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Lip on dome bricks using a tramell

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  • Giovanni Rossi
    replied
    I'm glad David summarized this for you. While researching this Forum for my build I keyed in on the distinction and importance of chemical curing and drying in this application and tried to be careful when documenting in my build thread.

    Just to capture your last question...this site typically recommends the vermicrete for dome insulation be 10:1. You need the stronger 5:1 for the floor because of the need to support the weight of the oven.
    I believe David often advocates making a more workable product by using a 50/50 mixture of fine vermiculite and medium perlite if you have access or a fine/med mix of either product alone.

    You can check out my build as I did something a little different. I used a build kit that used a 2" layer of 8:1 silicone coated pericrete (not available in the US as far as I know). But, I customized it by adding more blanket insulation so, with the greater diameter and resulting larger volumes required, I was going to run out of perlite. My solution was to do 3 layers: 8:1 then 5:1 then 0:1 thinking I would have greater insulating properties closer to the dome and increasing strength as I moved outward. I'll never know if this was a good solution, but it made sense to me at the time. The insulation of my pericrete layers probably didn't add much to 4 inches of blanket, but I believe I got a sufficiently hard shell on the dome.

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  • rsandler
    replied
    Another tool for drying out your oven after damp curing is to put a big halogen light in the oven. If you can find a strong enough light, and especially if you have at least a rudimentary door, you can get the bricks warm enough to evaporate off a lot of moisture if you're patient. On my second oven, I had a big halogen work light, and with the CF blanket insulation on and my insulated door around the cord, managed to get the interior above 100C after 24-48 hours. Can never know the alternative, but I think that's probably why I didn't end up with cracks despite my eventual drying fires getting away from me and raising the temperature faster and sooner than intended.

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  • UtahBeehiver
    replied
    David, as usual, gives sage as well as good technical info. This is the point where builders fire the ovens to fast and too hot risking the cracking of the dome. The turtle win the raise on this phase of the build. You can use briquettes or heat beads for the first few fires, gets you about 200 F and you can cook a dutch oven while you are at it. When you use wood, one extra piece of wood can really spike the temp quickly so don't over load. David sometimes places plastic over the insulation to watch of condensation formation which indicates water still exist in the build. If you see steam, you are firing too hot.

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  • david s
    replied
    There is confusion created with the word “curing” and therefore I try to avoid it. The term as applied to concretes and mortars refers to the hydration process in which a chemical change takes place including water. This is referred to as damp curing and why it is important to hold moisture in while this takes place. If a mix contains OPC, depending on temperature the hydration process will achieve sufficient strength in a week, but 28 damp days is the standard. If using CAC the reaction is faster, again temperature dependant, Specs say 24 hrs, I usually give it 48 to be sure)
    Once the damp curing is done that excess free water should be removed, if building an oven, or it can cause catastrophic problems of steam spalling.. Curing by heat means taking the material to a temperature where other chemical changes occur. This does not happen until the temperature exceeds 550C which is outside our service temperature range. So in the case of driving off mechanical water it is more properly referred to as "drying fires".
    Do not do these fires in an uninsulated oven. There have been numerous reports of ovens cracking when this has been performed. The problem is thermal shock and occurs when the inside temperature face is far higher than the outside face. The big difference in temperature leads to a big difference in thermal expansion. By insulating the outside it gives the material a chance to even out the temperature difference. Perhaps you could do the curing fires with just one layer of blanket. That way it’s easy to remove and inspect.
    So generally the safe procedure is a week of damp curing, followed by insulating the exterior, followed by 7 fires in 7 days getting progressively bigger and longer as well as trying to avoid too much flame impingement on the dome.
    Last edited by david s; 02-06-2024, 02:34 PM.

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  • TEDFB
    replied
    OK, I trust that you are all good well
    I have obviously not worked on the oven all the time, what with crazy high temperatures and I have had work to do on the property.
    Anyway I will most likely complete the dome this week.
    What I would like to know from you guys if you don't mind, is how long do I leave the oven to stand before curing it.
    I also believe that I must cure it before I insulate it, in case of any problems. Would this mean I would have to then cure it with a few more fires to dry out the insulating layers?
    I am going to put an insulating fibre blanket over the dome and then a vermiculite plaster before the final plaster layer.
    Is the vermiculite mix the same as below the oven floor. 5 to 1 cement.

