Pigments

Pigments (Lat. pigmentum, from pingere, to paint). It is convenient to distinguish between pigments and paints, the latter being prepared from the former by the addition of a vehicle or medium. Nor are pigments and dyes identical, although there are cases in which the same colouring matter which yields a dye or stain may give rise to a pigment. A pigment is, in fact, a substance which is insoluble in the vehicle with which it is mixed to make a paint, while a dye is soluble. Pigments exhibit various degrees of transparency and opacity, and ought to possess such qualities as these: ease in working, chemical indifference to each other and, generally, to the vehicles employed, also stability under exposure to light and air. As a rule, it is desirable that pigments should not be seriously affected in hue by the vehicle; at all events, whatever change does occur ought to admit of calculation. In the case of oil colours it should be remembered that a thorough drying of the paint is preferable to the formation of a surface-skin, and that a few pigments, notably white lead, possess properties conducing to this desirable result. It is scarcely necessary to add to these general observations concerning pigments that their artistic value depends primarily upon the nature and amount of the optical sensation which they are competent to produce.

Although the number of available pigments is great, the number of chemical elements which enter into their composition is not large. Very many richly-coloured compounds sources. cannot be employed because they lack the properties of insolubility, inertness and stability. Pigments are drawn from various sources. Some are natural, some artificial; some are inorganic, some organic, some are elements, some mixtures, some compounds. It is not unusual to arrange them into two groups, substantive and adjective. Amongst the members of the former group such a pigment as vermilion, where each particle is homogeneous, may be cited as an example. Amongst the adjective pigments rose-madder may be named, for each particle consists of a colourless base on which a colouring matter (alizarin) has been thrown. Most of the inorganic pigments, whether natural or artificial, belong to the substantive group; while there are many organic pigments, notably those of artificial origin, which are of adjective character. The following table presents a summary classification of pigments according to their source or origin: Mineral pigments S Natural; as terre verte. Artificial; as aureolin.

Animal; as carmine.

Vegetable; as madder-lake. Artificial; as alizarin-orange.

A variety of processes are in use in order to fit natural coloured substances for employment as pigments. The first step is, Organic pigments in many cases, to select, or " pick over," the raw material, rejecting whatever impurities may weaken or injure the characteristic hue of the product. It is occasionally Prepara- necessary to treat the finely-ground substance with tion, water by the method of elutriation or washing-over; the wash-waters will then deposit, on standing, various grades of the coloured body required. With rare exceptions native pigments need careful grinding, either by means of a muller on a slab or by edge rollers, or horizontal mill-stones, or special machines. The substance is usually ground in spirits of turpentine, or alcohol, or water; oil-paints are of course finally ground in a drying-oil, such as linseed oil or poppy oil; water-colours require gum-water, or gum-water and glycerin if they are to be " moist " paints. In the case of all pigments, whether mineral or organic, whether natural or artificial, it is of the highest importance to make sure that they are free from saline matters soluble in water. Such salts are removed by thorough washing with distilled water. A treatment of this kind is essential in the case of a large number of pigments formed by chemical reactions in the " wet way." Characteristic examples are furnished by Prussian blue, viridian and lakes. Sometimes it is necessary to remove dangerous impurities by solvents other than water, such as carbon bisulphide, which is used to extract free sulphur from cadmium yellow. Mention may here be made of another kind of preparative treatment which is adopted with some pigments: they are subjected to the action of heat - moderate in some cases, strong in others. Thus, a few substances, such as ivory black and yellow ochre, which in ordinary circumstances contain much non-essential moisture, before they are ground in oil may with advantage be gently dried at a temperature not above that of boiling water. Again, there are pigments, such as Prussian brown, light red and burnt sienna, which owe their hues to a process of actual calcination, the first of these being thus made from Prussian blue, the second from yellow ochre, and the third from raw sienna. The pigments known as burnt carmine and burnt madder are prepared at a much lower temperature, and ought to be described as roasted rather than as burnt.

The substitution of one pigment for another is rarely practised, but it is not so unusual to find that a costly substance has received an admixture of something cheaper, and Adultera- that an inferior grade of a genuine pigment has had tion . g g P g its hue exalted or enhanced by some unlawful or dangerous addition. In fact, these two kinds of sophistication are often associated. Thus vermilion is adulterated with red lead, with red antimony sulphide, or with baryta white and lead sulphate, and then the hue of the mixture is restored to the proper pitch by the introduction of the powerful but fugitive colouring matter eosin. Amongst other adulterations which may be named here are the addition of chrome-yellow (lead chromate) to yellow ochre, of green ultramarine to terre verte, and of indigo to ivory black; this last mixture being a substitute for vine-black, the natural blue-black. The detection of the above-named sophistications is by no means difficult even in the hands of persons unacquainted with chemical manipulation, but it needs a trained analyst when quantitative results are required. If we are dealing with an oil-colour, the first step is to 'remove the oil by means of a solvent, such, for example, as ether. The residual pigment is then allowed to dry, and the dry powder submitted to the appropriate physical and chemical tests. Thus a suspected vermilion, having been freed from oil, is heated in a small hard glass bulb-tube: it should prove practically volatile, leaving a mere trace of residue. In this particular case the presence of a red hue in the ether-extract affords evidence of adulteration with an organic colouring matter, such as eosin. Then, again, we may detect the presence in yellow ochre of lead chromate by pouring a little sulphuretted hydrogen water and dilute hydrochloric acid upon one portion of the dry pigment, and boiling another portion with dilute sulphuric acid and some alcohol: in the former experiment blackening will occur, in the latter the liquid part of the mixture will acquire a greenish tint. So also green ultramarine may be recognized in adulterated terre verte by the addition of dilute hydrochloric acid, which destroys the colour of the adulterant and causes an abundant evolution of the evil-smelling sulphuretted hydrogen. Moreover, nothing is easier than the recognition of indigo in vine or charcoal-black, for the dry powder, heated in a glass tube, gives off purple vapours of indigo, which condense in the cooler part of the tube into a blackish sublimate.

