Drewbacca

Ceramic blanket vs refractory

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I have a couple gallons of refractory but building a forge using ceramic blankets looks much easier. What are the pros and cons of each material? Would using layers of both be most effective? If i have an outer layer of ceramic blanket and an inner layer of refractory will the castable reftactory cast well against the blanket or will it look really dirty? Will it cause the wool on the bottom to compress over time and if so how does one combat that?

PS
Anyone in the chicagoland area have 5 feet of durablanket they would be willing to sell to me on the cheap.

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It's the difference between concrete and fiberglass insulation. Like concrete, castable refractory is strong, and very resistant to damage, but a rotten insulator. Like fiberglass, ceramic fiber is an excellent insulator but very fragile. A forge made entirely of ceramic fiber will be very fuel efficient, will heat very fast and will get quite hot (with the right burner), but it'll be extremely susceptible to mechanical and chemical (welding flux) damage. (And if you don't seal the surface, most ceramic fiber blankets will give off fine dust that's very bad for your lungs.) A forge made entirely of castable refractory will be darned near bulletproof, but it'll be an enormous fuel hog, it'll take a long time to heat up, and its maximum temperature may be somewhat limited.

So yeah -- ceramic fiber lined with a thin layer of hard refractory to protect and seal it is a pretty traditional compromise.

Edited by MattBower

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My first gas forge was double lined, 3/4" rammable for the fire contact and 1" of 8oz. Kaowool for the outer liner. It worked quite well for a good 15 years or so till I built my latest forge.

I rammed the hard liner between Sonotubes and simply wrapped the Kaowool around it to slip into the forge shell. I needed to compress the wool to get it to slip in so I wrapped it with a layer of newspaper and taped it. Think disposable piston ring compressor.

Frosty

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I question how much more efficient blanket is than castable if you are going to use the forge for a few hours or more, especially if you use a layer of ceramic blanket on the outside of the castable. Once the hard refractory is hot it acts as a heat sink so when you put in cold stock it does not cool the forge as much. Any insulation you put outside the hard refractory does not have to be rated for as high temperatures so is much cheaper.

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I question how much more efficient blanket is than castable if you are going to use the forge for a few hours or more, especially if you use a layer of ceramic blanket on the outside of the castable.


If you don't wrap the refractory in ceramic fiber, it's going to be less efficient than it could be -- period. Heat loss through the castable -- which is pure wasted heat -- will be far greater than if it were insulated, and that will continue as long as the forge is running. All you have to do is look at the insulating value of a castable vs. a good ceramic fiber. There's no getting around it. So unless your forge fuel is cheaper than your heating fuel, and you want to make the forge do double-duty as a space heater, insulation is always a good idea. The more the better.

I'm frankly a little skeptical of the idea that a big, thick castable liner is an advantage because it forms a heat sink. (Heat reservoir might be a better term.) I've heard that argument before, but I'm not sure the thermodynamics work out. Ultimately, it takes a given number of BTUs to heat a given piece of stock to a given temperature, and you pay for all of those BTUs. A well insulated, lightly built forge may experience a temporary temperature drop in the process of delivering those BTUs, but because it's lightly built it will recover quickly. A heavier forge may not see as much temperature drop because there are so many BTUs trapped in the castable, but the BTUs sucked out of the castable ultimately will have to be replaced -- and you pay for that. Either way, you're paying to put the same number of BTUs into the work piece. I suspect that at the end of the day, if you do all the complicated math, it's kind of a wash. Could be wrong, though.

It's certainly true that the longer you run the forge without shutting down, the less significant the cost of the heat trapped in the castable becomes.

Any insulation you put outside the hard refractory does not have to be rated for as high temperatures so is much cheaper.


Eh, that really all depends. Ceramic fiber isn't that expensive, the lower-temp alternatives aren't that much cheaper, and you'd have to pay for the extra thickness of castable required to achieve a significant temperature drop by castable alone. Plus the initial cost of materials pales in comparison to the ongoing cost of fuel, so it can easily be a matter of saving a dime to spend a dollar. Edited by MattBower

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I have a forge that I built with Insulating fire brick surrounded by fiber blanket.
I later coated the insulating fire brick with refactory mortar.
I have a very definite dislike of direct flame contact of with fiber blanket unless it is vapor barriered with something like ITC coating. One thing you should consider is is that fiber blanket become very much easier to damage after it has been to yellow heat.

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There is also insulating castable refractory, which I used in my forge to make am arched ceiling. I've never worked with hard castable, so can't compare it with that, but the insulating castable is similar to insulating firebrick. Not as tough as hard brick, but my forge floor is holding up real well. The IFB walls are mostly intact, with two of them having cracked in the past 5 years of occasional use. I use plain old furnace repair cement to fix them. The ceiling has held out very well, with only one corner chipped off.

