In equation  is constant. The relations 1 and 2 are valid only for temperatures and temperature differences within the conducting body. When conduction across a body is considered,  and  are the surface temperatures just within the surface of the body. Large temperature drops may occur between the surfaces of the refractory facing the combustion chamber of the furnace or environment. Newton’s law of cooling must be used to determine the surface temperature of the refractory.

Consider a simple series wall constructed of material of different thermal conductivities  and   having thicknesses  and .  is the furnace temperature and  is the surrounding
temperature.,  and  are the interface temperatures as shown in the figure: