A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Explanation

Ethyne is an unsaturated hydrocarbon containing high percentage of carbon in it.
(i) When a mixture of ethyne and pure oxygen is burnt, then ethyne burns completely producing an extremely hot blue flame which can be used for welding metals.
(ii) If however, a mixture of ethyne and air is burnt, then incomplete combustion of ethyne takes place (Because of insufficient oxygen of air), producing a yellow, sooty fame which is not hot enough to weld metals.