Originally posted by fillibuster
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Without calculating anything (which easily could be done) another way to think about it is that you can see a region of "red hotness" extending at least 1/2" from the center of the torch (i.e. a circle of diameter at least 1"). If the heat can cause red hotness 1/2" to the left and right of the torch, it certainly can cause red hotness 1/16" away in front of the torch, i.e. on the other side of the gasket.
Yet another way to think about this is the heat supplied by the torch really has no place to go other than into heating up the Cu. The heat transfers through the thin Cu gasket and then hits a brick wall of sorts, in that the air conducts the heat away much more slowly than the nearby torch is supplying it. That said, in addition to being conducted through the thin Cu gasket some of the heat also is conducted along it so the "cooling fin" action of having the entire Cu gasket to transfer heat to the air is why only a few sq. in. at a time is red hot.
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