31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

As with a brick and mortar wall, I don't believe you have any issues regarding structural integrity. Maybe others could comment on the possibility of spalling of the wider mortar joints in this high-heat environment. My research lead me to trying to make the mortar joints on the inside of the dome as narrow as possible. I worked each brick until I felt brick on brick contact. One of the most problematic things I faced was getting proper consistency with the various cementitious products I used. In order to get the tight joints I wanted, I had to keep the mortar on the wet side. Balancing that with the proper wetness of the bricks was challenging. My technique was to lay a mortar bed for 2-3 bricks and butter the side of the last brick I had set. It was too awkward for me to try to butter a brick before or after I clamped it in the IT. That mortar consistency certainly helped with getting brick on brick contact and squeezing mortar into those tight places. But, I believe I paid a price as the rows became more vertical since I had trouble keeping the bricks in place until the mortar cured.

Thanks for your input and I fully recognize your struggle with the correct consistency of the mortar and the correct wetness of the bricks. It's quite challenging. Further input below Randy's comment.

You want to make sure that you are packing the space between the bricks full of mortar. You do not need to have the super tight joints all the way back on the brick. The front edge is where you want to have the nice tight joint and it is not as critical as you move back. I believe that you do want to have the joint fully packed full though. I see your bricks do not look to have any mortar squeezing out of the joints. You want that then just clean up the mess after you are done for the day. Sometimes it is hard to get the mortar all the way to the front of the brick but it is worth a shot. You can use a deadblow hammer to tap the brick into place. I would definitely recommend going through and trying to pack as much mortar into the joints you have done already as possible. It will make for a stronger finished product.

Thanks for your input as well Randy, I"ll try and get all gaps filled in the coming rows. I just had another look at Ben's Firebrick video on the bricklaying and ordered a pointed trowel as well as a deadblow hammer, my large masonry trowel doesn't let met get close. Ben is also putting hte mortar on the existing row, instead of the new brick, that's what I'll also try.
My doubt from seeing Ben's video is that my homebrew mortar does not behave like that at all, I don't think it will flow as much, but instead just transfer the hammer force into the bricks underneath. He also hammers in the bricks sideways, which I accidentally did on my first row. This resulted in the whole row shifting a little bit, so I don't plan to do that at all. Didn't you suffer from your other bricks moving while trying to push in the current one?

[RandyJ]

Toiletman i just had a chance to look at your pictures colser. You want to make sure that you are packing the space between the bricks full of mortar. You do not need to have the super tight joints all the way back on the brick. The front edge is where you want to have the nice tight joint and it is not as critical as you move back. I believe that you do want to have the joint fully packed full though. I see your bricks do not look to have any mortar squeezing out of the joints. You want that then just clean up the mess after you are done for the day. Sometimes it is hard to get the mortar all the way to the front of the brick but it is worth a shot. You can use a deadblow hammer to tap the brick into place. I would definitely recommend going through and trying to pack as much mortar into the joints you have done already as possible. It will make for a stronger finished product. This is one of the reasons that if i ever was to cut another oven I would just bevel the sides and leave everything else square. So much easier to make sure the joints are full. The problem is it makes laying the bricks harder and cutting them harder to try to keep all the joints tight. I know a few have done it but short of that being your goal for personal reasons it will not make a difference in how it performs. As long as you are not using something like heatstop as a mortar as I believe that requires tighter joints but still the back side does not need to be as tight as the front. Make the front fit tight and don't worry about the back. No one will ever see it. You will also want to make sure the front side in the oven is packed full for the joints you have the just wash it clean when you are done. I know I need to crawl inside mine now and touch up a few spots. That is just part of the game.

Randy

[Giovanni Rossi]

So questions about these gaps:
Do they matter?
Structurally my feeling is that there's more than enough contact with mortar to glue the bricks down, as well as the weight and dome shape keeping everything in place?

I've now spent some effort to also fill the gaps on the top and on the outside of the bricks, but my feeling is that that might be a waste of effort? What do you guys think?

Toiletman There are fellows here with infinitely better masonry skills that I possess who can give you more peace of mind, but here's my 2 cents.

As with a brick and mortar wall, I don't believe you have any issues regarding structural integrity. Maybe others could comment on the possibility of spalling of the wider mortar joints in this high-heat environment. My research led me to trying to make the mortar joints on the inside of the dome as narrow as possible. I worked each brick until I felt brick on brick contact. One of the most problematic things I faced was getting proper consistency with the various cementitious products I used. In order to get the tight joints I wanted, I had to keep the mortar on the wet side. Balancing that with the proper wetness of the bricks was challenging. My technique was to lay a mortar bed for 2-3 bricks and butter the side of the last brick I had set. It was too awkward for me to try to butter a brick before or after I clamped it in the IT. That mortar consistency certainly helped with getting brick on brick contact and squeezing mortar into those tight places. But, I believe I paid a price as the rows became more vertical since I had trouble keeping the bricks in place until the mortar cured.

