Gas Forges:

Building Your First Gas Forge

Over the years Anvilfire has helped many people construct and troubleshoot gas forges. Here are tips and pointers for the more common problems.

Read Ron Reil's pages on ABANA thoroughly:
Don't make it too big: You may be tempted to make a forge big enough for every project you might want to make. Don't. No one gas forge can do it all. Gas forges are easy to make and in time you may make several. A 5" dia x 10" long chamber will do a LOT of forging, and is very manageable for a first project. This is about the size of R. Reil's mini - forge and is adequately heated by a single R. Reil EZ burner The rule of thumb is about 240 cu in of chamber for one EZ burner.

Use at least 2" of insulation around the chamber: The first inch or two should be Kaowool - the outer layer can be pearlite or vermiculite. Coat the inside surface with ITC-100, its well worth it. You will need about ½pt. for a typical minforge or a pint for a two burner forge. It is strongly reccommended that you use kaowool for the chamber of your first forge. It is very easy to work with and heats up very fast. The floor should be made of a piece of kiln shelf or half thick fire bricks. Later you may want to try a castable refactory which is more durable.

Close it up: Once the forge gets to red heat, the main heat loss is through radiant heat. The front and back are big windows that need to be blocked to reflect the heat back. A good idea is to make a small (2" dia) hole at the back, so that long work can poke through, and to leave a large open window at the front which can be blocked up with firebrick to suit the work. (Soft firebrick is preferable to the cheapy, heavy yellow things you buy at the hardware store). Don't be tempted to box it up too tight. Expanding hot gas needs to vent. This is especially true for atmospheric burners. The rule of thumb for the vent size is at least seven times the cross section area of the burner (about 3.5 sq in for a ¾" tube)

Tools: You don't need anything more than ordinairy garage tools. A sabre saw is very handy for cutting sheet metal - but not essential.

Construction: You don't need ½" walled iron pipe. If the forge is properly insulated, then only the area around the mouth will get to red heat. 14ga sheet metal - or even lighter is fine. If you do go for heavy construction, keep the weight down to something you can lift by yourself. Many forges are made from discarded freon bottles. Depending on your skill and resources, it should take one or two weekends to make a forge.

Your forge needs a porch: A lot of work gets done right at the mouth of the forge. Make provision to be able to pile up firebrick (the hard yellow cheap guys) in front of the mouth so that you can set small pieces there.

Burners: There are two types of burner "blown", where the air is driven by an electric blower, and "atmospheric" or "venturi" where a high speed gas jet is used to draw in the air. Atmospheric burners are easier to make but a bit fussier to set up. If you are near sea level then it is strongly reccommended that you start with a RR type EZ Burner (click on the PROPANE BURNER button). While not the most efficient design available, it can be constructed from off the shelf plumbing parts in an afternoon. Later you may wish to replace it with something more advanced. Ron's page is THE resource for atmospheric burners. He and the people mentioned there did a lot to advance this technology.

Atmospheric burners use higher gas pressures and smaller orifice sizes. Typically a #58 jet running 0-20psi. At the high end the forge will roar like a jet engine. The exhaust should be an orangey yellow - or perhaps just a little bit of blue if you want to run it rich.

If you are having trouble drilling tiny jet sizes, then the MIG wire contact tips are an excellent idea.

Plenty of air: A mini forge can consume a bbq sized propane bottle in a day. That's a lot of oxygen. If the forge is starved for oxygen, then so are you! Worse, if the forge doesn't have enough oxygen it will produce carbon monoxide. CO poisoning is insidious. You may not notice it until its too late! Make sure you have plenty of ventilation. A big fan is a good idea too.

No flammables: You are going to drop hot steel; there will be flying pieces of hot scale and there will be sparks. Red hot iron landing on bare wood will instantly burst into flames - no messing about with charring. Make sure that the area around the forge is free of combustible materials. A piece of cement board makes a good surface to set the forge on.

It is also a good idea to cover the gas line with some flexible conduit. If you drop hot steel on a rubber propane hose, you will not like it all! The same goes for the power cord to the blower.

- Adam Whiteson, 2003


In many shops gas forges are used unvented. However, they can only be used this way with VERY good ventilation, doors and windows open, preferably a garage door size opening. In a closed space they must have a hood and vent (chimney).


Forge Burner Ratio: We get a lot of questions about this. Here are some comments:
  1. We repeatedly have folks give dimensions for an area NOT a volume when asking questions. Think about the difference before posting a question related to volume. Also never assume we know the details of someone else's plans.

  2. There IS an area to consider and that is the minimum vent size for the forge relative to the burner. I believe the ratio is 7:1 (area to area).

