408 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS should you not have used a material of differing particle size and hardness but having the same chemical reactivity to the material you are examining? DR. WRIGHT: It would certainly have been better to have done this, but the data was acquired as part of a programme to study the abrasiveness of commercially avail- able dentifrices and the theoretical interpretation of the results followed later. The alumina type dentifrice was added primarily to study the relative behaviour of enamel and dentine tissues towards an abrasive of very high hardness. The question regarding the solubility of the tissue has to be answered in two parts: first, does the reactivity or solubilising effect of the dentifrice play a significant role in determining the wear rate of a dental tissue in the absence of radiophosphorus in its structure, and second, does the presence of radiophosphorus lead to real changes in solubility of the structure or possibly lead to a false indication of the actual wear rate as a result of ion exchange between the p•2 in the dental tissue and the phosphate ions in the slurry. In reply to the first of these possibilities, I doubt very much whether solubility effects enter into the wear process with the current dentifrices. None of the dentifrices was acidic in character, in fact they would all be expected to have pH values close to 8. The second possibility requires a more detailed examination, since the recoil action associated with the trapping of a thermal neutron may very well lead to highly 10calised changes in tissue structure. However, the total volume of the tissue affected in this way must be very small since the concentration of p32 atoms is only about 10-7% of the total number of phosphorus atoms present. One can therefore dismiss the argument that there is an overall real change in the solubility of the tissue. More permanent and extensive damage can result from excessive heating of the tissue during irradiation, but this is avoided by using a water moderated reactor. Although the overall solubility may not be changed, one does have to remember that the wear process is recorded by the release of p•2 into the dentifrice slurry and hence it is the local wear resistance or solubility of the regions around the radio- phosphorus atom that is relevant. On the short time basis, the rate of removal of these 10calised regions of tissue might very well differ from the rate of removal of the unchanged lattice and, indeed, perhaps exert some control on the latter. However, in the presence of mechanical action, insoluble products will not accumulate on the surface of the tissue and it is reasonable to assume that the solubility (and wear rate) of the unchanged tissue will not be affected, and will be adequately monitored by the release of p•2 in the slurry. Again the abrasive wear resistance of the tissue is unlikely to be changed by the presence of small quantities of tissue converted into a different structure by the recoil action. Most of the abrasive particles will engage groups of atoms greatly in excess of the small number associated with the recoil process. A third possibility arising from the use of p•2 as a tracer element is that ion exchange between the p•2 in the tissue and phosphate ions in the slurry will lead to a surface depletion of the radionuclide. The extent of the reduction will depend upon the nature of the slurry and the time of exposure. In order to counteract this possibility we recommended that all tissues be 'conditioned' in the dentifrices slurries prior to the performance of the actual test. In this way one establishes an equilibrium p•2 distribution in the surface layers of the tissue which is the same at the beginning of the test as at the end. The total amount of p32 entering the slurry, partly by

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