It was recently learned that the old lead, used in the medieval
time, had a lot of impurities. The new procedure is to extract
the more valuable metals in lead and save the "pure" lead. Not
only is the pure lead much more susceptible to corrosion, but
the lead is soft and lends little support to the window.
The cost is higher to have these "trace" metals put back in the
lead. A small amount of silver, copper and antimony make the
lead much stronger and more resistant to corrosion.