Quote:
Originally Posted by freebeard
How does the Sharpie work?
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It disappears at some temperature. Not sure if the ink oxidizes, or more likely just vaporizes.
The tried-and-true end-point indicator for annealing aluminum is soot, but the propane torch doesn't have a sooty setting (that I could think up), so the marker is a second-choice.
I think somewhere or other you can buy markers designed as specific temperature indicators. For all I know the sharpie could be off. It depends on the alloy, but the target should be around 600 or 650F.
Primarily I think it was right (or close enough) because the hammering steps afterwards felt noticeably softer.