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Help I used the wrong castable....suggestions? - Forno Bravo Forum: The Wood-Fired Oven Community


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Help I used the wrong castable....suggestions?

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  • Help I used the wrong castable....suggestions?

    Hey all

    Great site, just wished I'd found it before I started work on my oven!

    At this stage I have cast my dome over a sand mould. It was only after doing this that I realised from reading the posts on castable refractory cement that I have used the wrong product. I cast it instead of bricking for budget reasons- as you may be aware firebricks are not cheap here in Western Australia.

    The product I used is a medium weight insulating castable called Shiralite 130MW. Will paste some data below for those of you who understand it.

    The reason I used it was that the refractory stockist advised that it was the right product to use and that it was used by local commercial kit makers. Given my reading from this site I am now rather unimpressed with that advice

    My questions are these: Will it still work as an oven- what is the effect of using the insulating castable ( I had hoped to use it to make bread and other slow cook items so am wondering about heat retention)? Should I knock it down and start again? (cant really afford to )Or is there something I could do to improve its performance such as render the inside of the castable with non insulating refractory cement?

    I had been planning to use ceramic insulation and then vermiculite render on top of the dome I had cast but it would seem that my oven may be insulated enough....

    Any suggestions would be much appreciated. Yes I'm an idiot for not reading enough before I started...

    Shiralite 130MW

    Al2O3 ......................................... 46
    SiO2........................................... 38
    Fe2O3......................................... 1.5
    CaO........................................... 11
    TiO2........................................... 1.4
    MgO........................................... 1.1
    Alkalies...................................... 0.6

    Maximum Service Temperature (oC): 1300
    Nominal Shelf Life: 12 months

    Approximate Thermal Conductivity:
    oC 250 500 750 1000 1250
    W/mK 0.40 0.42 0.44 0.46 0.48

    Installation Procedure................... IP/001, 002
    Heat Up Schedule........................ HS/002

    Bulk Density (kg/m3)
    After Heating to 110oC.................. 1550 - 1610
    After Firing to 1000oC................... 1440 - 1500
    After Firing to 1200oC................... 1560 - 1620
    Cold Crushing Strength (MPa)
    After Heating to 110oC.................. 5.5 ? 8.0
    After Firing to 1000oC................... 3.0 ? 5.0
    After Firing to 1200oC................... 3.0 ? 5.0
    Modulus of Rupture (MPa)
    After Heating to 110oC.................. 2.7 - 6.0
    After Firing to 1000oC................... 1.0 - 4.3
    After Firing to 1200oC................... 1.9 - 5.2
    Permanent Linear Change (%)
    After Heating for 24 hrs at 110oC..... 0.0 to -0.2
    After Firing for 5 hrs at 1000oC........ -0.2 to -0.5
    After Firing for 5 hrs at 1200oC........ -0.5 to -1.5
    Maximum Grain Size (mm) 4

  • #2
    Re: Help I used the wrong castable....suggestions?

    The material is a little lighter then what we have had success with, but its not to far off.
    I think it is worth keeping intact. The lower density will allow the oven to heat up a little faster, but is will aslo cool down faster because of the lower mass


    • #3
      Re: Help I used the wrong castable....suggestions?

      That is pretty light. I converted 1500 kg/m3 and got about 93 lbs/cubic foot. Lighter than you would want.

      Does anyone else want to jump in here?

      I would want to think about the oven's heat holding capability. Is there any way of testing it before you go all the way through to complete it?

      Pizza Ovens
      Outdoor Fireplaces


      • #4
        Re: Help I used the wrong castable....suggestions?

        Piggybacking on James estimate, a firebrick is roughly 72 cu in, or about 1/24 of a 1728 cubic inches in a cubic foot. 93 Pounds divided by 24 is 3.875 pounds, or about half what you would expect a firebrick to weigh. Sounds like an insulating material.

        Since it's already built, I'd say try it with fire-in-the-oven pizza making. It may work great, let us know. For retained heat bread baking and roasting, I kind of doubt it will work.

        Why, oh why doesn't someone go into the discount firebrick business in Australia? Are there ruinous import duties or something? Surely someone in the import/export trade could do this as a profitable sideline at almost no cost. It seems that you could keystone firebricks, and still underprice the domestic prices by half.
        My geodesic oven project: part 1, part 2


        • #5
          Re: Help I used the wrong castable....suggestions?

          Thanks for the replies guys.

          At this stage I can test it as is before I add the insulation (ha didnt I already do that ) and flue.

          My oven is reasonably large (internal diameter of 1.2m or 47") so I had thought that if it is really bad I could stack up some firebricks around the inside for baking puposes. I only really need enough space for a few loaves of bread or a largish piece of meat in the middle so adding thermal mass as and when I need it might be a good solution.

          I used the inch thick firebricks (tiles) as a floor so there wont be too much heat retention there either I guess)

          I think that I will probably only be using the retained heat aspect of the oven maybe 15% of the time, so maybe its not too much of an issue.

          For pizza purposes the quick heat up time will be useful though I suppose.

          I guess making sure I do a good job on the top layers of insulation will make sure as much of the heat as possible stays in. Was going to go with 2inches of ceramic blanket followed by vermiculite render and waterproof layer. Is this worthwhile or will I be overinsulating then?
          Am thinking that the door will also need to be fairly well insulated.

