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I Forge Iron

Planning a new forge - comments and suggestions welcome


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Hi All. (warning, long read)

My friend and I are planning our next forge.  He works at a pulp mill and has picked up some pieces that we thought would make a fine, tough (but heavy) shell.  They are solid steel (mild, I think) U-shaped ... things.  The plan is to either use them as is for the front and back skeleton of the shell and weld sheet steel to the outside.  The inside will be layered with ceramic blanket insulation and refractory cement.  The whole thing will make an upside down U shape, and we'll use hard kiln brick for a floor. The brick pile forge floor we have been using is wearing well, and I like the idea of being able to lift the shell and replace the brick, since we have plenty now, with any that get too worn out from thermal cycling and/or flux etc.

These steel things are pretty big.  They are about 2" thick, and the "wall" thickness of them is about 3.5 inches.  The full radius of the semi-circle part at the top is about 6", so the inside radius (the space that would remain after insulation) is about 2.5"

After some thinking I realized that those steel skeleton parts are going to cunduct heat to the outside quite a bit.  Do you think a 1/2" of refractory cement on them will do enough insulation, or do we need to consider narrowing the inside cavity and put 1" thick kaowool over the entire interior including the skeleton pieces and then put refractory cement over that?  They will only be the front and rear "wall" of the forge.

We are considering a ribbon burner, or dual ribbon burner setup, possibly two 6" ones about 3/4 of the way up, where the radiused part of the shape is.  We figured that if we did 2 6" burners, on opposite sides and overlapping by about an inch, the burners don't come too close to the outside, with a length of 20".  

The legs of the skeleton pieces, from the radiused part downwards is 7".  That seems really tall to me, so we have discussed cutting them down to 5" tall.  That gives us a forge front height of 9.5 inches to the top of the curved roof.  The width of the opening is 7.5". (With the thickness of them, that means the entire front face would be 14.5" wide.)

My volume calculations @ 20" long tell me we'll have ~1246 cubic inches of internal space with the 7" legs.  With the 5" legs that brings us down to about ~946 cubic inches of heated volume.  I've done a pretty detailed spreadsheet to calculate the volume of internal space, the volume we'll have to insulate with wool and refractory, and the approximate BTU we will need to reach "forge welding" temperature.  I've used info I found on an old thread here (maybe from Frosty) that suggests 450BTU/Cubic inch is the minimum, and 540BTU/Cubic inch is the ideal ratio to achieve the temperatures we're looking for.  That means, using the above measurements, I think we want about 673000BTU for the taller size, or 511000BTU for the shorter size.

If anyone is a math wiz, I'd love to have someone look at my sheet and make sure I'm calculationing correctly.  Math and I have never been fast friends. Anyone with the link can comment. Link is: https://docs.google.com/spreadsheets/d/18t1R3EFrTOLovLSqt2eC2y53TRge55txo_V6TnoVyKQ/edit?usp=sharing

The forge we have now is 4.5Hx7Wx27.5L  We have trouble forge welding in it, as it's set up with a single burner in the center.  That burner is a blown DIY pipe burner using a leaf-blower as the blower.  It's a 1" pipe burner and we run it about 5lbs for regular forging and about 9lbs when we attempt forge welds. ( I know that doesn't mean a whole lot, because of mixture/MIG tip diameter/relational measurements etc)  Because the heat is all in the center, it's hard to get a "short" heat on a portion of a piece.  I don't know if this is a classic problem with gas forges themselves, or with large space gas forges that are underpowered.  We can forge weld, it just takes lots of luck to get the right amount of heat in the right place, as all the heat is concentrated about 13" in the center.

My ideal goal would be to have a forge that allows us room to make oddly shaped things (I've forged ladder rungs, weird hooks/spirals for hangers and lots of other strange stuff that uses up all and more of the space we have currently), allows us to easily forge weld for various projects (including eventually 'damascus' billets) and will allow us to put things only a little way into the forge to get a "short" heat, ie, heating tong bits up without having to heat up the entire thing 10" down the reins.  I figure this will make the stress of keeping things from getting all bendy where you don't want it bendy easier.  (We also make *a ton* of scale, again, this is something I want to understand better and improve)

I'll post another discussion with pictures of the current forge to see what suggestions people have for making some of our challenges easier with what we have until we decide on a concrete plan, source the parts and actually get a chance to build it.  This project likely won't start in earnest until the summertime, it's cold now and I don't want to try and cure refractory etc. in sub-zero (celcius) temperatures.

