Gas Mini Forge

Mini Gas Forge

The forge is used to heat steel. There are two basic purposes for heating, one being to shape the steel, say with a hammer and anvil or to heat treat the steel. One part of heat treating, known as hardening involves heating the steel to the appropriate temperature and then quenching it in oil or water.

Traditionally, the forge was a coal powered fire pit with some form of mechanically driven air supply to increase the temperature to get the steel hot to shape or harden. Bellows provided extra oxygen needed to burn the coal hot enough to heat steel. Similar results can be achieved very cleanly with electricity or gas.  For the DIY knife maker, propane gas is effective and readily available.

In order to heat treat the carbon steel used for knifemaking, the temperature needs to get to around 800°C (1470°F), which should be cherry red colour. At this temperature iron becomes no longer magnetic. For high-performance stainless steel, we need to get to the 1000°C mark (Austenizing temperature) and hold it for 30 to 60 minutes. This is the main goal. I reckon the amount of fuel for 45 minute burn at yellow hot is in one typical camping type propane fuel bottle.

The table below, (courtesy shows the colour the steel needs to be for hardening quench.

Temperature Color of Heated Carbon Steel
(oF) (oC)
2192 1200
2012 1100
1922 1050
1796 980
1706 930
1598 870
1490 810
1400 760
1292 700
1202 650
1212 600

To keep the heat in I am going to use firebrick rated for 1093°C (2000°F). The brick size (4-1/2" x 9" x 1-1/4") is convenient enough to make a small forge suitable for heat treating small knives and can still be heated using readily available propane gas.
I collected the parts from a few places, namely Home Depot and Canadian Tire. The torch is a hose type that will let me place the propane cylinder to the side of  the forge.

Also showing: Fireplace mortar (2000°F) and #10 x 2-1/4" concrete screws.

If you wish to build a similar forge, you may be able to find these things on Craig's List, Kijiji or yard sales. Good luck in your hunting.
Be sure the torch is a "swirl flame" or similar type. This design will not work well with pencil tip torches.
Dry assemble the blocks before mortaring.

Set the drill to hammer mode and drill the firebrick. It cuts very easily with the vibration from the hammer. I used a 3/16" bit that will accommodate the #10 concrete screws.

I put one fastener on each side. Next, I drilled the torch hole to 1/2" then enlarged it to 5/8" with a coarse round file. Firebrick cuts easily, although it's dusty. It's a good idea to wear a dust mask when drilling and cutting this brick. Be sure to mark all of the pieces as they will have to be disassembled for the mortaring.

With a hacksaw I cut some pieces to close in the ends. These 'caps' will help keep the heat in, yet still allow sufficient airflow to get the most out of the torch.

The high temperature mortar looks and feels like mortar. It's gritty. I squeezed a bead and spread it around with some scraps of wood.

Once the mortar is in place the screws go back in to hold everything together for the setting period. I have decided the leave the screws in, but they could be taken out and the holes filled with mortar and sanded when dry.

I cut the 2-1/2" steel pipe to fit into the brick box and used a pencil to mark the hole where the torch will go through. The torch tip should enter at a slight compound angle, that is upwards and forwards or downwards and to the back. This will aid in creating a vortex inside the tube.

After inserting the pipe, I marked the so it could be reinserted in the same spot again.

I drilled the torch hole to 5/8" with a step bit and elongated the hole with a round file.

A small amount of high temperature mortar at the front and back holds the pipe in place.
Test burn looks okay, but I am going to make a removable cover for the back to keep the heat in.

 With a few lengths of angle iron and some 1/4" threaded rod, I fashioned a frame to hold the forge. I will be able to mount this to a stand. Working with the forge requires the opening to be conveniently positioned so one can see inside.

A very light coat of black high heat paint. (The spray can was just about out of paint.)

To test the heating and heat retention of the forge, I choose a large 1/4" thick mill file. I was able to get the end to a non-magnetic state after a few minutes of heating. This should suffice for most of the knives I want to make. Overall I am happy with the results from propane gas.

An option for increasing heat is to switch the fuel to propylene gas (MAP). This isn't a problem as long as the torch you are using is good for MAP gas.

A short video shows the ultimate test for a carbon steel knifemaker: getting past that non-magnetic state. Sorry about the sound quality as the swirl tip torch is loud.

This should work well for 10xx carbon steels and O1 and some similar alloys which are of interest to knifemakers.

If I have any updates on the Gas Mini Forge, I will add them here.



Update: Someone asked about the torch. I recommend this one:


Peccy said...

Really enjoyed you site, I definitely will be back to watch the progress!

Apples said...

I love your site and willing to help attitude - I don't understand the 2.5 inch pipe - won't that melt - or does it just get hot and then cool off.