As for insulation, I think the castable insulates a somewhat less than fiber, but not too much. It gets glowing much faster than the steel in the forge and is hot, but touchable, after about an hour of forging. After much more than that, you can't touch it.

That said, it was kind of a pain to work with. Getting it to mold in an arch took some finagling with forms, where blanket would mostly be just stuffing it in. And figuring out the mix is iffy. The mixing is in water per bags of castable. In my small quantities, it takes some guessing. I would seriously think about using blanket for my next one. The other nice thing about it is I have my burner nozzles cast right in the roof.

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I used 2" ceramic blanket coated with refractory mortar rated to 3200*f. The mortar is about a 1/4" thick and very hard. It prtects the blanket without the long heat up time inherent to solid refractories. Hence, I have the best of both worlds. Much better than my first forge made with just castable refractory. Even blanketing that with ceramic didn't help much.

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I will soon build my first gas forge and I must say this forum is filled with useful information for anyone willing to read a bit. I looked a lot on internet for some blueprints and I recently found a model that got my attention. It said to be a furnace but I was thinking of building it has a forge with some little modification. Here is the link Home Foundry Design Plans

With all the useful information you guys wrote I'm wondering if it will be a good starting option since I don't know how often I'll smith and have no idea what fuel efficiency a good forge is. Also for the refractory cement I found a mix recipe at Homemade furnace refractories and I don't have any idea if that's a good mix at all. I was thinking of buying a high temperature specialized cement or mortar, but i don't know if that's a good idea also.


I found a couple of metal frame forge with just the kawool like insulation and I was wondering if it would be a better idea for a starting forge. Here is some I was looking at Plans to build a simple gas forge or Freon Tank Propane Miniforge

Anyone got some advice.
thanks

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There are plenty of good designs available and once you have an idea of how they work designing one to suit yourself is easy.

Don't mess with trying to make your own refractory, it's a LOT harder to get right than the available recipes make it look. For only a little more than 50lbs of fire clay you can buy 50lbs of high end castable or rammable refractory and not have to experiment.

Sure, lots of forges are lined with Kaowool and nothing more. I'm not a fan of uncoated ceramic blanket being exposed to the fire as there's a silicosis hazard from airborne particles. Coating it is easy enough though and many coatings will dramatically improve the efficiency of the forge. ITC-100 being the best known and a proven performer.

I'd also hold off on trying to modify one furnace to another use. While one for melting metal is in essence not much different than one used for heating it to forge there are real differences. Once you have a handle on how they work is the time to try modifying one.

The freon can forge is easy to make and economical if you're good at finding . . . Stuff. Believe me finding . . . Stuff or scrounging is a basic blacksmith's skill.

The Whisper Momma you showed in another post will work just fine. As will a Kaowool lined ammo box, mail box, length of SS stove pipe, (my preference for a pipe forge BTW) welded steel box, old freon tank, propane tank, etc. etc.

Frosty

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If your going to hotface kao-wool or dura blanket with a refractory cement then you only need a max of 1/2" thickness, any more than that all your doing is lengthening your heat up time which in return is a waste of fuel.
If you have the money then ITC-100 is the best route to go.
Wether I'm building a furnace or a forge I make my hotface 1/4"-3/8" thick and have no trouble with it holding up.

welder19

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Thanks for the info guys, I really appreciate the time you take to help someone starting like me. I was really looking forward making that furnace into a forge Frosty, but your the man and if you think I should stick with regular forge plan for a start, then a forge plan I must make ;)

thanks again!

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Imrie-Gielow out of St.Louis is a great place for Hi-temp riszer tubes. I bought 2 -7" tubes for 19.00 each and there sticky I-G Cerachrome caulk is great to fix cracks in the riser tubes. I use a product called itc-100 to line the tube and on the bottom of the forge I use a compound called Bubble Alumina Refractory. It works really well and is the best Ive used to date for flux drips in the forge. ITC is great for adding to the hi temp riser. It alone will give weld temps from propane at 5lbs on the regulator.My burners are simple straight pipe with a bell reducer welded to the fuel end with a small drill for the propane to pass. The need for expencive burnners are not needed.unless you have money to burn (get the pun) lol My burnners are 1" pipe with a 1x3" bell reducer. The gas pipe is weled accross the face of the bell reducer with the hole in the gas pipe pointing straight down the 1" flame holder pipe. Cheap and works well enough for me.

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Thanks for the info guys, I really appreciate the time you take to help someone starting like me. I was really looking forward making that furnace into a forge Frosty, but your the man and if you think I should stick with regular forge plan for a start, then a forge plan I must make ;)

thanks again!


Whoa! No must about it, just some friendly advice. If you were really looking forward to making the conversion then by all means go for it. Most of us here are into this for the fun of it. If making the tools and equipment is what gets you going then that's exactly what you should do.

