48O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This technique is obviously applicable to the study of the possible uptake of fluoride by enamel from a fluoride-containing product and some preliminary results which were obtained to prove the feasibility of the method can be described. Upper central incisors free from caries were selected and cut in half, one half to act as control and the other as test. The test halves were attached to the circumference of a disc fitted to the end of a stirrer spindle. When the stirrer motor was switched on, the test surfaces were arranged to brush lightly against the tips of toothbrush bristles set in the bottom of the beaker which contained the test solution. In the results which follow, the treatment was continued for 24 hr in an effort to ensure that some fluoride would be incorporated in the enamel. A light brushing with distilled water was given before the fluoride deter- mination was carried out as described above. Three consecutive 10 sec etches were given and the fluoride in each determined. In some experiments 2000- ABRADED TEETH 1:1 Toothpaste Slurry ETCHING TIME (seconds) UNABRADED TEETH 1:3 Toothpaste Slurry TREATED SURFACE ..... CONTROL SURFACE ----'-SIONIFICANCE LEVEL 2'o io Figure 13. The uptake of F- on enamel surfaces. the surface of the teeth was abraded to remove the layer of high fluoride content and so help to achieve more consistent results. Typical results, the average of six teeth per treatment, are shown in Fig. 13. The treatment in the first experiment was with a sodium fluoride/sodium phosphate

THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 481 solution as suggested by Brudevold (17). This is obviously more effective than when incorporated in a dentifrice though even then there is a sig- nificant uptake compared with the controls. The dotted line indicates the fluoride level above which the test teeth have acquired a significant increase in fluoride. TIN UPTAKE BY ENAMEL A similar type of experiment can be performed to demonstrate the presence of tin derived from stannous fluoride preparations though the analytical procedure is not so simple. The technique of electron probe microanalysis is applicable and the results can be obtained either as a photograph in which the atoms of tin appear as white spots on a dark ground or as a recorder trace obtained as the electron beam traverses the selected area. An example of the latter is shown in Fig. 14. A polished ENAMEL POLISHED • E NA.••/.,/- BACKGROUND Figure l•t. Tin uptake on polished and on etched enamel. tooth surface was etched with dilute acid to produce a white spot. A stannous fluoride treatment was given and, after rinsing, the area was