60 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



polymer and then probing into the bubbles with a clean dissecting needle 

 or a small glass rod to allow the air to escape. 



To bring about polymerization and hardening of the base layer of 

 plastic, the mold, with its covering in position, should be placed in a 

 constant-temperature oven heated by hot water or electricity. By no 

 means should gas or oil-burner ovens be employed; and if electric ovens; 

 are used, only those types with external thermostat-control contacts and 

 explosion-proof doors that cannot be positively latched should be con- 

 sidered safe for this purpose. The temperature should be maintained at 

 about 45° C. (113° F.) if methyl methacrylate sirup is being used, or 

 50° C. (122° F.) in the case of ethyl methacrylate, until the base layer 

 has become polymerized to a firm gel consistency, which usually requires 

 from 40 to 50 hours. The temperature should then be raised 5° C, either 

 by changing the thermostat setting or, preferably, by transferring the 

 mold to a second oven that is regulated to maintain a temperature of 

 50° C. (122° F.) for methyl methacrylate or 55° C. (131° F.) for ethyl 

 methacrylate. Slightly higher temperatures may be used for small cast- 

 ings, but in all cases care is necessary in heating the plastic while in the 

 gel stage to keep the temperature below the point that will cause vapor 

 bubbles to form and remain trapped in it. 



As a means of testing the hardness of the plastic as polymerization 

 progresses., a sharp-pointed implement such as a dissecting needle or a 

 scalpel should be pressed gently into it from time to time while it is 

 in the oven. When the upper surface finally becomes so hard that it is 

 no longer dented to a noticeable extent by the testing implement, it may 

 be considered that polymerization has progressed far enough and the base 

 is ready for the embedding operation. A convenient guide in determin- 

 ing the progress of polymerization as the plastic changes from gel to 

 solid state, is the temporary line or plane that becomes visible between 

 the gel and the denser solid portion below and moves gradually upward 

 as the hardening proceeds. When this line reaches the top and disappears 

 it indicates that the required degree of hardness has been attained 

 throughout the block. 



As soon as the base casting has become completely solid, the mold 

 should be removed from the warmer oven and gradually cooled down to 

 room temperature. This can be done by transferring it back to the cooler 

 oven for a few minutes, taking it out and setting it for a short time on 

 the slightly warm top of the oven, and finally placing it on a table or 

 bench until it has completely cooled to room temperature. If it is not to 

 be used at once for the embedding operation, it should be carefully pro- 

 tected from dust because otherwise a visible layer will be formed when 

 the remainder of the plastic is added. 



EMBEDMENT OF SPECIMENS 



The embedding operation consists of placing the prepared specimen on 

 the previously cast plastic base in the mold (fig. 14), then surrounding 

 it with fluid partial polymer or with monomer, and finally hardening the 

 entire mass through polymerization to form a solid block. In carrying 

 out this procedure minor variations are required, or optional, for differ- 

 ent types of specimens, depending upon their size and structural char- 

 acteristics and upon the methods employed for their preparation. 



