PIGMENTS, LAKES AND DYESTUFFS IN COSMETICS of shade on the skin. The important requirements of a pigment for face powder can, therefore, be summarised as: 1. High resistance to bleed in all solvents, 2. good dispersive qualities, and 3. other factors of minor importance. The most satisfactory colours for face powders are naturally occurring earth pigments selected for freedom from impurity and usually known as iron oxides. Their one disadvantage is possibly that of dullness of shade but by careful blending of colour this can be largely overcome. COLOURS FOR FOUNDATION LOTIONS, CAKE MAKE-UP AND SIMILAR PREPARATIONS From the view point of the coloured pigment, all these face colouring preparations present the same problems as those of a face powder. The remarks on colours for face powder therefore apply, with perhaps an emphasis on freedom from bleed in water, fats and oils. The cosmetic manufacturer may again have difficulty in preventing development of the colour on the face during application and this can be largely overcome by the use of semi- processed colours, often in paste form, in one of the basic media of the finished product. LIPSTICK COLOURS A lipstick may be described as a highly concentrated dispersion of pig- ment in fats, oils and waxes designed to be applied to the lip surface in a thin film. The short life of the product eliminates the need for high light fastness in the colour and, with the possible exception of exposure in the shop window, fastness to light of the pigment is not of primary importance. A fine and even particle size, which can be controlled to a large extent by the colour manufacturer, is of value to the cosmetic user but this must be balanced against the problems arising from occlusion of air by dry grinding. Efficient operation of wet milling will largely overcome difficulties given by pigmentary aggregates and air occluded in the lipstick mass can be removed in several ways. The oil absorption of the colour is an important factor as it controls the rheological behaviour of the finished mass during the final heating and moulding. High oil absorptions often give rise to thixotropic behaviour in the mass. If lakes are employed, it is important to select a base (sub-stratum) which will not react under any conditions of pH with the oils and fats in the lipstick base giving rise to soaps and similar compounds, thus causing gelling and difficulty in moulding. The freedom of the base (sub-stratum) from soluble salts is important and it is often wise to use a base such as aluminium

14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS benzoate which can be strictly controlled in manufacture, rather than alumina hydrate itself. The moisture content of the colour can give rise to difficulties in processing and should be kept to a minimum. The tinctorial power of the colour is of great importance. A lipstick must impart a thin, highly-coloured film to the lips and high tinctorial power is therefore an important require- ment of a good lipstick pigment. Opacity to ensure good hiding power is necessary and so, with lakes, one does not look for transparency but rather for opaqueness in the oil. Bleed of colour is of importance. To water, because of smearing on the lip and gradual migration of the colour away from the mucous membrane of the lip to other surfaces also because of staining the tongue and inner surfaces of the mouth. To oil, as a gradual change of shade in the mass and divergence from the accepted standard of shade will be caused and also with a slightly unbalanced formulation it will cause droplets of highly coloured oil to develop on the stick, which are most unsightly. It is also of advantage for the lipstick to have good mucous membrane staining qualities. This is usually obtained by the introduction of one of the eosin family of dyestuffs which, in the great majority of lipsticks, are used in the water-insoluble acid form. It should not be overlooked that, in this "acid" form, they possess certain pigmentary properties which modify the final mass-tone of the stick. The eosin colours have the special property of staining the lip membrane and hence giving permanence to the lipstick colour. They have the disadvantage that a small percentage of lipstick users are allergic to them but, with careful control during manufacture, the inci- dence of complaints can be reduced to a very small figure. For this irreducible minimum, lipsticks can be obtained with other staining agents from several manufacturers. To sum up, the requirements of a pigment for lipsticks are as follows: (1) Reasonable light-fastness, (2) small particle size, (3) stability to heat and pH, (4) low moisture content, (5) freedom from bleeding, (6) high tinctorial power, and (7) minimum allergic reaction. Many of these factors are controlled by the lipstick base employed to such an extent that the type of pigment to be used requires very careful choice. Returning to our colour definition, all three types may be used. A dyestuff of the eosin family to give stain and permanence to the colour a pigment and a lake of selected strength and st•itable characteristics.