I wanted to maximize thermal mass so I filled the gaps. I typically had a little mortar left over after setting the last brick, so I filled in where needed. However, I didn't have the up front labor for my mortar. Heck, I had enough mortar mix left over that I was able to apply a 1/4" layer over the entire dome.

You're making good progress. If there's anything I've learned from following this Forum it's that these projects are pretty forgiving. Like you said, yours is a hobby project, so enjoy the ride.

[Toiletman]

RandyJ
Giovanni Rossi ,
Thanks for your input and sorry for the late reply. I've been crazy buzy with work and tried to put every spare hour into the oven. Indeed I was wondering about the thickness difference between the bricks. It seems to be in the 1,5-3mm range. To prevent tolerance stackups going out of control I've gone and selected them to low, medium and high bricks, and made sure not too stack extremes on top of each other and indeed now on the 3rd row it nicely evens out. I'll bring the leveling laser in for the next round to see if I'm out of level, but eyeballing it so far it looks pretty good.

It's been slow going here, but now I've finally laid the first three courses in the mud it does feel 'real'. Learnings so far:

• It's quite a bit of jiggling with brick drying time after submersion & homebrew water amount. For normal bricks the recommendation was to soak and afterwards leave to rest for 24h, but that's way too much for my firebricks. Leaving them at rest for 2-3hrs seems more ideal. Otherwise I feel they immediately suck all the water, and therefore 'flow', out of the mud. Ie: it's impossible to bed the bricks down if they (and the mortar) are not on the wet side as compared to 'normal' bricks and mud. I think my first course was too dry, the second one was ok, and the third one slightly too wet, so probably I've now found a sweet spot.... until the angle gets too large and bricks start slipping
• The amount of 'overhead' that I'm going through now as compared to laying the bricks is quite high: sieving the ingredients of the mortar, cutting and selecting the bricks, detailing and cutting the arch bricks. It's slightly getting to me, but in the end it's a hobby project, so I'm just taking my time and enjoying the result. But for the people considering between a kit and the homebrew approach: definitely something to think about
• I've not fitted my IT with a clamp yet, so far it's working quite well without, after getting used to it.

In the attachment some pictures of the current status.

I do have some questions about the bricklaying, hopefully you guys can shed some light on it. See especially the 2nd and 3rd pictures:
To be able to bed the bricks down I'm trying to not put too much mortar under them. I also 'butter' them with a slight layer on the side, and then bed them down. This results in some gaps as you can see in the picture:
in Red: the 'thin wedge' under the brick doesnt get fully mortared in
In Blue and green: the vertical gaps are sometimes quite emtpy, without a lot of contact (this has gotten better in the third layer, with a slightly thicker 'buttered' layer, and a wetter brick and mortar.

So questions about these gaps:
Do they matter?
Structurally my feeling is that there's more than enough contact with mortar to glue the bricks down, as well as the weight and dome shape keeping everything in place?
Thermally I also feel that it would not matter a lot as the main heat flow is inwards->outwards. It might impact the thermal mass a little bit, but the majority of that comes from the bricks anyway.
Visually only the vertical gaps on the inside of the dome matter, so for those ones I've pointed them to look good.

I've now spent some effort to also fill the gaps on the top and on the outside of the bricks, but my feeling is that that might be a waste of effort? What do you guys think?

If the weather lets up I might have another course in on wednesday, but probably it's only going to be cutting the bricks.

[Giovanni Rossi]

Toiletman , it's sometimes tough seeing detail with photos because of lighting and angle, but it does look like there is a fair amount of difference in the height of some of the bricks 3-4 away from both sides of your IT. Randy is right, most often it will just work out with random placing. You'll just have more visible mortar joints here and there. Remember, you're going to be able to see only a small percentage of the inside of your dome unless you crawl in. Again, maybe it's not as dramatic as it looks, but I'm surprised at the differences in what is probably the same manufacturing run.

The one thing I'm thinking is, if you happen to have groupings of the shorter (or taller) bricks match up in 2 rows, it could throw you out of level enough to make difference. Would it be too crazy to sort your bricks so you have enough similar height ones for each row?

[RandyJ]

Hey Toiletman nice looking build. I am not entirely sure what you are asking in your question. I will take a stab at it. There is definitely a bit of a variation in hight or thickness of the bricks. My guess is they should be with in 3mm of each other so about 1.5mm +/- this is just how they are manufactured. Beyond that there is no need to be perfect on everything. Things will just work out and there will be a few slight differences here and there. None of that is a problem. Good luck with your build and I look forward to seeing it progress. Time for me to go throw more mud at mine.

