Keith is probably quite correct in what he says.  However, there are
a couple of relevant matters following from the subject.

1. Many chemical reactions take place very slowly and our industry
bible (Art's Book) is full of experiences in selecting materials for
repair that 'Don't Work'.  These materials, for one reason or another,
are unsuitable to the point where they do not maintain their properties
over a long time.

2. In the case of lead, as a newly extruded metal, its oxidation state
is zero.  After time, it will react slowly with the very weak acids in
the atmosphere (acids of carbon, sulphur) and these chemical reactions
will produce compounds which can further catalyse other reactions
involving the same compounds and or oxygen.  The oxidation state of
lead can be zero, +2 or +4, and the +4 state is less stable than the
+2 state.

My guess is that the 'products of oxidation' seen on lead tubing are
a mix of a range of compounds that have built up over the last 80
years.

Normally, people don't see these compounds because the product has gone
to the dump years before.  This is why building a reliable product
which is to survive for decades requires a lot of knowledge about the
chemistry of the aging process.  It can be very unreliable to try to
speed up this process to get a quick understanding of the expected
lifetime, because the degradation mechanism can change.

In Michael's case the absolute advice given in every case was "Don't
touch it if it is okay!"  That was the real message.

Just for what it is worth.
Paul Rumpf