Yesterday I ran another clay oven course at the Petersfield Sustainability Centre for 18 lovely people. It was a glorious sunny day again and everyone seemed to have a brilliant time. I certainly did.
During the day I asked one of the crew to make a note of the volumes of clay and sand we used (I’m not great at recording this stuff normally so thanks so much for doing this). Here’s what we used.
NOTE: 1 bag of builders sand fills a 15 litre bucket.
Sand Former – 10 bags of sand
First (oven) layer – 4 bags of sand, 3 buckets of clay (note that is a ratio of 2:1.5)
Brick arch, backfill and chimney – 2 bags of sand, 2 buckets of clay (we used a clay rich mix which made the material easier to manipulate). Also used 2 bags of sand to use as the former around which the arch was built. 11 bricks were used to make the arch.
Insulation Layer – 3 buckets of clay, 2 large bags of woodshavings.
The thing worth noting here is the ratio of sand to clay. Normally I prescribe a ratio of 2:1 but this is a rule of thumb. You need to aim for a material which holds firm, is not too soft (or wet) and not too dry such that it can’t be moulded very easily. If it is too wet (or if there is too much clay) it will slump around the base of the layer you are building. Clearly the plasticity of the material will depend upon the moisture content of the clay and the sand you are using. The drop test will help. I’d say always err on the dry side, you can always add a little water (or more clay) if you need to.