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Finally decided on 32in castable dome

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  • Hi Mullster, Don't get too down. Does the flue gallery stand up on its own? Is it too much of a worry if it is cracked but stable? Will it not be OK once it is covered in vermicrete?
    These are questions for more experienced member.
    IMHO, and take that for what it is as a novice, I don't see why you couldn't cut it off with a grinder and have another go at the flue gallery. The dome should be OK as others have said it is an inherently stable shape, maybe repack with sand if you are worried.
    I did the same thing with my entryway mould, big screws facing away from me, took me half an hour to carve my way in.
    Would you have enough fireclay left for another flue gallery?

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    • mesoiam thanks for the encouragement

      The gallery stands perfectly fine - I’m just worried about what happens when it starts to get hot? If it have cemented over the cracks do they get worse (my lack of experience here means I don’t know that answer).

      Used every last scrap of the fire clay so will need to order more assuming I’m re-casting at least something. I also need to re-read up on what sort of substance to fill crevices with - can’t remember if that requires fire clay but I assume so.

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      • I think it is repairable. The biggest problem about filling cracks and voids is that they need to be done while the casting is still moist. If it's dry the area to be filled needs to be wet down again, although the surface must not have water sitting on it. It needs to penetrate. Mostly the voids and cracks should be filled with the same material used for the casting but with the course material removed. This can be easily done with a fine sieve.Then mix the stuff to a peanut butter consistency and force it hard int the cracks.Try to hold moisture in iy again for a week.

        As you have learned trial and error are the best teachers. I'll bet you'll be itching to build a second oven for a friend with what you have learned from experience.Yes, casting the dome first, removing the oven mouth moulding plate, then casting the flue gallery in front of it is far easier.
        Kindled with zeal and fired with passion.

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        • Originally posted by david s View Post
          I think it is repairable. The biggest problem about filling cracks and voids is that they need to be done while the casting is still moist. If it's dry the area to be filled needs to be wet down again, although the surface must not have water sitting on it. It needs to penetrate. Mostly the voids and cracks should be filled with the same material used for the casting but with the course material removed. This can be easily done with a fine sieve.Then mix the stuff to a peanut butter consistency and force it hard int the cracks.Try to hold moisture in iy again for a week.

          As you have learned trial and error are the best teachers. I'll bet you'll be itching to build a second oven for a friend with what you have learned from experience.Yes, casting the dome first, removing the oven mouth moulding plate, then casting the flue gallery in front of it is far easier.
          DavidS for you to say it’s repairable makes me feel significantly better thank you! But do you mean both the gallery and the dome? The gallery has cracks but is also split in the top right side - is it possible to repair it strong enough then in your opinion?

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          • Sorry to hear of your set back Mullster. From the photos it doesn't look to bad so hopefully it will be repairable. I know how annoying it is to have a set back especially when you only have the weekends to work on the oven and you just want to get things done. Believe me you're not the only one, there have been several times when I've felt the same way during my build. It's a massive learning curve and like David says trial and error are the biggest teachers.

            From what I've read every oven has cracks and they will still function perfectly well. Some of yours look like they may just be surface cracks and most of the external voids should be able to be filled.
            If the structure is sound and the dome is in good shape by the time you have rendered the oven and the gallery is full of soot you wont even notice the cracks, your'll be enjoying the pizza too much

            The crevices you mention seem to be a common problem, at least from what I've seen and I'll admit I have a few in my dome.
            I think there might be a few reasons for this but david s could confirm

            The first is the wet newspaper.
            I found applying the newspaper really frustrating, I knew that if it didn't sit perfectly flat is would cause ripples in the dome.
            I made several attempts to lay the paper flat, getting so far then tearing it all off again as a crease would form in a lower layer. If this happens you have to start that section again as the crease will come through every subsequent layer.
            In the end I had to drown the paper in water to get it to lay flat and i had quite a thick build up of paper as I was doubling up the strips and I applied many.

            The second thing and one I wasn't expecting is how the newspaper can slide and crumple when applying the home brew. I believe this is the main cause of the crevices, the paper creases up in places and leaves voids that the home brew cant fill.
            Luckily I noticed this early on and was more cautious when applying the cast. If i saw the paper start to slip i would stop and adjust it so it was laying flat again.

            The last thing is the consistency of the home brew. I think a slightly wetter mix is easier to apply.
            I found if the mix was too dry it would stick to the newspaper and cause it to crease up easier.
            Keeping the newspaper really wet and having a wetter mix seemed to help prevent this.

            As you can probably tell I wasn't a massive fan of the newspaper method, I kept thinking to myself there must be a better way, then I'd remember I'm a complete novice and I'm probably just applying it wrong.
            I'd read some people have used cling film to cover the sand which I also tried but found it even harder to get it nice and flat so soon gave up with that and went back to the newspaper.