Randy

[Toiletman]

And another small update:

- Cut the soldier bricks and the ones for the first layer. The calculation sheet and mrChipster's jig work great! The saw is also doing a good job although I'm already getting the idea that it's getting dull. (had some pulling/slowing going on on a couple of occasions). Also there's a learning curve in getting the two half bricks out of one brick to really match.
- Mocked up the first two courses using wedges and the IT. Great lesson with the IT: I now know why people have clamps on 'em... I'll have to upgrade mine as well

Questions:
- if you look to the detail pictures: there's quite a bit of irregularity in gap size, but especially in brick height. Is that anything that I should worry about, or will it be easy to 'push it away' once the bricks are mortared in?

[Toiletman]

And then for today's real update:

The floor tiles have been cut to size, an IT has been made, and after a proper round of leveling the floor tiles, I should be ready to make a first ring this weekend. Only thing to prevent real progress is that the inner arch form still has to be constructed.

Again, attaching the pictures, as that seems to be more robust.

I'm slightly ashamed that I incorporated hot glue and tiewraps in the construction of the IT, but the 'brick end' has been properly welded. I also 3D printed a nice 'snap-on' attachment to guide a pencil for drawing the outer edge layout.

[Toiletman]

Hmm, looks like something went wrong with the pictures in the previous post?

let me try to repost them as attachments.

[Toiletman]

Finally a few days of good weather and time off so I could finally make some progress (see pictures).

I went with vermicrete for the low spots, and some loose vermiculite (seeved) for the high spots. All on top of Mosaic tiles and insect screen:

Afterwards the calsil boards... what a job to cut them to size while wearing breathing protection gear... guess I can write off my 'shop vacuum cleaner'.



Finally a first stab at the floor, just to have an initial feel for it. It'll need to be redone and properly cut to size after our holiday.


Ow yeah, I did also finish a chipster cutting jig, with a nice 3D printed angle indicator:

As that layer’s requirement is to insulate as well as to provide a firm enough substrate to apply the render/stucco on to, I find a 10:1 ratio better. While there is no strength in a 10:1 layer its insulation value is about the same a blanket. Strength wise it’s very weak, but firm enough to support a render/stucco layer. A 5:1 is stronger but insulates less.The higher the cement content, the greater the density and therefore strength, but the lower the insulation value. If you have plenty of blanket then the insulation value of the vermicrete layer is less important.

In this case the vermicrete didn't have an insulation function: I just used it for leveling, as the real insulation comes from the CalSilBoards on top of it. It turned out that the vermicrete that was covered by plastic was more robust than the leftovers: although I must say I didn't try to break it, but it held up well during cleaning and fitting the calsil boards.

[david s]

As that layer’s requirement is to insulate as well as to provide a firm enough substrate to apply the render/stucco on to, I find a 10:1 ratio better. While there is no strength in a 10:1 layer its insulation value is about the same a blanket. Strength wise it’s very weak, but firm enough to support a render/stucco layer. A 5:1 is stronger but insulates less.The higher the cement content, the greater the density and therefore strength, but the lower the insulation value. If you have plenty of blanket then the insulation value of the vermicrete layer is less important.

[Toiletman]

With my oven walls on the floor bricks and after measuring again I went the vermicrete route just to be sure. The equalizing layer of vermicrete is now sitting outside curing under plastic.

How brittle is the hardened vermicrete supposed to be? the leftovers that I made into a ball and left to dry (now 22hrs or so) are very brittle: Just a little squeeze and chunks start breaking apart. Regular mortar leftovers from sand/cement are much more robust? I used the usual 5:1:2 (vermiculiteortland:water) volume ratio's, so that should be ok no? Or could it have been to dry?

[MarkJerling]

Thanks for your reply. Any idea of how much unevenness you fixed in that way? It does seem to be the easiest way.

I think I have about a 1cm of difference between the highest point (center) and the outermost edges. During mock-up the calciumsilicate plates did seem to conform to that elastically, but I'm worried that they might suddenly crack or snap with the weight of the oven on top.

I used a thicker layer of sand than other users as I wanted the flexibility to be able to go to thicker floor bricks, once these ones wear out, in future. (Or if I found the ones I used were too thin, which they were not.) My slab was pretty even. So, I have a rather thick layer of sand and it's been absolutely fine. The sand works like another insulation layer in my case. Having said that, my oven floor is inside the dome, my dome does not sit on the floor bricks, so your experience may be different.

[Toiletman]

Either method will work. In my case, I did the leveling with a sand layer under the floor bricks.

Thanks for your reply. Any idea of how much unevenness you fixed in that way? It does seem to be the easiest way.

I think I have about a 1cm of difference between the highest point (center) and the outermost edges. During mock-up the calciumsilicate plates did seem to conform to that elastically, but I'm worried that they might suddenly crack or snap with the weight of the oven on top.

[MarkJerling]

Either method will work. In my case, I did the leveling with a sand layer under the floor bricks.