Page:EB1911 - Volume 21.djvu/626

 group. Besides the yellow and brown pigments, there is a magnificent deep green pigment in this group, known as emerald oxide of chromium or virician The blue copper preparation which goes under the name of bleu lumiére and mountain blue, a very unstable pigment, is also essentially a hydrate, though by no means pure It should be stated that all the earthy or native hydrates belonging to this group contain water in two states, namely, hygroscopic or loosely-attached and constitutional. Before grinding them in oil, the reduction in the amount of the hygroscopic moisture by means of a current of dry air or a gentle warmth often improves the hue and working quality of these pigments.

Carbonates.—There is but one really important member of this group, namel, the old and typical variety of white lead (2PbCO3, PbH2O2) Like green verditer (2CuCO3, CuH2O2), and blue verditer (CuCO3 CuH2O2), it is a basic carbonate. Purified chalk or whitening (CaCO3) belongs here also.

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 hae some claim to be ranked with the silicates.

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 somehat 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 M″CrO4; chrome red is basic, and is Pb2CrO2.

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)6, 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 meta phosphate of manganese, which goes under the name of lurnberg or manganese violet; and several mixed cobalt compounds containing arsenates and hosp hates of that metal, and represented by cobalt violet and Thénard’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.

Organic Compounds.—Most of the members of this large and unwieldy group of pigments possess this character in common, proneness to oxidation and conse uent deterioration in the presence of light, moisture and air. Succh 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 bein particularly maiked in the case of the yellow lakes. the cochinealglakes 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, stannxc oxide, bartya or lime; sometimes coloured bases containing such metals as copper, chromium, manganese or iron axe 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 'ud ment may be pronounced upon the large number of artificial] laies which owe their colours to coal-tar derivatives, with the single exception of the important class of pigments obtained from artificial alizarin, and from its conveners and derivatives. Of these, (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 decidedl brown. Many of the aliizarin crimsons sold as paints are not mad; from alizarin itself, but from the sulphonic acids of alizarin. These lakes present a wide range of hues. Another derivative of alizarin, known as -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 prohtably 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 modihcations 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 1900, 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