PRESERVATION OF SPECIMENS IN PLASTICS 



63 



Figure 15. — Bubbles in the casting sirup and entrapped air in the specimens are re- 

 moved after filling the molds by placing them in a vacuum desiccator. The small 

 quantity of vapor is removed from the closed desiccator by suction. 



the embedding layer has become a firm gel. The temperature may 

 then be increased to 50° C. (122° F.) or to 55° C. (131° F.), respectively, 

 either by changing the thermostat setting or by transferring the mold to a 

 second oven or water-bath regulated for operation at the higher tempera- 

 ture. Temperatures slightly higher than these may be' employed safely for 

 small castings, but they are not recommended for blocks much over 1 inch 



