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PIGMENTS (Lat. pigmentum, from pinger...

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Originally appearing in Volume V21, Page 598 of the 1911 Encyclopedia Britannica.
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PIGMENTS (See also: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 See also:medium. Nor are pigments and dyes identical, although there are cases in which the same colouring See also: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 See also:light and See also:air. As a See also:rule, it is desirable that pigments should not be seriously affected in See also:hue by the vehicle; at all events, whatever See also:change does occur ought to admit of calculation. In the See also:case of oil See also:colours it should be remembered that a thorough drying of the paint is preferable to the formation of a See also:surface-skin, and that a few pigments, notably See also:white See also:lead, possess properties conducing to this desirable result. It is scarcely necessary to add to these See also:general observations concerning pigments that their See also:artistic value depends primarily upon the nature and amount of the See also:optical sensation which they are competent to produce. Although the number of available pigments is See also:great, the number of chemical elements which enter into their See also:composition is not large. Very many richly-coloured compounds See also:Sources. cannot be employed because they lack the properties of insolubility, inertness and stability. Pigments are See also: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 See also:

groups, substantive and See also:adjective. Amongst the members of the former See also:group such a pigment as See also:vermilion, where each particle is homogeneous, may be cited as an example. Amongst the adjective pigments See also:rose-See also:madder may be named, for each particle consists of a colourless See also:base on which a colouring matter (See also: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 See also:character. The following table presents a See also:summary See also:classification of pigments according to their source or origin: See also:Mineral pigments Natural; as terre verte. Artificial; as aureolin. See also:Animal; as See also:carmine. See also:Vegetable; as madder-See also:lake. Artificial; as alizarin-See also:orange. A variety of processes are in use in See also:order to See also: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, recognized in adulterated terre verte by the addition of dilute hydrochloric See also:acid, which destroys the See also:colour of the adulterant and causes an abundant See also:evolution of the evil-smelling sulphuretted See also:hydrogen. Moreover, nothing is easier than the recognition of See also:indigo in See also:vine or See also:charcoal-See also:black, for the dry See also:powder, heated in a See also:glass See also:tube, gives off See also:purple vapours of indigo, which condense in the cooler See also:part of the tube into a blackish sublimate.

A word must be said here as to the See also:

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 See also:hydrate, and is wholly soluble in dilute nitric acid, while See also:barium sulphate, its most frequent adulterant, is wholly insoluble. See also:China-See also:clay and lead sulphate will also remain undissolved; but whitening or See also:chalk cannot be detected in this way—indeed, the thorough examination of white lead, not only for sophistications but also for See also:correspondence with the best type in composition, cannot be carried out See also:save by a skilled chemist. Pigments may be classified on two systems: (I) based on the chemical composition; (2) based on the colour. On the first See also:system pigments fall into nine groups, seven of which are fairly well defined, but the eighth and tion. assitica-~' ninth have a somewhat See also:miscellaneous character. The groups of elements, oxides, sulphides, hydrates, See also:carbonates and silicates See also:present this characteristic, namely, that each member of any one group is without See also: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 See also:copper (See also:verdigris and See also:emerald See also:green) which must be regarded with suspicion. The inertness of the members of the same group towards each other may be explained in the See also:majority of cases by the following See also:consideration. An See also:oxide does not See also:act upon an oxide, nor does a sulphide affect a sulphide, because all the pigment oxides have taken up their full See also:complement of See also:oxygen, and can neither give nor lose this See also:element to similar oxides; so also with See also:sulphur in the sulphides. A few details regarding the several members of the nine groups are now offered: rejecting whatever impurities may weaken or injure the See also:char- acteristic hue of the product. It is occasionally Prepare- necessary to treat the finely-ground substance with See also:lion.

See also:

water by the method of elutriation or washing-over; the See also:wash-See also:waters will then See also:deposit, on See also:standing, various grades of the coloured See also:body required. With rare exceptions native pigments need careful grinding, either by means of a See also:muller on a slab or by edge rollers, or See also:horizontal See also:mill-stones, or See also:special See also:machines. The substance is usually ground in See also:spirits of See also:turpentine, or See also:alcohol, or water; oil-paints are of course finally ground in a drying-oil, such as See also:linseed oil or See also:poppy oil; water-colours require See also:gum-water, or gum-water and See also: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 See also:free from saline matters soluble in water. Such salts are removed by thorough washing with distilled water. A treatment of this See also: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 See also:blue, viridian and lakes. Sometimes it is necessary to remove dangerous impurities by solvents other than water, such as See also:carbon bisulphide, which is used to See also:extract free sulphur from See also: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 See also:heat—moderate in some cases, strong in others. Thus, a few substances, such as See also:ivory black and yellow ochre, which in See also:ordinary circumstances contain much non-essential moisture, before they are ground in oil may with See also:advantage be gently dried at a temperature not above that of boiling water. Again, there are pigments, such as Prussian See also:brown, light red and burnt sienna, which owe their hues to a See also: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 See also: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 Adaltera- that an inferior grade of a genuine pigment has had lion.

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 See also:

antimony sulphide, or with baryta white and lead sulphate, and then the hue of the mixture is restored to the proper See also: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 See also: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 See also: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 See also:ether. The residual pigment is then allowed to dry, and the dry powder submitted to the appropriate See also: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 See also:mere trace of See also:residue. In this particular case the presence of a red hue in the ether-extract affords See also: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 Iatter the liquid part of the mixture will acquire a greenish tint.

End of Article: PIGMENTS (Lat. pigmentum, from pingere, to paint)

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