Been thinking about it for years...

Discussion in 'New member introductions' started by Fulmen, Sep 4, 2020.

  1. A friend of mine buys old pottery kilns off "for sale market" websites to get the high quality firebricks out of them for other projects. If the kiln is broken the prices can be very cheap.

    This is identical to one at my local recycling shop selling for $50 with broken heater elements and a rusty front door:

    Tetlow-Kilns.jpg
     
    Last edited: Sep 8, 2020
    Fulmen likes this.
  2. Fulmen

    Fulmen Silver

    That's very clever, I'll be sure to look out for broken pottery kilns.
     
  3. Petee716

    Petee716 Gold Banner Member

    The decision making on the refractory seems to be taking a turn in a different direction (all good). If you are going to burn oil or gas in your furnace then I strongly recommend you abandon your homebrew refractory idea altogether. It will absolutely not hold up to the punishment. It will turn into walnut sized chunks or will flow down the walls like lava in very short order. If you are using charcoal only you can use lesser materials, not because charcoal burns at lower temp than gas or oil (it does not significantly), but because of the way it burns in the furnace.
    One property that a charcoal refractory must have is abrasion resistance as the charcoal is mechanically abrasive and will erode anything that's not really hard.
    So the issue becomes sourcing the proper materials to do the job. This is one area where you may have to spend some money and to think outside of your normal avenues. You will not find refractory cement at a hardware store or even the typical fireplace shop. Anything in those places that says refractory is not going to work at our level. So where to go? Who uses industrial refractory and insulation? Furnace installers, refractory bricklayers, pizza oven enthusiasts, syrup makers, people who work at foundries. Ah... ceramics suppliers! These individuals may not carry the stuff in their inventories but have relationships with the pre-mix mafia so they can order it in. The fact that it gets damned cold in Norway also tells me that there is all kinds of that stuff around.
    Once you get over the "unavailable" hurdle then you can focus in on the details of your design. It may require some calling or visiting or shaking down acquaintances involved in the activities I mentioned above, but you'll find it.

    Pete
     
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  4. JCSalomon

    JCSalomon Copper

    And mortuaries.
     
  5. Fulmen

    Fulmen Silver

    Petee: I'm not committing to anything yet, other than the propane tank. But I'm leaning towards fireproof bricks as it won't be significantly more expensive. I could cut costs by throwing some crude DIY stuff together, but it's hard to get excited about half-ass builds. But even something half-decent will cost almost as much as the bricks, at least with the prices I've gotten so far.

    So if I go for bricks I'm thinking of something like this:
    Ovn 2.jpg
    The base would be cast from fireproof mortar with leca, and I was hoping that I could just stack the bricks and lock them in place with a loose leca/vermiculite backing . After all the leca should be pretty incompressible, and with beveled sides the bricks really can't go anywhere. In theory at least...
     
  6. Fulmen

    Fulmen Silver

    Good news everybody. I finally got in touch with someone at Borgestad which recommended their Borgcrete 50. It's a castable 1500°C chamotte, well suited for hand casting, and a 25kg bag costs less than 10kg of the 1000°C mortar that the hardware store sells.
     
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  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Coming into the discussion a little late but have a couple observations and comments for you.
    1. It's good that you have found an economical source for commercial castable refractory. If the source is close and the material readily available, buy one bag. If it is not close or unlikely to be further available, buy 3 bags.
    2. If you do buy castable, before you attempt to use it, come back and ask us about mixing and placing it. Your initial impressions will suggest it's very dry and difficult to work with but if even though they say it is for hand placement, vibration will make it flow and produce much better quality refractory castings.
    3. You don't want to make your refractory the entire thickness of your furnace wall. Most people will shoot for no more than 1 inch wall thickness. The more dense refractory the more mass and the longer it will take to heat the furnace mass. For charcoal, this wont matter because charcoal is self insulating, but for all other fuels it will.
    4. Honestly, I would not spend the money on commercial refractory for a charcoal furnace. You can literally dig a hole in the ground, put a grate in the bottom of the hole, and route/burry a steel tube under the grate for combustion air. Just use a piece of ceramic wool for the lid. With charcoal, it will melt just as well as a purpose built above ground furnace. When you tire of it (and you will), fill the hole back in.
    5. If you just want to get some melting and casting experience, I'd suggest you try the above with a steel crucible and some aluminum. Earthen furnaces and forges were successfully used for millennia. If you really are interested in using an oil burner and melting bronzes or iron, and are going to go to the effort of building a furnace with the expense of proper materials, I'd suggest you build a 10" bore furnace with 1/2" to 1" castable refractory wall and insulate it with ceramic wool. It will be durable enough to handle the oil burner. If just using a propane burner, you could skip the castable refractory and coat the wool with a thin layer of mortar making high performance furnace construction much easier.
    6. You'll need a proper crucible for higher melt point metals. Better get started sourcing those!
    Best,
    Kelly
     
  8. Melterskelter

    Melterskelter Gold Banner Member

    As, Kelly says, just a hole in the ground will work... Nothing new that we are doing. It is just a matter of convenience.



