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Relatively Lightweight Refractory Cement oven build

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  • #16
    Hi,

    So I cast my homebrew refractory layer last night approx 20 hours ago some pics attached. It is sitting in the garage at 14 celcius under plastic sheet and have been spritzing it with water to keep it damp. It has hardened to where you can't dent it with your finger but you can make a mark with a fingernail. Is the temp too cool for curing? I've seen other posts saying it has to be hot?

    Should I wait longer before removing the form and sand? Once I do should I keep it damp and covered for a week?

    Cheers,
    J

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    • #17
      Hi Jumbalaya,

      Have you removed the form and sand yet? If you have, I'd love to see the interior surface finish you were able to achieve. I'm in the early stages of my build and while I appreciate the simplicity of a sand form, I am very curious about the process and the resulting finish. It looks like the outer surface looks great, but I am curios as to how well you were able to control the interior surface.

      Thanks
      Steve

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      • #18
        Hey Steve,

        I waited 48hrs before removing the form and I am pretty happy with the results.

        The inside is not as smooth as the troweled outside and there are some mild wrinkles and creases here and there but it is structurally sound and no gaps or cracks.

        I did have some slumping on the lower back and right which caused the newspaper to bunch up into a crease. I soaked it and brushed it out then put in some more homebrew with a putty knife. You see the darker color where the patch is. I don't think I got quite as thick as I planned maybe 1.5" - 1.75" thick but seems very solid. I'm keeping it wet and covered for a few more days.

        A couple of things I might do differently is:

        - Be more careful laying the newspaper to avoid all the overlaps/creases in the newspaper and same with the tape over the wooden part.
        - Mix larger batches, I mixed around 8 batches in a large bucket and one of those was too wet which caused the slumping, the troweling on went surprisingly quick so next time I would use a wheelbarrow and maybe do 3 batches.
        - Make the outer arch thicker. The outer arch rebate is a little thin hopefully will not be too fragile when I bump it with the steel door. I will try to reinforce it somehow as I add the insulating layer.

        Thx,

        Jumbalaya
        Last edited by Jumbalaya; 04-13-2017, 05:13 AM.

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        • #19
          Good job, it's a hell of a lot easier and cheaper than cutting individual bricks. Damp cure it for a min of a week, then at least a week of drying, then insulate before putting any fire near it.

          Regarding the newspaper, you can eliminate the creases by covering the sand mould with thin strips. Wide strips won't conform to a compound curve. But it is only cosmetic.
          Last edited by david s; 04-12-2017, 11:12 PM.
          Kindled with zeal and fired with passion.

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          • #20
            That looks great! Thanks for posting the internal pictures and the tips. I have found a crazy deal on a paddle cement mixer and intend to pick it up over the weekend (and resell it when I am done later this summer). My castable supplier actually suggested a paddle mixer as the best way to mix his product. I should be able to mix quickly and spend the working time of the castable actually forming the dome instead of mixing.

            Did you add any clay to your sand mixture before forming your dome? If so, what ratio did you use and would you change that ratio if doing it again? What is the radius of your dome and how much sand did you end up using?


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            • #21
              The diameter is 24", the internal height is 13". I used some styrofoam pieces I had lying around to build up the inside core of the sand mound probably 1/3 of the volume was styrofoam. Then approx 3 66lb bags of regular moist builders sand, no clay. It didn't budge I was sure to wet it and tamp it down well.

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              • #22
                Originally posted by sriceky View Post
                That looks great! Thanks for posting the internal pictures and the tips. I have found a crazy deal on a paddle cement mixer and intend to pick it up over the weekend (and resell it when I am done later this summer). My castable supplier actually suggested a paddle mixer as the best way to mix his product. I should be able to mix quickly and spend the working time of the castable actually forming the dome instead of mixing.

                Did you add any clay to your sand mixture before forming your dome? If so, what ratio did you use and would you change that ratio if doing it again? What is the radius of your dome and how much sand did you end up using?

                What sort of paddle mixer are you talking about? I assume this is not something that goes into a drill chuck, but a full mixer?

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                • #23
                  It would be this: http://www.harborfreight.com/3-1-2-h...xer-67536.html
                  There is a 25% off coupon valid this Sunday only - http://www.hfqpdb.com/
                  If would be an hour round trip for me to drive and pick one up on Sunday, so I will probably just order online - they'll deliver for $6.99 (can't believe that I can get a cement mixer delivered for $6.99)
                  By the time I pay tax and shipping, I am looking at $165.99 for this to show up on my doorstep. I have no desire to premantly own or store a cement mixer, so after I pour my hearth, mix my castable and stucco, I'll put it up on Craigslist. If I can get $100, that would give me use of a mixer for a couple of months for $65 - single day rentals are around $50.

