This one is as close to a no-brainer as the tool end
gets. Solid fuel forges are among the
most basic and oldest of all metalworking tools. All you really need is a fire and an air draft to provide the
extra oxygen required to intensify the heat.
A campfire and a stiff breeze will do in a pinch, but would be
inconsistent and impractical for regular work.
You can play with this one until you get what you want with little harm
to yourself, unless you stick your hand in the fire!
If you want to go with a deluxe model you will want a reliable and controllable air draft. Any good squirrel cage blower will provide the flow and a simple butterfly valve on the intake will quickly give adjustable control. If you want a medium workout while running the forge, a little searching about old tool shops and farm actions could get you a hand crank blower and rivet forge. The hand cranks are a little more efficient in airflow because of the paddle design creating more pressure than volume. Also, if you don’t mind a little workout and have the room in your shop I would suggest the classical romance of traditional bellows. I made a set of bellows for doing demos at historical re-enactments and I love them! For absolute control and a pleasant tranquil working environment bellows are the best. But they require space and are not as production oriented as electric blowers.
From your air source you will want a sealed tube or conduit to carry the air to the fire pot. You will need a 90 degree bend with a space below forming a “T” so that ashes and debris from the fire above may fall safely out of the way without obstructing the air flow. A hinged plate with a counterweight handle on the bottom of this works very nice as an “ash dump” when things pile a little too high. Where the air enters the bottom of the fire pot you will need some sort of grating to keep the fire in the pot and out of the air duct. I highly recommend steel slats or bars instead of screen type perforations or many small holes. The small holes plug very easily and are impossible to clean out without destroying your fire.
In fact if you get the fire pot as a unit or a looking at an entire forge, the best ones have some slots with a slotted block of metal in the center that rotates on a handle that turns from the front of the forge. This is a “clinker breaker” and is a real joy to have. I have worked all day before with just an occasional turn of this handle to keep my fire burning clean for hours. The area where this apparatus is located in the grating at the bottom of the fire pot is referred to as the tuyere. With a little effort a tuyere with clinker breaker could be easily made with simple welding equipment.
The fire pot itself can be made of any heavy material that can withstand the intense heat of the fire. A certain amount of depth is desirable in order to get enough fire above the oxygen rich area at the tuyere. In a properly maintained fire there should be an oxidizing level or layer, a neutral area and a carburizing layer. The neutral layer is the best area for most forging. This is a good time to address the carburizing layer and the concept of coal forging adding carbon to the steel. Time and time again we in the forging crowd try desperately to inject some magical properties into the process that will produce a superior blade. I believe this is mostly a need for justifying all the extra trouble and work that goes into forging a blade. The much-touted idea of adding carbon to a blade in a forge fire is one of these attempts at justification. The carburizing area of a coal fire is fairly small and elusive in comparison to the oxidizing and neutral and you are only in that fire for enough time that it takes to heat the steel to working temperature. Any carbon that may accumulate on the thin skin of the steel will be lost when the thick skin of iron oxide peels off on the anvil. At best, if you are very careful, you may break even on the carbon content. But the odds are much more in favor of ending up with a healthy skin of lower carbon (decarb) on the outside of the steel, from forging.
The main body of the forge should be lined with some kind of refractory material. This can be a castible, fire clay, or cut firebrick. I have seen plenty of forge tables made of wood with a refractory covering that worked quite well.
You will absolutely want good ventilation for a coal or charcoal forge. Outdoors is best if you can tolerate it, but a really good drafting system indoors is more practical. The first idea that anyone goes with is a hood of some sort directly over the fire; this should catch the smoke most efficiently. But then you go to an old blacksmiths shop with a “side draft” forge and all of your ideas are turned upside down. The side draft is a chimney running beside the forge fire with a little hole cut in the side next to the fire- and they really work! Whichever way you go make sure that it does work and get plenty of black handkerchiefs since that is the color they will all end up regardless.
Coal or Charcoal?
Coal is not the original traditional fuel for smithing, that distinction goes to charcoal. Coal is a fairly late comer (16th century) to the scene, and was only adopted after folks in Europe decided that they had deforested enough of the land in producing charcoal. When you see how dirty and contaminate laden it is you realize why it was the second choice. But it is fairly inexpensive and is not consumed nearly as quick as charcoal is. It can also be more readily available than real charcoal, suitable for smithing.
Lets make something very clear, one does not forge in coal! We burn the coal to make coke and forge in that coke. Coal is a mass of carbon-based material that is saturated with hydrocarbons and contaminates such as sulfur and silicates. When we burn coal we are reducing it down to the basic carbon by using up the hydrocarbons in the combustion and hopefully boiling off all of the sulfur in the process. One should never forge in a faire that is still belching out all that yellow smoke. If you have globs of black crusty tar sticking to your work piece you are only half way through making coke and not doing the steel any favors, let it cook a little longer. A proper forging fire should have reducing coal around its perimeter with clean coke in its center giving off a pure blue flame. Yellow flame is a dirty flame.
As the coke burns away it will deposit a collection of semi-molten silicates and ash in the bottom of the fire pot, this is the infamous clinker. Clinkers will cover the tuyere and choke off your air blast. You will know when this has begun when you see little glowing globs flying from your fire like little meteors. Either rotate your clinker breaker and send it down the ash dump or probe with your fire tool until to hear the tell tale “clink” of a clinker at the bottom. Snag it and dig it out or you will end up with all of that crap stuck to your work piece and hammer it into its surface.
Charcoal
The first fuel that mankind harnessed was charcoal. Simply, it is wood that has been distilled down to its basic carbon constituents. Once again the yellow flames and smoke that you get from wood fires are all of the volatile components being reduced to gases. After these are removed, and before it is reduced to ash, you have the intensely hot, blue flamed, clean burning carbon. It takes a lot of wood to make charcoal and it takes a lot of charcoal to make a working forge fire (in comparison to coke). Charcoal is extremely clean in comparison to coal and the fine ashes can have a nice fluxing action keeping the steel much neater. It is a much lighter fuel so you will want to cut back on the draft a bit from coal. Intense air blasts have a tendency to throw lots of sparks and glowing embers for some ways.
Where to get real charcoal is a good question, indeed the main reason that charcoal is used so much less than coal is the availability. I must stress the words “real charcoal” in order to completely separate it from the charcoal briquettes used in backyard grills. If you are willing to resort to this stuff you would really be better off with coal. There are bits and pieces of burned wood in there but it is all ground up and mixed with ceramic type materials, glue binders and; you guessed it, coal! The stuff you are looking for is real “lump” charcoal and is beginning to become available for barbeque purists but it could take more than a few bags to do the job. Many people have taken to making their own charcoal in order to reap its incredibly clean burning benefits.
Some good charcoal information that I have found on the web is at Tim Lively’s site, Lively Knives. Tim seems to have a lot of this charcoal business figured out and has some resources available for charcoal forges and has links to a really great page on making simple forges and a charcoal making retort by Daniel O'Connor at Twin Oaks Forge