The early forges were charcoal fired. Charcoal is a clean burning fire, easy to maintain and capable of welding heat. The Japanese use charcoal to make their katana blades and steel. The problem with charcoal is that it takes a lot of wood to make charcoal. The use of charcoal in the preindustrial era lead to the deforestation of Europe and much of the east coat of the United States. Coal was introduced as an alternative fuel in the 1800's and continued as the main fuel well past the turn of the century. Coal has many problems and is not an ideal fuel for working steel.
A modern alternative is propane. Propane is clean burning, hot and controllable. I use propane forges exclusively for my welding, forging and heat treating operations. I have own several commercial forges over the years, but found that none were specifically suited to my applications and so I began to build my own forges.
I realized that building the universal forge, one that was perfect for welding, forging, heat treating, and general forging was an impossible task, so in my shop I have many forges each built for each operation. They are all built off one basic design however and on this page I will lay out the design and construction of a basic forge.
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Inswool-HP blanket, a 2400F ceramic fiber insulation. 1" thickness
Satanite, a high alumina, slag resistant mortar mix
All refractories are supplied by AP Green for local distributor call 1-800-523-5111
Black iron pipe fittings
2 - 1.25"x6" threaded nipples
1- 1.25" elbow
1-1.25" to 1" bell reducer
1-50cfm or smaller squirrel cage blower
A good source for blowers and many other parts is Surplus Center. Call for their catalog at 1-800-0488-3407
8" diameter iron pipe, I use scrap oxygen cylinders. Be sure that it is an oxygen cylinder and not flammable gas. Release the pressure in the cylinder by unscrewing valve or if frozen drill a small hole before attempting to cut.
With a cutting torch, cut off a section of the cylinder up fifteen inches from the base. You will need to lay out three holes in the cylinder, two rectangular doors 1.5"x3" on opposing sides of the cylinder, down approximately 4" from the top of the cylinder. At right angles to the door openings you need to cut an circular burner port. This port is approximately 3" from the base of the cylinder. Size the hole so that the smaller end of the bell reducer will fit comfortably into the hole, but not bigger than the bulge of the fitting.
Next line the cylinder with the Inswool blanket. By tipping the cylinder upside down on the blanket you can make two circular impressions that will form the top and bottom of the forge. The ceramic fiber insulation is hazardous and you should wear gloves and a respirator when handling it. Cut out the circles with a knife and set one in the bottom of the forge. Next measure and cut a length of blanket to go around the inside of the forge, leaving 1" space at the top for the circle you cut earlier. You can get an approximate length by rolling the cylinder and marking. It will be oversized because of the thickness of the blanket, but will get you in the ballpark. Roll it up slightly to fit down in the cylinder and then fit around the walls, making sure the seam does not fall on an opening. Next cut out the openings by pressing the wool into them and cutting with a knife.
The photo shows what your forge should look like at this point. As I said, the insulation is not safe to breath and I have found that coating it with refractory prolongs the life of the wool and prevents the fibers from becoming airborne. To do this mix up some of the Satanite with water to a creamy consistency. Spritz the wool inside the forge with water, this helps to wick the refractory into the fiber and makes a better bond, then with a wet paint brush coat the exposed wool inside the forge. You will have to put it on thickly and will find that the wool wants to roll up if you get the batter too thick or apply it too thinly. Go around the edges exposed by the openings. Coat the remaining circle and put it gently in place. Cut a piece of plate for the cover of the forge or use an iron skillet cover for a classy finishing touch.
Screw the pipe fittings together, bell reducer to nipple, to elbow to nipple. You need to adapt a connection for the blower and depending on the size and shape of the output on the blower.You will need to make a flap for the blower so you can control the air flow.
In the photograph, it shows a piece of 1.5" tubing that the burner outlet fits into.I have welded the nipple to the channel and welded shut. You can also weld the nipple to a piece of plate with a hole cut in and drill for flange mounted blowers. I have also added a hanger on this piece so that the burner can be mounted to the forge and slid in and out of the burner port.
In the nipple that goes between the elbow and the blower you want to drill and tap a hole to accept a barbed fitting. I use 1/4" flexible hose for by gas line and size the fittings to the hose. The burner is complete.
I recommend installing at least a 100lb cylinder to run your forges from. Small cylinders can be used, but under heavy use the evaporation rate of the propane will freeze smaller cylinders. From the tank you want to install a red head high pressure variable regulator and gage. You can buy this from you propane supplier or plumbing and heating supply. I run all my appliances from one main line and set the pressure at this regulator to between 5 and 7lbs. Run hard copper line, 3/8"or larger to a spot near your forge. Make sure the line is protected so that it can not be damaged while working. Terminate the line with a shut off and add a low pressure regulator at this point.
From the low pressure regulator add a needle value to control the gas flow and connect to a flexible hose leading to the burner. Give yourself some slack in the flexible line so you can move the burner and forge with some range, but not too much to be in the way or in danger of being burned or punctured. After all the fittings have been assembled check for leaks by spraying the fittings with Fantastic. The soap will bubble showing any leaks.
In the photograph above you can see that the burner has been mounted to a piece of angle that has been welded to the forge. There are many ways to mount the burner and I come up with a new one each time I build a forge, but this is one way to do it. Please notice that there is also a muffin fan mounted to the same bar. This small fan provides an air curtain diverting the hot flame and gases coming from the door. Using the fan allows you to work within inches of the door without getting burned and is recommended.
Working with propane requires some safety precautions. First, if you smell gas, shut down and find the leak. Propane is heavier than air and will accumlate and form a blivet of explosive gas. Second, vent the forge if it is used indoors. It is recommended that a hood covering the entire forge be place 1' above the appliance and be directly vented to the outside. These units produce volumes of carbon monoxide and all care should be taken to keep a flow of fresh air in the shop when it is running. Protect all lines and hoses from accidental damage while working. Think of all possibilities, it it can happen, it will happen. Operate the forge only when you are in the room, there are no safety features and it requires thay you be present while it is running.
The reason for mounting the burner on the slide is that you can light the burner from the outside and after it is burning slide it into the forge. To do this, slide the burner out of the burner port enough so that you can see the tip inside the forge. Turn on the air, then with a propane torch aimed at the burner tip, turn on the gas. It is helpful to think of the burner as a torch. Adjust the air and gas until you have a even burning flame. If you are getting blue flame out the door, back off on the gas, if it sounds like it is burning rough, try cutting back on the air by adjusting the air flap. In operation, my flap nearly covers the intake.
This little forge is what I use to forge all my blades including swords. It will provide a nice even heat over five inches of the blade, is clean and cheap to build and run. It will come up to working temperature in under five minutes and is capable of welding temperatures, though I do not use it for that purpose. My welding forge is a scaled up version of this unit. Look for more information and photos in Scott Hardy's soon to be released Knifemakers Directory.