TREATMENT WITHOUT FILLING 185 



The kind of excavation required in order to 

 prepare a cavity for tinning is in some ways like 

 an open system excavation, and in other ways like 

 excavation for an asphalt or concrete filling. 

 No provision need be made, of course, for the re- 

 tention of a filling. But fitting a cavity for tin- 

 ning is unlike treating it by the open system in that 

 a line all around the opening of the cavity must 



The correct way to cut an old callus in preparation for tinning 

 and manner in which one new callus will spread over the tin 



be laid out, up to which the tin shall come, and 

 all living bark and cambium inside of this line 

 must be removed. This line must keep as close 

 to the cavity as possible, so that the area to be 

 callused over may be minimized. The opposite 

 sides of the line must be as nearly as possible in 

 the same plane, so that the tin can be put on with- 

 out wrinkling it. And the line must be so located 

 that there shall be just inside of it a strip at 

 least three-quarters of an inch wide, of fairly 

 level, strong wood, upon which to nail the edges 

 of the metal. 