  3. For any given burner there is an optimum forge volume. Then there is also a minimum and a maximum. You cannot use just any size burner with any size forge OR adjust the forge outside of the operating range of the burner.

  4. When buying a commercial product the FIRST person you should ask about specifics is the manufacturer OR the seller if they are knowledgable. Rex Price can tell you exactly what his burners are designed to do and I believe provides technical infromation with the burner and on his web site.

  5. Also as noted, different burners have different performance levels. You cannot assume that a home made EZ burner with a poorly drilled, burred edged and crooked orrifice will perform anything like a professionaly designed and made product using precision machine tools.
Adam has answered the question (250 cubic inches per 3/4" EZ burner vs. 500 cubic inches for a 3/4" T Rex)

As noted others, "Your milage may very" - guru

Guru's Burner:

The best way to build a fool proof gas burner is to avoid making jets and custom fittings. The burner above will fire a small forge to welding heat.

Parts to make it: All these parts can be purchased at a good plumbing or construction supply house except the bracket and the welding hose adaptor. The welding hose adaptors are available from any welding supplier.

The bracket is the only part that must be fabricated. There are two ways to make it. One is sawn from 1/8" bar stock and welded as above. The other is to make it from a 1/2" block of steel or aluminium and screw it to the side of the intake pipe, the gas tube fit into a 7/16" or 13/32" (.4063") hole with a set screw holding it in place. The all brass fittings above will work in such a design. In the first method all that is needed is a hack saw, file and a way to weld the parts. In the second all that is needed is a hack saw, file, drills, taps and a small drill press.

This burner has no air adjustment and in the welded configuration above no nozzel depth adjustment. However, I have built a number of these burners and they have worked every time. They are the right size to fire a freon can melter or forge as shown below.

The Adjustable Burner:

This burner uses a piece of lamp pipe to create an adjustable burner where the depth of the injection orifice can be moved and an air damper used. The downside of this design is that lamp tube is straight pipe thread and the fittings will leak unless heavily doped with a filled hardening pipe dope (not one of the new annaerobic types). Note that nuts to fit lamp tube are also fairly expensive and the design requires four.

The bracket center is a piece of solid bar (round, hex or square) drilled length wise for the lamp pipe to pass through. It is welded to the burner the same as the design above. The damper disk is cut from any convienient piece of sheet metal. I used a couple of candle drip pans I had on hand. The dished pans made a nice flow consistant shape. All fittings are 1/8" pipe as in the first design.

1/8" SPT lamp pipe measures .385" diameter. A "W" (.386") drill is just the right size to make the hole. I used 3/4" hex stock drilling and facing the ends in a small lathe.

Welding Fixture

This crude fixture made from scraps I had laying around has been used to make about a dozen of these burners and they were all centered sufficiently well to see through the MIG orifice from the end of the 9" long mixing tube.

This was made from the scrap plug cut from a piece of 1-1/2" thick pine lumber with a 1-5/8" hole saw. It was the right diameter to fit 1-1/2" (1.61" ID) pipe and had a 1/4" pilot hole. A piece of 1/4" threaded rod was screwed into the wood (same size holes in soft wood are undersizd and a tight fit). The 1/8" schedule 40 pipe has a .269" ID so it was a little loose on the rod so I shimmed it with electrical tape. This made a mess when it got hot from the welding so I wrapped some shim stock around it the next time I used it. It was much looser on the thin wall lamp pipe so I just centered it by hand and locked it in place. SImple bushings could have been made for a better fit.

Freon can melting furnace Two crucibles and two foundry tongs

ABOVE: Freon can melting furnace with 2" of Kaowool lining and a half length full brick bottom (over Kaowool), lined with ITC-100. Using our guru's Burner #1. The same burner could fire a larger furnace melting aluminium or zinc.

This little melter uses a #1 crucible (3.2 pounds of brass) - ABOVE middle.

It melts a full 3 pound crucible in about 5 minutes or less once the furnace and crucible are hot (about 10 to 15 minutes)

We no longer recommend building melters this way. The lift off lid and vertical crucible lift requiring special tongs (above) are both remnants of the heavy brick lined furnace days.

Today, using light weight refractories, forge and furnace design can and SHOULD be different. For small melters the entire top and body can lift off and the crucible picked up directly with pouring tongs. This saves precious time transferring from lifting to pouring tongs. Larger melters can use lift off bodies with counter balances or the diagonally split clam-shell type similar to those used by potters.

More details later.

References and Links

Copyright © 2003 - 2005 Jock Dempsey,

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