Tallow: Flux for Leaded Stained Glass Windows
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 before application which lets the tallow help your solder flow and adhere.
Tallow is sold by the stick, these are candle shaped.