Short version: Anhydrous borax is a great flux for forge welding. It can be purchased in bulk or produced by melting regular borax (e.g., 40 Mule Team), letting it cool, and grinding it up. Drying regular borax in an oven isn't enough.
Long version: Borax is a great flux for forge welding. When sprinkled on the surfaces to be welded, it melts, dissolves the oxides that form during heating, and carries them out of the weld under the force of the hammer blows. It is cheap and easily obtained