Heck, I'd almost rather make the tooling than use it myself. It's why I started out spending a huge amount of time (months of trial and error) building my own forge and burner rather than just buying one. Tried making my own refractories too, several recipes and variations trying to get something that worked. My favorite was the 4,000f high phosphate rammable refractory I bought from EJ Bartells. That stuff worked like a charm. . . First try. ;)

I spent almost as much time figuring out how to make a simple burner that works.

Whatever you decide we'll be happy to help out.

Frosty

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 I just finished my first Forge and wanted to make sure that it will be OK with the heat blanket by itself or if I had to use  refractory coating on the inside as well. I did use  high temperature cement on the front I don't know if I need it on the inside .also do you guys think I need a front door or can I leave it open. I built a slide on the back that I could put a large fire brick to close off the back just wasn't sure if I needed something in the front as well

 

thx

marc

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On 7/24/2009 at 1:46 PM, Frosty said:

My first gas forge was double lined, 3/4" rammable for the fire contact and 1" of 8oz. Kaowool for the outer liner. It worked quite well for a good 15 years or so till I built my latest forge.

I rammed the hard liner between Sonotubes and simply wrapped the Kaowool around it to slip into the forge shell. I needed to compress the wool to get it to slip in so I wrapped it with a layer of newspaper and taped it. Think disposable piston ring compressor.

Frosty

Have you ever published a picture of your first gas forge here at IFI?  

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On 7/24/2009 at 1:16 PM, MattBower said:

I'm frankly a little skeptical of the idea that a big, thick castable liner is an advantage because it forms a heat sink. (Heat reservoir might be a better term.) I've heard that argument before, but I'm not sure the thermodynamics work out.

Circumstances alters cases. This is usually the bottom line behind progress. It's the bottom line here too. Back in the bad old days of natural gas for fuel and crude burner designs, heat reservoirs weren't just a good idea; they were necessary. Than propane came into the marketplace, and crude burner designs hung around for a few extra decades longer than the would have otherwise. Fuel costs were rising, but space age insulation replaced heat reservoirs, to keep crude burners viable. About twenty years back the price of propane in portable cylinders increased sharply, and hotter burners became a necessity. That's how we got here. Today heat reservoirs are a very bad idea; they were a bad idea even fifty years ago, but nobody did anything about them until it hurt wallets sharp enough. Hot glass artists lead the way for the most part because their trade goes through a lot more fuel than blacksmiths.   

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15 hours ago, mariom2 said:

Have you ever published a picture of your first gas forge here at IFI?  

No, it's not a particularly good forge unless you want a catalog of mistakes. I'll make a short list of my personal typical beginner's mistakes. #1, the shell was a piece of steel pipe with a wall thickness on the order of 5/16". It was under insulated with only one inch of 8 lb. Kaowool. The refractory  inner liner was way too thick at 3/4". Doors? What doors, it was open at both ends and about 6" ID x 10" L.

I put a number of burners on it till I finally got a successful T and it was a cobbled together mess. It was a well tuned 1" T heating a chamber under 400 cu/in.

It got HOT for sure and made short work of everything I put in it but boy what a gas hog. The best part was the look on some old timer blacksmiths who KNEW you couldn't weld in a NA gas forge when I turned the psi up a little and started melting their work if they didn't pay attention.

Anyway, my first successful gas forge is NOT an example of anything but mistakes that can be MADE to work if you throw enough fuel into them. 

Frosty The Lucky.

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12 hours ago, Mikey98118 said:

Today heat reservoirs are a very bad idea

I think you are ignoring your own advice here on circumstances altering the optimal design.  While I certainly don't think a heat reservoir is needed for most hobby smiths, if I were building a forge for a production shop that was using larger stock at a decent throughput (say Uri Hoffi making hammer heads, Gransfor Bruks axes, or Al Paley working 4" square stock) I'd certainly want a thermal battery to even out the load on the burner over time.  Even in my forge with a thin wall refractory layer (that I'm planning on updating soon),  I've noticed a marked cooling of the interior when I forge 3# hammer heads.  You can clearly see the stock pull the heat right off the  floor when the stock is put in for heating or reheating.

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I stand corrected. My viewpoint was too narrow. Thinking only of hobby forges, I tend to forget the other end of the spectrum exists. No doubt I will forget again; please remind me when I do :D

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On 7/27/2009 at 8:30 PM, Dodge said:

I used 2" ceramic blanket coated with refractory mortar rated to 3200*f. The mortar is about a 1/4" thick and very hard. It prtects the blanket without the long heat up time inherent to solid refractories. Hence, I have the best of both worlds. Much better than my first forge made with just castable refractory. Even blanketing that with ceramic didn't help much.

Dodge, where did you get the refractory mortar?

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