A goldsmith uses the outermost zone of a flame, which is non-luminous and is the hottest zone of the flame thus providing required temperature for melting gold and silver.
A candle flame consists of three different zones. Each zone has different temperature and different colour.
Due to complete combustion, the outer zone is blue in colour. This zone is the hottest zone as compared to the other zones. This blue coloured zone is the non-luminous part of the flame.
The yellow colour region which is the bright part of the flame is the middle zone. It is moderately hot and exhibits partial combustion of fuel.
The least hot region of the flame is the inner zone. This inner zone is black due to the presence of unburnt wax vapours as it undergoes incomplete combustion due to lack of oxygen.