Traditionally, lead tubing is sealed to the wood of the stack with
burnt shellac.  I have previously discussed this marvellous substance
on MMD.  Removal of tubes can be done without damaging the tubes, but
this can be a bit hit and miss.  Sometimes you will damage the tube
and need to replace it.  Obviously, one should never replace lead
tubing with anything other than the like.

Use a temperature controlled soldering iron, fitted with a suitable
tip that you can make to fit around the curve of the tube so as to
apply heat as evenly as possible.  Run the iron at a temperature low
enough to not melt the lead.  Gently heat the tube and it will pull
out.  Make up new burnt shellac to refit or fit new tubes.  It really
is an easy and satisfying job.

Nicholas Simons, GB