Michael Shoshani wrote:
To my knowledge, the general practice, at least in England, was to heat the
wax to nearly 100 degrees F in a purpose-built oven, rather than heating the
stylus. Both HMV and Columbia (and later EMI) were doing this in the studio
and in mobile recording trucks, although I have no idea whether this was
done by their remote engineering crews traipsing all over Europe, Asia, the
Near and Middle East, and Africa.
According to the 1930 Columbia Gramophone Co. Recording Expedition
Instructions, "If no heating cupboard is available, one should be made, or a
cupboard hired for the purpose, and carbon lamps used for heating elements."
The document is online here (see paragraph 55):-