ArtMetal

Sounds like you have the tip of the flame too far away from the puddle. When that happens, the velocity of the flame draws in atmospheric oxygen which causes the burning and popping. The tip of the inner blue cone of flame should be just touching the edge of the puddle. Holding the torch at too low an angle can also cause this same phenomenon - the torch should be at an angle of 60 degrees or steeper to the surface.

Hope this helps.

Rich