Devitrification is most common between 1350° and 1400° F. Avoiding that temperature range if you're heating and cooling multiple times will probably give you better results. Where that occurs depends on your torch and the size of your flame, but on a hothead, that's typically about 6 - 12 inches from the torch. Get in, heat and shape, and get out. Also, you might want to try pickling your glass. It could be contaminants or dust causing your problems.
To use your hot head with a large propane tank, just get the Mr. Heater propane hose. You can find it at Lowes or Walmart, and I'm pretty sure Devardi sells them on their website as well.
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