The variation of absorption coefficient with wave length and temperature has been measured for a number of glasses containing iron oxide and chromium oxide. From these results, an effective mean free path F has been calculated, in which all the information on the variation of absorption coefficient, at a given temperature, is compounded. This value of F then has been used to calculate the coefficient of radiation conductivity Kr. It has been shown that there is a correlation between the mean free path F and the iron oxide content of soda‐lime‐silica glasses. Examples are given of the method of estimating furnace bottom temperatures and temperature gradients through a mass of glass.