Disaster! Please help

[debbpic]

Last summer my husband and I started construction of a large pizza oven. We had a great base but the roof did not work so we went back to basics and stripped it back to the base and left it until a few weeks ago. We decided to restart the project using clay this time. The base is fire bricks with a three tier brick wall with an opening at the front. As the base measures approx 1 metre square we decided against the pile of sand as a mould for the clay. What we did instead was put a frame of aluminium bar and chicken wire across. We put aluminium flat bar across from side to side using the holes in the bricks as a start. This made a kind of rib cage. Then we put a double layer of chicken wire across the rib cage and fixed it onto the ribs with thin wire. Next we bought a load of pizza oven clay and covered it all over. This is where it started it's demise. We started off by putting a double layer on the back wall (meaning a layer inside and a layer on the outside. It was pretty clear then that we would not have enough clay to cover the whole oven in such a thick layer so we proceeded with just the outside covering (even though it did obviously go through the chicken wire and there were no gaps or spaces anywhere. We have the chimney placed centrally and this was also sealed properly. After a few hours drying off we put a small fire in the oven and that's when the cracks started appearing. After an hour of burning it was cracked all over. We drove the three hour round trip to get another block of clay and filled in all the cracks. Again, after firing up it again cracked all over. We managed to get another block and a half of regular clay which we mixed with sand and covered the whole oven again. By this time we needed to use it as we had promised my daughter a pizza party for her friends. AT first it fired up brilliantly and reached great temperatures. But as the night wore on the cracks appeared everywhere and it started to pop pieces off! It ended up looking like an octogenarian after a night out! Although the fire behaved exactly how it was supposed to (curling up and over the curve of the oven) it has finished up looking sad and forlorn with huge cracks.
Does anyone have any idea why these cracks are appearing? And, more importantly, how can we stop it happening again? WHat are we doing wrong and should we strip the whole thing back to the ribcage and start again? I would really appreciate any hints or tips you can give. Thank you in advance.
D

[chas]

Well, D, 'ideas' is all I have, not having built an oven from clay, but until someone else better informed comes along, here goes:

any build incorporating metalwork in it may be prone to stresses due to expansion - clay and aluminium probably expand differently
clay contains water, and until it's uniformly dry, the heat inside will create uneven stresses, worse, it may create steam that forces its way out
clay mixed 50/50 with sand is more stable (like bricks)

If you were to consider starting over, a barrel-oven formed from bricks stuck with clay mix instead of mortar may be a way to go - the bricks seem more likely to hold in position without collapse and a sand former may not be necessary, it wasn't for my dome...

Chas

[cobblerdave]

G'day
Again not a clay oven expert , but I think Chas is one the mark with the metal frame expansion bit. Again with the mix, 50/ 50 with sand and trodden throughly though seems to be the mix that produces the least cracking.
Interesting with the frame because of read of ovens in Canada in the 1880s being built on a wicker Frame but I'm sure they still applied the clay in the form of cobs, cricket ball size lumps of clay mix that were molded into each other as more were applied.
I think it's back to the investigation bit. The clay is reusable I'm sure as it can be pounded to sand size pieces and remixed
Regards dave

[chas]

May 19, 2015 21:30:42 GMT cobblerdave said:

G'day
Interesting with the frame because of read of ovens in Canada in the 1880s being built on a wicker Frame but I'm sure they still applied the clay in the form of cobs, cricket ball size lumps of clay mix that were molded into each other as more were applied.

that's an interesting insight to old oven builds Dave... I guess with the clay just pushed into place from one side (and plenty of it) the framework would burn out leaving the dome. A bit rough on the inside, but so what? It's a sort of extension of wattle and daub technique, and something the old timers would be familiar with.

Maybe Deb would have a go...

Chas

[chas]

Just a thought... and maybe someone with experience of barrel ovens would comment, but while rebuilding you might consider bringing the chimney towards the front to make heat travel over the bake before escaping. JMS sent me a really good diagram in my build, page one. I've attached it, but it makes a good read on the thread.



Cheers,

Chas

[brennanpincardiff]

Dear Deb,
An interesting set of problems.
I wonder if we might ask a few more questions to clarify.
(1) Is your bar/chicken wire frame still in place. As such do you have a kind of clay metal fusion to the oven?
If so, this is quite unusual. Usually we go for the structure being clay or cement.
(2) Also, how are you applying the clay and are the cracks appearing where you are doing this?
If you applied the clay in balls pressed into the frame, are the cracks appearing at the 'joins' of your balls?
(3) from the picture, your oven seems to be quite a barrel shape. Is that the case?
(4) did you use pure clay or did you mix your clay with other materials e.g. sand?

When we build our first clay oven, we used the approach of Kiko Denzer as shown in his excellent book (Build Your Own Earth Oven). There is also videos available online.
One of the keys to the structure of this is pushing the clay mixture together down the sand structure. This binds it together really well. Also getting a good mixture for this oven dense layer makes a difference.
Then a key point is the insulation layer. This protects the clay from the elements - wind, rain and frost which can have a terrible effect on the structure.
Finally a cob layer on the top for protection can really help.

I have to confess that my instinct is that you should strip it back and start again. I know this sounds a bit depressing but it's quicker and easier second time around. Check out some of the other builds here and see what you can learn from others. Learning is a great part of the process.
By the way, your base looks really cool so you'll be starting from a good foundation :-)
Hope this all helps.
Not an expert here either - just an enthusiastic amateur.
Don't be disheartened. You'll make a great oven very soon.
Best wishes,
Paul

[rivergirl]

Can't add much but also did you dry it out internally as well? Very slowly! Mine is on a sand and bottle base , my new build will also be built on those lines... Busy collecting bottles at the moment!

[debbpic]

Well thank you guys for all your replies and suggestions. Unfortunately things have moved on. After a night of heavy rain the oven has completely collapsed. The rain just seemed to soften and dissolve the clay.

Gutted (

[rivergirl]

What was the recipe for the cob that you used? I always tarped mine until we built him a shelter.

[oblertone]

I bought a 2m x 2m waterproof gazebo on eBay for about £65 to cover my bare dome while I applied a waterproof finish; unlike a tarp it allowed me space to carry on working while it was raining. Tried to give it away on here when I'd finished but had no takers so it went to the tip.