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Starting new build ~32in Gas homebrew castable

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  • Starting new build ~32in Gas homebrew castable


    Found this forum from the pizza making forum and found more castable info here.

    Did a stand with 4x5 stack concrete block on corners and 6cm thick reinforcement slab. Watered it for a week on a timed system. Was first time I did this...Click image for larger version

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    Now doing the 10cm perlite 6:1 for under bricks today.

    Didn't buy firebricks as they were 12x+ the cost. Just going to use red clay bricks for now for a month or 2 (or more) to afford the needed 25 firebricks.

    Can't afford wood to burn here so going with a gas burner system. Couldn't find a "real one" so still figuring out how I will run the natural gas to fire it.

    The castable will be homebrew 3:1:1:1 sand: hyd lime: concrete:fireclay or the .5 version of whatever the one thing is in half I read a while ago, dunno if fireclay or something else will check later.

    Found only glass fiber anti crack for castable here. Couldn't find SS or polypropylene burn out.

    Question: can I use dried thin grass or basil stems for burn out/moisture removal after first or second firing a week or so after the castable is set? Or there is no need for this and do the glass fiber


  • #2
    Solid red bricks are generally unsuitable for an oven floor, but it does depend on the clay body composition. Clay bodies with a low percentage of alumina and a high percentage of iron (usually indicated by the red colour) do not stand up to prolonged heat well over time. If you lay them loose you may be able to replace them fairly easily later. You also need their thermal mass, so make sure they’re solid, not modern house bricks with holes in them.
    Gas burners are required to be fitted and supplied by a licensed gas fitter. The burner and regulator need to be matched to the oven’s size and shape and must also be fitted with a flame failure device. The discussion and promotion of home made set ups is not allowed on this forum for safety reasons so you won’t get any help here regarding that topic.
    The fibreglass reinforcing fibres are suitable for a homebrew castable as they don’t melt until around 900C which is well north of our temperature range, but you must use the AR alkaline resistant ones.
    Regarding the burnout fibres, seek a Sika agent to access them. They are widely used to reinforce concrete and usually come in a double pack with a thicker longer polypropylene fibre for strength and the much finer fibres which are the ones you want, for controlling early shrinkage cracking. These fibres melt at only 160C so any thing else you use to replace them should be tested first otherwise you risk destruction from steam spalling which usually occurs between 200-300C. Any added burnout fibres need to be mixed and very well dispersed into the wet castable mix to do their job.
    Last edited by david s; 09-29-2023, 12:35 PM.
    Kindled with zeal and fired with passion.


    • #3
      Perlitethank you David, Ive read many, many of your useful posts.

      for the bricks they clay solid for oven exteriors, and I will lay them on fireclay/sand loosely for floor and see how it goes for a couple months, and if needed replace with 25 white firebricks. I plan to use the oven about once per 2 weeks.

      There are many gas companies in the city so I will contact one to set something up to make sure there are safeties involved and not promote or ask about the setup here. Otherwise will purchase some wood for fireups but not cheap(can u use coal firewood?). Edit,.found a supplier wholesale... maybe cheaper than I think so I'll check Monday, would be easier and safer.

      I will contact sika online, I went to some dealers here last week and before who didn't have any. Worse case I'll leave them out. Sand mold will start in a few days as just laid the 1:6 perlite.... weird stuff, also first time using it.

      Dims of the perlite base is around 34" plus the 20in or so front. The cast will sit on it. Then I have 2in of rockwool or so, and the rest perlite at 10:1, tho the way 6:1 was I dunno how it will stick...

      Last edited by AlPizzaMan; 09-29-2023, 01:46 PM.


      • #4
        So if your perlcrete slab is 34” are you intending a 30” inside diameter ove?
        if you intend to make your floor bricks removable and replaceable down the track, then you would be better to cast around the floor rather than on top of it. This will require you to cut and fit pretty accurately the red floorbricks now, prior to building a sandcastle over the floor and casting in situ. Just wrap a strip of cardboard around the outside of the bricks so the castable won’t bond to the brick edge This will leave a slight gap to allow for floor brick expansion and the space will simply fill with ash anyhow.
        Regarding wood, on principle I never buy wood and have only broken the rule once in 17 years when I was scint for time. There’s plenty around if you look. I collect most of mine from a nearby park where there are always fallen sticks and sometimes larger branches. I only collect hardwood, council areas with trees that are neglected are great and you’re doing them a favour. The fork of a tree works really well to break small branches and as the rule is ‘“ nothing thicker than your wrist”. This method eliminates any cutting and splitting. In addition it’s using carbon that’s already in the system rather than adding to it by using fossil fuels.
        Last edited by david s; 09-29-2023, 06:14 PM.
        Kindled with zeal and fired with passion.


        • #5

          The perclete slab is around 35-37in and the front area 22 or so. I left it u covered for about 9 hours now watered it and covered with a light spray every hour on a watering system .

          I had planned a ~32in ID and then 18" door width as i would like to use my 2 16in pans for New York style other than high temp neopolitan. I am not fully sure of the height yet, will confirm in a few days. I wanted it a bit lower for higher temp on the pizza and less to burn.