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                  • #24
                    I've been waiting for that one to go on sale as well, exact same philosophy. I may keep it around awhile as I have some planter bed projects I hope to someday get to... Thanks for the coupon info!

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                    • #25
                      Hi David regarding this:

                      Originally posted by david s View Post
                      Damp cure it for a min of a week, then at least a week of drying, then insulate before putting any fire near it.
                      I've damp cured it a week, I will soon remove the wet blanket etc:

                      - Any danger of applying gentle heat to assist in drying like from a small electric space heater or a tea candle?

                      - I plan on using 10:1 vermicrete to insulate, any special considerations on that process? I suspect I would have to wet the refractory layer again and just apply the vermicrete directly onto the refractory layer?

                      - I have some chicken wire, would it help prevent cracking if I embedded some in the insulation layer?

                      Thx,
                      J

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                      • #26
                        Do you have the thing outside and exposed to sun and wind, because it will greatly assist drying. For vermicrete to insulate well the mix needs to be lean. The problem with that is it is less workable and results in lower strength. I find 10:1 is about as lean as is workable with enough strength to provide a firm substrate to render against. A small handful of powdered clay for every litre of cement added helps make the mix more workable. Mix the dry materials together well in a barrow before adding the water. For every 10 parts of vermiculite add 3 parts of water and one part cement. Chicken wire in this weak mix would do little to strengthen it and is very fiddle to apply. Do not wet the casting prior to vermicrete application because you are only adding more water that will have to be eliminated and there is no requirement that it be bonded well to the casting anyhow. A 10:1 vermicrete has lots of airspaces between the grains and has little strength. If you are not using blanket first then it is a good idea to do the vermicrete in layers of around 1.5" thick and at least a week to dry between layers. Start from the bottom and make a flat ledge that you can build on top of. Go all the way around the dome then start again building on top of the wall you've just laid. Start with tennis ball sized handfuls and press and hold the mix in place. Once you've done the whole dome tap the surface lightly with the flat of a brickies trowel. The use of a cheap garden moisture meter will assist in drying the vermicrete before rendering over it. It will turn white when partly dried but don't be misled it still contains lots of moisture. Another method is to throw some plastic sheeting over it when firing. The moisture will condense on the underside to let you know it's still purging water. If this process is done too fast the vermicrete will swell and crack so best to take it slow. I use "heat beads" which are brickette bbq fuel. They don't flame up much and provide prolonged gentle heat. Use some small kindling with them to keep them alight. For your sized oven you'll need at least 10 kg of the stuff to dry your vermicrete. Sorry if this sounds a bit involved, but if you proceed too fast wet vermicrete can swell and crack your outer shell. If using a layer of insulating blanketbetween the dome casting and the vermicrete then it acts as an expansion layer and the problem is partly reduced.
                        Last edited by david s; 04-17-2017, 01:06 PM.
                        Kindled with zeal and fired with passion.

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                        • #27
                          very nice. I may have to build another one.
                          Texman Kitchen
                          http://www.fornobravo.com/forum/f8/t...ild-17324.html

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                          • #28
                            Thanks Texman!

                            I finished insulating the dome. I did a 1" layer waited a week and then another inch layer. I'm a bit concerned that the second layer will not stick or delaminate. We shall see...

                            J
                            Last edited by Jumbalaya; 04-29-2017, 01:11 PM.

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                            • #29
                              Well, my mixer came and it's missing tons of bolts, screws, and has extras of others. The directions for assembly are so poor, that I don't know that this will work. I don't plan to spend a lot of time putting it together if I get frustrated. Some damage also occurred on the shipment, as noted by Fedex. Here it goes.

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                              • #30
                                Originally posted by cnegrelli View Post
                                Well, my mixer came and it's missing tons of bolts, screws, and has extras of others. The directions for assembly are so poor, that I don't know that this will work. I don't plan to spend a lot of time putting it together if I get frustrated. Some damage also occurred on the shipment, as noted by Fedex. Here it goes.
                                I have this exact mixer and assembled it about 3 weeks ago. If you have any questions feel free to ask. Yes, the instructions are nearly worthless, but its not a very complicated machine so that helps.
                                You'll need some gasket in a tube stuff (Permatex) and I'd recommend some Loctite on the threads of some bolts.
                                This thing sure beats mixing everything in a wheel barrow.

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