Terry - that's a good thought. With the test bricks, when they got wet, the clay got all sticky. Would the clay/sand get fired hot enough to not do that? That was my concern. I also wondered why it's standard with cob ovens to use firebricks, rather than lay a clay/sand base, given that one of the main aims is low budget. I also thought today that a lot of sand is probably already in the cracks from the sand former. But hopefully I'd be able to brush a lot of that out.
Did a bit more work on the sand former today and was much happier with it. Also, cut away a bit at the front to make room for the arch.
Turns out my friend has quite a few tools and he helped me make a former for the arch AND even cut a couple of bricks for the arch. I wasn't expecting to be able to do this. I remembered since then the issue of the chimney. Seeing as he has a grinder, I think I will cut the middle bricks to fit the chimney into the arch rather than behind it. I still don't know what to use for a chimney. Can a handmade clay chimney be my fallback?
Dunno what Terry might think but i've always wondered whether a sand/clay/cement mix may be fine to use. You might need to get the clay to a slurry by mixing first with the sand and then add the cement later. Building sand over here used to have a good amount of clay left in the sand to give it some "life" but tends to be washed more nowadays.
Its thought that the cement in "homebrew" eventually loses its strength through the heat cycles and the lime and fireclay take over the binding of the bricks. You may have to prove this theory and be a pioneer, but from the picture your arch will be mechanically fine and is most of the battle anyway. Try mixing up a small batch of mortar and set two bricks or pieces of brick together and come back to it after two or three days? cheers, Danno.
spinal - Thanks. This is such an encouraging forum. Last year I wrote on one of the US ones, and one of the first replies was saying that unless I had my pizza balling technique perfect, I shouldn't be teaching the guys here how to do it (that is the older children of the children's centre on which we live). I think that is taking your pizza a little too seriously!
Did the thermal layer today. I've been dreaming of doing this for more than a year, so it was a good day.
I had a couple of helpers and I lost track a few times of whether it was anywhere near the 3" we were aiming for. I think it is pretty thick in a few places. I just hope it's not thin anywhere else.
Had a slight panic when I noticed some cracks in the arch mortar on the end bricks:
This join doesn't feel loose and didn't get any worse, but then the centre brick kept getting worse and by the end was completely loose:
I'm not sure what's gone wrong, though I am an absolute beginner so could be anything. I couldn't see any movement off the vertical from the arch walls, but I put a couple of bits of iron on either side embedded into the clay dome for extra support. I also put the wooden former back in for now.
- clay (not fireclay) in homebrew means the mortar shrinks? - took the wooden former out too soon? (was in for about 18 hours) - didn't mortar the arch well enough / missed some step needed to bond the bricks and mortar well together? - my mortar not strong enough to hold this type of arch (not entirely self-supporting)?
What to do now? With the arch corner bricks should I leave those or re-mortar? With the centre one shall I try to get the mortar off its neighbouring bricks so as to start again fresh or just put a thin layer on?
Now I'm a bit nervous about the clay dome staying up! Might start taking out a little sand tomorrow as drying is so quick here - or do people think that's too soon?
That looks quite normal (the crack). Arches are held together by gravity, as odd as that sounds. Gravity pulls downwards, the wedges redirect the force sideways and voila - the arch stays up. My keystone brick can almost be removed... it's been a year and hasn't fallen yet... though I have meant to add some clay to stop it jiggling.
Personally, I would add a cosmetic layer of cement+clay and push it into the crack. Add something on the sides of the arch to hold it in place as I reckon part of the issue is the "columns" are moving outwards.
This will get worse when the chimney goes on unless you "buttress" the sides of the arch, as the chimney will add more pressure. M.
Thanks, that's good to know. I am thinking the insulation layer can come right around and buttress the side of the arch.
Mozambique's drying qualities have been out in full force. By 10am today (we finished the thermal cob layer yesterday afternoon) the cob was pretty hard and had some chunky cracks in it:
The cracks are about 5mm and seem to run all the way through the cob:
I had forgotten to cover it with the tarp which I had taken off at night to aid drying. I should have just left it on. The side not catching the sun was less dry and less cracked (though maybe will crack just as much in its own time):
I took out the wooden arch former again & the centre brick was so loose I took it out, but then the side 3 brick pieces were also totally loose so I had to take them out too
The mortar then came off really easily which makes me think it wasn't that good. I tried pulling the 3 brick pieces apart as a test and one brick came off:
It looks like our method was a bit lacking in that the surface contact was lacking. We kind of shoved mortar in once the bricks were in place. I am guessing now that it's better to take each brick out and apply mortar to its side and then put it back?
