Page:EB1911 - Volume 21.djvu/625

Rh

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

necessary to treat the finely-ground substance with 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 bi sulphide, 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 orrlinary 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 l ov 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 Adu“em received an admixture of something cheaper, and “om that an inferior grade of a genuine pigment has had 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 antimonv 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 sophistication's 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 Prepara-

tion.

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 Hake 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 sophistication's 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: (1) based o11 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 ffjfinca 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 useivory black, lamp black, charcoal black, Indian ink, and graphite, less correctly termed black-lead and plumbago-consist of or contain carbon, an element not llable to chan e. The metallic pigments, gold, sllver, aluminium and platinum, Telong here; of these, sllver 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 llable 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 (C0O, nZnO), cobalt blue (C0O, nAl2O3), coeruleum (C0O, nSnO2), Venetian red, light red, Indian red and burnt slenna (all chiefly composed of ferric oxide), and red lead (Pb3O4).

GROUP III. Sulphzdes -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 (As2S,), antlmony 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 umber, raw sienna and

their colours mainly to

belong here, notably yellow ochre, raw

Cappagh brown. These substances owe

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