          Thanks again for your time.


          • #6
            Re: Help I used the wrong castable....suggestions?

            The conventional wisdom around here is that you can't be too thin, too rich, or have too much insulation.

            But hey, try it the way it is, first. If it's only warm to the touch with a raging fire inside, you have all the insulation you need.
            My geodesic oven project: part 1, part 2


            • #7
              Re: Help I used the wrong castable....suggestions?

              Thanks Dmun

              Have to go through the curing process yet but will let you all know how I go.

              Here's hoping!


              • #8
                Re: Help I used the wrong castable....suggestions?


                Well the curing of the internal shell is finished. I got a series of hairline fractures (which I was told were unavoidable-and are only open while it's hot) but the dome seems to have maintained structural integrity.

                In terms of the shell heating up, on the final curing fire the whole structure was so hot I could barely touch it (1 and a half hours after lighting). So I guess that although the castable has insulating qualities it seems to be heating up appropriately.

                I am a bit concerned about whether it is reaching the right temperature though. The inside didnt turn white as I have seen with other ovens in terms of indicating cooking temperature. Can anyone advise whether this is likely to happen before the internal layer is insulated or is this an indication that the castable is not functioning correctly or that the measurements in terms of opening to dome (currently at 63% of dome height)??


                • #9
                  Re: Help I used the wrong castable....suggestions?

                  Originally posted by dmun View Post

                  Why, oh why doesn't someone go into the discount firebrick business in Australia?
                  Plenty of stuff for sale at reasonable prices here.
                  refractory and ceramic
                  And here.
                  Darley Refractories Australia, Bacchus Marsh, Victoria
                  The English language was invented by people who couldnt spell.

                  My Build.



                  • #10
                    Re: Help I used the wrong castable....suggestions?

                    The castable refractory I use is around 2000 kg/cu m so yours at 1500 is a bit lighter but should probably still work ok. You'll have to go through several firings to eliminate all the water. Don't be in a rush to get the dome white yet. Read the sections on curing in the forum.
                    Kindled with zeal and fired with passion.


                    • #11
                      Re: Help I used the wrong castable....suggestions?

                      This sounds like a mixed blessing type problem. WFOs are made out of just about everything imaginable. I can't imagine it being a big problem for pizza where you want flames and heat loss is not a big deal and a shorter heat up time is nice.

                      One question is what the floor is made of. If it is your refractory mix you may have to pull coals out to heat soak it more often but...whether it is every pizza or every third pizza is not a particularly big deal.

                      I would tend to put an inch or two of additional denser cement over the dome IF I could (and I did on my FB oven - just to give it some extra mass) but again it is not going to be critical.)

                      Putting some extra bricks in when doing bread MIGHT be beneficial but it will make cleaning out the oven a bit more tricky.

                      My bet is that it is well within the range of WFOs that work reasonably well. Biggest issue may be that you can only get one batch of bread per heating or that your temp drops such that you won't have as big a window for baking roasts, etc, after doing pizza or bread, and that is a good reason to insulate, insulate, insulate. But I will bet it is not a big problem for you.

                      Each of our ovens have their own peculiarities and yours will too and you will learn the oven and make it work!

                      Good Luck!


                      • #12
                        Re: Help I used the wrong castable....suggestions?

                        Thanks for the responses.

                        Yep have been through the 7 day curing process at this stage to get rid of all moisture. Was just wondering if at the end of this curing process I should be getting the dome white?

                        Jay, my floor is made of one inch think refractory bricks or tiles as they are sometimes called and they are sitting on 2 inches of calcium silicate board. Thanks for the encouragement- its a bit frustrating starting and then learning that you recieved some bad advice on the product to use. Will go to town on the insulation since i have purchased it all anyway.

                        If I dont get the heat retention i need then i will likely get inside the dome and render it with some denser refractory cement-it seems like this is the most sensible place to put it if the other castable has insulating qualities.

                        Will post some photos when I return from holidays in about a month!


                        • #13
                          Re: Help I used the wrong castable....suggestions?

                          Bill1, whose product did you use? We have had success with SILA brand ex W.A. Also the height of the dome internally, what is it? Ok, just saw it was Shiralite 140 MW. This is Shinigawa's medium weight 1400 grade insulating castable. NOT a good dome material. If you choose to redo the dome, get some Silacast 135. Density of 2200kg/m3 vs 1400kg/m3 that you have used.
                          Last edited by Johnny the oven man; 04-29-2009, 05:42 PM.


                          • #14
                            Re: Help I used the wrong castable....suggestions?

                            Pretty sure it was the Sila brand- will have to check though. It seems to be working well anyway- when the walls of the oven were too hot to touch after 4 hours of a largish fire on the final stage of curing there was a just noticeable warmth on the underside of my slab- but that is more likely because the walls of the dome were poured directly onto it. As you'd expect the walls are quite thick at the bottom anyway- maybe 100mm.

                            The dome height internally is 43cm, and the opening height is 27cm. The internal diameter is 1.2m.


                            • #15
                              Re: Help I used the wrong castable....suggestions?

                              Dome height looks OK. PM me if you need any info.