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1 hour ago, JCloss said:

After some thinking I realized that those steel skeleton parts are going to cunduct heat to the outside quite a bit.  Do you think a 1/2" of refractory cement on them will do enough insulation, or do we need to consider narrowing the inside cavity and put 1" thick kaowool over the entire interior including the skeleton pieces and then put refractory cement over that?  They will only be the front and rear "wall" of the forge.

By the time conductive heat reaches the outside of insulation, metal thickness or type simply doesn't matter.

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I'm not sure I understand...or I do, but maybe I'm explaining incorrectly. 

Our original plan was to use those thick metal pieces to make the front and back wall, and sheet welded to the outside of them.  The gap would be filled with insulation/refractory, and we weren't going to cover the metal on the U shapes making the front and back wall. Then I realized the metal will get really hot, so I thought we should cover those metal pieces with refractory cement on the inside.  Would 1/2" of refractory be enough to keep them from conducting a ridiculous amount of heat outward and making the outside of the forge glow, or would we need to narrow the front and back so that we can put 1" or more of wool over the inside of them and cover that with refractory cement?  I still don't know if I'm explaining well enough, maybe I need to draw it. ;)

The difference between the "skeleton" end pieces and the outside is about 3.5 inches, so we were planning on 2" of wool and 1/2" of refractory.  That still leaves an inch of bare metal, so we were going to add refractory to cover that.  When I said:

"do we need to consider narrowing the inside cavity and put 1" thick kaowool over the entire interior including the skeleton pieces"

I meant making the wool thicker throughout so that theres at least 1" over those end pieces, and then 1/2" of refractory over that, which narrows the opening and makes it actually smaller than the current forge we have now.  I just don't know how much insulation we need and of what types, to keep those metal pieces from making the skin of the forge do things we don't want it to do, like glow. ;)

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You need to understand the difference between refractory cement and castable refractory. Cement is designed to stick/mortar bricks together and is not used as a flame face insulation. While castable refractory will withstand the flame face it is not very good insulation like fiber blanket (K-O-Wool) which needs to be rigidized and coated with castable refractory, to prevent wool fibers from becoming airborne causing health problems.

I strongly recommend reading through Forges 101 and Insulation and Refractories.

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2 hours ago, Irondragon ForgeClay Works said:

You need to understand the difference between refractory cement and castable refractory.

Sorry, my bad for using the improper terminology.  I have been reading through those two forums, mostly the pinned discussions.  I do understand the difference between the two different types of refractory.  I accidentally used the term cement when what I meant was castable refractory. (I haven't found refractory cement used much in gas forges in my research, usually just in lower temperature applications like pizza ovens.)  I should have been more aware of the terminology I was employing. 

I understood castable refractory to perhaps be a better insulator than it is, I thought it was used as an insulator as well as being used to coat ceramic blanket insulation (which I do know needs to be rigidized and coated and am cognizent of the health concerns).  I guess I was hoping that covering those metal parts that are at the front and back inside of the forge with *just* castable refractory when we do the interior would be enough to mitigate enough of the heat transfer outwards to not have to narrow the opening further with ceramic blanket insulation.

1 hour ago, Mikey98118 said:

What I;m saying is that the metal shell ONLY provides physical protection; it does almost nothing temperature wise, You could employ chicken wire, and it would do no less.

I totally understand that.  What I'm not explaining clearly enough, I think, is how the pieces of metal that form the outer ends of the forge would go through into the internal space because they are so big/thick, and thus transfer heat to the outside of the forge if they aren't protected, becuase they are both the inside *and* the outside - and then how much insulation I'll need to realistically think about applying to those pieces to keep that from happening.

I'm going to see if I can make a diagram that helps ask the question I have.

Edited by Mod30
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My sketching skills are not good, but this is what I cobbled together.  Hopefully that helps illustrate my purpose.

Exactly!  Thanks Thomas.  I've since come to the conclusion that some insulation will be needed and am trying to figure out how much will be needed, and as such, how much I'll need to shrink the opening.  

Perhaps this forge design is not going to work as well.  It's kind of my buddies baby though, I'm just trying to be the realist where possible. (Like explaining that the bare steel structure inside the forge will conduct heat to the outside.)

A smaller forge may be the better option, or two forges.  We already have one burner that we could use for a smaller forge and probably reach consistent forge welding temperatures, the problem is space more than materials.

forge sketch.jpg

Edited by Mod30
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There are a lot of different types of castable refractory.  Some are just as tough as hard firebrick, and just as poor insulation.  Others are softer than insulating firebrick, prone to melting at forging temperatures and prone to dissolve when in contact with molten flux.  You need to choose a compromise castable refractory insulation with high alumina content that is rated for the anticipated temperatures inside your forge (i.e. Kastolite 30).

