398 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS this short test period cannot always be taken, however, because the initial detailed surface condition of a test specimen can have a very large effect on the recorded wear rate. In effect the tissues have to be 'run in' until a surface texture is created which is characteristic of the particular dentifrice under investigation. A particularly marked effect is observed, for instance, if one changes from a coarse to a fine abrasive dentifrice, since it is necessary to obliterate the highly roughened surface before one can establish the true intrinsic wear rate of the finer compound. For this reason it is recommended that all wear test measurements be preceded by a fairly lengthy con- ditioning period often exceeding the actual duration of the test measure- ment run. Conditioning in this way not only allows a physical equilibrium to be established between the dentifrice and the surface it is creating, but also assists the surface to attain chemical equilibrium with its environment. The latter may be important in tracer techniques, since ion exchange between the slurry and the tissue is a possibility. 2 (a) •a • 3 Dentine •: 2 Enamel -- I 2 3 4 5xlO • Number of brush sfrokes, enomel I 2 3 4 5 I0 3 Number of brush strokes, denfine (a) Dentine and enamel brushed with 'foreign' dentifrice 1 igure 4 (b) 'XexEnome I \ e\ I 2 log, Strokes (b) Logarithmic curve for enamel after decalcification in lactic acid Conditioning curves for specimen Examples of two 'running in' tests are shown in Fig. 4 (a) for dentine and enamel specimens. It will be seen that in these cases it was necessary to remove at least 50 g of dentine tissue and 8 g of enamel tissue, before a constant equilibrium wear rate was obtained. The exact amount of material and time taken to remove it will obviously depend on the initial state of

(a) before etching {b) after etching for 10 seconds in 2.5• lactic acid Figure ,5 Electron micrograph of enamel specimen after polishing with ¬ micron diamond paste followed by brushing in water for 5,000 double brush strokes Facing page 398