Earth Oven 3

Build Overview

I am going to give a run through of what I do when constructing an oven. Don't worry if you feel that you do not completely understand the details of the process, I will go into those in subsequent posts. I think it is important to see the whole process unfold, so that you can plan for what comes next. You shouldn'r slavishly follow every particular of what I do, instead  look at what you want to achieve and the materials available, and amend to suit your purposes.

Where?

Cooking and eating are fun communal activities, unless you stick your oven in a distant corner and then using it becomes a chore. But remember an oven is a fire hazard (It has got a fire in it) and it will smoke, so don't necessarily place it too close to the house.
Think about-

• Access to the kitchen, 
• Nearby seating, 
• A place to store fire-wood, 
• The oven doorway should ideally face away from the prevailing wind so that it draws well, 
• Where will it look best from?

The Base


You could build your oven straight onto the earth but apart from the fact that you would have to crawl on the floor this would also mean losing heat into the ground, and your oven will act like a wick drawing moisture from the ground. For a temporary oven this may be ok but for a more permament oven it is best to build a base. From personal experience this is the most time consuming process of construction. It also uses huge quantities of stuff; sand, rubble, bottles, bricks, clay, straw/wood-chip. This is the point where you might give up, if you don't do some planning. The excellent Build your own Earth Oven  book focusses primarily on oven building and leaves you to decide the best way for the base. The base serves two functions, one,  to raise the oven door to a comfortable working height, and the other, to provide insulation under your oven floor. Here I will outline the way I build a base.

I start by marking out a circle 3-4 feet in diameter. The oven I am describing will have a 22 inch diameter cooking surface, this is surrounded by 3-4 inches of clay/sand 'oven mix' , which is surrounded by 4-7 inches of cob 'insulation mix'. The oven mix could be thinner but it is harder to use, and the oven is going to be trickier to make. The insulation could be thinner but more insulation means a more efficient oven. A smaller base is easier to build but leaves little wriggle room in the final stages, on the other hand a larger base requires a lot more work but you end up with space to add additional insulation or extra features; a seat, preparation area or wood-dryer.

I might remove some of the topsoil and tamp the soil down. Into this depression I put gravel/rubble for drainage. In the centre of the circle I build a brick column. The column is designed to support the load of the oven and stop the walls of the base buckling. At this point I use cement. I tend to avoid cement in favour of cob for the majority of the construction. It is nice to work with and it is very forgiving. Cob in contact with the ground will pull moisture from the ground into the oven. I try to avoid this by using cement for the support column, and the first course of bricks in the outer wall of  the base.

The photo above shows how things proceed. With some chicken wire I make a tube just inside the where I will lay the outer course of bricks. I find something (old copper pipes, poles from old estate agents signs), anything to weave in-between the chicken wire and  bash into the ground. Next the last use of cement. This is used to lay the first course or bricks. After that rubble/hard-core is added around the column.

From this point on it is all cob. The bricks are mortared together with cob, and cob is added on the inside surface to bind the chicken wire to the outer wall. Rubble is added to the centre until it is level with the top of the support column. At this point you will have a cob/brick outer wall, built as high as needed for a comfortable end working height.

 Inside this walled enclosure the rubble will come around half-way up you base. It will look a little like this photo.

This hole is going to be filled with insulation. We want to keep the heat generated by the fire trapped inside the oven. The outside is going to be covered with insulation, and this under-level must also be insulated. I use glass bottles.

I prefer to stand them head-to-tail with the bottoms up, as this leaves a level area to work up from but on their side is fine. 

If you have a mix of sizes I suggest the large ones on the outside as a ring around the soon-to-be oven floor. 

In-between the bottle I used to make straight cob (clay, sand, straw) to hold the bottles in place but I now use a mix of sawdust and a slip (clay and water mixed to consistency of cream). It is really easy to make and I  have easier access to sawdust than straw. This insulation layer to built up until you have a level surface upon which the refractory bricks are to be laid.

The building continues in the next posting.

Toodle pip!