9. The chemical core of the formation pathway

The roasting process described in the patent specification is based on four factors:

1. a chromium source,

2. a basic calcium and/or sodium compound,

3. an oxidising atmosphere, in particular oxygen, and

4. sufficient temperature and reaction time.

In simplified form, the central sub-process forming calcium chromate can be represented as follows:

2 Cr₂O₃ + 4 CaO + 3 O₂ → 4 CaCrO₄

For sodium chromate, the corresponding oxidation pathway when using sodium carbonate can be formulated schematically:

2 Cr₂O₃ + 4 Na₂CO₃ + 3 O₂ → 4 Na₂CrO₄ + 4 CO₂

These equations are mass balances of the chromate formation pathway, not a claim that every real plant surface corresponds to an idealised pure system. They do, however, show the long-known chemical relationship: a Cr(III)-containing chromium source, a base and oxygen form a chromate Cr(VI) pathway under thermal stress.

A calcium content stated as CaO in glass or silicate does not automatically mean that free calcium oxide is present. Reactive availability is decisive for the application-related assessment. Calcium silicates can also supply calcium for chromate formation; Sayano describes corresponding reactions. [11]