The mass of the sample increases due to the oxidation of copper by atmospheric oxygen: 2Cu + O2 = 2CuO and 2Cu2O + O2 = 4CuO

Deepa Rani The graph means the material is reacting with the atmosphere in the furnace, most often through oxidation. Cu2O may first undergo thermal decomposition or structural changes. Once metallic Cu is exposed, and if your TGA purge gas contains even small amounts of oxygen, the Cu rapidly oxidizes to CuO.

• At high temperatures in air, Cu and Cu₂O oxidize to CuO, which has a higher molar mass due to oxygen uptake:

If this run was under nitrogen, then the mass gain could only happen if there was oxygen leakage or another reactive gas uptake (like CO₂ reacting with an oxide surface).

It might be oxidized, in the case where the sample is in an atmosphere containing oxygen.