196 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS very much weaker and modified fragrance when the same amount is mixed with Italian talc. This is a case of changed conditions giving a changed odour and it is immediately apparent (i.e. not a deterioration with time). The differences in talcs are probably due to differences in crystal structure rather than any impurity--although complications may occur due to occluded traces of iron oxides. In talcs with platelet structure there may be varying weaknesses in the crystal structure in some planes, conferring a reactivity not usually associated with "inert" minerals. Question 6: What in the pane!'s opinion are appropriate and sufficient accelerated shelf life tests for emulsified products ? Answer: An old, often-debated subject! It is certain that relating temperature against time--on the principle that slow chemical reactions are speeded up by increasing the temperature--is of very limited applica- tion. In emulsions many possible changes are physical, not chemical, and it is misleading to subject emulsions to temperatures higher than would be encountered in practice. A temperature as high as 60øC is quite unlikely to be met with in practice, except perhaps in a shop window in England. People in the tropics are more accustomed to keeping products away from sunlight. Similarly at the lower end of the scale, some emulsions will break if the aqueous phase has been solidified, others will not. Shelf life testing should normally be limited to 10-50øC. An emulsion may have a threshold temperature for deterioration, i.e. it may deteriorate very rapidly above say 50øC and not at all below that. It is impossible to lay down any "printed form" of storage tests. For any particular emulsion system it is necessary to consider, often from previous experience, what is likely to go wrong and investigate that--by accelerated tests where possible. Transit tests should not be forgotten-- vibration and shaking can have ill-effects on emulsions. Some information on stability of emulsions can be gained by the addition of small amounts of reagents. Admittedly this changes the system some- what. But stability of an emulsion to acid or alkali, would in some cases be useful information. Not enough use is made of the microscope in making a rapid assessment of emulsion stability. In some cases it is possible to see an emulsion breaking. In others, it is certainly possible to see whether or not an emulsion conforms in pattern to previously satisfactory products of the same kind. Much work has been done on centrifuging as a quick method of assessing emulsion stability. It might be useful in special cases but it must be remembered that temperature rises often occur in such instruments while they are running. It is useful where there is a solid dispersion, but for

QUESTION AND ANSWER 197 oil-water systems, the density differences are not great and viscosities are usually high. These conditions are not conducive to reliable results. The consensus of opinion seems to be that a general scheme of acceler- ated shelf life testing can be very misleading. Something can be done for particular products provided the conditions to be met are known. As a routine procedure it can be useful when there is already a good history of a certain product. Question 7: Is the use of silicones of any value in coating the hair and in hair preparations ? Answer: Silicones have some use in coating the hair. They improve its appearance, but mineral oil will do the same thing at very much less expense. What is possibly more interesting is something which has so far been restricted to the textile industry because of the difficulty of actually carrying out the reaction on the hair. This is the use of the various silicone monomers which still have reactive groups on them, which will combine with a good many of the groups in the keratin of the hair, and actually give a certain degree of water repellency to the outside cuticle of the hair. This can be done in the laboratory and there is no doubt that the hair becomes more water repellent and retains a set longer in the rain. But the main difficulty is that it has so far been impossible to carry out this reaction without doing some damage to the scalp as well. There are, of course, a large number of silicone monomers beside the simple ones like the chlorosilanes and some of these have been tried with varying success. So far none of them have been ruled as completely safe for use on the skin, but it is quite possible that there may be possibilities of doing this sort of reaction when more refined silicone monomers become available. Question 8: What does the panel recommend for the practical definition and control determination of the consistency of products with non- NewtonJan flow properties ? Answer: This subject was treated by Dr. •larriott in Chicago a short while ago. The subjective effect of a cream being applied to the skin is difficult to describe or to measure. Properties such as flow and resistance to flow, contribute to the effect, and to this extent can be measured. Some form of rotating cup viscometer will give values which can be used as a criterion in the control laboratory for checking viscosity and thixotropy. Consistency of results will give some confidence in consistency of usage feel which the customer has found satisfactory. Question 9: What is the panel's opinion of medicated shampoos ? Answer: A medicated shampoo can be formulated to have a bactericidal of bacteriostatic effect. Taking the case of hexachlorophene in soap, it is