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Thread: Bentonite as an ok alternative to fireclay?

  1. #1

    Bentonite as an ok alternative to fireclay?

    So i have a question regarding on whether or not Bentonite can be used as an alternative to a homemade refractory recipe calls for fireclay. Manly why i am saying this is a have a very good and very cheap source of Bentonite clay at my disposal where my supplier provides it for a cheap price i think, which is usually around $8 for 30- 45 lbs.
    I want to run a better furnace for as cheap as i can get it and would gladly like to know if it would be ok to use Bentonite as an alternative. I have read that because Bentonite is so hydrophilic that there could be cracking in the refractory from the clay when drying and a tip to help prevent that if it is ok to use would be much appreciated.

    *I am a new member and am not sure if i have posted in the correct section or not or should looked for an older thread dealing with this?*

  2. #2
    Senior Member Essej's Avatar
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    there could be cracking in the refractory from the clay when drying and a tip to help prevent that if it is ok to use would be much appreciated.
    For anything clay shrinkage can be a problem, grog (crushed firebrick) in the mix will help with this. A 1:1 ratio I think would be a good place to start. Sand would do the same thing as grog, but I've read that sand in refractories can be problematic.

    As far as the bentonite, I can't tell you if it'd work or not, but my gut says that it would be less than ideal...there's got to be a reason that I haven't seen anyone use it for a furnace lining...haven't seen any bentonite crucibles either

    Commercial castable refractories are the way to go in my opinion, but they can be pricey...for diy refractories, check out this thread

  3. #3
    Thanks, that thread has alot of good information.

  4. #4
    I tried to form a bentonite crucible, shrank and cracked horribly. I also tried it as a surfacing in my furnace, it worked ok, but tended to shrink away from the wall and flake off. As a mixture component probably be ok.

  5. #5
    Ok, i will test it out when i get the chance it is going to be just used for melts of aluminum around 5 or less times a month.

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    Administrator Site Admin Anon's Avatar
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    Bentonite can be used as a binder in a refractory recipe, but it is not a substitute for fireclay. I have a recipe in my book that uses 5% bentonite as a binder--and with bentonite, 5% is enough to make a workable refractory. A fireclay shrinks much less, but it's also much less plastic (the two factors are related!), so you would need 25-30% fireclay for the same result. The resulting bulk material, depending on what else is in it, will have approximately the same shrinkage if you control clay amounts for the same plasticity.

    Bentonite is a flux, so you can't use large amounts of it in any refractory recipe that you want to be refractory. In addition, if you try using it in larger amounts, it's so plastic that you'll have horrible shrinkage and drying properties, and cracks everywhere. Fireclays (and kaolins, and other less-plastic clays) can be used in higher amounts to bulk out a recipe, and they're a pretty cheap source of alumina. The disadvantage is, if you have a recipe that needs a lot of alumina (e.g. for mullite), they're not a very concentrated source, and adding the pure stuff is expensive. Using bentonite as a binder leaves room for medium-priced nonplastic fillers such as kyanite, which have a higher alumina content and also contribute very good drying and firing properties. Of course, you can get the best results if you use some of both, but then you have a more complicated recipe and more ingredients to source. Ceramics is a complex subject, and it can get very complex very quickly if you start doing stuff like this.
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  7. #7
    Mind = Blown, that was very informative and thanks Anon.
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    Senior Member SolarFreak's Avatar
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    Quote Originally Posted by Essej View Post
    Sand would do the same thing as grog, but I've read that sand in refractories can be problematic.
    I've had very good results with sand+fireclay+grog as a refractory. Not the best insulator, but surprisingly durable at brass / bronze temperatures (~2K*f)
    I seem to have accumulated so many things that I need, that I have no space left to use them in...
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  9. #9
    Anon like a recipe like this (did a little math after seeing 5% bentonite as a possibility no idea if it could work) :

    Ratios
    4:1 Foam beads to Fire clay (for air pockets when furnace is fired and the foam beads vaporize and then create better insolating properties)
    1:1 Fire clay to Grog (to add strength to the week structure of the air pockets)
    20:1 Fire clay to bentonite clay (flux and add alumina)
    20:1 Fire clay to kyanite ( non-plastic and flux + added alumina)

    Percent’s %
    60% Foam beads
    15% Fire clay
    15% Grog (crushed fire bricks)
    5% Bentonite clay
    5% Kyanite

    Part's recpie
    6 parts foam beads
    1.5 parts fire clay
    1.5 parts grog
    Less than .5 parts bentonite
    .5 parts kyanite

    Add water till workable and let set for the clay to absorb water for overnight?

    Could drop the ratio of 4:1 foam beads to fire clay and use more Fire clay and Grog for strength?
    Also add stronger more dense hot face with out foam beads?
    Last edited by GreySpark; 12-09-2012 at 06:54 AM. Reason: grammer and spelling errors
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  10. #10
    Senior Member Essej's Avatar
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    Percent’s %
    60% Foam beads
    15% Fire clay
    15% Grog (crushed fire bricks)
    5% Bentonite clay
    5% Kyanite
    That might be ok...I'd probably use as much grog or sand as there is clay, 1:1... you may be a little high on the foam too...I think most use 50%...I learned with the foam...my next furnace I'm not even going to mess around with it, it'll be ceramic fiber...Perlite could also be an option for you, from what I understand it will melt and flux, but only @ higher temps, I believe it's ok to brass/bronze temps? Burning the foam out creates gasses and those gasses need to escape, when they do, they cause damage (microfractures), this further weakens your mix...

    Add water till workable and let set for the clay to absorb water for overnight?
    You want to use as little water as possible, just enough to make it workable, more water = more shrinkage...I don't think there'd be any benefit to leaving it sit overnight...mix it up and put it in place...

    Also add stronger more dense hot face with out foam beads?
    Yes, you want a separate dense hotface, something that can hold your insulating layer in place and take some mechanical abuse. Anywhere from 1/2" to 1" thick is pretty standard...

    I applaud the diy spirit, but looking at all the materials you've listed, I'm wondering if by the time you source all of them you could just as easily spend that money on some commercial castable refractory. I made test pucks of different fireclay/grog mixes when I was going to build my furnace, then switched to castable...no comparison, for me the castable was superior in every way.

    I'm not saying that they're aren't good diy refractories or that you shouldn't try your own, I'm just saying do the math...if you can get a castable for about the same money, that'd be the way I'd go. My local masonry supplier has 50lb bags of alsey high-cast, good up to 2700*F, for $50/bag...

    If you're dead set on a diy mix, I'd use something tried and true, if you want to make your own, do small test samples first, before mixing large batches...
    Last edited by Essej; 12-09-2012 at 12:35 PM.

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