I worked as a Journeyman lndustrial Blacksmith at ASARCO inc. Omaha plant A non ferrious metal refinery for abt. 14 Years, Where forge welding was done every day in a coal forge.
Most of the forge welding was done in the morning while the coal fire was nice and clean. Some forge welding was small stuff usually 1/2 " round stock . Then later around mid day the fire would be large enough to weld heavier stuff ..1-3/4 " .. 2 1/2" usually round or square stock . Supplying the plant with necessary tools and implement .
The "Forge welding compound" was made by or called EZ Weld ( if I am remembering correctly.) It's been allmost 40 years ago.
It came in a paper bag inside a cardboard, flip top box 5 lbs.
It had borax mixed with what looked like a silica & play sand mix & cast Iron filings.
Just guessing it look like 75% with 20% and 5% respectively.
After upsetting and faggoting the two pieces to be welded they were placed in the fire and heated to a good forgeing temp , bright cherry red. Applied Compound with a long handle spoon tool while in the fire continuing to heat evenly .
The borax would boil cleaning the metal then cast iron would melt in as the sand turned to a glass like coating sealing or sheilding the oxygen off the weld area..
When the metal pieces started to liquify or flow simultaneously they were quickly taken from the fire, set in place on the anvil & immediately struck hard . The first two or three correctly placed blows would complete the weld . The welding Compound was as I described . Borax ,Cast Iron filings & sand