Different types of solder blocks seem to have different advantages/
disadvantages so it is one of those personal preference situations.
I have a soldering board that looks like the old asbestos type but
has no asbestos in it at all. I like it because it holds heat from
the torch and can bounce the heat back up into the piece. This works
really well for large pieces where I have had difficulty getting
enough heat into such a piece when using other surfaces to solder
Fire brick - same as above really, in that you can use its heat-
holding properties to your advantage. Also easy to carve so that you
can support components being soldered using custom-made holes.
Charcoal blocks. I’ve not used them yet myself but from other
people’s comments have gathered that the best ones are the dense
kind made from hard woods as opposed to the cheaper, less dense type
made of soft woods. They can be bound with wire so that they last
longer - otherwise they fall apart after a while. Advantage of
charcoal is that the charcoal reacts with the oxygen in the
atmosphere, thus helping to prevent excessive oxidation of the piece
being soldered and thus going a long way towards preventing
firestain. I’m not sure of their heat-holding properties. I’m sure
others will comment.