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!

Earth Ovens 2

Heat


There are three types of Heat-

1. Convection
2. Conduction
3. Radiation

How does this work in an Earth Oven? (I hear you cry) You stick a heat source, like a blazing fire, in your oven. The heat from the fire is absorbed by the (well-insulated) walls, and into the refractory brick floor. Some of this heat is conducted through the direct contact made by the Pizza/Bread/Food with the refractory brick floor. Some heat absorbed by the walls and is radiated back from them. And some of the heat swirls around, or is convected. inside the oven in the air and steam. End result; crispy pizzas and crusty bread. The oven will remain warm for around 6 hours without rekindling. At the start of the cooking cycle , after the oven has been fired for about two hours, the temperature can reach 700 degrees, or so I'm reliably informed, I place my fist at the oven door to guesstimate. This will cook a pizza in minutes. After pizzas, the oven has cooled for Sourdough  bread, then cakes and biscuits, roast meat/veg, fish, fruit, etc., maybe end with yoghurt.

The activities can last all day, if you want. It encourages sociability; why make pizza just for yourselves? Why waste the heat? Invite neighbours to bring their favourite toppings for a pizza brunch. Bake bread with friends, and everyone goes home with a loaf. Roasted peppers, anyone? Once a month for the full moon come together, make some heat, bake and celebrate!

Your indoor oven is not so much fun, and it doesn't cook food in the same way in its use of heat as the Earth Oven (see above and look at your indoor oven), therefore it does not taste as good.

Earth Ovens 1

Ok, I am not totally obsessed with Earth Ovens but I do spend a lot of time thinking, planning and building them. I am not an expert by any means but I thought I could share my thoughts on them.

What do I mean by Earth Oven? 
Literally, an Oven made of Earth. I build a particular design (a sort of pizza oven) but the techniques can be adapted to what you have available and what you want to create; a Tandoor for example. When I say earth, I mean a mix of Clay and Sand.  Someone once said to me "There is a layer of clay a mile thick encasing the planet". Now, I don't know whether this is accurate or not, I think though that the point is correct. Clay is abundant and fairly easily found. Dig a hole, and depending where you are, you will eventually find clay. Soil  is no good; It is full of seeds and micro-organisms, and it ain't sticky enough.  Clay when dry becomes solid but it will develop cracks, hence the addition of sand.

The sand (probably) can't come straight off the beach. Beach sand is too round and smooth. What you need is coarser grained; sharp sand (or builders sand). The coarse particles will bind the clay.

The other material used in construction is Straw (and/or Wood-chip). This mix of Clay, Sand, Straw, also known as Cob  has been used since the beginnings of time to make buildings. This video about Shibam, so-called Manhattan of the Desert, shows the power Cob in action. In the oven the straw is principally used for insulation.

Put these together with some Refractory Bricks, and you can make a domed oven, that looks like a bit this one I made earlier-

You might ask yourself, "Why would I want to build one of those? I have an oven in my house." To which I reply-

• Communal activity, both in its construction and use. Couldn't you do with a bit more quality time with people? 
• Your indoor oven doesn't cook like this oven does. (see next post)
• It is very hip and people will charge you £1500 to build something equivalent but not as treasured as the one you could have made. (If you do want to pay someone to build an Earth Oven for you please get in touch I wouldn't charge anywhere near that.)
• When Civilisation has collapsed how are you going to bake without gas or electricity?

.....Next, a short digression into heat, then back to construction with an overview of the building process.

Toodle pip!