Instructables

How to Build a Forge (Gas)

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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.

 
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Step 1: Materials

Picture of Materials
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home depot torch shot.PNG
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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.

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handy1573 years ago
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.
garthn handy15716 days ago

Wrong

MAPP = 2010 (c)

Propane = 2392

makingcustomknives (author)  handy1573 years ago
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.
Skalblaka2 months ago

Cool! I will have to build this!

cfreitas1 year ago
Very interesting instructable. I see a pic where you are using a common gas tank ?
I want to do a furnace using normal butane gas, the same one we use in kitchen or boilers, is it possible ? I´m very new to this but almost got most of materials.
I´m living in mexico and around here is not that easy to find a place where I can use charcoal to melt metal, because people ( and police ) dont like to see people burning things....
Propane ( like benzomatic ) is very expensive here, and I must find a cheaper way of melt.
So my option would be use common gas, is this possible or I´m missing something ?

Btw, your furnace is a nice inspiration to me.

Thankyou for share it.
black hole1 year ago
I got my bricks from a website called "Shefield Pottery"
mookster2 years ago
Hi, I am just in the process of making one of these. I love the idea. To make it easier, as I'm not used to chiselling stone, I've taped a masonry bit, and drilled pilot holes down the centre of the cavity to the depth I need. This helps me get the right depth all the way down. Also I've drilled loads of smaller holes to give a 'swiss cheese' effect, to make the chiselling easier. This way the brick just flies out, and as these bricks can be very fragile, lessens the amount of force needed each blow of the hammer. Hope this is helpful, and I'm not doing anything stupid!
DV Customs2 years ago
You can buy these bricks on Ebay, both sizes. Just type in fire bricks
TALLJ293 years ago
are you pumping in air ( atmosphere air) into the furnace? I would make it hotter.
a blowtorch does that for you. Otherwise it would be a normal flame...
(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.
Would adding a forced air intake to the back be a good idea?
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.
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
makingcustomknives (author)  TALLJ293 years ago
No, only the torch is supplying fuel to the forge. Thank you for your comment!
man normaly i do it old style (fire and wood) but will it heat the metal faster?
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.
Wade Tarzia3 years ago
Nice technical writing, especially using sound as a cue, ie., "most hollow sounding note." We former tech writers appreciate such details!
makingcustomknives (author)  Wade Tarzia3 years ago
Thank you.
Love the instructable have seen this design before and used one similar and it works great for small stuff.. For anyone attempting this or working metal for the first time please consider first finding your local abana (www.abana.org) chapter.. The chapters are full of blacksmiths and other hobby metal workers who are an amazing wealth of information. i played at garage forge for over a year and ended with a lot of mashed metal (art??) then i met up with my local Michigan chapter was able to head over to a nearby smith and practice with a real setup to get the feel of it. They also helped in maximising my homebuilt forge (didn't get hot enough) and was able to buy a real anvil for dirt cheap (never again harbor freight... never again).. Definitely took a few years off of the learning curve.. now i make knives and other things not just art..
Excellent advice, thank you.
I'm guessing the eyes were an afterthought, and the original intention was a different internet meme :P
lol!
drewgrey3 years ago
I love the simplicity and flexablity of your design. A forge like that could be assembled in any shape to match a project.It could even have multiple torches for a larger work. I made my propane "carb" on my forge by welding the end shut on a piece of 1/4 " pipe , drilling it with a small #70 bit, and then spinning the pipe while grinding it to gently taper it for improving the venturi
effect. Tthe propane pipe slides into a fluted air horn and by sliding it in and out I can change the air fuel mix.
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makingcustomknives (author)  drewgrey3 years ago
That is an impressive setup!
Thanks, I am trying to make it work with the piazo electric push button start but I need some tungsten rods first. . I hope that if I put the tungsten rods in the flame path they will help keep the flame going on cold start ups . Once the forge is a liitle hot it stays lit real good but ignition can be tricky till then.
makingcustomknives (author)  drewgrey3 years ago
Could you give me some details on the inside of the forge? Are you using kaowool? refractory cement/clay? Im gathering materials to build a new forge with a Ron Reil style burner and any advice would be helpful.
I used the kaowool and the ceramic liquid and topcoat. It was pricey but seems to work good. I got it at seattle pottery supply. Your local pottery supply may also carry it.
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anvil_man3 years ago
Fire brick...?? I don't see what kind.. Looks like hard in photos,and the way you are cutting it..

I would use the hi-temp 2500℉ soft fire brick, would be better and a lot easy to cut,
also a lot better in holding your temp, less heat loss and comes up to temp faster than hard brick.

Nice little forge thou

RE stuff on the hose scale not to bad.. but a red hot bar is not good, first hand experience.

LPG is heaver that air so be careful
makingcustomknives (author)  anvil_man3 years ago
It is hard fire brick. I know it isn't ideal, but it is dirt cheap and was able to find it locally.

Thanks for the advice!
Tim Temple3 years ago
Using a drill with a level bubble on it can give you a straight hole almost as good as a drill press.
J-Ri3 years ago
The stuff that lands on the hose is fine, it all bounces off. The stuff you have to worry about is what hits the floor, it can bounce onto the hose :-D

Great instructable, thanks!
Tanmcnew3 years ago
HAHA funny how the fire extinguisher signs are sold nearby :)
dgallimore3 years ago
Could this be modified to use charcoal instead? I live in a rural area.
Thanks
makingcustomknives (author)  dgallimore3 years ago
I would suggest a design like this for charcoal.
Thanks!
Civicalized3 years ago
I have been finding parts for a project I'm working on, and needed a burner for. I too found that one at Home Depot, but couldn't bring myself to spend that much. I was on Harbor Freight's site last night and saw one that takes camping propane tanks (I figure they could be hacked to fit a large propane tank) for $20 regular price. Just a thought!

EDIT: After I typed that I double checked the price and saw they have the same type as the one you listed, but an off-brand, for $25 regular price. Hope that helps at least someone!
Torch hacking is needed if you can't find a torch that will hook to a bulk tank. If you venture into the camping area of most big box stores or a decent camping/outdoors place you should find hoses used to connect most camping equipment that uses small bottles to bulk tanks. You may need an adapter for the bottle to hose, or some hoses come with a regulator in them.
weldor3 years ago
For larger work, the big brother to this one can be made using a "weed" or "brush" burning torch. BE CAREFUL with this as it creates a whole lot more heat in way less time than you would think!! I have used one and it really works awsome. The only drawback is it also consumes a whole lot more fuel. Also on the plus side is that it is super eaasy to adjust the fire box's dimensions to add another burner or to reconfigure it for use as a blast furnace for foundry work. Remember at all times to ensure that you have a contingency plan in place in case of an emergency!
Fun Fact #16: did you know that the scientific term for the opening is the glory hole?
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