The steel plate ends of the forge will reflect the radiant heat of the forge interior, but will be massive heat sinks as well as heat bridges to the forge exterior (this will lead to significant waste of gas, especially at forge welding temperatures).  Mike is correct that there is no good reason to use such thick steel for a forge skin. the heat loss through the steel plates is proportional to the surface area of those plates exposed to the forge temperature.  If you have such a thick frame you are essentially extending your forge 2' on each end of the forge without any insulation.  2" plate will hold up for a long time, but may be prone to warping and certainly will lose heat.  The forge extension will make it more difficult to direct the heat where you want it on your stock.

You can try to cast some refractory insulation over the inner face of these steel forms and/or the door openings, but you will have two issues.  First the castable will likely have to be at least 2" thick to provide effective insulation without an additional layer of refractory ceramic blanket.  Second the differing rate of thermal expansion of the steel and castable will require some form of retention for the cast layer to avoid having it just split off.  This will be difficult to achieve using 2" thick steel.  If possible I would have at least 3/4" thickness of castable to protect the door openings and 2" thickness on the inner face of the front and rear of your forge if any of that is exposed.

Gas forges have drawbacks that solid fuel forges don't.  The key one is that there is a trade off between ability to spot heat, heat irregular shapes, and heat both large and small projects efficiently.  Many smiths end up with multiple gas forges for this reason.

Note: a hard brick floor with no surrounding insulation will be a big heat sink as well.  Might be good for a production forge, but not for a hobbyist.  Split hard bricks with an outer layer of insulation is a better idea.

You proposed forge is extremely long.  The only forge I've seen that size that was used on a regular basis was Albert Pailey's (look up the kind of work he did) and he didn't have to worry about making enough money from his work to be concerned about paying the gas bill.


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Thank you Latticino for a really comprehensive reply.

With the feedback I've gotten so far and the ideas that were floating around in my head already, I'm going to be recommending some changes to our plan.

The examples of what you would look for in thickness of insulation over the metal was exactly the sort of experienced tip I was looking for, and gives me something to go with for planning, or convincing to change plans.

The suggestion to account for differences in thermal expansion is also helpful.

Yes, I'm learning the drawbacks of gas forges vs solid fuel forges.  I would like to have access to a coal forge, but I don't at the moment, nor do I have an appropriate location to put one, so I'll have to make do.

Oh, and yes, DOORS!  Doors will happen no matter what, however they might be stacked firebrick for a few years. ;)


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This forge plan isn't really just mine - it's more the brainchild of my forging partner, but I have some input.  I'm coming here to get more experienced input, as I've successfully helped build 2 forges, together the members here have built hundreds at least.  I'd like to benefit from that knowledge.

To help clarify, I thought I would share the shapes that he salvaged and wanted to use as the ends of the forge.  I'm coming to the conclusion that it's a bad idea, but I need to talk to him and have a brain storming session. His plan may still work if he was planning to round the two mouths, and in that case there would still be 2" of blanket and 1/2"+ castable refractory insulation over the end pieces, with just the "mouth" not covered. I'm still not convinced it won't create a really bad heat bridge though.


He's pretty convinced on this because he has been looking at these shapes for a year in the salvage yard and thinking about how they're perfect for him to turn them into a forge. However, I think I can convince him that we can use something like 10ga sheet cut on the plasma CNC that I have intermittent access to as the ends of the forge skeleton instead of these beefy things.  Makes the whole shell about 80lbs lighter too, and mostly eliminates the heat bridge to the outside as the 10ga would be more fully insulated.



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Soon as the ground un-freezes you can build a coal forge in less than 15 minutes for probably nothing!  (hole in the ground, pipe to bottom of the hole, hair dryer as air supply.)  The JABOD is actually a bit fancier and can be done with moistened cheap clay kitty litter rather than dirt.

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You aren't wrong about how simple it can be to build a coal forge.  I'd like to cobble together something in the intermediate zone - brake drum in a table sort of thing.  However, the shop we have access to isn't conducive to burning coal inside, and we don't have access to the outside of it - it's in a friend's yard in a residential area. 

I have great plans for a pole-barn on my property eventually, but that's a few years down the road as electrical will need to be run first. (Well, I *could* do without electrical, but I don't actually *want* to.). Previously our local antique machinery museum asked me if id be interested in spending some time at their coal forge, as they don't have a demonstrator anymore.  However, I don't want to learn to run a fire in front of spectators without practice, and then covid hit so they're shut down now.  Perhaps this summer/fall I'll be able to work out some access there.

