632 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS It is for these reasons that many attempts have been made to assess the lathering characteristics of shampoos by laboratory techniques in order that formulations can be screened prior to lengthier checking in the hairdressing salon. In the early days the Ross and Miles (1) test, where a standard volume of shampoo or detergent solution was run from a burette into a measured quantity of the same solution in a glass tube--the resulting foam height being measured, was used quite widely. Unfortunately the Ross and Miles method of testing, while giving repro- ducible results, very often leads to products being assessed in an order which is not similar to the order obtained as a result of salon testing. Various other methods of assessment have been used. For example the use of a standardized shaking technique where a solution of the product under test is shaken in a stoppered measuring cylinder--the foam height being measured. Myddleton (2) devised a method of producing foam by bubbling a standard volume of air through a sintered glass disc--the rate of collapse of the foam being measured. A modification to this method has been reported by Dutton and Reinisch (3), where the use of a nylon web enclosed in a cylinder is proposed, in an attempt to overcome some of the disadvantages of other methods. In my view the most important disadvantage in any method of foam assessment using a cylinder or tube is that the foam tends to be stabilized by mechanical support and the fact that the air above the foam is of high relative humidity. There is no doubt in my mind that attempts to substitute a comprehen- sive series of laboratory tests, designed to replace the assessment of the various characteristics of shampoos in the salon, may well lead to a situation where the laboratory test methods would take as long or longer than the salon evaluation. All this probably without certain advantages which are obtained from in vivo tests. Nevertheless, some acceptable method of screening shampoos at least for their major characteristic--lathering--would be of great value to check whether or not they are worth further testing in the salon. New (4,5) has described foam assessment methods, and his principles have been adapted in our laboratories in an attempt to produce a suitable screening test for new formulations. New originally (4) suggested that foams could be categorized by measure- ment of various physical properties, and described methods of producing foams using a Sunbeam Mixmaster. The physical properties of the foams so produced were checked by measuring the specific foam volume, viscosity, the change in viscosity with the age of the foam, light transmission, and foam drainage.

MODERN TRENDS IN THE ASSESSMENT OF SHAMPOOS 633 A considerable amount of investigation was carried out on the method of generating the foam, and it was suggested (rightly in my opinion) that a Sunbeam Mixmaster set to give a constant speed of 720 rpm was a satisfac- tory method. Bearing in mind that the original paper dealt with the foam characteris- tics of different groups of surfactants, and also that our work on developing this method has been largely aimed at the assessment of shampoo formula- tions with fairly similar performance characteristics, we decided to attempt to improve the reproducibility of the results, at the same time keeping the number of tests down to an absolute minimum. We commenced our investigations by concentrating solely on foam viscosity determinations. There is no doubt that the more tests that are carried out on the foam produced by the Mixmaster, the more information is obtained. Neverthe- less, it is time-consuming to carry out all the various tests suggested in the original paper, the only advantage being that by so doing one may get a better assessment of a shampoo characteristic than could be obtained by washing a few heads in the hairdressing salon. In the original paper on this technique of foam assessment, the method of determining specific foam volume was described and we adopted this method and find it an extremely useful and simple process. We also have measured the viscosity of the foams produced, and carried out a considerable amount of work to investigate the possible use of a Ferranti VM Type Viscometer in this connection. The use of this instrument for measuring foam viscosities at different shear rates tends to over-complicate matters, due to the non-Newtonian properties of foams. We have decided, therefore, to use the recommended standard Techne Viscometer. Our experiments showed that although it was useful to examine samples of foam from the Mixmaster after various periods of beating, in general the results obtained after a three-minute beating period at 720 rpm were the most generally useful, and we therefore standardized our tests on this basis. One further variable which appeared to be of considerable importance was the temperature of the foam produced in the Mixmaster, and variations in temperature were found to be one of the main causes of poor reproducibility of results. At present, therefore, our normal procedure for producing foam is to take a shampoo diluted to 25%, 12.50% and 6.25% with water, subsequently heating a 50 ml aliquot of these solutions in a beaker to 45 ø C with 0.5 g of standard soil. This mixture is then placed in a previously warmed Mix- master bowl, and agitation is continued for three minutes at 720 rpm when the temperature is again checked. We find that reasonable reproducibility can be obtained so long as the