Old plumber's lead could have solder in it too. Some drains and sink plumbing made under the older 'Lead Codes" as an old plumber friend calls them, were 'wiped' with solder to join them together. It was a big thing with plumbers years ago. Solder was used because it melts at a lower temperature than lead and stays semi-liquid longer than pure lead, allowing the plumber to wipe the lead joints. Plumbers aren't stupid, so they won't throw away a little solder contaminated lead, and will recycle it into their next cast iron joint or whatever. Mistakes go back into the pot. Yes the solder wouldn't help by being hard, but it could, and has, ended up in cast iron drain joints. If it's soft to a fingernail, it's probably mostly lead, if it's harder, it's got solder in it. Get a piece of pure lead and compare. As to the pipes being dirty, it always good to observe hygiene with gloves and face protection, and washing frequently. Melted lead has no germs. :wink:
P.S. Plumbum is Latin for Lead, gives us the chemical symbol Pb for lead and the name Plumbers for those who work lead pipes.