How to Build a Forge (Gas)

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Watch the video version.

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.

Step 1: Materials

Materials.

Firebrick

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 2 1/2x4x9”.
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.

Burner

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 click here to get them from amazon.

   The brass torch tips that screw onto the tops of bottles do not get nearly as hot as the bernzomatic torch. Also, if you do not already have a torch, investing in a quality model will add an extremely useful piece of equipment to your shop.

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.

Step 2: Chiseling Out the Forge

   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. 

   For more detail on chiseling out the forge, check out the video.

Step 3: Drilling

     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.

Step 4: Finishing Touches

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.

Step 5: Usage and Safety


Usage.

WORK IN A WELL VENTILATED AREA

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.

IMPORTANT

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.

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55 Discussions

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handy157

7 years ago on Introduction

If you only need to use one of the smaller bottles and not a 20# bottle, MAPP gas burns much hotter than propane and will heat your project steel much faster.

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JeremyM175garthn

Reply 1 year ago

Mapp gas burns about 3700 degrees fahrenheit a couple hundred degrees more than propane. It would not be a good idea to use as a forge gas due to the high concentration of hydrogen in the flame will make the steel more brittle.

[de

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michaels989garthn

Reply 2 years ago

However, MAPP, with the proper setup, will burn hotter, esp. if you have an oxygen feed.

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ShonaO1garthn

Reply 2 years ago

Not to be rude but, you are wrong. Mapp gas burns much hotter than propane. Not sure where you are getting your information but, you should look elsewhere.

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CobraTesterhandy157

Reply 4 years ago on Introduction

Heating up your project quickly isn't the only thing to think about. If you are trying to heat treat metal, certain temperature ranges are a target, and in a little home built furnace, you likely can't properly measure the temperature of your item. You can select a gas that cannot over heat the item by selecting a gas that doesn't burn hotter than you want. (just a thought....speed isn't everything in a furnace)

You are right about the heat difference, but a bottle that size will only burn for several hours before it is empty. I haven't needed to replace my 20 lb tank in months.

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michaels989steelswordman

Reply 2 years ago

It does heat faster with less fuel, but it has no carbon content in the gas, so the wood/coal/coke would be ideal, a slow heat is less stress on the metal, and improves the form, while solid fuel will increase the carbon content over time. Also slow cooling in coal/coke powder will harden and increase the carbon content.

I honestly don't know if it will heat faster. The reason I use gas is because I live in a dense residential area, and a gas forge reduces the fire hazard.

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(thought I posted this but I guess not...hopefully this doesn't become a dupe)
No that's wrong. If you turn down the gas flow of a blowtorch it will be a normal flame. All a blowtorch does is allow more air into the mix, it doesn't actually pump it in which makes a hell of a difference. The difference is akin to a blowtorch versus a cutting torch.

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michaels989WhittVT

Reply 2 years ago

The blowtorch is a form of forced air intake. It works similar to an airbrush. Instead of pressurized air it's pressurized gas, and instead of paint it siphons in air.

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ehrichweissWhittVT

Reply 7 years ago on Introduction

Depending on what you mean by "to the back", yes, it could be a very good idea or at least just an alright idea. To clarify, if you're talking about injecting air on the back side of the burner, before combustion takes place, then yes it's a good idea. If you mean pumping it into the back side of the forge itself...maaaybe, there's a lot of factors there.

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That is what I meant, but when i turn down the gas flow, unless I turn it down almost all of the way to off, it stays a non-normal flame

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TweakGeek1

2 years ago

Great ible! I've been looking to make a forge and something better than a coffee can forge. This might be up my alley.