PIGMENTS By J. K. BARRACLOUGH, B.Sc., F.R.I.C., and H. WEBB* IN' THE course of this paper we propose briefly to review the general properties of pigment classes, emphasislug where possible those particular aspects most likely to interest this Society. Description of manufacturing methods will only be referred to if they can be shown to be directly responsible for some unique property of a pigment or group of pigments. IXlATURAL COLOURED MINERALS It is perhaps easiest to begin with the naturally occurring coloured minerals which are cheap enough, and suitable after treatment, to be used as pigments. These all depend for their colour upon the presence of oxides of iron and are known by such names as ochre, Persian Gulf red, Indian red, umber, sienna, etc. Because they are naturally occurring products they possess certain in- herent disadvantages they show greater variation in colour and tint- ing power than one gets with manu- factured pigments the nature, and amount, of impurities in them may vary, and may, from time to time, be of such a nature as specifically to exclude their use in any particular * Lewis Berger, (Great Britain) Ltd. industry, e.g., the presence of sulphur in pigments will delay the drying of paints containing them, the presence of over ten parts per million of manganese will render them Unsuitable for rubber just as the presence of traces of antimony, arsenic, lead and many other metals would considerably perturb a cos- metic chemist who wished to use them. Again, these naturally occur- ring earth colours are frequently less finely ground and (due to the frequent presence of silica or silace- ous material adventitiously present) harder than their manufactured rivals. "SYNTHETIG" IRO•q OXIDES With the exception of raw and burnt umbers and siennas, the ea•:th colours can be replaced by what have come to be called "synthetic" iron oxides, and which we have hitherto referred to as manufactured pro- ducts. The transparency and clarity of the siennas and umbers, however, cannot be achieved by manufac- tured products. The term undertone is used to designate the colour seen when the coloured pigment is intimately mixed with an opaque white pigment. Al- though the blended pigments are 141

]OURNAL OF THE SOCIETY OF COSMETIC CHEMISTS generally dispersed in some sort of liquid medium, the definition holds for dry powders. The yellow and yellowish-brown earth colours contain the hydrated form of ferric oxide whilst the red and reddish brown ones contain the (largely) non-hydrated form. In the case of umber, appreciable amounts of manganese are also present. All these materials possess excellent light fastness and, except for the relatively transparent umbers and siennas, good opacity when used in oily media. In aqueous media, of course, their opacity is much greater and in such conditions all would be satisfactory. Generally their tinc- torial power, i.e., their ability to import their own hue to other materials, is lower than that of the corresponding manufactured pro- ducts. MANUFACTURED INORGANIC PIGMENTS Leaving the naturally occurring pigments, we come to the manu- factured inorganic ones we shall omit mention of ultramarine blues, chromium oxides and the chromate pigments which we do not think are of interest to you, and again limit ourselves to those depending upon the oxides of iron for their colour properties. These are prepared either by precipitation from solution (usually of ferrous sulphate) or by calcination of solid ferrous sulphate. The colour and general pigmentary properties of the products of the former method are controlled by "seeding" before precipitation corn- 142 mences and, of course, by careful control of the physical conditions during precipitation' whereas the properties of those prepared by the calcination process are obtained by careful control of the calcining con- ditions. Probably the major differ- ence between two similar pigments prepared by these two methods is that those resulting from calcination of ferrous sulphate very frequently contain appreciable amounts of ad- sorbed sulphur trioxide and give markedly acidic extracts, whereas those prepared by precipitation are generally more nearly neutral. It may be interesting at this point to note that all the so-called synthetic oxides which we have examined, whatever their colour, have given more or less acid aqueous extracts, whereas almost all the naturally occurring products give aqueous extracts of alkaline reaction. This point can, of course, be extremely important when nicely balanced emulsions are to be pigmented. The manufactured oxides can be produced in all shades from a rather '• dirty yellow through marigold ochres :•: to reddish browns, and very pleasing : browns of relatively soft texture, and...: all show the same high light fastness !:!i both as sole pigments and as minor? ingredients in a mixture which one'.:: associates with the natural products. :':" Some of the brown synthetic oxides'i of pleasing self colour are, however/(•'i disappointing when used as tinters their undertone lacks life and liance and in no way compares with the umbers. The relative cheapness of theses. :