          • #6
            Whilst a lower dome reduces the chamber volume and therefore reduces fuel consumption somewhat, it’s disadvantage is the dome being lower reduces height for tall loaves roasts or cooking pans close to the sides, particularly for small ovens. The hemisphere has proved to be the best all round chamber form, but it also depends what kind of cooking you will be doing mainly. Because you are doing a cast oven, the departure from a hemisphere does not affect the structural integrity as much as a brick build so you could alter the form of the mould to have both a low dome and taller sides at the perimeter. Let me know if you don’t understand this and I can post a diagram.

            Regarding pizza cooking, my own preference is to cook smaller pizzas as they’re easier to make, load and cook, but again, we’re all different. Most operators prefer to cook the pizzas directly on the oven floor rather than in pans because they require double handling because the wet dough traps moisture against the pan resulting in soggy bottoms. The usual technique in this case is to remove the pizza from the pan when 2/3 done and replace it on the floor to crisp the base. As the very conductive metal also requires heating before it starts to cook the bottom, it is drawing additional heat from the oven floor. This is likely to lead to more regular floor recharging. But do whatever works for you.
            Last edited by david s; 09-30-2023, 05:02 AM.
            Kindled with zeal and fired with passion.


            • #7
              About the castable ratios, I have read 3-1-1-1 and then 6-1-1-1 (but I think this was from a forum post about mortar used between bricks not a castable), plus another mention where the fireclay ( I think was in half and mentioned by you David) so 3-1-1-.5-1 for sand, cement, fireclay, and hyd lime.

              Can you let me know which ratio is best to use for a castable oven in a hot dry environment (where I will be curing the cast with automated watering on a timer) ?

              I will be adding about a kg of fiber glass needles, and am trying to get the sika burn out micro fibers delivered, they were not in stock in store.



              • #8
                I am going to move this thread to Other Type Ovens instead of being in Introductions.
                Google Photo Album []


                • #9
                  The 3:1:1:1 recipe by volume would be1:5 of clay to the other ingredients. Because clay imparts shrinkage owing to its extremely small particle size, I’ve found it sometimes creates shrinkage cracking when used as a mortar, so I dropped the clay content to 0.5 in the recipe. This condition can be exacerbated when casting over a sand mould, because it’s got nowhere to shrink from like an outside mould, although sand may provide a little give, but the reduction in clay volume is a better recipe IMO. We also live in a hot tropical climate so in summer I always leave casting work for the late afternoon to avoid early shrinkage cracking. Chilled water also helps.
                  Regarding the damp curing, a constant supply of water for the hydration process is far preferable to a wet and dry cycle that a sprinkler gives, particularly in a hot dry climate. Commercial precasts like large concrete pipes are often placed in a water filled trench to achieve constant hydration. I use large wet hessian sacks to wrap around Portland castings then place it in a large plastic bag.
                  Unfortunately the filling of any voids on the inside needs to be done as early as possible to get a good bond, so removing the sand after 24-48 hrs would be best, then sealing in the moisture with plastic for a week.
                  One of the advantages of CAC is that it undergoes a different and much faster reaction than Portland and therefore extended damp curing which slows down production, is not required., However, for the home builder a bit of extra time combined with the difficulty in using CAC and it’s prohibitive cost, make the homebrew a far better solution.
                  Last edited by david s; 10-01-2023, 04:54 PM.
                  Kindled with zeal and fired with passion.


                  • #10
                    I found the sika fibers I think, they are 12mm by 0.034mm (here I dont think they have other small size) / 34micro meters and come as 600 grams. They have ones that are 48mm long and .3 or .5mm (and another company has 30cm/0.3mm) thick but I should get the really small ones right? One is order out of town, the .3mm I could buy tonight.

                    The percrete after 4 days or so seems to be getting harder, was worried a couple days ago, even now a hard finger press crushes the perlite... I plan to place some ceramic tiles on the sides to be able to make the oven wall directly on top of the perlite and not the concrete slab. I want to be able to remove it later if need be or modify. I do wonder if the 4in percrete will hold or give. From calculation my 32" inner diameter should be about 0.03m cubed, so around 70kg, plus the front section+percrete on top after (still alot lower than the 100 psi that the 6:1 percrete is near rated).

                    The bricks are ~3.5kg each and will be placed on top of sand on the percrete and make sure not to bind to the cast (again so I can remove it later if I need, 25 red clay was ~14usd, while 25 firebricks are 180usd).

                    I saw this thread where I think I will try to replicate the build of a circle straight up, then the roof on sand, and the door area made separate.

                    ​​​​​​​I appreciate your input and feed back David


                    • #11
                      Because cement in a perlcrete mix makes the mix stronger, at the same time it reduces insulation value. So for under floor insulation that has to support the floor and the dome, we recommend a ratio of 5:1. So at 6:1 it might be on the weak side, but as it’s pretty close, I think you’ll get away with it. For insulation over the dome, far less strength is required and it only needs to be strong enough to apply a cement render to it, so a 10:1 mix works well, providing good extra insulation value.

