If they're pre-cast, they should be mostly pure (maybe 1% tin). -(precast donut, set into the joint)
If they're pounded seals, they should be pure. -(Flat or round piece of lead, pounded into the joint.)
If they're poured seals, they'll probably be sweetened with some tin. (2-3%) -(molten lead is poured into a makeshift mold around the joint)