Diacetylene time-temperature indicators

In a recording device including a substrate having deposited thereon at least one indicator, said indicator comprised of at least one acetylenic compound, containing two conjugated C.tbd.C groups, capable of undergoing a 1,4-addition polymerization upon thermal annealing or exposure to actinic radiation, thereby undergoing an irreversible, progressive color change, wherein the color produced at any time during the color change represents an integrated time-temperature history of thermal annealing or integrated radiation-dosage history of exposure to actinic radiation, the improvement which comprises said acetylenic compound in an inactive form, positioned for exposure to the conditions to be recorded and capable of in situ conversion, by melt or solvent recrystallization, to an active form capable of undergoing 1,4-addition polymerization;

• wherein said active and inactive forms exhibit different X-ray crystallographic diffraction patterns, and said inactive form being incapable of undergoing 1,4-addition polymerization upon exposure to gamma radiation at room temperature or thermal annealing below its melting point.