The thermo crayon is used to determine the temperature of already hot test surfaces. The CelsiStick® is dipped on the hot surface. If the dip (= contact) does not really melt off material from the CelsiStick® end and deposit on the hot surface, then the surface temperature is still below the specific melting point of the specific CelsiStick®. If a material melting deposit is left on the surface, then this clearly indicates that the surface was at least at the melting temperature level of the dipped CelsiStick®. The user of the CelsiStick® must feel that material melts from CelsiStick® end, similar like touching with a candle a hot surface. CelsiStick® deposited melt material will quickly decompose under heat and chemical reaction with the ambient gases like oxygen or others when flame heating is applied. CelsiStick®s are brittle and hard and not thought for a premark deposit on the cold surface. Such marks do not well adhere and may splatter off. Pre-marking on cold surfaces is not accurate.