A word must be said here as to the adulteration of white lead, and the examination of this most important pigment. The best variety of white lead or flake white contains two molecules of lead carbonate to one of lead hydrate, and is wholly soluble in dilute nitric acid, while barium sulphate, its most frequent adulterant, is wholly insoluble. China-clay and lead sulphate will also remain undissolved; but whitening or chalk cannot be detected in this way - indeed, the thorough examination of white lead, not only for sophistications but also for correspondence with the best type in composition, cannot be carried out save by a skilled chemist.

Pigments may be classified on two systems: (r) based on the chemical composition; (2) based on the colour. On the first system pigments fall into nine groups, seven of which are fairly well defined, but the eighth and tion fica- y ? g tion.

ninth have a somewhat miscellaneous character.

The groups of elements, oxides, sulphides, hydrates, carbonates and silicates present this characteristic, namely, that each member of any one group is without action upon the other members of the group; any two or more may therefore be mixed together without fear of mutual injury. The same statement may be made with reference to the various inorganic salts of Group VIII. and to the organic compounds of Group IX., although in this large final group there are two pigments containing copper (verdigris and emerald green) which must be regarded with suspicion. The inertness of the members of the same group towards each other may be explained in the majority of cases by the following consideration. An oxide does not act upon an oxide, nor does a sulphide affect a sulphide, because all the pigment oxides have taken up their full complement of oxygen, and can neither give nor lose this element to similar oxides; so also with sulphur in the sulphides. A few details regarding the several members of the nine groups are now offered: Group I. Elements. - All the black pigments in ordinary use - ivory black, lamp black, charcoal black, Indian ink, and graphite, less correctly termed black-lead and plumbago - consist of or contain. carbon, an element not liable to change. The metallic pigments, gold, silver, aluminium and platinum, belong here; of these, silver alone is easily susceptible of change, tarnishing by combination with sulphur.

Group Ii. Oxides. - The oxides have generally been formed. at a high temperature and are not easily amenable to physical or chemical change; they are, moreover, not liable to affect other pigments, being practically inert, red lead only being an exception.. The oxides include zinc white, green chromium oxide, burnt umber (a mixture of iron and manganese oxide), cobalt green (CoO,nZnO), cobalt blue (CoO,nAla03), coeruleum (CoO,nSnOa), Venetian red, light red, Indian red and burnt sienna (all chiefly composed of ferric oxide), and red lead (Pb304).

GROUP III. Sulphides. - Some of the members of this group are liable to contain free sulphur, and some may give up this element to the metallic bases of other pigments. Thus cadmium yellow blackens emerald green, producing copper sulphide. Another pigment of this group, vermilion, is prone to a molecular change whereby the red form passes into the black variety. This change, frequent in water-colour drawings, is scarcely observable in works painted in oil. The sulphides comprise cadmium yellow (CdS), king's yellow (As2S2), realgar (As 2 S 3), antimony red (Sb2S3) and vermilion (HgS). It is convenient to give places in the same group, to the various kinds of ultramarine, blue, green, red, violet and native, for in all of them a part of the sulphur present occurs in the form of a sulphide. It may be stated that the sulphides of arsenic and antimony just named are dangerous and changeable pigments. not suited for artistic painting.

Group Iv. Hydrates or Hydroxides. - Several native earths belong here, notably yellow ochre, raw umber, raw sienna and Cappagh brown. These substances owe their colours mainly to hydrates and oxides of iron and of manganese, but the presence of a colourless body such as white clay or barium sulphate is usual with the paler pigments. A false yellow ochre from Cyprus is really a basic ferric sulphate, and does not properly belong to this group. Besides the yellow and brown pigments, there is a magnificent deep green pigment in this group, known as emerald oxide of chromium or viridian. The blue copper preparation which goes under the name of bleu lumiere 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.

Group V. Carbonates. - There is but one really important member of this group, namely, the old and typical variety of white lead (2PbCO 3, PbH202). Like green verditer (2CuCO 3. CuH202), and blue verditer (CuCO 3. CuH202), it is a basic carbonate. Purified chalk or whitening (CaCO 3) belongs here also.

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.

Group 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 M"Cr0 4; chrome red is basic, and is Pb2Cr05.

GROUP 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 (K 3 Co(NO 2) 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 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 (BaS04) and lead sulphate (PbSO 4); 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.

Group 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 madderlakes, 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 of the 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 1900, and is represented by the formula K2NaCo(N02)6, H 2 O. 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 wallpapers 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.

Authorities

J. Bersch, Manufacture of Pigments, translated from the 2nd German edition by A. C. Wright (London, 1900); Cennino Cennini, The Book of the Art, translated by Mrs Herringham (London, 1899); Sir A. H. Church, Chemistry of Paints and Painting (London, 1901); G. H. Hurst, Painters' Colours, Oils and Varnishes (London, 1901); S. Mierzinski, Handbuch der Farben-Fabrikation (Vienna, 1898); Riffault (and others), Fabricant de couleurs (Paris, 1884). (A. H. C.)