428 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS structure as hectorire and capable of forming colourless translucent gels is a distinct advantage. Such a material is Laponite. This paper considers the measurement of rigidity in gelatin gels con- taining glycerin and the possibility of using the same method of measure- ment on Laponite gels. It also examines the much more important para- meter, for the latter, of static and dynamic yield values. EXPERIMENTAL Gelatins. Alkali processed hide materials from a single source supplied by Richard Hodgsons, Beverley, Yorks., were used. The characteristics of these materials have been given previously {1). Synthetic hectorite. This was Laponite B supplied by The Fullers Earth Union Ltd., Redhill. This material is similar to mica in that it has a variable layer lattice structure of Si, Mg, Li, O, (OH), and F ions. The soluble fluoride was less than 0.5% and the pH of a 2% solution was 8.3. Bentonite. This material complied with the specification given in the British Pharmacopoeia. Glycerin. A nalar material was used. Preparation of gels Gelatin and distilled water were allowed to imbibe at 5 ø for 1 h before solution at 50 ø . The glycerin, also at 50 ø , was added and the mixture gently, but thoroughly, stirred. Gels were allowed to mature at 25 ø + 0.1 ø for 16 h before rigidity measurements were made Bentonire and Laponite gels were made by sprinkling the material on the surface of distilled water and agitating with a non aerating high speed stirrer for half-an-hour. Ageing was in sealed containers at room temper- ature for at least two days. Measurement of rigidity The Saunders and Ward (2) rigidometer method was used. The volume displacement of a gelatin column, maintained at Tø+ 0.1 ø, on the appli- cation of air pressure was measured by means of the movement of a mercury thread in a side arm (Fig. 1). This allowed very small changes in volume to register as large movements of the mercury meniscus. The adhesion of the

SOME APPLICATIONS OF RIGIDITY AND YIELD VALUES 429 Air pressure 2a ____ (o) (b) Figure I Arrangement of apparatus. (a) at rest, (b) under applied air pressure (2). gel to the wall of the tube was sufficient to prevent slip. The rigidity is calculated using the formula PR4 c,, = 8La2h where G----rigidity modulus, Nm-2. P=net pressure (corrected for the back pressure of displaced mercury). R=radius of tube, cm. L--length of gel column, cm. a=radius of mercury capillary, cm. h=displacement of mercury, cm. The formula is valid for the condition that L R. Rheological measurements A Ferranti-Shirley cone and plate viscometer fitted with an automatic programming unit and an X-Y recorder was used to obtain shear stress versus rate of shear curves. The cone was enclosed in a special vapour hood to prevent evaporation of the sample (3). Measurements were made at 25 ø + 0.1 ø using a 7 cm cone (0.3 ø) and a 600 g torque spring. A standardised procedure was used for plotting the flow curves of the gels. All were sheared from zero to 1 759 s-t and back to zero in 120 s as a preliminary. The flow curves were then recorded after allowing the gel to age in the viscometer for varying periods. For assessing the rate of breakdown of the gel, the decrease in shearing stress at a constant rate of