

PRESERVATION OF SPECIMENS IN PLASTICS 59 



venient, although glass jars or tumblers might prove to be more economi- 

 cal in most instances. Electric light bulbs with the neck portion cut off 

 can be used for making globular castings. Molds for rectangular castings 

 can be made by binding together glass plates of the proper size with 

 suitable adhesive tape on the outer edges and then sealing the seams on 

 the inside with gelatin, casein, egg white, or polyvinyl alcohol. A 10- 

 percent solution of gelatin, plasticized with 5 to io percent of glycerin, 

 sorbitol, or polyethylene glycol and applied while warm with a small 

 round brush will be found to be suitable for this purpose. The gelatin or 

 other sealing substance should be thoroughly dried by baking in a mod- 

 erately warm oven before the mold is used. Glass refrigerator dishes are 

 serviceable for larger specimens, and in some cases it may be found con- 

 venient to embed several specimens in one shallow block and then saw the 

 casting into separate mounts after hardening. 



Molds made of plaster of Paris, or of wood or cardboard, may also be 

 employed if their inner surfaces are coated with gelatin or other suitable 

 material to prevent absorption of monomer and facilitate release of the 

 hardened casting. Gelatin capsules have been used as molds for embed- 

 ment of very small specimens. 



To cast the supporting base for the specimen, pour into a dry, clean 

 mold a layer, up to one-half inch in thickness, of the most viscous grade 

 of partial-polymer casting sirup that has been prepared. In deciding upon 

 the thickness of this supporting layer, it should be borne in mind that 

 a shrinkage of approximately 20 percent will occur during hardening, 

 together with an additional loss of from 2 to 5 percent through uncon- 

 trollable volatilization. It is also necessary to allow for the possible re- 

 moval of 0.1 inch or more from the bottom of the block by sanding and 

 polishing when finishing it. 



Pouring the casting sirup for the base layer, and for the subsequent 

 embedding layer as well, should always be done in a fume hood or other 

 enclosure from which the air is continuously withdrawn to guard against 

 diffusion of the escaping vapor into the room. For the pouring operation 

 it will usually be found convenient to place the mold on an elevated 

 stand or platform so that it will be nearly level with the eyes and thus 

 facilitate judging the true depth of the sirup in it. When working with 

 the more viscous grades of partial polymer, it is necessary to use extra 

 care in controlling the flow in order to avoid pouring too much into the 

 mold. This can be done by gradually tilting back the flask as the sirup 

 begins to approach the desired level in the mold, and finally "cutting 

 off" any surplus sirup with a glass rod and returning it to the flask. 



The mold into which the base layer of partial polymer sirup has been 

 poured must be well covered with tinfoil or cellophane suitably pressed 

 down or tied in place so as to prevent escape of vapor as effectively as 

 possible during polymerization. In the case of large castings, the covered 

 mold should be allowed to stand for an hour or two, preferably in the 

 refrigerator, before it is heated in order to permit all impounded air to 

 rise to the surface and escape. This step may be omitted with small cast- 

 ings, however, provided they are inspected two or three times during the 

 first hour in the oven and are transferred promptly to the refrigerator 

 if air bubbles appear to be trapped in the semifluid plastic. In this case, 

 elimination of bubbles can usually be facilitated by pouring a small 

 amount of unthickened monomer on the surface of the viscous partial 



