PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 227 



except number 5 is shown in Fig. 3d. Zone 5, vvhicli represents the 

 ahnost completely destro)'ed enamel, is beyond the scope ot this 

 study, but can be obtained by carrying out sufficiently long ex- 

 posures. 



The progression of the lesion began with zone 1 or zone 3 (dark 

 blue or purple band near the surface, Fig. 3e), these zones being 

 indistinguishable from each other. The relatively sound outer layer 

 was formed next. This was followed by formation of zone 2 (gold 

 band), which then separated zones 1 and 3 (Fig. 3f). Zone 4 (light 

 blue band above zone 2) developed as the lesion progressed, fol- 

 lowed by zone 5. 



Although a detailed stud\ of the formation of these zones as a 

 function of the man\^ \ ariables of the decalcification was not made, 

 a few relationships were discovered. The most significant was that 

 at very high decalcification rates the yellow or gold birefringence 

 colors of zone 2 became almost white with overtones of yellow, 

 green, or blue (Fig. 3g). As the rate of decalcification decreased, 

 hints of yellow and finally the familiar gold color appeared. At high 

 decalcification rates, zone 2 was usually very narrow (Fig. 3f). 

 Periodic interruption of tlie decalcification tended to broaden zone 



Figs. 6 to 13. The effect of increasing exposure time on the incipient carious 

 lesion formed by exposure of enamel to a medium at 4°C consisting of 0.05 m 

 lactic acid and 0.03 m CaCL plus 6 per cent hydroxyethyl cellulose adjusted to 

 pH 3.5 is demonstrated with light micrographs (Figs. 6, 8, 10, and 12) and 

 microradiographs (Figs. 7, 9, 11, and 13) of sections from enamel samples ex- 

 posed to the medium for different time periods. 



Figs. 6 and 7. After a 16-hour exposure, decalcification at or near the sur- 

 face, extending about 10 microns deep, was visible. (X 200.) 



Figs. 8 and 9. After a 3()-hour exposure, the lesion had progressed to 

 about 25 microns and the relatively sound outer layer had appeared. ( X 200.) 



Figs. 10 and 11. After 90 hours, the lesion had penetrated about 50 mi- 

 crons and the relativelv sound outer laver was about 5 microns wide. ( X 

 200.) 



Figs. 12 and 13. After 120 hours, there was a further increase in loss of 

 subsurface enamel material and an enhancement of the relativelv sound outer 

 layer. About 50 per cent of the enamel had been dissolved from the lesion, 

 according to calculations from the measured dissolved phosphate shown in 

 Fig. 26. The surfaces of all the samples were almost untouched by the de- 

 calcification medium as determined microscopicallv. The depth of the lesion 

 across each sample was also quite uniform. (X 200.) 



