Further to my previous post, I now revise my statement in that the
white powder is probably tin oxide and not lead carbonate. I say
this because when I was 14 and started in the piano shop, I replaced
key leads that had swollen and were full of white powder. My master
piano builder, who taught me, told me that all this lead was a mixture
of tin and lead and that the tin corrodes like that.
The same is probably true for lead tubing -- it will have tin in it to
make it pliable and less prone to cracking. Tin oxide forms much more
easily than lead carbonate and it is not really toxic. If anyone has
some of the powder from tubing handy, please check if it dissolves in
water and let me know.