Permaculturing in Portugal

One family's attempts to live in a more planet-friendly way

Cob bread/pizza oven

As part of the renovations of the smaller building on the quinta, we intended to create an outdoor kitchen under the new roof extension with all cooking to be done using energy sources on the quinta – ie. with wood-burning or solar. First on the list – mainly because I’ve been itching to build one – was a cob bread/pizza oven.

I followed the guidelines and proportions recommended by Kiko Denzer in his “Build Your Own Earth Oven”, the seminal reference book on the subject.

Outdoor kitchen counters under construction

First, we built a schist stone retaining wall for a kitchen counter, backfilling with rubble and stamping it well down, leaving enough depth for a substantial insulation layer beneath the oven.

Outdoor kitchen counters under construction with site for bread oven

Leca an clay slip base insulation layer for clay oven

Into this space I added a good 15cm/6″ of Leca (lightweight expanded clay aggregate) coated in clay slip.

Beer bottle insulation for base of cob bread oven

Then a layer of 20cl beer bottles on end.

Sand layer for base of earthen pizza oven

The bottles were then covered in dry sand and levelled.

Clay mortar bed for brick base of clay oven

Next, a layer of clay mortar (sand, earth and clay in the proportions 3 measures sand, 1 earth, 1 clay) in which to bed the firebricks forming the base of the oven.

Base of cob bread oven

The oven base laid, with a stick cut to the internal height of the oven centred in a circle marking the circumference of the oven. Temporary bricks mark the width of the doorway and another stick marks the height of the doorway.

Creating sand form for earth oven

The sand form for the oven under construction. Building this was an exercise in patience. Just as I was adding the very last handful of sand and smoothing over the shape, part of it collapsed. This happened no less than 6 times. I came to the eventual conclusion that the mixed particle size sharp sand (recommended for the purpose by Kiko Denzer) I was using didn’t have enough fine sand in it to hold its shape, no matter how carefully I adjusted the moisture content, so mixed in some much finer sand which finally did the trick.

Complete sand form for clay pizza oven with beginnings of thermal layer

I didn’t have any newspaper to lay over the form (I gave up newspapers some years ago) so it had to be damp kitchen roll instead.

Thermal layer of cob oven complete

The thermal layer of the oven – the same mix and proportions as the clay mortar used for bedding the firebrick base – was then built onto the sand form. Carefully. I didn’t want it collapsing again. This layer is around 8-10cm/3-4″ thick. Rather than cut the door later, I shaped it at the time, using another stick cut to the correct height to get the oven proportions correct. The relationship of oven diameter to height and to door width and height is, according to Kiko Denzer, crucial to good oven performance. The oven height should be 70% of its diameter and the door half the width of the oven diameter and 63% of its height.

Doorway and chimney being built for cob bread oven

I wanted to add a doorway and chimney to the oven rather than stick to a simple opening. Neither are necessary for the performance of the oven, but a doorway protects the vulnerable oven edges and a chimney takes smoke away from the entrance and out of your eyes when you’re firing the oven. Firing is done with the door placed to the outside of the chimney to allow smoke to escape, but baking is done with the door set to the inside of the chimney to conserve heat in the oven. I made the doorway with an arch of firebricks and created a chimney from old broken roman roof tiles.

Thermal layer of cob oven complete

The doorway was completed by a schist wedge (set well forward of the chimney so as not to crack with the heat from the oven) and the chimney tiles were surrounded with the thermal layer clay mix.

The thermal layer was left to dry for a few days. We had a lot of rain and high humidity at the time and I didn’t want to risk a slumping inner layer once I added the insulation.

Insulation layer of cob oven being added

The insulation layer, around 10-12cm/4-5″ thick, was made from sawdust mixed with clay slip. This is a great mix to use! Easy to form and it keeps its shape surprisingly well.

Insulation layer of cob oven being added

Insulation layer of cob oven being added

Insulation layer of cob oven complete

The insulation layer was again left to dry out a bit for a few days before adding the final cob layer. This was the same mix as the thermal/mortar layers, with the addition of straw, covering the oven, chimney and doorway.

Cob layer of earth oven complete

The finished oven has been left to dry slowly since to minimise cracking. I removed the sand form after another week, and after allowing the inside of the oven to dry a little, finally set a slow fire inside. This will be repeated a couple more times as the weather improves. The pizza party won’t be long now …

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