Often it is necessary or preferable to heat a piece of glass from both sides. A crossfire torch accomplishes this by having two opposing flames of equal size, with enough of a gap between them to fit the desired piece of glass without blocking either orifice. I made one out of copper tubing, with a plug soldered into each tip containing a 1/16 inch orifice. I initially had the two orifices aimed at each other, but despite the flame itself appearing to deflect, the heat from one tip would be picked up by the other, overheating both and turning the flame green from copper oxidation. I unbent the tips slightly so that neither tip is in the flame path of the other, and it now works well.
This torch can also be used to eliminate the need for a glass lathe, at least for small parts. Rather than rotating two pieces of glass as they are joined together to keep them evenly heated, the parts can be held stationary while the flames from the crossfire torch envelop the joint area. In the center picture above I use this method to join a stem to an envelope, which had previously been punctured at the top with a 3mm glass drill. This method of assembly is very easy, and the result is that both components are perfectly aligned. This is far superior to joining the two by hand, which I had done for previous vacuum tubes. This torch will also be useful for making pinch seals; since the seal will no longer need to be rotated, the feedthrough wires can be held in a jig in the tailstock of the lathe. This should make the feedthrough placement much more reliable, and will eliminate the risk of accidental shorts due to the wires falling out of position during the sealing process.