LABORATORY MODELS FOR DEPILATORY PERFORMANCE 375 Thioglycollic acid. Depilation time Neutralizing base pH (min) Sodium hydroxidc 9 15.0 Sodium hydroxide 10 12.0 Sodium hydroxide 11 5.0 Calcium hydroxide 9 15.0 Calcium hydroxide 10 15.0 Calcium hydroxide 11 11.0 Calcium hydroxide 12 7.0 Conclusions. Depilation time is decreased as pH is increased as would be expected. In practical terms a pH of 9 would be too low to give a reasonably fast-acting depilatory. In the range of pH of 10-12 the choice of thio compound and neutraliz- ing base can have more effect on speed of depilation than the pH. Effect of concentration of thio compound Using the basic depilatory cream formula previously described, de- pilatory creams were made up containing (a) 2.5•o w/w, (b) 5.0• w/w and (c) 7.5•o w/w of each of the following thio compounds: (a) 2-mercapto ethanol (b) thioglycerol (c) thioglycollic acid. Each cream was neutralized to a pH of 11 with (a) sodium hydroxide and (b) calcium hydroxide, and the depilation times were determined for each formulation. 2-mercapto-ethanol. •o w/w of thio Depilation time Neutralizing base compound (min) Sodium hydroxide Sodium hydroxide Sodium hydroxide Calcium hydroxide Calcium hydroxide Calcium hydroxide 25 5O 75 25 5O 75 8.0 3.0 3.0 8.0 3.0 4.0

376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Thioglycerol. Yo w/w of thio Depilation time Neutralizing base compound (min) Sodium hydroxide 2.5 14.0 Sodium hydroxide 5.0 5.0 Sodium hydroxide 7.5 5.0 Calcium hydroxide 2.5 10.0 Calcium hydroxide 5.0 8.0 Calcium hydroxide 7.5 6.0 Thioglycollic acid. % w/w of thio Depilation time Neutralizing base compound (min) Sodium hydroxide 2.5 10.0 Sodium hydroxide 5.0 5.0 Sodium hydroxide 7.5 6.0 Calcium hydroxide 2.5 12.0 Calcium hydroxide 5.0 11.0 Calcium hydroxide 7.5 15.0 Conclusions. Whilst an increase in concentration from 2.5•o to 5.0•o with all three thio compounds gives a decrease in depilation times, a further increase to 7.5•o does not have any further effect (probably due to solubility limits having been reached). An optimum concentration of thio compound of approximately 5• would seem to be indicated for practical purposes. DISCUSSION This series of results enabled us to formulate a range of practical de- pilatories which could then be compared with competitive products to give a measure of relative performance. Although speed of depilation is probably the most important factor in the development of a commercial depilatory, other characteristics (particu- larly potential irritation) have to be considered. When skin irritation studies were carried out using the most rapid acting depilatories, it was found in general that speed of action and depila- tion were inversely proportional.