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Thread: Homemade insulation refractory?

  1. #1

    Homemade insulation refractory?

    I hope to be building a furnace soon. I have a well pressure tank I can use for the shell. It has an OD of 22".

    Initially I was thinking, a 12" bore, 1" hotface and 4" insulation refractory. But because of the size of this thing, I'll need a LOT of refractory which is going to be pretty expensive.

    Now I won't skimp on the hotface, I still plan to use a 1" thick layer of 3000 degrees commercial refractory.. but I'm wondering if I could substitute the Kast-O-Lite 26 LI that I originally had in mind for the insulation layer, for say, fireclay (40 mesh Hawthorn bond) mixed with foam or sawdust. Would that hold up to cast iron temps? Fireclay is soooooo much cheaper! If fireclay alone isn't gonna cut it, would adding some aluminum oxide help? They sell it as abrasive at Harbor Freight.

  2. #2
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    1" of hotface is too thick, it will absorb too much heat, 1/2" is a better choice. Also for insulation, I would recommend ceramic fiber, it is a lot less trouble than trying to get a clay mix right and fired correctly...

    you should also consider the fuel costs involved firing a clay based refractory properly. If you keep this in mind and the amount of extra effort clay refractories involves, something like ceramic fiber becomes a great deal more appealing not to mention the superior insulating properties of ceramic fiber...
    The voices in my head may not be real, but they do have some good ideas...

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    Administrator Site Admin Anon's Avatar
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    First thing to do: Build a smaller furnace (unless you already have a smaller working one). It'll take less material and be easier to use. I recommend something about 8" bore (A6 crucible) for most castings. You can save the big shell for when you actually need to cast something large, then build the big furnace using the same design (if it worked) or an improved design.

    If you insist on building a 12" bore monster (which will need a mechanical lift setup or a two-person team to safely pull large crucibles) then the first thing to do to cut costs and improve the design is to make your insulating layer more insulating. Fill that insulating refractory with foam, or substitute it entirely for ceramic fiber blanket.

    A 1" hotface is more than enough for this size furnace. You could cut that to 3/4" or even 1/2" if it's supported by the insulating layer. 1" will work, and last a long time, at the expense of taking some more fuel to heat up.

    Fireclay alone won't work, and neither will fireclay mixed with coarse grog and nothing else. (Plus, I'd be skeptical of the purity of alumina abrasives at HF. Not that it won't necessarily work, I'd just be skeptical.) Take a look at my book for some clay-based recipes that have been tested to work. Bear in mind that they're not significantly cheaper than commercial refractories by the time you include firing costs.
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  4. #4
    Well, I'm just in the planning stages for now. I'm open to any and all suggestions. The reason for wanting to use the shell was because it was given to me by a friend and not wanting to offend him I thought I'd go with it.

    But I'm starting to realize that this thing is too dang big and will cost me a fortune to build, which is why I was looking to save money on the refractory. But I doubt I can make a refractory that will withstand cast iron temps. So it looks like I don't have a choice but to make a smaller furnace.

    Just wondering, but what kinda savings am I looking at if I were to reduce the bore from 12" to 8"? Math never was my thing, I can't seem to figure out how to calculate how much material I'd need. If someone could give me a link to a place that explains how to calculate how much material I'll need, that would be appreciated.

    Btw, A6 seems a little small though. I think I could use an A8 and still keep the 8" bore.

    @Heimo: If you have a cheap source for ceramic blankets, by all means let me know. Any idea how much it would take if I were to go with a 8" bore?

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    Quote Originally Posted by wikisi
    @Heimo: If you have a cheap source for ceramic blankets, by all means let me know. Any idea how much it would take if I were to go with a 8" bore?
    well I bought an entire roll of blanket, and it is enough to build several furnaces, I have build a 10" bore furnace with 3" insulation, a 6" bore for my dad to harden his knives in, and there is still enough left to build me a small burnout oven for lost wax... since I live in South Africa my sources wont do you any good, but I suggest you check Ebay... in fact I already done it for you :P

    link

    link

    this is the brand I bought

    link
    The voices in my head may not be real, but they do have some good ideas...

  6. #6
    Administrator Site Admin Anon's Avatar
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    Take a look at the furnace design in my book, which is an 8" bore. A roll of blanket and one bag of refractory should be enough to build about five of those furnaces.

    Check eBay for ceramic wool. I bought mine from the seller hightempinc, but there are other sellers for the material.

    An A8 would fit in that furnace, yes. It's a little tighter than I'd like, but if you make your tongs suitably compact, you won't have problems.

    You could make a muller out of that tank, or save it for a later monster furnace build.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

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  7. #7
    Sorry Heimo, didn't notice you were from S. Africa. Thanks for the links though. These prices are a lot better than what I found.

    Just downloaded your book, Anon. I better start reading maybe I'll finish it today :shock: :lol:

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    Senior Member joel's Avatar
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    Quote Originally Posted by Heimo
    1" of hotface is too thick, it will absorb too much heat, 1/2" is a better choice. Also for insulation, I would recommend ceramic fiber, it is a lot less trouble than trying to get a clay mix right and fired correctly...

    you should also consider the fuel costs involved firing a clay based refractory properly. If you keep this in mind and the amount of extra effort clay refractories involves, something like ceramic fiber becomes a great deal more appealing not to mention the superior insulating properties of ceramic fiber...
    I have to disagree. I'm pretty rough on my stuff and i like mine rigid. I used well over an inch of hotface on the last furnace i built and still able to melt in the 15-20 min range. oil and wood combo. propane may be a different story.

    Like Heimo says, consider the fuel costs, mine was free because i used waste oil(WO). My new furnace uses ~.75 gal of diesel an hr so it costs ~4.50$/hr to run... fairly cheap. and i could convert it to an oil/diesel mix i think too.
    The Difficult Anytime, The Impossible By Appointment Only

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  9. #9
    with something that big couldn't he use regular old cement(yes i said it) for the outside 2" then put 2.5" insulative with a .5" hot face? or would it still get too hot by the time it got to the concrete?
    mark

  10. #10
    Senior Member joel's Avatar
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    probably, I think machinemaker has portland behind his, and its several years old.
    The Difficult Anytime, The Impossible By Appointment Only

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