Alchemy was a very strange mixture of real science, primitive as it may have been, mythology and mysticism. For example, each metal was associated with a planet or heavenly body: gold with the Sun, mercury with Mercury, copper with Venus, silver with the Moon, iron with Mars, tin with Jupiter, and lead with Saturn.
Paracelsus seems to think that lead was ‘metaphysically dead’. In Coelom philosophorum, he says ‘The other six have cast me out as their examiner. They have thrust me forth from them and from a spiritual place.’ (If you follow the link, note that Paracelsus refers to the metals by their mythological names, so you’d have to refer to the list above or to an equivalent list.)
Note that the alchemists may not have really believed in the supernatural aspects of their nascent science as much as they often seem to have believed. The alchemists believed that their work was to be kept secret from the uninitiated (much like modern scientists, in a way =)), and so they used extensive references to the supernatural to hide the real meaning of their publications. So, what may seem like a description of a supernatural power may actually refer to a real scientific observation. Something like ‘to lift the essence of fire from Jupiter and promote Mars together with Mercury’s Hadean encumberment’ may mean ‘to remove oxygen from stannous oxide and form ferrous sulfide from cinnabar and metallic iron’. (That’s not a real example, though; I don’t understand much about alchemy.)
This name always makes me laugh. It is a pen name meaning “better than Celsus” (Celsus was a highly regarded ancient natural philosopher). It’s like a modern physicist going around calling himself “Super-Einstein”.
Lead is the heaviest non-radioactive element (bismuth having recently been shown to be very, very, very slightly radioactive.) However, one particular isotope of iron is the most stable nucleus - either fissioning or fusioning it creates a higher energy state, so eventually, all the matter in the universe might turn into iron. Plus, even the smallest quantities of iron are sufficient to bind fairies and elves.
In ancient Greece you could write a prayer to Hecate on a lead tablet, asking for a curse on someone and then bury it. The weight of the lead was supposed to cause it to sink down into the underworld where she lived. Cemeteries were popular burial places.
Typical cursees included unrequiting lovers, competing actors, and the other guys lawyer. The actors and lawyers were supposed to lose their voices. Don’t remember off hand what the spurners were supposed to get.