New build - 1.2M dome

[JRPizza]

Wow, nice tight joints! The trick is going to be avoiding joint "creep" as you don't have much room to play with and don't want to have to shave bricks down as you get to the top. Are you planning on marking their location on the template as a guide?

[jonv]

A little light work before the pub beckons... Put the inner arch form together and played around with the bricks. I still need to shim it when I put it in place - there'll be two flat bricks above the floor level. Very happy to find that the stock 3"/2" side arch bricks work out almost perfectly; at least until I try cutting them to meet the dome!

[jonv]

Any views on whether there might be advantages to building the whole inner arch first, or is it better to construct that as needed as the courses progress?

[jonv]

I shall give it a go. I rechecked the position of my arch and found I had brought it forward 1/2" too much for the top brick to reach far enough back into the oven... had to lift the front hearth bricks and trim an inch or so off them. Hopefully I'll be sticking bricks together next week

[thevance53@gmail.com]

The best thing to do is just cut a couple of bricks and see how far they are off. You might waste a couple, but you will see how it goes together pretty easily. I couldn't figure it out until I just did it, and then it just comes together. Bring the dome to the arch brick, use your IT to mark where the dome brick will intersect the arch brick, nad then cope that back to your arch form.

[jonv]

As I had a pile of firebrick scraps from cutting the floor, I thought I'd see whether the mortar was any good, since this sort of mortar is new to me. I know many builds have used the homebrew entirely successfully, but I elected to experiment with ciment fondu. I won't need much mortar with the arch bricks I'm using for the dome, but they do dictate thin joints.

For the aggregate, I had some mullite, but it was a bit too coarse with not enough fines to be worth sieving. For something finer, I was offered some finely ground bauxite. It's a fair bit finer than sand, so ideal for thin joints.

Anyway, mixed at 3:1 it formed a nice silky paste that I think will be easy to work with once I get used to it. I soaked a few scraps for a minute or two, smeared on a little of the mortar and gave it a squeeze. They seemed to have a good grip and the next day they had set rock hard (actually fairly solid after a couple of hours). Managed a couple of pretty thin joints; the mortar is more like glue than any sort of mortar I've used before. These suckers aren't coming apart any time soon. I'm hopeful.

[jonv]

Thanks! I'm not even dreaming of the moment I get to plug the dome... the inner arch transition is what occupies all my thoughts. Still can't get my head around it!

[thevance53@gmail.com]

The fiddling has just begun, and there is a lot more to come. I probably should have fiddled a little more, but now I am close to plugging the dome. Good luck.

[jonv]

Finished setting out the hearth today. Like others, I used a mix of sand and fireclay to deal with my somewhat variable calsil layer. I did try to do it dry, but found it much easier to mix it to a thick paste and apply to the blocks first with a notched trowel. I was surprised just how much fiddling was needed to get it more or less flat and there's not a lot of working time before the calsil has sucked the moisture from the paste.

Following earlier feedback, I dealt with the possible problem of rising damp with three thick layers of waterproof paint, hence the now black slab. I wanted to raise the calsil off the base a little, so put down a layer of 6mm tile backer boards, which I treated with three coats of sealer, although the third coat seemed to evaporate more than soak in. With that, and the two holes drilled through the boards and slab, I think the moisture issue has been dealt with

[jonv]

Thanks both for your thoughts on that. The wooden brick idea is a great one, but when I got hold of some 12" x 12" x 3" blocks, which I hope will make for a great hearth, that idea went out of the window. They are really heavy and I didn't fancy trying to drop one in at the end without much of a chance of getting the level right.

You're quite right, my oven with be a little taller inside than the radius of the dome, but over that size I figure it will be ok and saves me having to do any adjustment as the dome progresses. I am trying to minise that extra height though - using 12mm ply instead of the usual 18 and using a mini tripod ball joint instead of a proper sized one.

I didn't think about the strength of the bond being better without a soldier coarse Having seen them used on so many builds it didn't occur to me that there were disadvantages. How much difference do you think it makes?

I need to think about this a bit more. I only have a few flat bricks, and they are needed for the start of the arches and the vent area. The rest are all arch bricks, which makes it a bit tricky to lay a vertical section to bring the level above the floor...

[deejayoh]

Impressed that you figured it out in practice Gulf, I didn't figure it out until I built the spreadsheet. Took me lots of maths...

[Gulf]

Jonv, have you considered pulling out your "center" floor brick and using the wooden brick trick? I don't know if Gulf invented it, but I copied his . You adjust the thickness of the wood to put your pivot point right at the floor level. If you do that you can have the hemisphere without having to do any adjustment.

I probably did come up with the wooden brick. But, I was too green to think of lowering it below grade to accomodate for the difference in the pivot for the IT as you did. I actually started out with a plywood floor covering the floor brick and mounting it to that. But, it was as Jonv calls "wonky" so I came up with the wooden brick .

[Gulf]

Thanks. At the moment I am thinking of fixing it to some 1/2" ply that I've cut to cover the hearth bricks, putting the pivot point an inch or so above the hearth. To compensate I'll cut a course of 'half soldiers' at the right height and angle to put me back onto a hemisphere with the IT. If that makes sense.

If you do as you describe, it will be a perfect hemisphere in a horizontal plain from the top of your soldier course up. There will still be some difference in the ID of the dome. From the center point to the walls of the dome and from the finished floor height to the apex of the dome will be different. No biggie, your oven will cook just fine.

I understand that the soldier course is not normally mortared down, but then it will be rather wonky sitting straight on those boards, especially along the joints between sheets. What should I do? I'm thinking I will have to apply at least a thin bed to create a flat and level basis for the subsequent courses?

I don't think that it will matter that much if you are mortaring to the CalSil. There is enough residual dust on top of that stuff to prevent a solid bond. Imo you can use what ever mortar you need to start the dome wall level. It might, if you were setting the dome on top of the floor brick. But, if you are setting your dome outside the floor use some cardboard to set the distance between the floor and the dome. It will keep the mortar from filling in that expansion joint.

As for as soldier courses go: You don't need a soldier course. If you are after vertical height before starting the curve, you can do the same thing by laying the first row (or two or three) with the bricks in the same orientation as the rest of the courses. You will have a stronger bond if you do that way.

[JRPizza]

Jonv, have you considered pulling out your "center" floor brick and using the wooden brick trick? I don't know if Gulf invented it, but I copied his . You adjust the thickness of the wood to put your pivot point right at the floor level. If you do that you can have the hemisphere without having to do any adjustment.

[jonv]

I have a question about setting the soldier course. Although my hearth was flat and level the calsil varied somewhat in thickness so, as I learnt here, I levelled with a mixture of sand and fireclay. I'm about half way through that job.

I understand that the soldier course is not normally mortared down, but then it will be rather wonky sitting straight on those boards, especially along the joints between sheets. What should I do? I'm thinking I will have to apply at least a thin bed to create a flat and level basis for the subsequent courses?