New build - 1.2M dome

[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.

[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]

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

[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]

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!

[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?

[SableSprings]

Jonv, as JR noted you are going to have joint creep... When I laid out my arch as you have done, my joints were tight and I didn't think anything could go "wrong". What I did not realize was that no matter how thin you apply your mortar, the accumulated mortar thickness will affect the dry layout (tight joint) plan. The end result is that your arch will be bigger than you planned and won't fit together the same way as it is laid out "dry". Either put in a little space between the arch bricks (1/16"-1/8") or build the arch up and plan on cutting the keystone to fit at the top of the arch.

[jonv]

You're right, those joints don't leave much room for error. Will definitely mark their positions on the template - would hate to have to cut down the top brick.

The outer arch won't be quite so nice - the stock arch bricks don't match the radius so well but still better than flats and easier than cutting them myself.

[JRPizza]

Just curious, how thick are those shims? You might want to try mortaring a few bricks together with your target joint thickness to see if it is going to be doable. Don't know what kind of sand you are planning on using, but for those joints it probably should be pretty fine

[jonv]

SableSprings - I tested the mortar I'm using and it seems pretty good for thin joints; the aggregate is nice and fine and it squeezes out fairly well. If I can't keep the arch under control I'm going to have trouble with the dome too as the arch bricks for that are not far from my radius of 600, so it will be a good test. Either I get the joints tight or I get the grinder out!

If I'm honest, I am a little adrift on my numbers (can't hide mistakes from you guys!); I had originally drawn out a 1.1M dome and only later changed it to 1.2M. I made a quick check that the arch bricks were still ok at that radius, but when I rechecked everything a couple of days ago I found it is closer than I thought. Keeping the inner edges close together will dictate pretty thin joints. I will make it work... or adjust the bricks.

The average width of the joints for the arch as laid out is around 1/16" - the black spacers on the outside are 2mm and the green ones on the inside are 1mm. If I have to, I'll trim the keystone to fit.

[JRPizza]

What is your door to oven height ratio? There is sometimes a little wiggle room there (the FB plans go from 59.5% to 66.6%) and if you made the door a little higher you could gain some joint width. Not trying to redesign your oven, just wondering if you had any leeway in the design.

[jonv]

I do share your concerns - I could have made this job easier if I picked flats to work with and hadn't wanted to try and make everything such a tight fit, but that's me I guess - take an already tricky job and make it harder!

I'm aiming for a 500mm width and something around 390 in height. I have a radius of 600mm plus an inch or so for the height of the pivot point above the floor. Should work out in the 61-63% range.

I have some finely ground bauxite for the mortar - it's finer than any sand I've come across. There's a couple of scraps stuck together with it in post #46, with joints as thin as I need - of course, it's easy when just slapping a couple of scraps together, it will be more tricky on the arch/dome itself, but seems doable. Time will tell...

I'm leaning towards building the arch first, unless there's a good reason I should build it as the dome progresses?

[JRPizza]

If you look at two of my "benchmark" builds, UtahBeehiver's and Gulf's, one built his arch first, and one didn't, so it's whatever makes you more comfortable. I had a little trouble visualizing the angles that would intersect with the inner dome so build as you go was the way for me, and having the dome bricks to give extra support seemed like a good idea. When I later built my outer arches I found that the extra support was not something I really needed.

[jonv]

I think I'm having the same trouble figuring out the intersection of done and arch, but various posts have given me some clues... I will at least set up the arch in place and see what sense I can make of it with the IT.

[UtahBeehiver]

I always attach this photo (done by Mr. Chipster) showing how the IT is used to determine the various angles of a tapered arch. Even though this is a partial arch, the concept is the same. But you start the the top dead center of the arch and work your way back down. Each brick is slightly different to you cannot cut identical bricks. Also you need to start with a full brick not a half brick.