            If any one has used any other method to cover the sand I'd like to hear it.

            Good luck with the repair, hopefully after a few days of rest you can come back at it with fresh eyes and sort it out with out to much trouble.

            Where abouts are you? I may have a bit of left over clay.

            Comment


            • Hattori-Hanzo thank you - I’m hoping I will feel it’s not so bad after a few days. I’m in Wigan - thank you for the offer on the clay - but it’s no trouble - I’ll order some more - it’s a small percentage of the total build cost at this point as we all know

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              • Regarding the newspaper technique, I've actually only used this method for the first three ovens that i built back in 2007. I don't recall having difficulty working over it, but if there's a breeze it would certainly interfere with the placement and the stuff drying and blowing away. As I suggested the addition of flour to the waterwheel applying the wet strips (or diluted PVA) will make them stick, making in effect paper mâché. You must start adding the castable to the mould from the base up, making a flat ledge at the top to take the next layer, wriggling a handful of mix against the mould is usually sufficient to eliminate the formation of voids.

                Re the repair, it's your call, but from the look of the pics, if it were mine I'd be bogging up those voids and cracks ASAP, before the casting has a chance to dry out.
                Last edited by david s; 06-28-2020, 02:47 PM.
                Kindled with zeal and fired with passion.

                Comment


                • Originally posted by david s View Post
                  Regarding the newspaper technique, I've actually only used this method for the first three ovens that i built back in 2007. I don't recall having difficulty working over it, but if there's a breeze it would certainly interfere with the placement and the stuff drying and blowing away. As I suggested the addition of flour to the waterwheel applying the wet strips (or diluted PVA) will make them stick, making in effect paper mâché. You must start adding the castable to the mould from the base up, making a flat ledge at the top to take the next layer, wriggling a handful of mix against the mould is usually sufficient to eliminate the formation of voids.

                  Re the repair, it's your call, but from the look of the pics, if it were mine I'd be bogging up those voids and cracks ASAP, before the casting has a chance to dry out.
                  David,

                  What do you use now? A better built form that can be reused? I remember making paper mache being a kid using newspapers and water with potato starch. But that needs to be dry. Do you think it is OK to glue newspaper all over the sand mold, let it dry and then cast over it? Maybe even painting over the dry paper before casting? I kind of like the idea because the mold will be firm (so the castable may be pushed onto it) but I am not sure if it will create any problems with the castable concrete.

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                  • I use a fibreglass mould for making my ovens, but the work required to do that for one oven would not be worth the extra labour and expense. If you do a paper mâché layer allow it to dry then dampen the surface before casting on to it then it should still all hold together. any few bits that stick to the inside of the casting and proves difficult to remove will simply burn away once you start the fires.
                    Kindled with zeal and fired with passion.

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                    • David

                      Thank you! Sorry, another question. What if I paint the paper mache will I be able to lay the castable material on/around it? Not worried about removing it, it will burn off.

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                      • Yes you could do that, more work of course. The whole purpose of the newspaper is simply to get a nice smooth surface on the inside, free of sand grains. As there maybe some voids on the inside to fill, the easy removal of the paper saves time. If you decide to paint it, coat it with a thin oil to make its removal easier. I use 50/50 kero and motor oil but spray cooking oil or similar also work ok.
                        Kindled with zeal and fired with passion.

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                        • David, thank you. I am trying to understand how much work I am willing to put in during preparation to help make the casting process easier... Or trying to make up for lack of experience.

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                          • Ah gutted for you Mullster , but at least the pro's think it's still salvageable. All I can say is thanks very much for sharing though mate, genuinly, as I had just ordered a load of MDF from B&Q!! I'll have a little check on here what the best gallery form material would be.
                            Hope you get it sorted, best of luck.

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                            • You can still use MDF just wrap it really well in cling film, that's what I did for some of my formers.

                              Laminated chipboard is another good material to use and doesn't require wrapping.

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                              • Repair job done - fingers crossed

                                So the additional bag of clay arrived today and I took advantage of the dry evening to do the repair job. Made a small batch of the home brew to peanut butter consistency and troweled away.

                                Well - truth be told I spent an hour trying to rectify another mistake which was using a full metal paint can as the mould for the chimney. As I mentioned before - for no good reason I hadn’t put any cardboard or anything around it, so it was wedged in there real good. Angle grinder, chisel, washing up liquid, all played a role until an hour later it finally was set free. I had worried that the whole flue gallery might collapse getting it out so I’m glad that didn’t happen!

                                Repaired the cracks and breaks - it’s not pretty so I’m not sharing the pics . You can see it when I have a blanket covering my mistakes!

                                Just a question regarding timings now. Assuming I spend this weekend on the decorative arch and it is the following weekend (10 days) from now before I start with blanket, etc - is that long enough?

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