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I would think you could preheat the platens and your horn, then just tighten outside of the oven.

1. No clue. But I would think a one-time tightening would work.
2. You should still be able to...

I really don't think you need to worry about oxidizers. Horn is a thermoplastic, so you can just heat it until it becomes pliable, then work, then cool, and the working will be set. I don't think...