Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/96

Rh 8(3 PIGMENTS affecting these or being themselves deteriorated, and per manence and inalterability of tone after long exposure. Pigments being so numerous and so diverse in their origin, the industries connected with their production and preparation are of necessity varied in character. Many of the substances employed being used in large quantities in other important industrial relations, as well as for paints, are manufactured on a large scale and constitute the basis of considerable chemical industries, as, for example, the manufacture of white lead, Prussian blue, ultramarine, the chrome materials, &c. In other cases the materials require no preparation other than that given to them by the paint-grinder or the artists colourman, according to the purpose for which the substances are to be prepared. The colour trade embraces two distinct departments: that of the paint-grinder, who manufactures and com pounds the pigments used by artisans, house-painters, and paper-stainers ; and that of the artists colourman, who prepares and supplies the finer, more brilliant, and exten sive assortment of pigments used for artistic purposes. The pigments employed for pottery painting and glass and enamel work are a special class of preparations to suit the requirements of these trades. Leaving out of account the chemical reactions involved in preparing raw materials, the ordinary manufacturing operations connected with the preparation of painters colours are simple, and consist essentially of a careful system of grinding. Formerly, when painters ground their own colours, a stone slab and muller formed the entire apparatus ; but now, when paint- grinding has become a separate industry, efficient machinery has been devised for grinding and its collateral operations. Bulky and rough colours such as whiting and common ochres are dry-ground under heavy edge stones which revolve in a strong iron bed. Ordinary dry colours requiring to be pulverized with more care are mixed to a thin cream with water, which is fed into and ground principally between a pair of millstones dressed and mounted like the ordinary horizontal stones of a flour mill, but smaller in diameter. For fine colours the pig ment so ground is levigated, or floated into a vat in which the heavier particles sink, and the lighter, more finely divided portion is run into another vessel at a lower level, where it is deposited as a fine sediment. The sediment is dried in a uniformly heated stove, and when thoroughly dry is again pulverized under a pair of edge stones, and sifted or winnowed ; so treated it is ready for use as dry colour. The greater proportion of the white lead and the other common oil paints are ground in oil. For this purpose the raw material is mixed in a machine with oil (sometimes boiled) to the consistence of a stiffish paste, and in this state it is ground in horizontal paint millstones, after which it requires no further preparation than the necessary thinning with oil when to be used for painting. There are many varieties of apparatus used for grinding both dry and oil colours. The artists colourman grinds his pigments with much greater labour, and selects his materials in a more care ful manner, than is necessary in the case of the ordinary paint-grinder. Pigments for artistic painting in oil are ground in that medium to a definite consistency, and are put up for use in convenient compressible tubes of tin. For water colours the pigments are prepared principally in the form of small indurated cakes or as &quot; moist colours &quot; contained in small porcelain dishes. Water colours may also be obtained thin in tin tubes like oil colours, or as &quot;pastilles,&quot; which are thin round cakes intermediate in condition between cake and moist colours. In enumerating the principal commercial pigments it is usual and convenient to classify them according to their tints. They are not, as a rule, definite chemical com- pounds: many indeed are mixed substances prepared by processes and according to recipes known only to their makers ; and, while the same commercial name is fre- I the confusion is further increased by applying man} different titles to substances which are practically identi. cal. Thus white lead is known by at least a dozen names, and distinct and even conflicting qualities are by autho rities attributed to this one substance under its various aliases. It would be impossible to catalogue all the paints met with in commercial lists, and it would serve no good purpose to enumerate the whole of the pigments which might be and are occasionally used. Premising that details regarding many of the substances will be found under the heading of the metals, Arc., whence they are derived, we shall here simply classify, according to their colour, the principal well-recognized pigments of commerce, adding brief remarks regarding each class. WHITE PIGMENTS. The whites are the most important pigments used by painters, forming as they do the basis or body of nearly all paints, excepting only certain dark lines. Good available whites are limited in number, and all of real importance are included in the following list : white lead, a carbonate of lead (chiefly); zinc white, oxide of zinc, called also Chinese white ; antimony white, oxide of antimony ; fixed white, sulphate of baryta ; &quot; silicate &quot; white, sulphate of baryta, or strontia and sulphide of zinc ; mineral white, powdered gypsum (with alumina it forms satin white); chalk or whiting carbonate of lime and china clay, silicate of alumina. White Lead (see LEAD, vol. xiv. p. 378) is the most important of all pigments, and forms the basis of nearly all ordinary oil paints, which, when coloured, consist of white lead tinted with the necessary coloured pigments. It possesses the greatest amount of body or covering power, and works beautifully in oil, with which it partially combines, drying as a hard homogeneous adherent plaster. On the other hand it is a most poisonous body, very injurious to the persons connected with many of the processes by which it is prepared. As an oil colour it darkens gradually in an atmosphere containing traces of sulphur ; it cannot be used at all as a water or distemper colour; and it acts injuriously on the colour of several important pigments. Notwithstanding these drawbacks no white has yet been made that can compete with white lead, although paint manufacturers go far to provide a substitute by adulterating it to such an extent that the white lead frequently bears only a small ratio to the adulterant. Baryta white is the ordinary adulterant, and among respectable manufacturers the intermixture is a well-understood fact, and the relative proportions of white lead and baryta are regulated by a scries of grades passing from &quot; genuine &quot; to No. 5 or No. 6 white lead. Many efforts have been mado to substitute for ordinary white lead lead carbonates made by other processes, and other lead salts such as the oxychloride (Pattinson s), sulphate, tungstate, antirnoniate, &c. ; but none of these has proved permanently successful. Zinc White. Next in importance to white lead, is an oxide of zinc prepared by the sublimation and combustion of metallic zinc. The pigment is deficient in covering power and it dries but slowly when mixed with oil. On the other hand it is not injurious to health, its purity of tone is not affected by sulphurous air, it does not ail ect tints added to it or with which it comes in contact, and it can be used in water as well as in oil. Like white lead it is very much adulterated, and generally with the same agent baryta white. Baryta White plays an important independent part as well as acting so extensively as a sophisticator of other pigments. It is prepared by grinding to a fine powder the pure white native sulphate of baryta (heavy spar), and the same substance artificially prepared is known as permanent white or llancfixe. The artificial preparation is much superior, as a pigment, to the powdered spar ; but both are deficient in body, notwithstanding which they are of great value to paper-stainers and for distemper painting. Under the name of Charlton White or silicate paints, Mr J. I. Orr prepares a range of white paints which have come into exten sive use. The pigment as originally prepared under Mr Orr s patent of 1874 consisted of an intimate mixture of artificial sul- by the double decomposition of solutions of barium sulphide and sulphate of zinc. In 1881 a patent was secured by Mr Orr for a combination in which strontia takes the place of baryta. It is claimed for these pigments that they possess body greater than j white lead, that they are non-poisonous, and that with certain modifications in the manufacture they can be made quite as valuable for distemper painting as for oil colours.
 * quently given to substances quite dissimilar in character.
 * phate of baryta and sulphide of zinc in certain proportions, made