in a method for forming castings made from a polymerizable hydrophobic monomeric material wherein a liquid hydrophobic polymerizable plastic monomer material is pumped into a cavity , which is defined by a pair of spaced apart smooth surfaced plates that are sealed about the outer perimeter , and polymerized therein to a solid , self - supporting casting , the improvement comprising the preliminary step of applying the relatively thin thermoplastic sheet across at least a portion of at least one of the spaced apart smooth surfaces , said thermoplastic sheet stretching when in contact with the liquid polymerizable monomer material during polymerization of said material and during cooling of said polymerized material , the sheet shrinking and distorting in random fashion so as to produce a randomly textured surface on at least the corresponding adjacent portion of the polymerized casting .