What substance is burned in excess to produce an oxidizing flame?

An oxidizing flame is produced when there is an excess of oxygen in the combustion process. In this particular scenario, oxygen acts as the oxidizing agent that supports the complete combustion of the fuel involved, which is typically acetylene in brazing applications. When oxygen is in surplus, it allows for a hotter and more efficient flame that can provide the necessary heat for brazing metals effectively.

When producing an oxidizing flame, the balance of fuel and oxygen is crucial. This type of flame can result in higher temperatures than a carburizing flame, which has an excess of fuel and can lead to undesired effects like reducing the surface of the metal.

The other options are less relevant because they do not directly relate to the condition of having an excess that would create an oxidizing flame in a brazing context. Acetylene serves more as a fuel, hydrogen can also serve as a fuel but typically does not denote the creation of an oxidizing flame on its own, and carbon dioxide is a combustion product rather than a reactant. Thus, the presence of excess oxygen is the key factor in creating an oxidizing flame.