He does so because the outer zone of a flame is the hottest part of the flame with it undergoing complete combustion there. It is blue in colour.
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Sreeram.m.s answered this
Because it is the hottest zone of a flame.it is also known as luminous zone.it burns with complete combustion.it means that it burns with the supply of air.The hottest zone is used for melting purpose of metals.It is blue in colour.It produces carbondie oxide.
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Sushma Kumari answered this
a goldsmith blows the outermost zone of a flame with metallic blowpipe for melting gold and silver because it is the hottest zone of the flame is non-luminous in nature.
