Andrew Mawson wrote in message ...
It's not just the mechanical retention of water in the soot, the sulpher
compounds that are produced when coal burns leave salts in the soot that
are
hygroscopic - these are absorbed into the brickwork and cause problems for
years after the flue has been used for open fires. For this reason avoid
using reclaimed bricks that have come from chimneys as they will cause
problems even when built into a new wall.
Thanks, Andrew. Interesting stuff.