Glass melts are usually not in a chemical equilibrium with a coexisting gas phase, it does not adjust before maintaining the melt at a constant temperature and a constant gas phase for hours. However, the one-phase equilibrium between several polyvalent elements in a melt above the glass transition temperature always exists, it adjusts spontaneously. Below the transition temperature the redox state is invariably frozen-in. The calculation of redox states must be based on the matter and charge balance (stoichiometric equations) and equilibrium relations (mass-action law). Such a calculation of Schreiber's experimental results was consistently possible. Accordingly, cooled glasses are free from Cr6+ if the glass containes Fe2+. To achieve this one has to melt oxidized chromium-containing glasses with sufficient iron content.
