Question

Why do you think goldsmiths blow the outermost zone of a flame with a metallic hollow pipe in order to melt gold and silver?

Answer 1

It's a complicated question with a very easy answer.
Why do goldsmiths use the outer part of the flame for burning gold? Because it's the hottest region of the flame. A flame has 3 major parts. The innermost black part is the least hot. In fact, it's not hot at all. The middle portion is the reddish yellow part, something we all are familiar with. However, the outermost part is the hottest of all. It's the blue part. A thin layer of bluish tinge engulfs the other two parts and it's the hottest part because it's the part where Complete combustion of particles takes place producing a blue flame whereas in the reddish yellow part, not so much oxygen is available and hence partial combustion takes place (giving it that reddish yellow appearance).