I can't add anything to what has been said about forge welding in coal fires.
But, I do all of my forge welds in my propane forge. Things that work for me: make sure your forge gets hot enough. This often requires 2" of kaowool and a good coat of ITC. Burners must be properly adjusted, a slightly reducing flame works best for me. If you are welding two separate pieces together, tie them together with some wire to hold them in alignment. Cleanliness is most important. 20 Mule Team is a fine flux, but I often use Sure Weld or EZ Weld. Wire brush, flux, heat, wire brush, flux, and heat to welding heat. Watch the flux amount, it will eat your kaowool. A ceramic kiln shelf for a floor helps a lot.
Leave the work in the gas forge till it is the same color as the interior of the forge. That is as hot as it is going to get. Remove quickly and lightly tap to "set" the weld, tap quickly to finish the weld. Speed is highly important as is a minimum of force. When working the welded area later in your project keep it good and hot, close to a welding heat.
Make lots of welds. It's just like arc welding. Your welds look and hold terrible till you burn a few pounds of rod.
