Why does copper brazing melt only with oxidation flame?

when welding brass, the oxidation of weak oxidation flame is adopted, and the oxide film generated is covered on the surface of the molten pool, which can prevent the evaporation of zinc and tin.

due to the high temperature of oxidation flame, oxidation flame is often used in flame heating to improve efficiency. When gas cutting, oxidation flame is usually used. Brass contains zinc. In order to inhibit the evaporation of zinc, brass and silicon bronze welding wires with high silicon content are selected. This is a senior engineer's question.