G’day The way a charcoal (carbon) block works as a help to avoid
firestain is that when the block is strongly heated with a flame to
solder carat gold or sterling silver, a reaction begins between the
carbon and the oxygen in the air, to form a mixture of carbon
dioxide and carbon monoxide gases. It is the carbon monoxide gas
which stops the copper which is alloyed with the metal from
oxidizing to black copper oxide, which is the dreaded firestain.
At the same time some of the charcoal will of course burn away and
disappear. Graphite slabs work as well, but tend to conduct heat
far mor easily than porous charcoal, so a hotter flame is needed.
Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZