The black solid gradually turns into a red solid

Quick answer：

This phenomenon could be observed in various chemical reactions or phase transitions. For instance, it might occur during the oxidation of a compound, where a black solid is converted into a red one due to the formation of a new compound with a different color. Another possibility is a physical change, such as the transition of certain metal sulfides from black to red upon heating. The exact cause would depend on the specific substance and conditions involved.