CHARACTERIZATION OF DENTIFRICES 191 which was neutralized with sodium hydroxide. Another consisted of a 20•o suspension of a synthetic silica thickener in sorbitol. Dentifrice 1 contained 14•o of an abrasive, dentifrice 3 contained 32•o of a mixture of abrasives, Seven different hydrocolloids were used at the stated concentrations, with corresponding minor changes in water content, in order to achieve acceptable consistency and different rheological pro- perties (Table I). Methods of theological characterization Initial force of extrusion (corrected) Vasic and deMan's method (16) was adapted for use on an Instron tensiometer. Extrusion rheometer attachment. Figs. 1-3 illustrate the apparatus, which consisted of a motor crosshead rigidly joined to the Instron crosshead. A stainless steel rod fitted to an aluminium piston was attached to the motor crosshead. The piston was fitted at first to a metal tube bearing a brass cap, ' Aluminium framel I Piston ,1•-•- It ••--•-ScP2 • I I bRahrer:•neter • •'RuDDer 'o'- ri• I I (aluminium) •• I I Brass cap • I I (threaded) • •50ml beaker I • INSTRQN I I • I CROSSHE i I Figure 2. Mk I rheometer barrel.