392 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS slightly rounded to prevent the brush fibres encountering abrupt changes in height. The brush head of the machine is equipped with four perforated vanes conforming closely to the internal dimensions of the trough and so orien- tated as to force the slurry to pass through the brush fibres when the head is in motion. The presence of these vanes also assists in preventing sedi- mentation of the dentifrice abrasives. The specimens, after mounting, are thoroughly scrubbed with a dentifrice slurry and distilled water, and then placed in a polythene capsule which is filled with distilled water and a trace of bacterial antiseptic 1% Hibitane. The capsule and contents are irradiated for a period of five hours. After removal from the reactor, any Na 24 activity is allowed to decay before the specimens are inserted in the rig, but the level of this ¾ activity can be reduced by avoiding direct handling of the specimens whilst cleaning. To perform a typical brushing test, a known volume of dentifrice slurry is placed in the trough and at the end of the prescribed brushing period, 1 cm 3 samples are withdrawn with the aid of a pipette fitted with a micro- filler. The samples are transferred to 2.5 cm diameter aluminium planchets and dried beneath an ir lamp at a surface temperature of 105øC for one hour. In cases where excessive frothing of the slurry has occurred, it is preferable to pour the contents of the trough into a beaker and break the foam with a drop of ether. After careful restirring of the slurry a non-aerated sample may be withdrawn. The level of [i activity of the dried samples is measured with a 5 cm diameter mica end-window Geiger-Muller counter with the specimens placed within a millimetre or so of the window. With this geometry a high counting sensitivity can be obtained and errors arising from a non-uniform deposit eliminated. This arrangement also reduces the small back scattering errors which can occur if a comparison is made between dentifrices con- taining elements of widely different atomic number (6). The dentifrice samples are usually taken from slurties having a dentifrice concentration of 20% or less and thus containing only 100-200 mg of solid matter when dried. At such mass thicknesses (20-40 mg/cm 2), [• absorption errors can be neglected. In other tests where a high concentration of the dentifrice is employed the slurry must be diluted before taking a sample. The measured figures for the [i activity are corrected in the usual way for counter 'dead time' followed by background activity, and the final result multiplied by the volume of slurry originally added to the trough. The latter may vary slightly from one dentifrice to another if weight concentrations

MEASUREMENT AND INTERPRETATION OF DENTIFRICE ABRASIVENESS 393 are used in making up the slurries. Corrections for the decay in [• activity of the source may be necessary if a long time elapses between the measure- ments of different samples. The dried samples are rarely active enough to give rise to any health hazard, but the irradiated tooth specimens should be handled in accordance with the approved recommendations for a high energy • source of 1 or more mCi activity. Satisfactory shielding is obtained with 1 cm of water or Perspex, whilst tongs of at least 25 cm length and rubber gloves are recom- mended for handling the irradiated samples. Perspex goggles should be used to protect the eyes. By employing a water-moderated reactor, such as exists at the Scottish Research Reactor Centre, East Kilbride, for the specimen irradiation, it is possible to irradiate the specimens at temperatures below that of the human body and to keep them immersed in water for the whole irradiation period. Under such conditions of irradiation and low neutron dose, major changes in the physical properties of a tissue can be avoided. In view of the fact that human dentine and enamel contain different proportions of hydroxyapatite, the specific activity for a given radiation dose will not be the same for the two tissues. For comparative measure- ments on the same tissue this is unimportant, but for experiments com- paring the wear behaviour of different tissues any difference in specific activity must be allowed for. The ratio of specific activities for dentine and enamel is about 1:2, but it is usually desirable to include small fragments of the actual tissues with the main specimens so that the ratio can be accur- ately determined. The size of these fragments must be kept very small if the [• particle absorption error is to be minimized. A mass of less than 0.5 mg is recommended. RELATIONSHIP BETWEEN ABRASIVE WEAR RESISTANCE OF A DENTAL TISSUE AND ITS HARDNESS The technical problem of cleaning and polishing a surface by mechanical means presents many difficulties if it is composed of a number of elements of varying hardness. Thus the abrasives incorporated into dentifrices must assist not only in removing the relatively soft bacterial plaque and any other pellicle present, but also assist in repolishing the tooth enamel together with any cement or dentine exposed at the cervical margin. The most efficient manner of achieving the required cleansing action