Doug,
Actually, if you want to get the spine to temper all the way through, that's about the only way you can do it...otherwise with the duration of heat required, you wind up softening the cutting edge.
This is not any kind of hard and fast rule, but for me to successfully get a spine to tempered all the way through, its generally 30-40 mins with the heat applied to the spine. It's difficult to do, which is why I favor using the depth of an edge quench... just more predictable and controllable.
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