8 



Physical Chemistry of Enamel Dissolution 



JOHN A. GRAY and MARION D. FRANCIS, Miami Valley Labora- 

 tories, The Procter & Gamble Company, Cincinnati. Ohio 



INCIPIENT dental caries in enamel begins with the appearance of 

 an opaque lesion, commonly known as a "white spot."' The formation 

 of this stage of caries is the result of chemical dissolution of enamel 

 substance and, therefore, should be amenable to description in terms 

 of physical-chemical principles. Such a description requires an iden- 

 tification of the factors affecting the process and a measurement of 

 the magnitude of their effect on the process. The objective of this 

 exposition is to present the physical and chemical factors affecting 

 or controlling formation of an incipient carious lesion and to de- 

 scribe the process on a physical-chemical basis. 



Recognition of the fundamentally chemical character of caries 

 formation, beginning shortly before 1900 (Magitot, 1878; Miller, 

 1905), provided the inspiration for many investigations into the 

 mechanism of this process. Concurrently with these investigations, 

 a variety of systems, some of which included bacteria, were devised 

 for producing the lesions in vitro, and, indeed, these methods were 

 eminently successful. The essential process consists of the diffusion 

 of the reactant to the reaction site, which in this case is the solid, 

 enamel, followed by reaction and diffusion of the products away 

 from the reaction site. Such an interaction between a solution and 

 a solid is known as a heterogeneous reaction. In most cases, the rate 

 of heterogeneous reactions is controlled by the rate of diffusion of 



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