If the flame has too much oxygen, an oxidizing flame is produced. When the amount of oxygen increases, the flame shortens due to quicker combustion, its color becomes a more transparent blue, and it hisses/roars.^[2] With some exceptions (e.g., platinum soldering in jewelry), the oxidizing flame is usually undesirable for welding and soldering, since, as its name suggests, it oxidizes the metal's surface.^[2] The same principle is important in firing pottery.

A reducing flame is a flame with insufficient oxygen. It has an opaque yellow or orange color due to carbon or hydrocarbons^[3] which bind with (or reduce) the oxygen contained in the materials the flame processes.^[2] The flame is also called carburizing flame, since it tends to introduce carbon soot into the molten metal.

The flame also produces carbon monoxide, a poisonous gas which burns on the outer envelope of flame into carbon dioxide.^[4]

Reducing flames with no carbon edit

Reducing zero-carbon fuel flames, such as reducing hydrogen flames, are exceptions. They don't have an opaque yellow or orange glow, nor do they produce soot or carbon monoxide.

• Oxy-fuel welding and cutting § Types of flame for further details about the above types of flame in oxy-fuel burners
• Oxyhydrogen
• Redox
• Oxygen
• Flame test
• Spark testing