]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. :

PIGMENTS products depends very largely upon their bulk production from readily available, cheap raw materials (usually by-products of some other process), and impurities, although small in comparison with those of the natural earth colours, can be quite large when expressed in parts per million. Indeed, the authors have been unable to find a synthetic yellow ochre with arsenic lower than about 25 parts per million. When, therefore, limitations of the quanti- ties of scheduled poisons are con- cerned, it is advisable to buy on bulk sample which has been ap- proved as being within the limits set. Before leaving the subject of iron oxide pigments brief mention must be made of black iron oxide. This material is a by-product of the manu- facture of aniline, iron being used as a catalyst in the reduction of nitro benzene. It is not a real black, and possesses a distinct brownish cast, and being a by-product is somewhat subiect to variation in strength. It is, however, very inert and requires stewing with strong hydrochloric acid to bring it into solution, and is less absorbent than the carbon blacks. So far as we know, there is no residual nitro-benzene in the pigment, which is completely innocu- ous. LAKES, TONERS, PIGMENTS Before beginning to discuss organic pigments it is necessary to define with some precision the terms lake, toner and pigment dyestuff. These terms have quite definite meanings to pigment manufacturers, but are often used loosely and misleadingly by pigment consumers. Lakes: When a water-soluble dyestuff is removed from solution by precipitation upon, or adsorption by, a substratum which is a necessary carrier, it is said to be "laked," and the resulting product is a "lake." A certain amount of this substratum is essential to confer pigmentary properties upon the complex formed and is not employed as a diluent. Toners: Certain dyestuffs contain water-solubilising groups and can- not be used, as such, as pigments if, however, the water-solubilising groups are converted into metal salts such as barium or calcium, the dye- stuff-salt (called a toner) loses its solubility characteristics and becomes suitable for use as a pigment. In pure toners there is no substratum. Pigment dyestuffs: Certain dye- stuffs, chiefly of the dis- or tetra-azo class, possess pigmentary properties as first produced by reaction of their components--these are called pig- ment dyestuffs and, •vhen pure, contain no substratum. Before leaving definitions it may be of advantage to point out that a "pure" pigment is merely a lake, a toner, etc., to which no diluent has been added a "reduced" pigment contains added extender or diluent. Tkis diluent is not necessarily added merely to reduce the cost of the pigment a small addition of a, suitable extender will frequently confer improved properties such as ease of incorporation into a medium, or improved "working" properties upon the finished product. 143