The minimum thickness of a lining material that must be inserted into a cavity in order to afford protection to the dental pulp from thermal trauma has never been determined. By taking the diffusivity ratios of the hard dental tissues, amalgam and lining materials and equating them with the situations incurred in the transmission of thermal energy by the unfilled, as against the restored tooth, it becomes possible to produce a simple table from which measurements serving as guidelines for adequate protection can be taken. An experimental method of confirmation of the hypothesis is described.