                      The fine polypropylene fibres are finer than human hair and used to control early shrinkage cracking, but also conveniently melt at 160C so work as burnout fibres for our application. Keep asking around or try the net.
                      Kindled with zeal and fired with passion.


                      • #12
                        I remember in high school kids would say "oh man I did so bad on my test", I'd be like, me too, what's your mark. "Only 93%, what's yours?", uh dude that's good, I got 60.

                        Anyway. I saw some other guys saying their ovens looked bad upon making it. Lolol. Wait till you see these pics. Anyway. I'm missing some tools here and didn't wanna buy any, and didn't wanna buy any wood so used what I had, an old desk.

                        Man it took about 5 hours. And I guess 180kg of mixed cement.... anyway. It's also about 15 5 gallon buckets of sand.... that's with some extra stuff I put in the mound like some bricks, large 12l empty water bottle, and some styrofoam I had....

                        Now, questions.

                        1) I did this 12 hours ago, watered it and covered it. I read that within 1 or 2.days I should open it and look for parts I messed up inside to fill the spaces, right?

                        2)do I let it dry for a week covered and/or watered? Then I put the insulation on?

                        3) do I start small fires only after I put the rock wool + perlite layers?

                        4) should I run an auto sprayer over it? Ie spray mist for 30 secs every hour or 1/2 hour and just leave it uncovered all week?

                        Can't think of anything else now.

                        Attached Files


                        • #13
                          Here are the pics after I was adding the 3-1-1-1 mix. I reduced the clay a little and a couple times added more sand when it was too wet.

                          I used about .5kg fiber glass total.
                          And about 200g of burn out fibers. Maybe I shoulda used more but was too hot/too late... anyway we'll see how it goes.

                          The tiles around my perclete were because, I miscalculated the size. 32" oven requires about 36" space. So I went about 31.5 or so and added the tiles on sand/clay dust on the sides to make sure I wasn't over and tearing up the percrete. Ie more surface area while the hard tile is taking the extra mass spread over its larger surface outside of the percrete. I plan to add percrete around the outside all later on and should remove the red clay bricks which were supporting some weight before the castable dries and has strength.

                          This was alot of work. I am glad I have an idea now how one is made so I pay some workers to do all the hard stuff if I do again... it was hard to get near the 50mm I planned originally, some areas more some areas less. Hard to get mix right too, too dry to too wet is a fast change I did
                          ​​​​​a couple times. Hydrated lime wasn't fun to work with, so must dust, esp when adding water.

                          Cost was (prices local to about USD equi)

                          Rebar : 15$
                          Concrete blocks = 25$
                          Cement= 6$
                          Sand= 6$
                          Non shrink med str cement 50kg = 17$
                          Rocks= 10$

                          Total 74$
                          Plus about 5 hours making and pouring cement (all manually)

                          Oven so far +percrete
                          Fiber glass free
                          Burn out 20$ (bought 1.2kg)
                          Red clay 20$
                          Fire brick (2 only) 9$
                          Perlitex2 18$
                          rock wool x2 12$
                          Sand 10$
                          Lime hydrated 40kg, free
                          Fire clay 40kg 23$
                          red clay bricks (25) 14$
                          Cement 50kg 6$
                          Extra steel wire 2$
                          wooden frame stuff free
                          Hours fooling around planning and making 10 hours.
                          Percrete probably took 2 hours as well
                          Total 135 USD.

                          didn't Include gloves, mask, bucket,.shovel, trowel etc, which would add another 50usd or so

                          Still need to see how much wood Is here. The oven shop I was at today said they have a guy that does the gas blower stuff if I want. Dunno yet if I'll bother.

                          Bought 2 fire bricks today I found that were half price what I saw 6 months ago. Still 7x red clay price. Will try side by side and see bottom of pizza to compare. Might have to buy another 15 anyway...
                          Attached Files
                          Last edited by AlPizzaMan; 10-07-2023, 01:34 PM.


                          • #14

                            door hieght 12 from sand/clay base, from clay bricks -2.5in.
                            6in straight up sides. Then ~11.5in more so oven hight ~18in to base or -2.5to bricks which are removable. Castable sides sit on thin ceramic tiles which sit on percrete.
                            door width is ~18.5in at bottom around 15at top. Oven bricks are 44cm wide for 4. The 2.5 in up from door gives around 45cm.

                            Distance from oven to door is about 9in. 1.5in spaces between for chimney 6in.

                            Last edited by AlPizzaMan; 10-07-2023, 07:47 AM.


                            • #15
                              Ok, I removed the sand just now, one hole from the door to the chimney, was pretty small, sprayed water around, filled it. Most seems to be 2in thick, I added a bit on a couple spots outside and inside.

                              if you wanna ask someone, to make sure they will do this work, just ask the. "How much do you like sand?" If they answer "yes", then you can recommend them to make a pizza oven. Nothing like piling up 300kg of sand, then a day later move the pile back lol.

                              watered outside of oven and covered with plastic and other stuff to cure.

                              The tile support I did will need to be re Insulated around bottom with some 5:1 or just near 10:1.... I guess then just cover with a cement layer then tiles or paint.

                              Attached Files