Tallow is a natural flux for use on lead came in leaded window work. Stained Glass artists have been using tallow for centuries to flux their solder. Simply rub a little tallow onto your lead joints 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. We use this Bazil Brush
Tallow is sold by the stick, these are candle shaped without a wick, they are fragile so can break in transit but this doesn't affect their use, its often easier to hold a small piece rather than the full stick.