In this blog series, I will take you through the step-by-step process of building a Cob Oven, an Earthen Bench, and a Rammed Earth oven base. These items were built in a semester course I taught for the American College of Building Arts. Everything you will see was built in a total of 45 hours with 12 students, including time spent teaching and learning as well as site clean up.
Step 1: Making Earth Blocks
An Earth Block, also known as Adobe, is a quick and easy way to make modular building units, which could then be used for constructing a bench, a wall, an oven base, etc.
- spread out your tarp and place 1 (5 gal) bucket of clay soil
- add sand and/or pea gravel if needed (see below)
- foot mix until all clay chunks are broken apart and all material is fully mixed into a homogeneous mixture.
- begin to add water slowly, little by little, and continue mixing, until the soil mix is the consistency of a thick pudding
- create a flat area on the ground to lay your mold (see below); a concrete slab works well
- place mold flat on ground and begin to throw soil mix into the mold. Start with the 4 corners and then fill the middle. It is important to throw the mix in with some force so that you are filling all micro-voids.
- use a screed (a scrap piece of 2×4 lumber works well) to level off the top
- in a quick, wiggling motion, lift the mold straight up. it’s important to lift both ends evenly. if the soil mix is sticking to the mold, use very quick, jerking motions to help it unstick.
- allow blocks to dry in flat position until it is stable enough to sit it up vertically, then allow it to dry in the vertical position until fully dry (if you are in a time crunch, adding a small amount of portland cement or lime will speed up drying time significant). make sure your drying station has a roof, or the ability to cover in the event of rain.
- wait until blocks are fully dry and then they are ready for building!
Notes:
Soil mix is based on roughly a 70:30 (sand:clay) ratio. You may need to add more sand or aggregate depending on the content of your soil.
Adobe molds can be made a variety of ways. If you plan to make large quantities of blocks, or use it frequently, building a metal form will last indefinitely, which wooden molds will eventually break. The size of the mold shown here is 14″x 14″x 4″; you can make the mold the size you need for your particular block.
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