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Understanding the inside of a forge. Insulation vs reflection


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In researching for a fairly small single burner forced air forge i have come across firebricks, kaowool, refractory cement and other things like plistix and mizzou (which may be types refractory cement … I’m not sure without looking it up). Anyway.. here’s what I’m trying to understand ..


What causes the jnside of a forge to get so hot?


1. Do Firebricks insulate? And I’ve seen firebrick only forges. Does the outside of the firebrick get hot? Is that a good thing? Does the firebrick just keep getting hotter and hotter until it reaches >2000F while excess heat bleeds off the outside (shell area)?


2. on the other hand I’ve seen kaowool painted with refractory. So .. instead of heating up to temp like the firebrick, it reflects the heat thereby building up temp without really having to heat up a sink. 

3. What about casting the entire forge from the refractory cement (or whatever you would cast a ribbon burner from) itself? 

So is the forge heating up due to the heat sink eventually coming up to temp, or due to reflected heat? Or both?? Then would it be best to use a double layer of firebrick (to prevent heat loss to the outside of the bricks get hot) coated with refractory cement on the inside? Or wrapping the outside of the bricks in kaowool? 

just trying to get a better grasp on the fundamental thermal workings here. In the three examples i used above, I’ve read people doing it in various ways. There’s LOTS AND LOTS of instructions telling me HOW, but very little telling me WHY. 

Edited by Nicholai
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Forges work by radiant heat.  With solid fuels like coal, the steel is in contact with the burning coke, getting mostly radiant heat with a little conduction. With gas, the idea is to contain the heat of combustion as efficiently as possible, which usually means an enclosed space. Insulation helps efficiency since it reflects heat, or at least keeps it from dissipating.  


Hard firebrick is a heat sink, and does get very hot indeed.  They rarely (the thick ones, anyway) get hot enough to glow on the outside since clay is such an inefficient conductor of heat, but they heat up very slowly (thus using fuel) and stay hot for hours after shutdown, wasting the heat you burned fuel to get the brick hot in the first place. Extremely durable, at least the thick ones. Also very resistant to flux.


Soft firebrick, aka insulating firebrick, is as advertised.  Soft enough to shape with hand tools, insulating enough that the outside doesn't get hot for a long time. Not as efficient as ceramic wool, but more durable.  Comes in several grades of differeng efficiency and heat resistance. Not resistant to flux.


Ceramic wool is a great insulator, but once fired it's brittle and very easily damaged. No resistance to flux whatsoever.  


Castable and rammable refractory cements some in a variety of insulating and heat-reflecting properties.  Mizzou is the equivalent of hard firebrick, but much tougher. Flux-proof. Kast-o-lite is more like soft firebrick in that it insulates quite well, and it is also fluxproof, and just as durable as Mizzou.  It does come in several temperature ratings, though, as Tim noted in another post, so pay attention.  


Refractory mortars like Sairset and Satanite are easier to use, since you just paint it on.  Not as durable as castable/rammable, fairly flux resistant if not cracked. It is possible to accidentally jam the point of a blade through a mortar lining that's less than 3/8" thick.


IR coatings like ITC-100 and Plistix are ceramic coatings that actually do reflect heat, raising forge temps by anywhere from 100 to 250 degrees F hotter than the forge without the coatings.  Not durable enough to be a lining alone, somewhat flux resistant. Expensive.


The way I see it, it's a tradeoff between speed of heating up and durability, and where you want to be on that continuum determines what you need in your forge.


The example I like is to look at industrial kilns.  Those used for hard duty (abrasive use, impact possibility) are solid firebrick or castable refractory cement depending on the process temperature, kaowool outside if needed for worker/facility safety. Why? If you run 24/7, durability is key. The solid refractories retain heat, thus being more fuel efficient to run (once hot) until a relining becomes necessary.  Those industrial ones used in a less abusive environment with the potential for intermittent operation (i.e. not 24/7, low risk of puncture/abrasion) are mostly kaowool, usually not lined with anything, often sewn into an accordion fold to gain thickness.  If the furnace isn't going to run more than an 8 to 10 hour day, it needs to heat up fast, use little fuel, and not be a fire hazard if left unattended overnight. 


