PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 225 



hydroxyethyl cellulose and lactic acid adjusted to pH 3.5, along 

 with a microradiograph (Fig. 5) of the same section of enamel, 

 illustrates the type of result obtainable. Lesions produced by this 

 system definitely had a more sound layer, albeit not very deep, 

 overlying the subsurface decalcified region. The surface of the 

 enamel sample was only slightly changed, as determined by micro- 

 scopic observation using a metallurgical microscope with top illumi- 

 nation. 



The depth of the relatively sound outer layer at the surface can be 

 enhanced by several devices: (1) increasing the exposure to the de- 

 calcification medium, (2) removing and returning the enamel 

 sample to the decalcification medium at intervals, (3) adding cal- 

 cium ions initially in the decalcification medium, (4) adding both 

 calcium and phosphate ions initially in the decalcification medium, 

 (5) increasing the buffer concentration, and (6) including one of 

 many select cations, such as zinc, in the decalcification medium. 



Examples of the lesion that resulted from adding calcium as 

 CaCL to the decalcification medium are presented in Figs. 6 to 13 

 for exposures of 16, 30, 90, and 120 hours. Figures 14 and 15 illus- 

 trate the effect of including both calcium ( as CaCl2 ) and phosphate 

 (as NaH2F04) to the medium (all at pH 3.5). There was a pro- 

 gressive increase in the depth of the relatively sound outer layer and 

 in the loss of material in the decalcified region with each successive 

 addition to the decalcification medium. Also, the enamel surface was 

 now unchanged as determined microscopically. Adding phosphate 

 (as NaH2P04) alone caused severe surface damage. An even more 

 dramatic improvement was found by doubling the lactate buffer 

 concentration (Figs. 16 and 17) or by interrupting the exposure 

 periodically (Figs. 18 and 19). The similarity of these lesions to 

 natural lesions can be seen by comparison with Figs. 1 and 2. Add- 

 ing zinc as ZnCl2 had the same qualitative effect as CaCL, although 

 higher concentrations of ZnCL were required for quantitative 

 equivalency. 



The various zones observed in natural lesions using polarized light 

 with a full-wave plate, first order red, have been well described pre- 

 viously ( Gustafson, 1957 ) , and a diagrammatic representation is re- 

 produced (Fig. 3b). A chemically produced lesion with all zones 



