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



in diameter or where the embedding layer is the full one-half inch in 

 thickness. 



Carrying out polymerization of masses of methacrylate resin under 

 controlled temperature conditions that permit removal of exothermically 

 produced heat, as well as provide the heat needed for initiating the 

 reaction, was described in a patent by Loder (30 \ 6 . 



When the first embedding layer has become fully hardened, the mold 

 should be removed from the oven. Determination of the extent to which 

 polymerization has progressed can be made by testing the hardness of the 

 plastic with a dissecting needle or other sharp-pointed implement. After 

 careful cooling to avoid breakage as explained in the section on the casting 

 of bases, the mount will be ready to receive the next layer. In applying 

 this, as well as all remaining ones, the same procedure should be followed, 

 and the same polymerization temperatures employed, as for the initial 

 layer. The amount of time required for completing the embedment may be 

 shortened somewhat in the preparation of small mounts of durable speci- 

 mens by using thicker embedding layers and by raising the final polymeri- 

 zation temperatures by 5° or 10° C. (9° or 18° F. ) ; but on the other hand 

 when unstable or fragile specimen material is to be embedded, it will often 

 be found worthwhile to proceed even more slowly, using thinner layers and 

 lower temperatures, in order to insure more uniformly satisfactory results. 



Annealing to Prevent Crazing 



Unless special precautions are taken, the development of minute surface 

 cracks or crazing is almost certain to occur within a few months in blocks 

 of methacrylate plastic cast in the manner here described. This undesir- 

 able condition can be prevented, however, if the cast blocks are annealed 

 or heat-treated before removal from their molds. Prevention of crazing 

 in cast methacrylate polymer by prolonged heating at suitable elevated 

 temperatures was described in a patent by Kuettel (28 V. The procedure 

 for this treatment consists in subjecting the polymerized plastic to pro- 

 longed heating at as high a temperature as the embedded objects can 

 tolerate. In the case of durable specimen material capable of withstand- 

 ing at least 75° C. (167° F.) , heating at this temperature for from 10 to 

 20 hours, dependent upon the size of the block, is usually sufficient to 

 eliminate all likelihood of crazing. With blocks containing specimens that 

 are not stable at temperatures much above 50° C. (122° F.), heating for 

 3 weeks at 50° C. is generally necessary. If they can tolerate 55° C. 

 (131° F.) heating at that temperature for 2 weeks should be sufficient. 

 Specimen mounts having a maximum temperature toleration between 

 55° and 75° C. call for heat-treating periods of intermediate length. 



Upon completion of the heat-treating cycle, the casting should be 

 cooled cautiously to avoid breakage of the mold from uneven contraction. 

 This can be done conveniently in most cases by setting the mold with its 

 casting in a vessel of water that has been heated to about the same tem- 

 perature as the casting and allowing it to cool gradually to room tempera- 

 ture. 



6 Patent under assignment to the du Pont Viseoloid Company, and cannot be em- 

 ployed except by consent of the assignee. 



7 See footnote 5, page 62. 