If I were a full-time maker producing loads of damascus on a commercial scale, I'd go with a solid castable forge lining, preferably an insulating one, maybe with a layer of wool around the outside and an air-gap heat shield.  Castable because it's durable and fuel efficient once up to heat, preferably insulating because it heats up faster, wool outside if it's getting the shop too hot, heat shield (aluminum flashing held about an inch off the surface of the forge) to minimize heat risks after shutdown. Maybe an IR reflective coating, especially if I were welding without flux exclusively.


If I were a weekend hobbyist who didn't want to weld often, but just liked to bang on hot steel for an hour or two on Saturdays, I'd go with a kaowool forge lined with 1/4" of Satanite.  Cheap, easy to patch, the mortar protects the wool for slight accidental pokes and snags.


In between those extremes, two inches of wool with an inch of castable in the floor, half an inch elsewhere, IR-reflective coating probably.


Personally, right now I'm still mostly burning coal because solid fuel is great for axes.  My gas forge is made from two soft firebricks set in a steel frame and mostly hollowed out.  With a 1" venturi burner it gets to forging heat in two minutes, welding in five, runs for hours on very little gas, and cools off in an hour or two. It's tiny, but big enough to do any size blade up to about 2 inches wide, length unimportant but the hot spot is only three inches long.  Great for small blades, hooks, small ornamental thingys, and quick jobs where I don't want to fire up the coal forge, wait ten (twenty if no leftover coke) minutes for it to be ready for forging.  Like when someone's kid asks to see me forge a hook/leaf/steak flipper/other quick project and I don't have the coal forge running anyway, out comes the gasser. Light, talk for two minutes, forge, turn it off and walk away.


When or if I go to mostly gas, I'll be in the intermediate range above, with a blown burner. For one forge, anyway. 




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Wow man thanks for taking the time to pen that. Lots and lots of great information too, right there in one place. Now i understand why wool->refractory->IR is such a popular method! I originally intended to make the forge out of soft firebrick but there’s no real sense in doing it that way (for my purposes).. wool sounds like it is the way to go with maybe 1/2” Kaolite and I’m thinking a casted disposable floor “pan” make from more kaolite to protect the bottom. 

this was incredibly helpful for tying together all the bits of info I’ve been gathering. 

.. off topic now, but for a small forge (single burner) would a ribbon burner be of any real benefit over a regular forced air pipe? Any idea if there is a minimum volume of forge for a ribbon burner? .. it just looks like fun to make lol

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It's taken me enought time getting that figured out I like to save others the trouble. :lol:


As for a ribbon burner, I don't know how small a volume they like, but a plain blown burner acts a lot like a ribbon burner if you squash the end of the tube into an oval. If using big pipe.  Like a flame spreader on a plumber's torch.

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Yeah, great write-up and synopsis, Alan.  Sticky this somewhere?

A few thoughts from my experiences, both with my forge and using other designs.

1 hour ago, Alan Longmire said:

In between those extremes, two inches of wool with an inch of castable in the floor, half an inch elsewhere,

This is probably as close to an ideal forge set-up as you can get, for anywhere from full-time shop use to any forging beyond the "couple hours a couple times a month" crew.  When not abused, this type of build can last many, many years with minimal maintenance.

I went big on my first forge build 11+ years ago, a 18"dia pipe with 1/4" walls and 2" of castable refractory.  It takes a while to get to welding heat, but because of all the thermal mass, it holds the heat well.  

1 hour ago, Alan Longmire said:

heat shield (aluminum flashing held about an inch off the surface of the forge) to minimize heat risks after shutdown.

Not sure if this is necessary.  After a full day of running, the outer shell is hot, but my anvil gets hotter by the end of the day sometimes.  You can hold your hand an inch off the shell and not get burned.   

RIP Bear....be free!


as always

peace and love




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On 6/9/2021 at 10:46 PM, billyO said:

Sticky this somewhere?


"The way we win matters" (Ender Wiggins) Orson Scott Card


Nos qui libertate donati nescimus quid constat

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56 minutes ago, Alan Longmire said:

Thanks for the vote, guys.  I'll put it in the Tools and Toolmaking section, there's more than enough pinned topics in the Beginner's Place.

And I’m just ecstatic that i asked a good question and you took the time to answer. :) 

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  • 4 weeks later...

