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Forges 101


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after reading through this section for a good two hours or so, I have settled on a few things that would best help me build my forge. 1. Using a helium or freon tank for the optimum size. 2. 2 rigidized 1" layers of kaowool or similar substance for the first layer, followed by a 1/4"? thick layer of castable refractory, then a 1/4"? thick layer of ITC. Some things I'm still wondering about is what size burner to use in this setup, and whether I should try to make it able to stand up to put a crucible into for casting, or if I should just build a foundry separate from my forge. Also, How do you decide how big the vent hole in the door should be?

Edited by Ernest_U
forgot to type "rigidized"
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Why small burners anyway?

Below is the introduction to a book I'm writing:

Why construct miniature burners at all? There aren’t many people wanting a miniature forge or casting furnace. But there are plenty of people who want to get more heat from air-fuel hand torches than are available in commercial models. In the face of rising energy prices, many more are looking to maximize efficiency.

    Naturally aspirated burners have large turn-down ranges. So, it would seem that a wide selection of burner sizes isn’t needed to accommodate heating equipment. But fame efficacy is about more than how well fuel burns.

    There is another factor to get control of, and that’s the flame’s exhaust path. It’s easy to see what goes wrong when there is too much or too little flame. It is harder to appreciate what goes right when you get superb control of the equipment’s exhaust velocity. That's because doing so involves balancing two conflicting combustion issues; they are flame and exhaust speeds. Fast flames burn hot, but a fast exhaust wastes heat.

    The reason burners are aimed on a tangent, whenever possible, is to cause their combustion gasses to swirl around equipment interiors, creating a longer distance from flame to exit. A longer exhaust path increases the amount of "hang time” for flame heat to be deposited on equipment surfaces. That seems obvious doesn't it?

    What isn't so clear is that most of that increased time isn't made by the gases running a little farther at a given velocity; it’s provided through a considerable drop in velocity over that added distance. A bunch of little flames will decelerate much faster than a single large flame.

    The smaller flames of a pair of 1/2" burners will drop velocity faster than a single 3/4" burner on a five-gallon forge, greatly increasing efficiency; because they can be turned up faster/hotter without creating a tongue of fire out the exhaust port. But, what about the guy who wants to build a two-gallon knife maker's forge? He is going to need two 3/8" burners to do the same trick. Someone who wants to forge hand tools in a one-gallon forge is going to need two 1/4" burners to run it with maximum proficiency; these same figures hold true in casting furnaces. The law of diminishing returns makes maximum fuel savings a minor issue in miniature equipment, but time savings and portability remain significant advantages.

Of course the other side of the coin is ribbon burners, about which I'm not qualified to speak. Frosty and others should feel free to jump in here, and complete the discussion :)


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Hello Ernest.  A couple of big points first.  Do not use 1/4 inch of ITC.  It is to be painted on in thin layers.  Maybe 1/16 inch total.  

ITC does not work well with a forge. Most here recommend Plistex or Matrikote.  Both are less expensive than ITC and work better.  I prefer Plistex because it has higher temperature ratings.  

You can make a dual purpose forge/foundry but most don't recommend it.  Forges and foundries have different operating requirements so a dual purpose will have to compromise on one or both functions.

My first forge was a freon cylinder mini forge.  I made it full length (12") with a 3/4 inch modified sidearm burner.  The 9 inch cylinder, minus 5 for insulation/refractory yields a 4 inch diameter by 12 inch long chamber.  In my case, with a central burner.  I hated it.  In order for metal to be in the hottest zone, 6 inches of metal had to be in the forge.  The section I wanted to hold was always hot.  If it was in the middle of the stock, 12 inches of metal was being cooked to heat the section I wanted heated.  

If I were to build it today and wanted to use a freon cylinder, I would cut the length to 7 inches and use a 1/2 inch burner.  Less fuel used without loss in much useable forge volume, less over cooked metal, less hot parent stock to hold onto.  

