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Natural Building – Earth Oven

earth-oven
Your finished oven can look basic or lavish, it’s the ratios which are important. Photo By Dani Wolff

Whether it’s for pizzas, bread or slow-cooked roasts, earth ovens are delightful to use and made from natural and breathable materials.

Building your own earth oven is a great place to start your natural building journey and an excellent way to test the clay content of your subsoil. The following is a guide for a medium-sized oven. You can increase or decrease the size of your oven, but it’s important to stick to the ratios which not only ensure the most efficient burn, but means you’ll e1liminate the need to incorporate a flue.

Prepare Your Base

Your base will need to be at least one metre square to accommodate your oven and having it raised one metre off the ground will make it easier to both build and use once completed. There are no rules as to what you construct your base with, as long as it’s sturdy enough to hold what will be quite a heavy oven; we’ve opted for besser blocks, but you can use recycled bricks, gabion rock baskets, concrete or a hardwood frame.

Know The Dimensions

To ensure an efficient oven, it’s important to ensure the internal height of the oven falls between 60 and 75 percent of the internal diameter. The height of the door is probably more important because as well as supporting an efficient burn, this ratio is what negates the need for a chimney for smoke to escape. The door needs to be as close to 63 percent of the height of the inside of the oven at its highest point as possible. In this case, the internal diameter is 660 millimetres, the internal height is 430 millimetres while the height of the door is 271 millimetres.

Prepare Your Materials

You’ll need around 100 litres of clay-rich subsoil, as well as half a cubic metre of bricklayer’s sand and a bale of straw – not hay. You’ll need a small bag of fine, washed sand to ensure the base layer of 24 bricks – you’ll need 34 in total if you’d like a brick arch at the opening – is completely level.

Firebricks will withstand a high temperature without cracking, but they’re not essential and regular bricks will work. You’ll also need a spirit level, newspaper, sponge, tape measure, a pencil and some string, as well as tarps, buckets and a shovel for cobbing.

Sourcing The Clay

A simple way to determine if there is enough clay content in your soil to build with, is to collect a sample and wet it slightly, mix it with your hands, squish it between your fingers and roll it in your hands. If you’re able to shape it into a ball, then into a log which is able to hang from your hands without breaking it is likely to have enough clay. Be aware that you are not confusing clay with silt – silt can look and feel similar but it doesn’t have the same sticky structure that clay has.

earth-oven

Start The Build

You want to get the basic structure in place before you make your cob mix. Lay a ten-millimetre layer of washed sand over your base to lay your bricks on. This will ensure the bricks are perfectly level once they are laid.

Start with the four bricks at the base of the oven opening which will sit directly below your door, add one either side, then lay the remaining 18 bricks to form four rows of six to complete the brick base layer. There is no mortar to stick them together so you must ‘kiss’ the bricks together as you lay them. This means you need to ensure there’s no grit between the bricks; have each one touching the neighbouring brick before sliding it down its surface.

Next, locate the centre of the rear 18-brick pad and draw a 660-millimetre-diameter circle which marks out the interior of your oven. An easy way to do this is to hold a 330-millimetre-long string at the centre point and, with a pencil at the outside edge, run the string around in a circle like a compass. Using a level and a mallet, ensure the brick base is perfectly smooth and level.

Plan And Mix

The completed castle will require three layers: a 100 mm mass layer of clay and sand mix, followed by a 120 mm thermal layer of clay, sand and straw and finished off with a thin, finishing layer of clay and sand.

Lay out your tarp and, using your predetermined ratio based on the clay content of your subsoil (see breakout), bucket four batches of your clay and sand on top. Add a little moisture and stomp it with your bare feet to mix. You want the sand and clay to be completely combined and with enough moisture for it to form a ball in your hands, but not sloppy. If it’s a hot day, cover with a tarp so it doesn’t dry out.

Supporting Sandcastle

Now you’re ready to build a dome of sand which will eventually be removed to form the interior of your oven. Measure and cut a stick to the 430-mm interior height of your oven, stand it in the centre of your oven and keep it in place using some sand. Use this as a guide to how high your sandcastle needs to be and build a dome shape down to the pencilled-in circumference. If you’ve opted for a brick archway at the opening – it’s not necessary – now’s the time to lay it using your mixed cob as mortar ensuring the all-important height of the door is adhered to. Cover the sandcastle with a thin layer of wet newspaper strips to keep the cob separate from the sand form.

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From top: A timber door will help you lay the arch bricks; Mix the cob with your feet; Wet newspaper will stop the sand from sticking to the mass layer; Working with clay has a tactile quality that, with patience, produces a beautiful finish; Dani opts for an insulating layer of cob around her brick base. Photos By Dani Wolff

Layer Up

Depending on how you want your finished oven to look, you can opt to add a insulating layer of cob around the outside of your base bricks at this point. Now, working from the top down and using a tape to ensure it’s even, cover the dome and the top of your bricks with a 100-mm-thick layer of cob to form your first mass layer.

If you haven’t built an arch at the entrance, use a butter knife to cut your door out of the cob. Your door opening should be four bricks wide and 271 mm high. Cover with a tarp and leave overnight.

Day Two

If your mass layer has dried overnight, mist it down with some water to give the thermal layer something to stick to. To make it, make another four batches of cob as before, mixing in three or four big handfuls of straw. You want this layer to have decent straw content but with enough clay to still stick together.

This layer will be 120 mm thick and is applied the same way as the cob layer, ensuring again it’s laid evenly over the whole dome. Finally, either use leftover cob from your mass layer, or make another batch to cover the thermal layer with a thin finishing layer.

Once completed, the sand form needs to come out within a three-day period. It can be done immediately, but dig out just a little and make sure the mass layer isn’t going to cave in. If it is holding together well then dig the whole sand castle out and allow the oven to dry. Naturally is ideal, but if you are eager to use the oven you can make a small fire inside to help it dry out.

Know your ratios

HOW TO DETERMINE YOUR CLAY AND SAND RATIO WHEN USING YOUR OWN SUBSOIL

Subsoils have varying degrees of clay content, so four tennis-ball-sized tests will reveal what you’re working with. First mix one part clay soil with one part sand, add a little water and make a ball. Repeat this with increasing ratios; one part clay to two parts sand, then three parts sand, then four parts sand before labelling them and leaving them to dry. Observe the balls, if they shrink a lot and crack there is too much clay, if they are weak and dusty, it means there’s too much sand. The strongest, crackfree ball will be your ratio for your mix.

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