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Gas Forge Build


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So there's a lot of talk about building burners and which design is better, but not so much about the box you put the burner in. To me, the box in the more important part. No matter how efficient your burner is, it is the box that will largely determine how hot the forge gets, how quickly you can get to work, how long you wait for heats, and how scaly your metal gets. 

Here are some of the things I want in a gas forge, and why I want them (Your desires may be different, and you'd want a different design. The design I'll show is not the one I'd build for example if I were a knife maker or if I only had a half-hour at a time to forge.):

It should get hot and stay hot when I fill it with metal. Insulation is key, but so is thermal mass. I want enough of a heat sink to make sure the metal heats and re-heats quickly rather than the forge cooling significantly when I add the mass of metal. 

It should be durable. Who wants to spend lots of time building and fixing forges instead of using them?

You should be able to adjust the atmosphere, at least from neutral to rich. Too much oxygen makes heavy scale and makes welding hard to accomplish. The burner matters here, but also, a forge should have doors. Without doors, it is hard to control the atmosphere except by gas pressure, which is wasteful.

Doors should not prevent access to the whole chamber when necessary. Why build and heat a volume you can't access?

If you use ceramic fiber blanket, it should not be where it can be abraded. 

If your forge or doors use a steel shell, keep the steel out of the flame path as much as possible. This goes to durability. when your steel burns up, does it expose fiber insulation? Does it open a gap around the burner? Will your roof buckle and cave in when heat gets between the insulation and the shell? None of these are good in my book. 

Quiet is nice. The shop is loud enough.

For all of the reasons above, I cast forges from commonly available refractory materials and use ribbon burners. In the pictures attached, the arch is cast from 3000 degree lightweight insulating castable and the floor and burner head are cast from 3000 degree dense castable. The entire box is surrounded by 2" of fiber insulation. You can't see it on the arch because I made the castings slightly proud of the frax to keep the fiber out of the flame/abrasion path. The casting is also proud of the steel shell so that there is no steel to erode in the flame path and open gaps. 

This may look like a lot of work, but with assembled materials, it all took well less than three days, elapsed including the stand and plumbing. Not so bad when I have other forges built this way that have lasted close to ten years. For this rather small forge I did push the limits of casting with sections as thin as 1" for the floor but I think it'll hold up and I didn't want it to take forever to heat up. 

I did build the ribbon burner but there are other people here more expert than I am. It is 2" x 6" with 11 crayola size jets. I used a needle valve on the gas and a homemade butterfly on the air and it is very tunable and quiet. 

Image notes:

1 have a plan.

2 the base for the casting form. Note 3/8" blocks to lift steel shell off of floor and allow casting to extend beyond the steel shell.

3 the outside shell. Holes drilled are to wire the frax to the shell. Flange secures mold and will ultimately be bolted to stand.

4 outer shell in mold. Note that top of mold is also proud of shell so that casting will go beyond steel.

5-6 the outside of the shell mold. 2x4s support the shell and keep it from bowing out.

7-8 the inner shell- arch form. This will not be part of the finished forge. Don't forget the mold release.

9 the two halves of the mold mocked up.

10 the "core" for the burner.

11 the core mounted and caulked to the inner form.


13 the (not yet caulked) burner core fit to the inner arch and through the outer form.

14 opening for the burner core cut into the outer shell.

15-16 the frax wired onto the outer shell, and with the burner opening cut out. Note that the frax is shy of the top and bottom of the steel shell so that in the finished casting the frax will all be encapsulated in castable.

17 the nearly ready assembly. What this doesn't show clearly is that there is a little more than an inch of clear space between the frax and the inner arch. I will ram the castable onto the frax which will compress and compromise it some but will still leave me with more than 2" of insulation all around. 

18 the casting in the mold. Be sure to really ram the castable into the bottom to fill in the lee of the burner core.

19 very simple mold for the floor. 1/2" plywood core makes the lip on which the arch will rest and gives me a seamless floor.

20 the floor casting sitting on 2" fiber board. (forgot mold release: cracked casting slightly. Sairset to the rescue.)

21 dry stacked and ready to burn in gently.

22 Ta daa. Bright lights mess with photography but it really is that hot. 

Forge floor is 8" x 12" and the opening is small enough to use a brick for doors. I'll probably eventually build doors that are easier to move around but I can get to work for now. 








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Nice build and how to. Well done!

I've done similar double lined forges and my main difference in tooling is to use Sono tube for the forms. A vault or mailbox forge chamber can be cast with a Sonotube form easily enough and doesn't require metal working tools like a brake. A piece of plywood the width of the forge floor tacked inside a sonotube the right diameter, 8" in this example. Cutting angles to match the sonotube on the plywood makes construction much easier but isn't necessary. Repeat with larger sonotube around 14" in this example. Tack both forms to a base board and you're ready to cast the liner.

