388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tissues and perhaps impair the cosmetic properties of the product, it also includes lumps of material which the sensory tissues of the mouth dis- tinguish as discrete matter and which destroy the sensation of smoothness when the dentifrice is in use. A satisfactory test for dentifrice abrasiveness must therefore assess the overall wear likely to be caused to the main dental tissues and at the same time sense the presence of 'gritty' matter. The former is simply a quanti- tative test of the total amount of wear, whereas the latter is a form of quality control and rejects the dentifrice unless it achieves the measured level of wear without recourse to visible scratching of the surface. Two tests are therefore required. First the product should be screened to find whether it contains gritty particles, and second, if it does not contain those, its overall abrasiveness towards specific dental tissues should be measured, either absolutely, or by comparison with some reference dentifrice. The problem of grittiness is not discussed further in this paper, since the well known metal disc (coin)/glass-slide technique (1) provides a sufficiently precise assessment of its more damaging aspects. Such a test can readily detect coarse gritty matter in concentrations of 0.1% w/w, although the presence of such particles may have little effect on the overall wear. On the other hand, it is also perfectly feasible to formulate a dentifrice that will pass the glass-slide test, but exhibit a very high degree of tissue wear. Despite this possibility many National Standards Specifications still only specify the glass-slide test as the abrasiveness' control. The establishment of acceptable limits of overall abrasiveness presents many problems since the actual cleansing requirements will vary widely from one individual to another. Kitchen and Robinson (2) made an attempt to assess this variability by visually examining a large number of students' teeth which were cleaned regularly with certain proprietary dentifrices. After close examination it was found that the tooth enamel of 20% of the students was susceptible to heavy staining, but that these stains could be eliminated in 95% of this heavy-staining group by the regular use of a dentifrice capable of removing 1 mm of dentine in 100,000 brush strokes. Unfortunately, the equivalent enamel wear was not measured but judging from the type of dentifrices employed one can assume that it would be about one hundredth part of the dentine wear. It follows that during the course of a daily brushing of 15 strokes with a dentifrice of the above abrasiveness, the maximum amount of material worn away would be 1.5 x 10-6 mm of enamel or 1.5 x 10-4mm of dentine. Although the dentine wear is rather high, 1.5 x 10-6mm of enamel only corresponds to about four

MEASUREMENT AND INTERPRETATION OF DENTIFRICE ABRASIVENPZSS 389 atom layers of the tissue and can be considered a perfectly safe level of wear. However, the fact that most commercially available dentifrices seem to be capable of removing 0.5-1.0 mm of dentine in 100,000 brush strokes implies that these dentifrices have been designed to cope with the heavy staining proportion of the population. If this is so, then the degree of dentine abrasion is possibly too high for many people, especially those showing advanced cervical exposure. The increasing use of motorized toothbrushes, although beneficial to gingival health, necessitates greater care in selecting a dentifrice which avoids excessive wear of dental tissues. The measurement of dentifrice abrasiveness has always proved dif•cult in practice, especially with enamel surfaces for which prolonged brushing is often necessary to obtain an accurately measurable amount of wear. This is aggravated by the fact that dental tissues absorb and release water so rapidly that precise weight loss measurements are not easy to make. In addition the tissues vary considerably in composition, not only from one tooth to another, but also over the surface of a single tooth. For these reasons numerous attempts have been made to find alternative test mater- ials (3), but the results have not proved encouraging. Some experiments, made by the present authors with glass surfaces of similar hardness (370 ]i-IV 2.5) to enamel, indicated that some proprietary brands of dentifrice chemically attacked the glass and yielded results which correlated badly with those obtained with human enamel. The particular glass employed was a medium barium crown glass containing 6% Tin20 3 for radiotracer purposes. Dentine has usually been the popular choice of test material, but its use in isolation can, as will be seen later, lead to gross errors in the assessment of the abrasiveness of the dentifrice to harder tissues. For the same reason, ivory has little to recommend it, even though it can be obtained in large and convenient pieces. Most investigators prefer a brushing test for evaluation purposes, since this clearly reproduces the normal oral cleaning action, but others have utilized polishing machines, based upon a wax lapping plate to retain the dentifrice (4). The latter method was favoured some years ago as it avoided the variability experienced with natural fibre brushes. However, the intro- duction of synthetic fibres has largely overcome this objection, and lapping plate machines are now rarely employed. Tissue wear may be recorded by measuring either weight loss or changes in dimensions. In spite of the problem presented by a non-uniformly