As to the vent hole size, as Mikey states, variability is great.  Movable baffle walls are very versatile and accommodating.



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If you've already paid for it then mix the ITC about the consistency of latex paint and apply it in a thin coat. If you can buy Plistex or Matrikote both are better products for a forge. A single burner forge wants to be reasonably monodimensional, same measurement in all 3 dimensions. Long and narrow like Mr. Frankenburner describes is pretty useless except for special cases. 

Frosty The Lucky.

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So I finished a forge that was a bit of a qualified success and have already been thinking putting what I learned to use building something slightly larger and a bit more refined.

I have been reading and re-reading through parts of the Forges 101 thread today and I had a question about a forge design I saw. There are a couple instances that someone has built a flat-bottomed, half-round forge where the burner port came up through the bottom of the floor, adjacent to where the arch wall meets the floor, and at the back of the forge. 

In looking at it the thoughts that pop into my head are:

- Within reason, half-round/oval shaped forges tend to maximize floor space while reducing excess "open" volume... at least as compared to rectilinear forge bodies (Thanks Frosty:D)

- The burner location induces the flame to follow a path along the ceiling and create a swirling atmosphere that may increase flame/hot gas hang time.

- As a corollary, I want to believe that such an arrangement would lend itself a less turbulent environment and as a result create less back pressure than if the flame were to impinge on a wall in a perpendicular fashion.

- Burning propane creates a chemical active environment and locating the burner in such a manner would make it difficult to use any replaceable materials (kiln shelf) to protect the refractory at the point where the flame impacts the wall.

- There might be some logistics to navigate in mounting a forge to a stand when the burner is projecting out the bottom.

I don't want to bash anyone's efforts, because I think it is a really neat design..... especially because swirling hot flames/gases are cool to watch. (Don't judge, I'm a prescribed fire junkie and any day is a fun day when you can watch fire whirls from a safe distance). I was hoping for a bit better understanding towards any additional pros and cons I might have missed in such a design.


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I call the flat floored cylindricals a "Vaulted" forge others call them mail box shaped. I think Vault sounds better and is architecturally accurate. 

Forges with burners firing up from the floor work pretty well, one of the guys in our club has been building them since I showed him how to make a T burner. They work really well and a good kiln wash, Plistex, takes care of the flame impingement issue. What surprised me was how little fell in his burners. I figured they'd be magnets for any loose item in the forge just because Murphy's law says it will. 

I think all his forges are up drafts now. 

You're over thinking the turbulence it's not an issue normally and if back pressure is an issue you'll know by it's tuning characteristics.

So you have to build it a stable stand 10"-12" tall, or make a hole in the table. You're a blacksmith, making holes in things is a problem? Were you a driller I'd be mocking you in public, even calling drillers I swore I'd never speak to again. :lol:  :lol:  :lol:  :lol:  :lol: !!

Frosty The Lucky.

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LOL, never been a driller, well a well driller anyway.... worked with a bunch as a project scientist for a chemical remediation company straight out of college.... I was a pretty sheltered kid and it was an fun education working with them, especially since I was technically supposed to be in charge. Kinda the rougher version of a staff sargent dealing with a lieutenant straight out of OCS.

You're not the first person to accuse me of overthinking.... it's a blessing and curse:lol:

Not a blacksmith yet... more dilettante at the moment but interestingly enough when I bought a 2nd hand planer it came with a beast of a stand that is perfect for supporting an overhanging top. Things to think about definitely... just would need to not get caught up in watching the flame swirl in the forge without actually doing anything.

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I didn't drill wells, we took soil samples, set instruments and did infield tests. You would've been the person taking samples and logging the holes. Puts you in charge of where to drill and when to move on.  Ayup, we did a couple of those kinds of jobs, though we were more interested in foundations for bridges and the like.

I don't think I've ever met a blacksmith that didn't like fire. Maybe bring out a comfy chair side table for snacks and beverage, turn on some music and watch the flames swirl in your forge. Doesn't sound like a bad thing to do, especially if the honey do list is too long.

