knife making

[Mortblanc]

Borax works great and has been the standard flux for may years but there was a time when it was not available.

What did the Romans use?
What did the Celtic iron workers use?
What was used during the "dark ages"?

There are other natural materials that make a good flux after you learn how to use them.

Clean sand is one. Sand will work as well as Borax after a couple of experimental trials to get the temperature correct.

Agricultural lime is another. Lime production was a big deal all through history so lime has been available forever.

Over here we have a black wasp we call a "dirt dobber". They build long tunnels of clay and lay their eggs inside the hollow tunnel. During the frontier days when any production item was scarce on the edge of civilization the blacksmiths would gather these mud nests and grind them to powder and use that clay as a flux.

I am not certain, but I bet a mud barn swallow nest ground up to a fine powder would work.