486 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3'8 pH . 3'7- 3'6 gROUP BASE LINE TEST 1 TEST 2 Figure 18. 17 persons on sequence- control, control, fluoride. to produce caries under controlled conditions. Von der Fehr (25) described a technique in which he induced lesions in premolars which were to be extracted for orthodontic reasons. The buccal surface of the tooth was pumiced, rinsed with water and dried. One half of the surface selected as the control was covered with wax while the other was treated with the test solution. The wax was removed and a gold onlay cemented in place over the test and control areas in such a way as to leave a space about 0.5 to 1 mm wide between the enamel surface and the gold. The teeth were extracted after periods of three to five weeks and the test areas examined micro-

THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 487 ,o.I BASE LINE CON FLUORIDE 3'5 3.6 / I I I I I I 3.7 3.8 3"9 pH Figure 19. pH distribution of fluoride and control users. scopically and macroscopically to assess the degree of carious attack. This seems a valid method but is impracticable for studying large numbers of test materials. THE ARTIFICIAL MOUTH , Numerous attempts have been made to produce carious lesions in an artificial mouth system for the purpose of studying the carious process. Pigman et al (26) quote some twenty references, the first dating from 1878, to work involving the formation of carious lesions in 'a laboratory appara- tus'. Some report merely general decalcification while others claim to pro- duce lesions identical to natural ones. Such a system is attractive as an assessment technique in that the overall effect of an agent, whether it be on enamel structure, enzymes or bacteria, could be assessed without understanding in detail the mechanisms involved. Pigman (27) has developed an artificial mouth apparatus in which a nutrient medium is fed to a culture of oral organisms in contact