A note of caution:

I once bought a small forge body that I was to line myself. It was only 8" or so inner diameter. Unlined. I lined it with two inches of kaowool and sealed it with rigidizer, and then I put some satanite on it. 


The opening was about 4" when I was done. It never worked well, because there wasn't enough volume inside to trap the gasses/heat. You don't want to overdo this idea, but you do need a certain amount of volume to make sure you trap the heat in a gas forge. I am finishing one now from an old propane bottle with a kast-o-lite floor and 2" of kaowool with colloidal silica rigidizer. I may put some ITC on it to reflect more, but maybe not. The soaking of colloidal silica has gotten the wool pretty rigid. I did the wool in two layers, and soaked each.


Shape the inlet for the gas like a cone with the big end pointed into the forge chamber.


Alan, thanks for putting all of that in one place.

please visit my website http://www.professorsforge.com/


“Years ago I recognized my kinship with all living things, and I made up my mind that I was not one bit better than the meanest on the earth. I said then and I say now, that while there is a lower class, I am in it; while there is a criminal element, I am of it; while there is a soul in prison, I am not free.” E. V. Debs

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  • 2 years later...

Hello everyone. Thanks for the extensive info. you guys are awesome.


I have limitiations on what I can find locally so I had to imporvise. I built my forge from refractory cement (alumina silicate) and perlite with a t burner which works fine. not the most efficient but still does the job for the knives I make.


Now I need to build an electric heat treat oven. 

My question is, has anyone built a Forge / Oven using perlite mix? wondering on how efficient it is as a material (perlite with cement) vs just refrectory cement cast.

I cannot find soft firebrick here, and my first attempt with clay fire brick is it takes too long to get up to temp. 


Thanks a lot. Kamil

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In my experience, heat treat ovens should be made of a lightweight refractory material rated for at least a couple hundred degrees above the maximum temperature you plan on using it at.  This can be ceramic board insulation (with proper rigidizing) or light-weight firebrick.  I wouldn't go with a home made castable refractory unless I had no other choice.  You might want to check at ceramic supply shops (I found two with a quick google search in Amman) or industrial refractory supply house that serves the HVAC or production industries.


Good luck

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Thanks Dan. I didn't consider ceramic board insulation . I know some are in boilers. gotta check the fire rating on those, I think it was 1250C which should be good for what I need. 

Thanks again. 

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I used (2) 1" layers of board insulation on the first heat treat ovens I constructed.  The inner layer rated for 2,300 deg. F and the outer layer for around 1,200 deg F.  Worked quite well, but the difficulty is building a "ceiling".  At temperature the board tends to sag a bit if not supported.  The industrial insulation place I got my board at had special stainless steel clips just for this purpose, and they provide a clamp between the oven frame and the internal board.  Might have lost a little heat through these, but worth it for keeping a ceiling intact.  Of course this depends on how large a span you build.  For a smaller oven it may not be an issue.


The other challenge for a board interior is supporting the electrical coils.  These do need to be supported every 6 inches, or so, or they will also sag under heat and if they touch each other they will burn out quickly.  It doesn't work well to cut channels in the board like you would in soft brick.  I have used a couple of different strategies:  pin coils into the board with nichrome wire "U" shaped pins (be careful not to pin all the way through to the metal oven skin), use ceramic coil holders from discarded electric duct heaters and pin same with nichrome wire through to the skin, use coils with high temperature coil supports from discarded ceramic kilns and mortar the supports in place.


Finally you need to deal with wiring same to power through a controlled contactor or SCR.  The coil pigtails have to go through the oven metal skin without any chance of contacting same and join robustly to high temperature rated wiring for the power supply downstream of the contactor or SCR.  Hopefully you have a plan for this.

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Hi Dan. Thanks for the advice. really appreciated.

I was thinking of using 2 layers joined by refrectory adhesive. the inner layer cut to create the channel for the coil. since the boards I found are 1 or 2cm thick I will use 2cm (rated to 1250C / 2280F), will probably add a blanket layer then the casing. gonna attempt using wood style joints hoping it will support the structure a bit more if I am able to cut it cleanly. 


My friend will be helping me set up the electrics as I'm not well versed at that, but yes, definately, safety first... will experiment with the material over the weekend and see how it works. 


Thanks again

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