    Denis
     
  9. You should be able to fit the brick in without mortar. The full circle of loose brick can't fall into the center.
     
  10. Fulmen

    Fulmen Silver

    Al2O3: I'm building an oil fired furnace, but I do expect to run it on charcoal (and maybe propane) at first. I'm not really interested in any improvised setup where things can collapse or topple over, and besides I don't have an inch of soil where I was planning on setting up shop. I was planning on casting an outer shell mixed with leca (perhaps 1-1.5" thick) and perhaps 1/2" of solid hotface, that should be just about doable with a single bag.
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Ok, sure seems like most of the first page of this thread was spent talking about a charcoal build and then around post 13 you tossed in the idea of propane or an oil burner. As mentioned by several people, there are different design considerations for solid fuel furnaces versus gas/oil.

    Who said anything about that? You do understand that with charcoal furnaces, the crucible usually sits on and is surrounded by a bed of coals, right?

    I had to look LECA up.

    https://en.wikipedia.org/wiki/Expanded_clay_aggregate

    It's probably a decent insulator and of sufficient refractory, similar to fire clay. You could mix the LECA with something to bind it but would likely be better without and just pack/vibrate it into place between the outer furnace shell and hot face. Not sure what you are planning to mix it with but you are in a for a rude surprise if you were planning on using the castable refractory of even mortar. Why not just ask you source of castable refractory for some ceramic wool? -To each his own.

    Best,
    Kelly
     
    Last edited: Sep 11, 2020
  12. rocco

    rocco Silver

    I don't know about the rest of you but this is my context for leca:
    [​IMG]
     
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  13. Fulmen

    Fulmen Silver

    Re: Fuel.
    You're right, I wasn't very clear on that part. And there is the inevitable mission creep as you guys poke holes in my plans. Of course I want something like an oil burner eventually, but I also know it could take a while. So I hope it will handle a few runs using charcoal. Of course if I see unacceptable wear I'll have to reconsider that plan.

    Re: Simpler constructions.

    I just have a bad track record with makeshift solutions, and a healthy respect for hot liquid metal. So I want something made from steel with three wide legs and a hinged lid. Remember I have to learn this on my own, rookie mistakes will be made.

    Re: Leca.
    That's the stuff, rocco. It's burned at 1200°C and a decent insulator. Around here it's used as a lightweight construction material: https://no.wikipedia.org/wiki/Leca
    The plan was to make a similar material using the castable refractory, why won't that work? It sets hydraulic and while I can't find any air-dry strength it should be at least 40MPa after "firing" at 100°C. It only has to support itself until it's covered with a layer of straight refractory... What am I missing?
    Casting the hotface first seems harder, or am I overthinking it? I worry about the de-molding of unfired refractory, as you have pointed out yourself it shouldn't be too thick. What about making curved bricks? Sounds like even more work, but if the results are favorable I might be persuaded.
     
  14. Tobho Mott

    Tobho Mott Gold Banner Member

    Demolding a castable refractory hot face isn't so hard. Setting up the forms is trickier. You would have had to try pretty hard to break mine in the demolding.

    Click this link to see the post that shows how I set up the forms for mine.

    That's my big oil furnace, insulated with 2" of ceramic fiber. What makes it an oil furnace rather than an overbuilt overinsulated charcoal furnace is the tangential tuyere. But I still think if you use the grate above the tuyere idea to suspend the coals, the tuyere angle won't matter.

    One of our members, Rasper, uses similar furnaces but his have a mix of sand and clay rammed up behind the hot face instead of insulation. He says the refractory never cracks and his oil burners still melt bronze quickly despite the furnace walls being less insulating and more of a heat sink.

    There are old threads about using leca to build furnaces on the alloyavenue forums, but they are hard to view right now. My recollection is of mixed results, but behind a 1" hot face some kind of leca mix might work ok. But I would consider it fairly experimental so no guatantees... I doubt the failure mode will be demolding the hot face.

