C

Borate ions and other components leaching from alginate/agar hydrocolloid impressions interfere with the setting of gypsum, yielding a soft, chalky cast surface and lowering stone hardness, whereas waxes, impression compound, and elastomeric (rubber-based) materials do not show this effect [Jarvis & Earnshaw 1980, PMID 6940533; Teteruck et al. 1989, PMID 2667721].