Having the right heat reflecting board beneath the object you are
soldering or annealing is very important. You don’t want to have the
heat you are generating being absorbed into the board, which
diminishes or defeats the heat you are trying to develop. The board
you are currently using may not offer enough reflective properties.
Charcoal blocks are reflective, but don’t come in large sizes, and
can smolder for some time after heating. They tend to disintegrate
over time, and are fragile if you drop them.
Pumice is a fairly good heat reflector, but must be contained in a
pan, and doesn’t have a flat surface. Solder chips fall into the
pumice and are lost.
I recommend and use Solderite boards. They are highly heat
reflective, super flat, long lasting, and won’t break if you drop
them. They don’t have any ceramic fibers or asbestos in them, so they
are not dangerous to your health. I had a T.A. of mine once tell me
that soldering on Solderite was “like using a second torch!”.
We’ve been doing some “tufa stone casting” in our studio using the
flat Solderite boards instead of the tufa, and it is not only easy to
carve, but stands up well to the molten metal, too.