The Sonotube allows the castable or rammable refractory to dry and peals off easily.

I'm offering zero criticism to your build plans just an alternative to maybe speed things up some. You really put a big smile on my mug using Pam for a release agent, I have a can in the shop, Pledge spray wax works really well too, Pam doesn't work as well in some situations. Either makes a decent finish for metal work too. Win win all round.

Thank you for posting this, I'll be saving it in my forge build folder.

Frosty The Lucky.

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Thanks for the comments/compliments guys.

Scrambler82: you could use that tank, and maybe you should if your shop has a real big hammer, a crane, and a manipulator so you'd need a forge that big ;). I know if I had a 500 gal tank I'd be designing a whole hog smoker. Seriously, that's huge and heavy. Frosty's sonotube idea is a good one. The shell isn't structural in the finished forge. 18 or 20 gauge is plenty. 

Frosty: Good ideas. With a sonotube I'd still want to contain the fiber insulation but I guess rigidizer would probably be fine on the outside. Truth is I had a slip roll and brake handy but not a sonotube so for me the way I did it was the cheap and easy way. The half octagon was a purely aesthetic choice, but I am partial to the doghouse forge. 

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I encase the Kaowool but when I was making pipe and vault forges I used stove pipe. I have enough shop equipment to use heavier stock but the guys I was showing had hand drills and tin snips so stove pipe was in their league. Since then I've decided stove pipe is plenty heavy structurally and SS stove pipe is a nice IR reflector and shiny.

Frosty The Lucky.

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The extended floor is a most excellent feature, maybe extend it farther even so you wouldn't need a helper up close for moderately long pieces.

I'm also a fan of the built in ribbon burner. It may not be practical for most of us, I'd wait till I had a ribbon burner build down pat before incorporating it in the forge wall itself. However once you do have it down pat building it into the forge is sweet, it eliminates mounting issues.

Frosty The Lucky.

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Scrambler82- glad to help. To be clear, the inner steel shell is a mold for the casting which is not part of the finished forge. A steel liner on the inside would just burn up. But you could take Frosty's idea and use a sonotube or stove pipe for the inner mold and be on your way.

Frosty- a longer floor has crossed my mind, but I worried about the temp differential/ heat sink causing potential problems. I usually go with an adjustable sliding stock rest built onto the stand. Just some square bars sliding in square tubing welded to the underside of the table.

Also to be clear, the burner isn't exactly built in. I made a plywood blank- the "core" - that was a bit bigger than the burner and fit it to the inner arch form to create a void in the casting so I wouldn't need to break it out later to fit the burner in. The burner is independent and bolted to the stand. For that matter, the whole design is "modular" in the sense that the burner, arch, and floor are all independent and may be rebuilt/ recast separately at any time without disturbing the other parts other than to unstack the unit. The finished forge is dry-stacked and only the bolts through the flange of the outer shell to the stand hold everything in place.

Anyway, good ideas all. I'm no expert I just wanted to share what has worked for me and encourage people to think at least as much about the box as we do about the burner. Think less about how fast your forge heats, and more about how fast your metal heats and maybe save yourself from breathing ceramic fibers and building disposable forges at the same time.

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MY helpers are all telescoping arms on the forge frame or body too.

Okay so you've planned for replacing damaged, worn components with minimum hassle. I see we have similar design philosophies, put more work in the front end to make modifying, repairing or replacing problems as easy as possible.

Every time I get over confident I build myself into corners, more than one usually. I did just that with my most recent down sized forge. I not only changed more than one thing at once I (I can't BELIEVE I did this) cast my mistakes in refractory. It's really good refractory too, my pneumatic chisel barely chips it. And BOY would I like it to just bust it out where I need it busted out. <sigh>

Frosty The Lucky.

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One of the particulars I like about ribbon burners is that they are recommended for use with ceramic fiber packing between the burner head and its opening in the forge wall. Every other ceramic burner head I've read about requires a 1/4" or larger gap between the burner head and forge wall opening; a gap that allows in a ruinous amount of secondary air. Uncontrolled secondary air will lower efficiency about 20%; How can this be? Because, unlike the exhaust opening(s), which can't draw in ambient air so long as a running burner's flame provides increased internal pressure, but any opening around a burner head will induce outside air into the forge.

This is called a "vaulted" forge? Never heard that term before, but it sounds like a good one.


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A more common term is a mailbox but in architectural terms it's a vault, a barrel vault to be specific, goes back to Minoan architecture, Rome used hydraulic concrete and traveling forms to make it a common form.

So as far as forges are concerned vault or vaulted is the term I use. "Vault" looks up and is applicable.