Look how many times John has posted pics of the flame swirling in his forge. 

Frosty The Lucky.

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22 hours ago, Another FrankenBurner said:

I have a good swirl picture.  There's a little green in my flame. 

Aside from drooling over the pretty (rich), vortical forge atmosphere, I was wondering if you were using wool in that forge or is it a solid cast of refractory?

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The tubing in that photo is copper.  :D 

Copper tubing in a cold forge is spectacular for a minute.  I was annealing copper tubing to flatten out and make soft jaw covers for the vices.  


Welcome Panik.  Overthink away.  You are in good company.

The forge does have ceramic blanket.  Solid refractory would not be nice to the fuel mileage and I want as many miles as I can get.  All of my forges are blanket, Kast-o-lite and Plistex any more.  This particular forge was an experiment.  It is a monolithic spool shape.  Forge chamber with flanged ends.  Blanket wrapped around/in the spool.  As you see in the picture, the ends cracked which then travelled into the forge.  We half expected it with the mono cast.  We have another experiment in the works to solve that.

This forge was built a while ago and I had forgotten Frosty's advice of buttering the refractory before applying Matrikote.  What happened... the dry refractory wicked the moisture from the Matrikote.  Frosty got to be right again.  If you look at that picture, you can see flakes of the ceiling Matrikote on the floor.  Morals of the story, butter the refractory and listen to Frosty.  I've also since switched to Plistex but not because of this.  It's less expensive and has a higher temperature rating.  

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Hello all

I just wanted to post an update from the burners 101 thread to here considering I'm finished the forge... Well... Sorta.. 

I had some large set backs and made stupid mistakes, it runs great (I think..) but the longevity of the inside castable wall is definitely in question.. 


So this is after 10 minutes of warm up at 15/20 psi just to get up to temp and then it will run at about 8 psi when the burners begin to slightly jug.. I get a bit of yellow dragons breath if I'm running at 8 psi but alot less if I run it at 12 and up which doesn't make sense to me

Frosty, I took your advice and filled the void on the bottom floor, I used hard fire brick but I still used 3 inches of the kaowool in the main body and just 2 inches on the door (there will eventually be a door at the front with a small cut out) 


This is after 15 minutes of both burners running full out at 20 psi, I'm pretty sure it's welding heat but because of my lack of experience I'm not close to testing that theory by actually welding, once it's at this heat I can maintain it with one of my 1 inch burners at 8 psi no problem. 20200528_233816.thumb.jpg.fe18c388fab582356409806b4ddee467.jpg

This is the same but just a differant shot with the back door closed and a brick in the way to impede heat loss. 

Here is where I hit a bit of snag... So when applying my vesuvius refactory it would not spread easily at all and even with the blanket rigidize it started to take it apart on application  think hard butter on cold toast (I was also using a gloved hand as I misplaced my trowel, I should have just bought one but I was impatient) so I had the GREAT :wacko: idea of mixing the refactory with a bit of water to make it easier to spread, it worked but here is a picture where you can clearly see the refactory bubbling in many spots 20200523_131126.thumb.jpg.26580ebdf7ef788be4619f57097618de.jpg

Then there is all the cracking inside which I assume is from uneven application (tried to stay at 1/4" thick but it was tricky) as well as the added mix of water, and now the whole thing looks like tectonic plates covered in bubbles... 20200528_233343.thumb.jpg.c7dc559b367490e2947cd03eb9f3031b.jpg

Here you can clearly see the 1/8" or more wide cracking. This picture is also taken 15 to 20 minutes after shut down so it retains its heat well (I think) 

So.. What do you all think? 

Should I just trowel over the cracks (found my trowel) with the non water mixed refactory and that should do or would someone recommend I pull each "tectonic plate" off inside until the first layer of kaowool is completly exposed and apply a whole new layer? 

Any advice and criticism is greatly appreciated!

