative See also:work, where permanence is of secondary moment, one is obliged to exclude from the See also: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, See also: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 See also:Antwerp blue, can scarcely be spared, but care should be taken to choose a See also: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 See also:

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 See also:artistic capacities of asphalt, madder brown, and the old bituminous Vandyke brown. Cappagh brown, burnt sienna, and raw and burnt See also:umber may be employed safely. Little need be said as to the selection of black pigments, for all are permanent. The See also:soot from burning See also: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 See also: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 See also:hand, many alterable or weak pigments are commonly employed. In paints intended for the See also: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 See also:influence, which is not observed in the case of the vast See also: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, See also:anti-fouling paints, metallic paints, See also:damp-See also:proof paints, and See also:asbestos and other See also:fire-proof paints.

End of Article: GROUP VI

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