From Fig. 6, it can be seen that the conversion of CuS into Cu2S starts to take place at a relatively low temperature
of approximately 300 ◦C. CuS decomposes into Cu2S and elemental sulfur according to Eq. (3) (listed in Table 1).
The gaseous elemental sulfur (Sx) that is liberated during this step is oxidized into sulfur dioxide (Eq. (4)) when it comes
into contact with the oxygen from the gas surrounding the sample. Only when the conversion of CuS towards Cu2S is
completed, Cu2S is oxidized further to CuO·CuSO4. As can be seen in Fig. 3 CuO·CuSO4 is an unstable component at
temperatures above 527 ◦C, but only at temperatures above approximately 650 ◦C the decomposition of CuO·CuSO4 actually takes place at a rate that is accurately measurable with the experimental set-up used in the present study.