There is a product sold commercially called Versene.  Usually it has
a number after it, e.g., Versene 67 or Versene 100.  I think it is
made by Dow Chemical Co. and it is available from chemical suppliers.

Mix the liquid or crystals in very hot water (140+ F.), until the
solution is slippery, and soak the pipes.  They should be completely
submerged or you will get a permanent ring where they stick out of
the water.  It will remove all dirt including smoke.

The solution is effective as long as it is slippery.  This is best
for spotted metal and higher tin content pipes.  Not very good for
zinc.  For pipes with higher lead content, a phosphoric acid solution
and neutralizer can be used if you know how to handle these products
safely.

John Nolte