"As regards the Saturn/lead equation, it should be noted that although astrologically Saturn is a malefic planet of whom only the worst is expected, he is also a purifier, because true purity is attained only through repentance and expiation of sins. Thus Meister Eckhart says in his sermon on the text, "For the powers of heaven shall be shaken" (Luke 21:26): "Further we must note how (God) has decked the natural heavens with seven planets, seven noble stars which are nearer to us than the rest. The first is Saturn, then comes Jupiter, then Mars, and then the Sun; after that comes Venus, and then Mercury, and then the Moon. Now when the soul becomes a spiritual heaven, our Lord will deck her with these same stars spiritually, as St. John saw in his Apocalypse when he espied the King of Kings seated upon the throne of the majesty of God, and having seven stars in his hand. Know that the first star , Saturn, is the purger... In the heaven of the soul Saturn becomes of angelic purity, bringing as reward the vision of God, as our Lord said, "Blessed are the pure in heart for they shall see God." (Evans, I, p. 168)
It is in this sense that Saturn should be understood here. Cf. Vigenerus ("De igne et sale," Theatr. chem., VI, p. 76): "Lead signifies the vexations and troubles wherewith God visits us and brings us back to repentance. For as lead burns up and removes all the imperfections of the metals, for which reason Boethus the Arab called it the water of sulphur, so likewise tribulation in this life cleanses us from the many blemishes which we have incurred: wherefore St. Ambrose calls it the key of heaven"
pg 335n Mysterium Coniunctionis