How to build a gas forge
Having the ability to bring steel to a temperature high enough to change its physical state is essential in knifemaking / bladesmithing. I am going to show you how to build a forge that is simple, safe, and effective. There are countless gas forge builds documented on the internet, I want to set this one apart by showing exactly where I got the materials and how they go together to result in a forge that work will work for you too!
Gas v Charcoal
I have made both gas and charcoal forges/ furnaces. Both types are relatively simple to build, but for my needs, a propane fueled forge is the only option. I live in a residential area. I have neighbors on all four sides of my shop within 100 ft. Using gas lets me run a simple setup inside my garage with minimal fire hazard. Propane is much more space efficient and cost effective in my situation. However, if you live in an more rural area, you may want to consider using coal.
Firebrick is different that ordinary red brick because of its ability to withstand high heat without breaking down. Shipping brick is not exactly cost effective, so only order it as a last resort. Search your area for a distributor. Google Maps makes this easy, search keyword phrases like “Firebrick”, “Fireplace” or “Refractory”. Call ahead to your selected business to make sure they have firebrick and what sizes they offer. The ones I used for this forge are 2x4x9”.
I also found some thinner bricks at Tractor Supply Co. They are 1 1/4” thick and I used them on the top and bottom of the forge to insulate it, but if this is all you have access to, try wiring 4 of them together for the proper thickness.
The burner is the assembly that applies heat to the forge. You can find plans and parts online to build high pressure high heat propane burners. To make it simple, I suggest you buy the Bernzomatic Hose Torch Propane Kit. Home depot carries them in store for about 65$.
You can also find it on Amazon
To run the torch off of a 20 lb propane tank, you are going to need a hose adapter. I got an 8′ hose with the proper fittings from the camping section of walmart. You may be able to buy the female to male fitting on its own, but I chose the hose adapter because it added an additional 8′ into my setup.
Buy on Amazon:
I cut the forge cavity into the firebricks with a hammer and a chisel. I started with a paper template – a 2” x 9” rectangle, the ends taper down to 1 1/5”. Next, I cut it out and traced it to both faces of the bricks.
Take your time with the chiseling. Wear safety glasses, a dust mask, and gloves. Always angle the chisel so that chips fly away from you. Approach the template marks from different angles through the chiseling process. The cavity doesn’t need to be perfect, but try to get as close to a half circle in profile as possible. Watch the video for more detail on the chiseling.
I designed the shape of the inside of the forge with the intention of focusing the hot gas in its center.
With this in mind, a wanted to cut the gas inlet hole so that the gas would swirl inside of it. So take note that the hole is offset so that the gas leads into the contour of the cavity.
I marked approximately where the bit would enter the inside curve of the forge cavity. Then I transferred the marks to the top with a square and started drilling.
I used (sacrificed) a 1/2” paddle bit to cut the gas inlet hole. It worked, but it took a long time, and the bit was totally destroyed. If you have a few duplicate 1/2” paddle bits in your tool set, then you could do the same. After all, it is good tooling steel so you can forge a knife from it when you are finished. Otherwise, it would be faster to use a proper masonry bit. The hole didn’t come through exactly where I wanted it, so I used a round file to adjust it.
Over time, the bricks will crack. As long as they are fastened securely and don’t come apart, there is no effect on the forge’s performance. I used two of the thinner bricks on top and bottom to improve the insulation. Use STEEL wire to fasten them together. Use 8 or so ties across the width, and at least two across the length. If desired, add a handle.
The last steps are optional, but recommend for safety. Make a simple stand to keep the torch upright. This reduces the fire hazard if you happen to bump the torch out of position during usage. Next, cover the rubber propane hose with a piece of pipe or conduit so that if hot material flies from the forge and lands on the hose, it will not burn your shop down.
Open the valves on the propane tank and the torch and ignite the torch end. The model of torch I have uses a locking feature so that you don’t have to hold the trigger down continuously. Push the lit torch into the forge and look into the forge FROM A SAFE DISTANCE to observe the flame. When I am using the forge, I push the torch in to the point where the flame follows the circular profile of the forge and produces the most hollow sounding note.
when you are shutting down the forge, close the valve on the propane tank first, and allow the gas in the hose to burn of completely instead of slowly leaking into your shop.