I'm getting closer and closer to taking a lash at ribbon burners I just need to find a little 120v blower around the right output for cheap. I need to go with cheap.

Frosty The Lucky.

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One of the open questions he answered with this build is that ribbon burners are practical on smaller forges; I was certain they'd be great for larger structures like day tanks and glory holes, but my interest is small equipment; and since his burner looked kind of large for the forge, it is probable that ribbon burners have an excellent turn-down range. It's been a great week for learning things (way more fun then teaching things).


You probably already have a compressor in your shop. Since one of the requirements for the fan on ribbon burners is more air pressure than is provided by most fans, a guy looking for low cost performance might consider employing a compressor for the air source, at least while hunting for a bargain fan?

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Mikey98118: I am nobody's expert on ribbon burners but they seem very forgiving. After asking around a bit for specs on forge volume vs burner port area vs mixing volume and not getting much (I'm sure it's out there) I just winged it and I lucked out. I'm very pleased. I had a Pine Ridge ribbon burner that's 4 1/2" x 6 1/2" that's not supposed to be used for smaller volumes than .75 sq ft. I basically made one about 2/5 that size. 11 crayon size orifices in a 2" x 6" face. A baffle at the gas-air mix inlet is just a piece of flat stock about 1/2 as long as the burner centered with a notch in one long edge. A needle valve on the gas and a homemade butterfly on the air and I am very tunable. I say go for it. 

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I can't really tell from the picture.  Is your ribbon burner tapered or radiused to fit the contour of the forge?  I'm curious as to how that would affect the burner function and if any part of the burner head sticking into the forge would decrease the life of the refractory due to thermal shock.


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Buzzkill: it isn't. Ribbon burners aren't supposed to be in the forge. It is just a flat face. The face of the burner is about an inch and a quarter back from the arch face on the top and about a half inch back at the bottom. The refractory is designed for thermal shock but to be safe I always run the air for a while after I turn the gas off. The insulation of the arch wall plus the air blast keep all but the face of the burner pretty cool. 

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When I began with Ribbon Burners I too would let the fan run on after shutting the gas off but then considered, the fan is pulling in cold air and might be worse than any chimney effect of heat being drawn up into the burner and beyond.  I now shut off the fan immediately after turning the gas off.  I have felt the pipe that feeds the gas/air mixture and can not feel any gain in heat.  Just remember, if your blower is lower than the burner ALWAYS turn on the fan first, then the gas.  Propane is lighter than air so if the gas is turned on first it will fall down into the blower and you might get a fire in the blower or across the floor.  ALWAYS turn the gas off first when shutting the forge down.  BTW natural gas is lighter than air but still follow the same instructions.

Let me know if I can help you,




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Probably no big deal either way but I'll still keep running my blower after the gas is off for one simple reason. Think of it like your car, or maybe my 25 year old truck: the engine is warm and I'm driving around town. I stop, and when I turn the truck off, the fan stops and the coolant stops circulating and for a few minutes, the engine can reach higher temps than it does during operation. Same thing with the gas forge. The burner always has cool air and gas flowing through it when running. Only the face gets hot. You can look acros it in the hot forge and see the shadows in the orifices telling you it's cooler just behind the face. So on the one hand, there is always a tremendous heat differential there, but if I turn the gas and air off simultaneously, the balance of the burner casting will get hotter than it does during operation. It should be fine, but the burner is the least consumable part and I'd rather not shock it. 

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PS I misspoke.  I didn't mean for safety, I meant for durability. I always turn the gas off at the bottle and bleed it through the regulator and valve so I'm not worried about heat back through the system as a safety issue. It's just about the longevity of the burner. 

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  • 2 years later...

I know I'm late to this thread but I would like to comment on turning off the forge. I have a ribbon burner and set a timer for 2 hours to turn off so I don't burn out the blower. I also cast a preheat wall with a small diameter hole so heating up takes less time, and put it back on at the end of the day to prevent rapid cooling on the face of the burner. However, my forge is also used for blowing glass so that's why I cast the front. Some fire bricks in front will do just fine for forges. The reason I do this is to prevent thermal shock inside the cooler burner head that can lead to cracks between each port. Hope this helps!


Also best thread I've seen in a forge construction 

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Welcome aboard Funstein, glad to have you. If you'll put your general location in the header you might be surprised how many of the gang live within visiting distance.

What kind of refractory are you using for your burner blocks? I used Kastolite 30 a high alumina bubble refractory and see no signs of cracks after my first burner. This is a brief recounting of how and what I built. I did a lot of reading on glass kiln burner sites so I knew naturally aspirated multiple outlet burners were commonly available before I started messing around.

Frosty The Lucky.

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