The black copper oxide gradually turns red

Quick answer：

This phenomenon is due to the reduction of copper oxide (CuO) to copper (Cu). When copper oxide is exposed to a reducing agent, such as hydrogen gas or carbon monoxide, it is reduced to copper metal, which is red in color. This reaction can be represented by the following equation: CuO + H₂ → Cu + H₂O or CuO + CO → Cu + CO₂.