Do you know what, the thought of having to return a crate of beer bottles to get more full ones is something that ought to be the norm everywhere, instead of this throw away society we have, especially in what is considered to be "first world". Recycling through simple re-use, rather than more energy to melt down, etc.
Cracking place for an oven, but can't you just leave a flat stone out in the sun?
Did my brick tests. From left to right a 2:1, a 3:1 and a 4:1 sand:clay
After one day (left in the shade this time)
Then the next day
The lengthways shrinkage was minimal on all three - there wasn't much between them, which was unexpected. There is a slight bit of cracking visible in the 2:1 brick though.
Then I did a "knock test" where I held them in one hand & knocked on the upper half, like knocking on a door quite hard. This was the result:
Only the 2:1 survived, the other two broke fairly easily. (Admittedly, the 4:1 hadn't fully dried, but I expect it to be weaker than 3:1 anyway). I then tried a bit harder to snap the 2:1 brick and it did snap in half fairly easily. But I had knocked it quite a few times already without it breaking.
So I am inclined to go with 2:1 for strength. Does that sound good?
I figure that whichever mix I choose will be a lot stronger once it has been heated in the oven, so it's just a comparison relative to the other mixes, as well as to gauge shrinkage.
I am looking for a bit of help on the chimney. I think I will do a brick arch, as I love how they look. I can't decide whether to go for the chimney in the arch or behind the arch. I have references here of what I'm talking about:
I think I could cut bricks in half, but not achieve much more accuracy than that. I also don't have any chimney piece - it would be a cob chimney.
I could also use advice on how to decide the cross-sectional area. Also, if the chimney is behind the arch, the air doesn't have to go under the arch bricks and so is leaving at a much higher height. Should that be at 63% of dome height rather than for the door, as that is where the hot air will exit?
As long as the chimney is far enough forward I don't think it really matters if it is in the arch or behind it. When I built my first clay oven it was a bit rubbish and the chimney ended up too far back (behind the arch) so flames would come shooting out of it when the oven was at max fire. Obviously you don't want that because the chimney is to draw smoke out/pull oxygen in, not allow all the heat to escape directly .
Size of the chimney flue should depend on the size of the oven opening. Roughly 10% of the oven's opening size is the usual.
I had delayed the work for a few days focusing on laying a concrete floor in front of the oven.
I have decided on a 70% (of diameter) oven height. My oven will be 83.6cm wide (the width of 8 bricks), which means a height of 58.6cm and so (at 63% of that) a door height of 37cm. And with a door width of 4 bricks which is 45cm wide.
Amazingly, in laying out my bricks, they fit exactly to these measurements:
I realise it's a bit off-centre, I'll fix that.
My daughter also decided very quickly this was a play area for her:
Q - To mortar the bricks, do I just use the same as for the first layer, i.e. clay and sand mix? I don't have access to fireclay or anything else special - I just have clay, sand, cement, hay. I think I have read that you want to leave the cement out (unless you're mixing your own refractory mortar with a small amount of cement in it). What's the standard practice?
I really love those pictures....please keep em comin!!
Your build is very individual as you are limited to access of all the materials afforded to the likes of us in Europe or America. I would say "bite the bullet" and get that extra bag of cement for the entrance arch.....If you are trying to create a void for the chimney from the entrance arch...standard, in a Pompeii build, then the entrance arch will take quite a bit of heat....the inclusion of clay will help it take the heat, but cement in the mix will bond it all together.
I love those radius entrance arches, but for practicality of use I try to push the width to it's maximum (I go 18" on a 36" oven...you could probable go as much as 20" but any more and you will start losing the massed heat......which you will need for any type of baking. Although I love the semi-circular arch, I build straight sided arch supports as this gives the maximum area opening, which comes in handy if you want to put large casseroles or pots....or even the odd suckling pig in the oven for the long slow roasts.
So good to be finally doing it rather than just reading about it. Although it was some workout. We just made enough cob to cover the bottles today & with 3 of us it was hard work (1 bucket (20L) of clay, 2 of sand). I think I will bring in more feet next week.
Here are todays pics. I soaked the clay overnight in a bucket and that worked well. I wasn't sure how much straw to add - anyone have an idea how many buckets of straw say to one bucket of clay? (These photos are mid process, we added a lot more straw afterwards.)
I'm really enjoying watching this thing go up - if I'm ever heading northeast, I'll be sure to pop round, and if you just happen to pop into SA via Zim at Beitbridge, I insist you do the same. We can swap dough recipes or something.
Good luck with the rest of it, and please keep the photos coming. It looks awesome already!
somewhat missing making pizza while watching elephants
Yesterday we put the hearth bricks on. As you can see they don't fit quite as well together as your standard firebricks! But I used these bricks last year in another oven and they worked well (that's why some of them are covered in soot!).
I also had to reduce the planned size of my oven as the stone base got steadily smaller as it grew. I have also gone a bit to the left and off the front to fit the bricks on. It will now be a 31"/79cm oven with 3" thermal layer instead of 4". Pencil marks for sand and for thermal layer just about visible here:
Today we did the sand former. I used some bricks to encourage steep sides. We made the sand too wet so it was slumping a bit & I left them in place. Will do some more work on it tomorrow.
Your oven is really starting to shape now. I'm looking forward to your next set of pics.
We always butt the hearth bricks against each other as you've done without a mortar joint, but your bricks don't look as uniform as what we use......you have fairly big gaps between the hearth bricks. Normally the ash from the curing fires gets used to fill any small gaps between the hearth bricks, but I would be tempted to dry some clay and crush it up fine, mix it with sand and continually brush the mix between the bricks to fill the voids.
It's only a suggestion as your build is quite different to anything I have built.....