preservation of specimens in plastics 65 



Removal of Cast Blocks From Their Molds 



After the cast block in which the specimen is embedded has become 

 entirely cool, it can sometimes be removed from its mold by shaking or 

 tapping the open end of the mold on a smooth firm surface. In cases where 

 a block cannot be removed in this manner, it should be loosened as much 

 as possible by running a scalpel or strong-bladed knife around its rim to 

 free it from the wall of the mold, after which the shaking or tapping 

 should be repeated. If this fails to bring it out, the mold should be filled 

 with water and set aside for a day or two. After the mold is emptied and 

 dried at the end of this time, the cast block will usually be found to be 

 free enough to permit easy removal. Where a Pyrex-type glass mold has 

 been employed, easy release of the casting can often be accomplished by 

 first chilling the mold and block to a low temperature in the freezing 

 compartment of a refrigerator, and then quickly pouring hot water over 

 the outside of the mold to expand it while the block is still in a cold con- 

 tracted state. 



Satisfactory aid in removal of castings may also be obtained in some 

 cases by coating the inner walls of the mold with a suitable releasing 

 agent before pouring in the liquid plastic. Only such substances should 

 be used for this purpose, however, as are insoluble or not readily misci- 

 ble in the liquid resin and have no detrimental or discoloring effect on the 

 polymerized plastic. Thoroughly dried coatings of soap or of certain types 

 of surface active agents have proved to be suitable in some instances, as 

 have also various commercially available mold-release compounds pri- 

 marily intended for use in connection with pressure molding of plastics. 



In any case where none of the foregoing methods yields the desired re- 

 sult, it will be necessary to break the mold away from the casting. In 

 doing so it is advisable to protect the hands with gloves and to use 

 tweezers or forceps to pick off all glass splinters adhering to the casting. 

 The mold should be broken by striking it with a hammer or squeezing it 

 cautiously between the jaws of a vice in a manner that will not mar or 

 otherwise damage the cast block. 



FINISHING THE CASTINGS 



Upon removal from their molds, castings in which specimens have been 

 embedded ordinarily require a limited amount of finishing to take care 

 of surface irregularities and render all their faces uniformly smooth and 

 clear. The finishing procedure consists mainly of machining, sanding, 

 and polishing operations that are carried out much as in the case of brass 

 or hardwood, and may be performed with the tools and equipment gener- 

 ally used for work on such materials. 



Detailed directions that are applicable to work on methacrylate cast- 

 ings may be found in manuals 8 issued by producers of acrylic plastics, 

 as well as in various texts dealing with mechanical work on plastics in 

 general. For the benefit of those not having access to such publications, 



8 Among the publications that contain useful information in this connection are: 

 Lucite Methyl Methacrylate Resin Manual, issued bv Plastics Department, E. I. du Pont 

 de Nemours & Co., Inc., Arlington, N. J., Nov. 1942 (and subsequent dates); and 

 Plexiglas Fabricating Manual, issued by Pohm and Haas Company, Philadelphia, Pa., 

 Nov. 1941 (and subsequent dates). 



