<P> The yellow layer is the topmost one, sensitive to short heat pulses of high temperature . The magenta layer is in the middle, sensitive to longer pulses of moderate temperature . The cyan layer is at the bottom, sensitive to long pulses of low temperature . The layers are separated by thin interlayers, acting as heat insulation, moderating the heat throughput . </P> <P> Most direct thermal papers require a protective top - coating to: </P> <Ul> <Li> reduce fading of the thermal image caused by exposure to UV light, water, oils, grease, lard, fats, plasticizers, and similar causes </Li> <Li> reduce print head wear </Li> <Li> reduce or eliminate residue from the thermal coating on the thermal print heads </Li> <Li> provide better anchorage of flexographic printing inks applied to the thermal paper </Li> <Li> focus the heat from the thermal print head on the active coating . </Li> </Ul> <Li> reduce fading of the thermal image caused by exposure to UV light, water, oils, grease, lard, fats, plasticizers, and similar causes </Li>

At what temperature does thermal paper turn black