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  • TEDFB
    replied
    Hi guys
    Trusting that you are all doing fine
    Just letting you know the pizza oven is coming on well, although quite interesting joining the dome and the arch, but seem to be managing at the moment.
    Have a great day

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  • TEDFB
    replied
    What a pity I did not start off by first getting advice on the this site, as I was advised to begin with that I should stay away from home made mixtures as they tend to fail. Anyway we live and learn, I have spent the money so I will have to plod along.
    Thanks again for your input

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  • rsandler
    replied
    I was assuming you were using the Portland cement-based homebrew mortar made of sand, cement, lime and fireclay in a 3:1:1:1 ratio. That stuff can handle a large joint--I've heard of people sticking wedges of firebrick (offcuts from tapering the dome bricks) into the joint to use less mortar and have more strength, but it's not necessary, and surely more work than it's worth if you only have an angle grinder. Neither of my ovens, built with homebrew, used spacers, and if the one I built 11 years ago ever cracked I never saw it and it never affected performance. As with any Portland cement based product, the homebrew mortar will have more strength if you can keep it damp for a week or more, though it will probably have sufficient strength to keep the dome together without doing so. I put a wet blanket on top of the dome of my second oven once I closed it up, but didn't do anything to keep the mortar in the lower courses wet as I laid them (by the time I close the dome, those lower courses had been set for weeks).

    If you're using a Calcium Aluminate Cement (CAC)-based product, which typically are engineered for smaller joints, all bets are off and I don't know what to tell you :-)

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  • TEDFB
    replied
    Hi rsandler
    I was under the impression that the spacers were there to prevent cracking if the cement is too thick.
    I don't suppose adding a bit of fine sand to the refractory cement would be a good idea for that.
    Also is it a good idea to keep it damp for a day or so to allow for the curing?

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  • rsandler
    replied
    Won't help with the weather, but as far as spacers, depending on your design, you can use a 12" clamp (I used the squeeze action kind, I think maybe called a clutch clamp) to clamp the brick into the L shaped part of the trammel. Leave it there for a bit so the mortar can stiffen, then no spacers needed. On my tool, pictured above, the clamp sits against the back (outside face) of the brick and the smaller wood block.

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  • TEDFB
    replied
    Mmmm, ok if I look at what you guys said and see the green line explanation it should be common sense, obviously that evaded me for a while , any way I modified my trammel and all looks good.
    I have layed my soldiers /sailors but because of 104 degrees F I lost my enthusiasm for a couple of days.
    Really struggling to cut my spacers for between the bricks with only a hand held angle grinder, but never the less it is coming slowly.
    Have a great day, I have been greatly helped by your advice, THANKS

    ​​​​

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  • mongota
    replied

    You want the "L" (green line) that cradles the brick to cover more of the inside face of the brick so that your trammel arm (red line) is centered on the face of the brick.

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  • Giovanni Rossi
    replied
    Sorry Ted, I've been away. rsandler already gave you good direction and clarified my earlier comment: "AND, the center of the trammel shaft must line up in the middle of the dome bricks to achieve a hemispherical dome."
    The guys who have built proper ITs (trammels) are very clear that you need to be able to visualize a straight line from the pivot point at the floor through the shaft to the mid point of the inside face of each brick, both vertically and horizontally. Otherwise, each course will be out of alignment with the last.

    As far as your floor under the dome, I'm not the best source of info as my floor tiles were mortared in place. I'm not quite sure what you mean by "loose". But, from what I've read on this forum, as long as you've used enough dry sand/clay mixture to be able to get your bricks level and seated well (not rocking) under where your dome rests you should be fine. I've seen some builders recommend using a 3/8" - 1/2" notched trowel to apply a proper amount of the bedding. The weight of the dome will help to hold the bricks in place.

    I'll add to david s 's comment about how wet to make the bricks. I had good success with soaking my bricks for at least 15 minutes. When I was ready to lay the next group, I would lightly dry them and rest them in place on the dome. This allowed some water from those bricks to drain and dampen the bricks underneath. This keeps the bricks already in place from pulling too much water too quickly from the mortar.

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  • TEDFB
    replied
    Hey guys
    I am really grateful for advise and help, I am going to get stuck into this lot today and will give you an update on the progress,
    Have a great day

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  • rsandler
    replied
    Back on the trammel question, you need the pivot point of the trammel to be centered vertically on each brick. From your picture, it appear that the pivot point is in line with the top of each brick, leading to the substantial ledges in the picture. Lots of ways to get that offset; here's a picture of the one I built for my second oven. My bricks were 2.5" thick, so the bit of angle iron that the top of the brick sits against was 1.25" above the pivot on the castor. Click image for larger version

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