Use this stick for leading windows. Apply to join and then apply solder.
Tallow is a natural flux for leaded window work. Stained Glass artists have been using tallow for centuries to flux their solder. Simply rub a little tallow onto your joint prior to soldering to help it flow and stick. You can also use a small wire brush on your stained glass panel to help clean the lead and let the tallow help your solder flow and adhere.