Warming up indicator, for frozen food, has two pieces of cloth impregnated with potassium hexacyanoferrate solution and with ammonium ferrous sulfate solution, respectively, which react above their melting points to form blue color

摘要

Warming up indicator (4) has two pieces of cloth (1, 2) that are impregnated with potassium hexacyanoferrite solution and with ammonium ferrous sulfate solution, respectively. The pieces are frozen separately at respective salt concentration. The pieces are placed one above the other and are fixed together by welding with a transparent foil (3). The solutions react to form a blue color above their melting points.