D. Comeau said...

Hi, that's a good question. The pipe acts a container for the heat. It will get hot, but never hot enough to melt. Steel melts around 1350°C (2500°F) and that kind of heat is not going to happen with a little propane torch.

Unknown said...

Hello, thank you for the wealth of information you have shared on your pages!
What is the brand and model of torch that you use here? I am looking to start making knives from 1080 and believe this type of forge is just what I need! Thanks again.

D. Comeau said...

The one I use is a generic swirl tip torch from a Canadian chain store. It has a short length of hose making it quite useful. It is a reasonable replacement for a BernzOmatic TS4000, which I wholeheartedly recommend for small forging use.

Look for a "MAP" ready and "swirl flame" unit if you cannot locate a TS4000.



Dan said...

Would this design work with a slightly bigger pipe diameter that would still fit inside the brick box dimensions?
Is the torch strong enough for the increased diameter?
I would probably cap the back end of the pipe.
I love your website and this forge design, I am just starting out and it gives me a great starting point without the big cash outlay required by buying everything new.

D. Comeau said...

I'd go to MAPP gas if increasing the size a little bit. Blocking heat loss is a good idea. You'll have to try with propane and see if the volume/loading have a big effect. If you don't enough jam to make the steel non-magnetic, then try MAPP.

Unknown said...

Hey Dan, I trust you are doing well. Thanks for your wealth of information!

I want to setup the same kind of forge but where thinking of using refractory cement(temperature mortar) . I want to build a box to cast a forge and on the same manner add an LPG gas torch. When the mortar is dry I would remove the casting box. Would the mortar be strong enough to sustain over a long period or would I need to reinforce it with steel from the side and as you have done add a 2.5inch steel pipe inside?

What do you think, will an LPG gas torch be hot enough?


D. Comeau said...

Hi Walter.

It really depends on the mortar. Some will not hold up to 1100°C (2000°F) for very long. These will begins to flake and crumble, but you can re-apply and do repairs. Other mortars, even some homemade casting refractories will be okay for a while. It's expected that some maintenance will be required. The steel pipe section helps in equalizing the heat but takes more energy to heat up.

A swirl tip LPG torch should be enough to heat treat a fairly good sized knife. The same torch tip can be used with MAPP gas if you need extra heat.

Hope this helps.


Unknown said...

Hi Dan,

Do you know of a supplier of fire brick around Edmonton ?

Also, I have a dumb question - can you also use a forge for a simple stock removal knife? Are they for certain carbon steels or can you use them for tool steels and some of the more high performance steels?

Im trying to get into knifemaking but it certainly is an expensive hobby...especially if you just want to try it out. Im thinking i can get into and run a forge for cheaper than a heat treat oven but im guessing that some steels need fairly precise temperature control? I would apprecite your input if you can spare the time. Thanks...Shane

D. Comeau said...

Hi Shane,

Simple carbon steel blades can be heat treated in a gas forge. These are normally 10xx steel, O1, 15n20, W2 and so on. If you want to give heat treatment a try in a gas forge, see if you can get your hands on some 1084. It's great blade steel and dead simple to heat treat. Some makers work with old files and farrier's rasps to start. This can help with the expenses.

As you get into stainless steel with added chromium and vanadium for example, the temperature control becomes much more critical and an electric heat source is preferred.

Insulated Fire Bricks (IFBs) come in two basic types. The soft ones are rated typically for 2300°F or 2700°F. They are excellent insulators. These are the type of brick used in pottery kilns. Check any local pottery supply house. Pottery Supply House in Oakville Ontario sells individual bricks. Some commercial refractory places will sell a small number of bricks, while others are B2B only. It's worth giving them a call as it never hurts to ask. I lucked out and found a box of K23 bricks on Kijiji.

Another harder type of insulating brick can be found at most hardware stores for use in woodburning stoves. These are rated usually at 2000 to 2200°F. They work, but they are not nearly effective as insulators when compared to the soft IFBs. Another option is to make your own refractory. YouTube has lots of videos with recipes for castable refractory.

Budget wise, I'd recommend starting off with a basic gas forge. If you catch the bug, you may want to get into an electric setup. But, you'll always find uses for the gas forge for things that need quick heat, blacksmithing, making mokume gane etc.

I hope this helps,


Unknown said...

Thank you very much. Also thank you for taking time to document all this. Its such an incredibly helpful resource.

Unknown said...

Awesome walkthrough, great website! Just so you know, building a larger torch head with black pipe from any hardware store is pretty easy. There are quite a few very good venturi burner designs if you google it. you could then use a larger tank. Although, the potential for melting that pipe would go up quite a bit.

D. Comeau said...


Thanks for your comments. I've progressed a little since the old torch days. My new forge is detailed here and it uses common parts that are found in most hardware stores.

Best wishes,