B

Reversible hydrocolloid (agar) shows thermal hysteresis—a marked lag of about 10–20 °C between its liquefaction temperature (when it converts from gel to sol) and its gelation temperature on cooling. This temperature-dependent sol-gel cycle is unique to agar-based reversible hydrocolloids and is not seen with alginate, impression plaster, or metallic oxide pastes [Reed, 1990, PMID 2197671; Taira, 2002, PMID 12153461].