I will have another go at the brick arch. Maybe if I did it again I would stick with the circular arch which is more self-supporting, given my lack of access to materials to make good refractory mortar. Though I love this arch, and I'm sure it will hold together second time lucky!
As it was drying so quickly, I thought I'd take out some of the sand. I ended up taking out about 2/3's by accident as I had a bamboo stick in there to get the height of the dome right and when I took it out, the middle 1/3 of sand collapsed all at once! The clay is holding up ok so that's the good news.
Nick, i think you may have suffered from your mortar drying out too quickly through your extremely thirsty dry bricks drawing all the water from the mortar and the hot conditions. Don't worry, all is not lost, you've only gone backwards a small step. Take apart any brickwork that'll come apart with your hands. Clean down any remains of mortar. Do your preparations to lay your bricks as normal. Then...
Soak your bricks in a bucket of water for around a minute, take them out and then let them sit for around five more minutes before laying them. So that there is no water on the faces of the bricks. If any of your arch remains in place, give the bricks that you'll be butting up to a really good watering down, and again leave at least five minutes before mortaring up to them. Rebuild your arch, wait, point up and clean down the brickwork as usual. Try to "butter" your bricks when you lay them, you can fill the top most section of joint afterwards, but try to at least get 2/3rds of the joint right first. If your mix is good, it'll stick enough to let you get it into place. Give the brick a little wiggle or light tap towards the previous brick too. Try to keep it all in the shade, and mist the whole lot down as often as possible after the mortar has started to harden off a bit. Only mist everything, like with a fine hosepipe spray, or hand pump sprayer. You don't want to wash out the mortar. Then another good misting before putting it all to bed, under dampened sheets the best option.
Danno - thank you, that is very helpful. I will have another go at it.
I took all the sand out
What a great feeling when you first see the inside of your oven
I could see daylight through some of the cracks, but the dome did feel pretty stable
Over the weekend I patched up all the cracks - I tried using a plastic bag with the corner cut off to squeeze clay into the cracks like an icing bag. Kind of worked.
Then today, the insulation layer was done
I am wondering whether to try to add a smooth layer of clay/sand to the inside of the oven where it is a bit cracked? I don't care how it looks, I am just worried about bits of clay falling off onto pizza. But I don't know which is more likely to stop that, as extra clay might not bond too well now it's pretty hard and dry in there?
Could anyone help me with a timeline of how quickly I can get to full firing and cooking of pizza? I need to start selling pizza soon to the visiting students to pay for this thing.
It was helpful that the arch came down the first time as I had completely mucked up the chimney the first time round. Firstly, my dome height was not what I had planned it to be somehow. Secondly, the chimney height was much too high. I hadn't allowed for the chimney to be off the back of the arch and this requires the "neck" of the turtle shape of the sand former which I didn't have.
So we redid it all today, using advice given on this thread. I can't believe we've actually finished (well, apart from a render layer and a roof). I can start curing it next week, plus it has dried out a lot this week anyway.
I spent about a week curing the oven (after it had dried for one week). This wasn't that much fun and a little demoralising because it's hard to build small fires in wet ovens.
But things got better and I started building bigger fires which burnt much better.
I used up a lot of firewood but I was playing safe because I had already announced that I was selling 40 pizzas on the Friday night.
Miraculously, it all worked & we sold 40 pizzas and made another 10 for the team. Here the three boys from the center that I trained up.
Here is a pizza cooking
I must admit that I was a little bit disappointed with the cooking times which started around 2.5 mins and went to about 4 mins after 20 pizzas (so at that point we swapped sides in the oven).
I had fired the oven for about 4 hours to be safe and the soot had gone clear. One thing is that I haven't been able to get a brass brush so we had to wet the hearth a bit to clean off the ash which will take a bit of the heat out of it, how much I'm not sure. Also it may not have been fully cured yet. But it worked and I should be pretty happy about that.
I plan to get a roof up as the rainy season will come soon enough. I also plan to do a render layer, for looks as much as anything.