trying to build cob oven without experience

[addictedtodough]

Hey all,

Two friends fresh out of college decided to build mobile cob oven on trailer. First we made form from gravel with some wet newspaper on top and started first inner layer from mix grog clay and sand with proportion 2:1:1 30mm with thicker bottom. After we let it to dry for two days, then we tried to take out the gravel because everyone in forum takes out after max 1 day but as soon as we took out little piece of gravel it started to crack because it was still soft. As well we have the bottom part with the grog clay and sand 2:1:1 60mm thick with reinforcement inside. Bottom and top are in separate plywood.

The plan is to put inner layer on top of bottom part and put another layer of clay with another reinforcement . Tho still not decided what mix should we use to make it as strong as possible and also flexible. Would be nice to have suggestions. Also we making door 21inch in width to cook inside one 20inch pizza tho what height we should choose? 63% or less?

Any suggestions and tips how to continue building this mobile cob oven will be appreciated.

[david s]

Making a mobile oven on a trailer is asking a lot in terms of the strength of the material, i wish you luck. You should make your floor with fire bricks at least, otherwise you'll be cooking on a dirt floor which will abrade easily. For reinforcing the cob it is normal to add straw to the mix. An easy way to do it is to throw the clay, sand and straw onto a large sheet of plastic onto which you also spray water and invite some of your college girlfriends to stomp about on the mud, occasionally raising the corners of the plastic to tip the mix back into the centre. Alcohol also added to the experience, makes for a fun time too.I suggest you go for a wall thickness of around 3" after at least 3 weeks drying you can insulate it with lots of chopped straw held together with a minimal amount of slip (clay and water to a pourable consistency) around 4" thick.

[addictedtodough]

Thank you a lot! We'll try to follow your suggestions.

[addictedtodough]

Which platform you recommend to use that after it would be possible to transfer everything to trailer?

[david s]

If it were mine, I'd not be moving anything, but building the whole lot on the trailer. You don't say how big the oven is, but you'll probably need around 3" thick reinforced concrete slab held up by a welded steel frame sitting in the trailer, on top of which you must have insulation so heat from the floor won't be lost by conduction.

[addictedtodough]

Well the idea is to cook one 20" pizza inside so is it fine to make to thermal layer 3" thick and insulation 4" thick? We made dome heigh 20" is it enough? Is it fine to to use for constructing building clay powder or its better natural clay? Thank you.

[david s]

The normal floor thickness is 2" firebrick.
Normal height is half the oven diameter.
Natural clay is cheapest. Use 1 part clay to 2 parts sand

[addictedtodough]

So we have this thermal layer its too narrow... Mix is from clay sand water and chamotte. After two days we decided to cut the hole but it wasn't strong enough so we had to fill up that part with empty bags... This is the picture after one week of drying. Is it possible to put on top clay and sand mix or its better to start again only with clay and sand?

[david s]

Those big cracks are caused by the clay shrinking as it dries. You should dampen them and fill them with more cob mixture. Removal of the mould material can be done as soon as the casting has dried enough to be self supporting. Otherwise the mould prevents the casting from shrinking.