A few years ago, I experimented with refining low-carbon steel in the form of cut nails (aka square nails) into high-carbon steel. This meant building a small refinement furnace in the backyard and melting the nails down in a carbon rich atmosphere by using charcoal as the heat source. This type of refinement furnace is an ancient method used after smelting ore in a much larger furnace to create iron or low carbon steel.
The first thing to do is to dig a hole and cut a trench into the hole for the tuyere or air blast pipe. I don't have any pics of the hole, but it is shallow, maybe 8 inches deep and the trench comes into the hole about 2-3 inches above the bottom. Fill the trench back to the top and stack up some bricks around the hole another 8-12 inches tall. The finished furnace looks like this.
For the blower, I was using a shopvac with a jerry-rigged light dimmer to control the blast. You can use almost anything that provides forced air. A hairdryer or air mattress pump work well. The tuyere pipe doesn't have to be as long as this one is, but it does need to be metal and fairly thick walled. The end of it is going to melt during the process, so thicker is better.
For feedstock, I used 3 pounds of nails and an equal weight of charcoal I had made from spruce/pine/fir. Additional charcoal is needed to preheat the furnace.
Separate the feedstock and charcoal into small charges. Equal amounts of feedstock and charcoal by weight in each charge. I broke this down into X number of nails, and an equal weight of charcoal.
Start a small wood fire in the bottom of the furnace, fill it with charcoal and turn the blower on. This preheats the furnace.
When the charcoal is about halfway down, fill it with charcoal to about an inch below the top, add the first charge of feedstock and cover with the same amount of charcoal.
Repeat this burn down, add feedstock and charcoal process until you run out of charges.
Another pic of the furnace running
Let the charcoal burn down until it is almost gone and shut the blower off. Now you can either dig around in the hot coals to find your bloom, or wait until the whole thing has cooled down enough and remove it when cold. If you remove it hot, you can begin forging it lightly to consolidate the bloom. If you wait, you will need to heat it up enough to start forging it. I have multiple forges and decided to wait for it to cool down. You can always use the same furnace as a forge of sorts to reheat the bloom and forge it.
When I pulled the bloom(s) out (there were three pieces), they looked like this.
After cleaning them with a wire wheel they looked like this.
I cut through one of these with an angle grinder to check consolidation.
Satisfied with the solidity, I proceeded to forge the pieces into bars.
These were then forge welded together and eventually consolidated into a single bar. After the third forging, I had a bar about 6 inches long and 3/4 inch square.
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A spark test showed promise of high carbon content.
That is the basic process for a backyard method to refine iron or low-carbon steels into a high carbon steel useful for making tools.
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