Ps.. I know it's still too big :ph34r:


Phil the Welder 

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Why haven't you tried welding with it Phil? It's not magic, keep it simple to start. File or sand two pieces of square stock shiny clean and wire or tack weld them together. I like to dust the joint with a LITTLE borax/flux before heating but lots of guys do fine bringing the joint to orange then fluxing. Anyway, bring the test joint to high yellow bring it out and quickly apply a couple firm dead blows to set the weld. brush flux and repeat. 

I don't know if mixing water with the Vesuvius  was a mistake I'm not familiar with it at all. How long did you let it dry before firing it? I'm betting those are steam blisters but I'm not there to look at it.

Before you do anything else to the forge experiment with the Vesuvius. It won't take much, maybe 1/4 cup and add a little water, let it dry a day or two and fire it Dull red first, then hotter. Keep notes and keep us in the loop please. 

So long as it works as it is I wouldn't try repairing or modifying anything until I tested the refractory to see if you can thin it. I've only used "plastic" (that's pre mixed) refractory once. It was awesome refractory but a holy terror to work. I cast the inner liner between Sono tubes for about 3/4" thick 10" long and 8" ID. I had to use a single jack sledge on a piece of 1/2" square to get the stuff to the bottom of the mold. I WISH I'd experimented with adding water but I didn't have anything to test in and only thought and wished. I still have that forge and it's still bullet proof though I don't use cylindrical forges anymore. It's there just in case, dusty but there.

 Frosty The Lucky.

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Frosty, to be honest it's a couple of things, I've only had the forge running for 6 hours total so I'm just beginning to learn hammer techniques and get that better, and I'm making tools I know I will need like center punches and chisels. Lastly I guess I don't plan on doing any welding with it for at least a year until I have a firm grasp on the basics, that being said I understand the principles of how to do the welding and the steps, so I guess It wouldn't hurt to try just for the hell of it.. Just so I know it will do it. I have to get my hands on some inexpensive flux and I have no clue where to start...online? Farrier supply store that is up the road maybe? I don't see them needing flux though. 

About the vesuvius, I tried contacting the manafacturer but they were impossible to get them to call back and I couldn't find any guides online. I let it sit for an entire week before firing it.. But half way through the week a storm drove rain under my sliding shop door and soaked the one end of the Forge, so I peeled all that off of the end and reapplied the face,(it was so saturated with water that when it dried it was a really weird consistency that I didn't trust) then let it sit for another week, so technically the inner tube sat for 2 weeks

For testing, should I mix it in say a paper cup, then once dried rip all the cup off and throw it in the forge?

That sounded like alot of work, but I can just imagine how strong the finished product would be, definitely bullet proof! 

On  the plus side at least I know my 1 inch burners were more than enough for my forge volume :D

Just a pic of the chisel I made, my first tool I hammerd to 90% shape and then ground for finish, it started out as a little cut off of a RRR 1.5" square by 3.5" long



Phil the Welder

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Yeah, not wanting to try welding until you're good enough is EXACTLY why I make a point of showing folks how to weld during their first session. It's surprisingly easy IF you follow the steps.  Don't think you need "forge" welding flux, that stuff is too expensive and since EPA outlawed some of the ingredients for pretty sound reasons, the "real" stuff isn't any better than most any powder welding/brazing.etc. flux available at the local welding supply. I use Peterson's Blue, #2 I think, maybe #1. It's in the blue can and does NOT contain iron oxide powder. Lots of good forge welding fluxes contain black iron oxide so it may not matter.

The things to really watch for are on the label, Boric acid and Borax, those are the two most important ingredients. Don't worry if it doesn't "SAY" anhydrous borax, it will be or it wouldn't work very well and nobody would buy it.

When I bought my first (current) 1 lb. pint can of Peterson's a 1 lb. can of REAL forge welding flux by whatever brand was running over $100/lb Plus shipping. The Peterson's was on the shelf for $26.xx, shipping included. It works as well as any I've seen that didn't include some darned toxic fluoride compounds. 

 Frosty The Lucky.

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