Welding at low temperatures?

(Some people just ask good questions! A second comment based on e-mail from 'kimsey0000')

I heard a guy say you can actually weld with dull red heat... that must be hard, and you must have to be using borax and the surface must have to be perfect, ... Also, there's a guy on youtube,and you can actually *see* him welding with salmon heat....
kimsey0000

In actual fact, you can weld at room temperature - if you have the correct conditions.

What REALLY happens is that if the two surfaces are absolutely clean (no oxide) and absolutely flat, and touch perfectly, and no oxygen is present (like a vacuum) - the two surfaces will bond. The atoms will actually grab across the gap and fuse. They have this problem on space craft - its called vacuum welding there.
In practical effect, you have to force the two pieces tight enough, plus drive out all the oxide on the metal and all the available oxygen between the metals. So this is where both flux and hammer come in.

So yes - I have myself seen someone also weld at a bright red / low orange. You have to be extremely good in hammer control, start with clean metal, have the oxygen balance in the fire just right. If you are extremely extremely good, you can also skip the flux. It is a bit of a trick however, and I suspect the weld penetration is not the best either.
Of course often old wrought iron is basically self fluxing (there is glass slag embedded microscopically in the material).

For us mere mortals:
Clean the surfaces (grind off scale) before you start
Work in a clean, cavern style fire, with a neutral atmosphere
Flux (keeps stray oxygen out, floats out any scale formed during heating)
Move quickly to the anvil, striking fast and sweeping your blows from one side / edge to the other.

I personally usually double up my welds. The first is a 'tack weld', undertaken with a lighter (my normal 800 gm) hammer, which gives me speed and fine control. I then go over the section a section time, now with a heavier hammer. Although slower strokes, this ensures full penetration through the entire mass. I use normal 'washing soda' borax as a flux.
Works for me...