This review is aimed at a nonmathematical elucidation of the physical phenomena underlying radiant heat transfer in glass. In transparent materials, unlike the more familiar opaque materials, the emission and absorption of radiation are bulk, rather than surface phenomena. Interaction of the simultaneous emission and absorption of radiation throughout the volume leads to a dependence of the emissivity of transparent bodies on their thickness. The interaction also leads to a mechanism of radiative heat transfer in the interior of transparent materials entailing not the familiar radiation through them but internal radiant exchanges between nearby layers. The analogy of this mechanism with thermal conduction has led to the definition of an equivalent "radiative conductivity." The origin and limitations of this concept are briefly discussed, together with conditions at the boundaries of transparent bodies and applications to unsteady-state processes.