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Now do folks use and junk propane grills up there like they do down here?  You can build a jabod in the grill, junking the propane parts.  It can stay outside and not scare off the neighbors if they see it stored next to the garage...I see them at the scrapyard on a regular basis and when I lived in town I'd see them thrown out regularly too.  Note if you use real chunk charcoal there is no smoke to annoy the neighbors.  Charcoal was the forge fuel until (in western Europe), they started to use coal in the High/Late Middle Ages, ("Cathedral Forge and Waterwheel" Gies & Gies), charcoal has remained the major forge fuel all over the world to this day!

(My first forge was made in an old farm dry sink using clay from the backyard and charcoal I made myself in Oklahoma City back in 1981.)

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I can think of a lot of possibilities for those big U-shapes, not least of which would be as a combination swage/fuller/bridge tool. Use one as a template/mold for a sheet metal gas forge shell, but incorporating them into the forge itself would be a waste.

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J: There are a number of corners folk paint themselves into regularly, especially breaking into a new craft. There's one that applies to most everything. Just because you have a thing does NOT mean you have to use it.

There is nothing about those U shaped hunks of steel appropriate to building a forge. Nothing. If I had one I'd be wondering how to wrap a few thousand winds of rotor wire around it to make a junk lifter for a pick up truck hoist so I could salvage stuff without using my back. Otherwise I'd class them as "leaverights." That's, "Leave er right there." 

Even 10 ga. is excessive for a forge shell, mine are 14ga. and they're WAY heavier than I'd like but that's what I had on hand. If I do this again I might buy a sheet of 18ga. All the shell does is contain the refractory and fire. The legs only need to support the weight of the forge, fire and your work call it  50lbs. maybe. Stove pipe is heaver than necessary. 

The other general BAD corner to paint yourself into is. Designing tools or equipment you have no experience using. Oh we all do it, I've never seen a tool, machine or heck anything man made I haven't automatically started redesigning in my head. It's human nature but it's a good one to resist, especially when there are working things out there. Once you've use a thing a while you MIGHT be able to make modifications that ARE improvements. Honest, we ALL do it and it's not easy to resist.

How abut sending your friend here so he can look at PROVEN propane forge designs and ask the people who've developed them for feed back. We'll be gentle, honest.


20 minutes ago, ThomasPowers said:

Forge a large steel croquet mallet and some steel croquet balls?

Would you use a 100lb. axe to trim the weeds?

Frosty The Lucky.

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The advice here has been helpful folks.  The new plan (better than the old plan, but still rough) is to trace the shape onto some thin steel, shorten the height and shorten the overall length.  The floor will be ceramic blanket insulation with castable refractory insulation (we're debating Mizzou for this vs Kast-O-Lite) instead of replaceable brick and the internal insulation will be 2 1" thick ceramic blankets, rigidized and coated with about 1/2" castable refractory insulation. (This will probably be Kast-O-Lite 30?)  We're still wanting to do a multi-port (ribbon) burner or two, blown not NA at this point but I'm still researching burners in the appropriate threads.

I'll continue to update this as we continue to plan.  Right now the dimensions look like they'll be 16" long vs the 20" we were originally planning (we have 27" length now), the D-shaped opening likely to be 4" vertical plus 2.5" diameter and 7.5" wide.  (I'm still trying to convince my friend that the cubic inches saved going to 4+2 high, by 6 wide is worth it.)  Essentially, I wanna build it smaller and he wants to build it bigger. ;)  

He is very much against any exposed ceramic wool for obvious safety reasons, which I am too.  He's seen a lot of pictures on the internet of bottle forges etc that don't have the wool properly covered with castable refractory and for that reason has the idea that that's the only way they turn out.  I'm still trying to convince him that ours doesn't have to be like that. 

We haven't debated the floor completely.  He would like to do Mizzou (spelling?) as that's what we're planning on getting to cast the burner(s) out of, and we understand it's wear/impact resistance is better than kast-o-lite.  I've been reading about high alumina kiln shelves, and so will be doing more research in that direction.  I think either will be better than hard firebrick.  (I did like the idea of being able to pop worn firebricks out and replace them over time though, any reasons for keeping that idea that I'm missing?)

We have wanted to do a re-emissive coating in our current brick pile forge for a while now but haven't been able to get hold of any ITC-100.  Mikey pointed me toward some other options, so our new forge, when eventually built, may have a further re-emissive coating applied to the inside. (Although we're not likely to get fancy and mix it into the castable refractory insulation like some people around here have tried that are both smarter and more experienced than me)

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2 hours ago, JCloss said:

Essentially, I wanna build it smaller and he wants to build it bigger

Okay, I'm a big fan of smaller is better, but...bigger isn't so bad with a ribbon burner as with any other type. Plus, bigger is more useful if some height is lost, and replaced with floor width; especially as this allows a top facing ribbon burner at one side of the floor. Just some thoughts to keep in mind.

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