The next step was to fill the center with more stone (thankfully of any shape or size) and any other. Once the center was filled to within 25cm from the top, the next layer was added which was mixture of straw and soil, mixed to a mud, to provide an insulation layer. Small stones were used to fill in any gaps in the walls and finally thick stone tiles were placed into the center to provide the oven floor. The gap between the oven floor and wall edges were filled to the same level as the oven base with a mud mixture.
The next stage was undertaken in the Spring which allowed time for settling and when it was warm enough to start building the earth oven, allowing the mud mixture to dry well..
We finished the project in March 2012 and baked bread successfully. What follows is a step by step guide on how we built our earth oven.
Step 1 - Planning - deciding on the size of the earth oven. How big you build your oven is dependent on how often you are going to use it and what you are going to use it for. If it's for a occasional pizza it doesn't have to be that big but we wanted an oven large enough to make four loaves of bread. An oven's size is ultimately determined by the width and length of the base floor you build. Our base is 1m deep and 1.15m wide.
Our oven wall was built in three layers and is about 18.5 cm thick. This left the oven with an interior width of 68cm and a depth of 53cm and we decided on an internal height of 50cm for the oven. Apparently, for airflow purposes, the ideal height of an oven door should be 63% of the internal height of the oven and the door width of between one third and a half of the ovens internal diameter. So our oven door is 30.5cm tall and 34cm wide. Our oven internally is large enough to fit four loaves of bread comfortably.
Step 2 - Mould - We made a mould using two and a half wheelbarrow loads of wet sand to the height and width of the interior of the oven and this was extracted after the oven walls had completely dried. A length of bamboo was used to provide a guide for the height. The mould once competed was covered with layers of wet newspaper to keep the sand moist and to mark the beginning of the mud layer. This proved valuable when we dug out the mould. We made a template for the door which was also as wide as the wall, to avoid having to cut out an opening afterwards. This mould stage was completed on Tuesday 20th March 2012
Step 3 - Construction of the walls - we constructed the oven wall in three layers and each layer was allowed to dry before the next layer was added. Drying time is important as it decreases the likelihood of cracking. We used a mixture of 50% sand and 50% soil but included straw, which acts as an insulator, in the second layer. The soil was dug out after removing 40cm of the top-soil to access the more clayey sub-soil. The sand and soil were thoroughly mixed before adding sufficient water to make the mixture workable but not too sloppy. Handfuls of mixture were worked into a ball before applying to the mould and worked in to ensure there was no gaps between each handful applied.
The first layer - which is called the thermal layer was 8cm thick. It took us two hours to complete on Wednesday 21st March and we allowed this layer to dry for three days.
The second layer - the insulation layer of soil sand and as much straw as we could mix in was completed in two stages each 4 cm thick. It took two and half hours to finish on Saturday 24th March and this layer was allowed to dry for a week.
The third layer - known as the finishing layer was 2.5 cm thick and was completed on Saturday 31st March and took one and half hours.
Step 4 - Removing the mould - The earth oven was allowed to dry for a further week which was hot and sunny before on the 1st April the door template was removed and the mould dug out. This allowed air to circulate and speed up the drying process.
Step 5 - Finishing touches - Some surface cracking appeared so we made up some mud mixture to fill the cracks and also to build a rim for the door. We also used the door template to cut out a metal door and attached a wooden handle. The inside of the door will be covered with foil when the oven is in use to reflect the heat.
Step 6 - Drying - On Saturday 7th April we lit a series of three small fires to complete the drying and ensure no further cracking appeared.
Step 7 - Test run - On Sunday 8th April we lit a large fire and kept adding wood to permit the fire to burn for two hours. We then removed the ashes and mopped out the oven floor before we baked four loafs of bread which took about 30 minutes. After the bread was baked, we baked some ginger biscuits in around 10 minutes and cooked a chicken casserole which was left in the oven for quite a few hours.