    Jeff
     
  15. FishbonzWV

    FishbonzWV Silver Banner Member

    I've tried to stay out of this because I'm biased toward simplicity.
    My favorite furnace is a 20 pound propane tank using 2 inches of insulating wool and a Satanite coating.
    It is very easy and inexpensive to to build. It is portable and efficient.
    I didn't put a hinge lid on, just lift it off and set it down (I usually have ingots preheating on top).
    Your tank is 12" in diameter minus 4 inches of wool, minus 1/4" of coating leaves you with a 7 1/2" bore.
    An A6 Salamander Super crucible fits perfect when running with a propane burner (you'll get tired of charcoal after about two melts).
    Satanite is hard to find in the EU but Metallab, a member here, coated his wool with a product called Fermit Chamotte.
    You can build this with an angle grinder and a welder to tack the tuyere and handles on.
    To hold the wool on the lid I tacked four bolts on, pressed the wool over them and put a washer and nut on them then applied the coating. There's really no weight to hold up, the outside edge is supported by the side walls, so you only need the center held in place.

    Furnace1-700.JPG

    Coated with Satanite
    Furnace2-700.JPG

    Furnace3-700.JPG
     
    Last edited: Sep 12, 2020
  16. Tobho Mott

    Tobho Mott Gold Banner Member

    Not sure satanite would hold up well getting banged around by charcoal, though I do agree charcoal gets old fast. Do we know anyone who uses a blanket and satanite furnace with an oil burner who can say whether their insulation survives behind such a thin hotface? I really love my little blanket and satanite furnace and would definitely not hesitate to recommend tbat design for a firat furnace, but it melts so fast on just propane I haven't set up the oil tank for it since its maiden run, which melted the emergency-installed totally inappropriate 2700F box store fireplace mortar off before my satanite came in the mail...

    Jeff
     
  17. FishbonzWV

    FishbonzWV Silver Banner Member

    Here's the problem with trying to build a multi fueled efficient furnace.
    Charcoal and oil require at least two inches of space around the crucible, propane only requires one inch.
    With a 12" bore and appropriate insulation, that would limit you to an A4 or less crucible.
    Cast iron?...you need to build a whole different furnace to cope with those temps. One size does not fit all.
     
  18. Petee716

    Petee716 Gold Banner Member

    Mission creep has been the name of the game for me over my last 8 years or so when it comes to furnaces. I now have 3. I started with a Gingery style charcoal with solid 2300F solid refractory in a 5 gallon bucket. Then I got interested in traditional reverbs and built a scaled down version with the same refractory and some ceramic wool in a 10 gallon horizontal compressor tank split lengthwise. It gave me much greater capacity and a direct pour (no crucible). It was fired with anthracite coal in a firebox and it melted aluminum, but not effectively. So I launched into the realm of oil burners, removed the firebox and converted the reverb to a Hago/Delevan setup. (I dabble with propane as well.) It works famously but I limit its use to aluminum. Higher value and higher temperature metals aren't an option in it because of waste, refractory limitations, and contamination concerns, so I built a larger (10" bore) more robust crucible furnace in a beer keg with 1.5" 3000F refractory backed by 1.5" fireclay/sand mixture. It works well but takes awhile to heat up. For instance it takes a solid hour from startup to melt and pour a #10 crucible of bronze, but only about 20minutes for a second pour. This makes a good case for the use of insulation instead of a solid backing material. That is how my next one will be built if I decide to build one. The added time isn't really that much of a bother, although it's not terribly convenient, but I think it makes for a lot more unneeded punishment of the refractory, crucible, and metal quality from the extra time spent in the combustion environment. Oil is harsh.
    But through all of that I have not regrets. I'm a builder and a tinkerer albeit a not a great one, and Ive thoroughly enjoyed the process so far. I still use all of my furnaces for different reasons and although I've abandoned charcoal as a melting fuel I still use it occasionally for forging in my bucket furnace.

    Others here have followed similar paths, yet others have had the inclination to set goals, build their single furnace, and get on with the other aspects of the hobby with all of their furnace requirements met. There's nothing wrong with either.
    I personally come here, toss out ideas, get opinions (there are some damned good ones), and then proceed with my own conclusions. Win, lose, or draw, it keeps my feet moving, captures my interest, and let's me play with my toys.

    That's probably more than you were looking for. Lol

    Pete
     
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  19. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You must be trying to chum MelterSkelter into the conversation? I think the conclusion for that iteration of his diesel fired iron duty furnace was success but required maintenance every melt and if the Satanite became comprised, the ceramic fiber rapidly deteriorated and also required attention. Keep in mind this is extreme duty but the reality of an iron duty furnace. I think Satanite/Ceramic fiber furnaces have proven themselves in just about every other respect......the most efficient, light weight/portable, economical, easy to build.

    Best,
    Kelly
     
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  20. It's funny how one tends to promote what they have. That being said I couldn't be happier with my pure firebrick furnace. It was cheap and has lasted well through lots of copper and brass and a little cast iron. I would think with satanite on the face of the brick it would do well with charcoal but that has never interested me.

    Kelly is one of the few I have seen build with success on his first try.
     

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