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Or take an old crummy solingen blade. Try to file the spine and see what happens.
In principle, you could use sheffield blades as well, but I've had sheffield blades that had a relatively soft spine...

Given your description, I am guessing that you really don't get hot enough.
You should be able to make the entire blade glow a nice even orange along the entire length.
If that is not happening,...

Or perhaps the torch is not powerful enough to bring the entire chamber up to a high termperature?