PRESERVATION OF SPECIMENS IN PLASTICS (,] 



Figure 14. — The impregnated specimen is removed from the monomer liquid with 

 l tweezers (A); it is then placed in position on the previously hardened base in the 

 mold (B). 



At the time of embedment all specimens must be sufficiently dry to 

 insure no diffusion of moisture into the plastic, and. where dehydration 

 has been carried out through the use of alcohol or other volatile liquids, 

 10 appreciable amount of the liquid dehydrating agent must remain in the 

 specimen. If these precautions are not observed, the polymerized plastic 

 is likely to be clouded in the first case or to contain permanent vapor 

 bubbles in the second. Specimens that have not been impregnated with 

 monomer or partial polymer prior to embedment, either because they 

 have impervious surfaces or because they have been coated to protect 

 them from direct contact with the fluid plastic, should preferably be kept 

 in a desiccator until the embedding operation is started, although it is 

 reasonably safe in most cases to store them in the open air if the relative 

 humidity in the room is continuously lower than 30 percent. 



The first step in the embedding operation is to pour a small amount of 

 fluid plastic upon the base in the mold. Partial polymer of medium vis- 

 cosity is generally preferable for this purpose, except where extreme 

 porosity of the specimen material necessitates the use of unthickened 

 monomer or where impervious or coated specimens call for the highly 

 viscous grade. In the last case, where very viscous casting sirup is em- 

 ployed, it is usually advisable first to soften slightly the upper surface of 

 the base by covering it for a minute or two with monomer, which may 

 then be poured back into the stock container. 



When casting methacrylate resins in block form, it is important to 

 guard against excessive internal temperature rises, which tend to occur 

 because of the exothermic nature of the polymerization reaction and the 

 low heat conductivity of the solid polymer. Since temperatures high 

 enough to damage both block and specimen are frequently produced in 



