Active thermal conductivity of hot glass

For (semi)transparent glass, it is suggested to distinguish between an active and a passive thermal conductivity. Essentially, the latter is attached to the heat transfer through long-range photons most of which are both emittted and absorbed outside the glass volume, the former to the heat transfer through phonons and short-range photons which have an intense energy exchange with the glass. As the temperature distribution in the glass volume can be influenced only by the heat exchange in which the glass is actively involved, it is determined by the active thermal conductivity. The sum of the active and the passive thermal conductivity, which is identical to the so-called apparent thermal conductivity, indicates the overall heat flux through the glass. Like the apparent thermal conductivity, the active thermal conductivity can be obtained from the prototype of thermal conductivity measurements where the heat flux through a sample between two heat reservoirs of different temperatures is measured. The apparent thermal conductivity follows from the usual reduction of the measurement data, the active thermal conductivity is derived via a differentiation rule. Comparing the calculation and the measurement of the temperature inside a cooling block of glass, this differentiation rule is verified