There are two ways to make the adobe mix that you will use to create the first and last layers of the oven
The traditional method involves puddling the clay and sand together with bare feet. Lay a sheet of tarpaulin out and tip 2 buckets of clay on it, spreading it out. Then add chunks of clay from a bucket full, getting rid of sharp stones etc which will hurt. Then do the twist and shout. Occasionally lift one end of the tarpaulin to tip the mix back over itself. Keep going until it is thoroughly mixed.
The famous River cottage (did I mention you should but it!*) drop test is to drop a squeezed handful from shoulder height – the ball should hold together – if it it splats flat it is too wet and will need a little more sand. If it explodes it is too dry and you’ll need to add a little water. In practice on a hot day – spreading the mix out on the tarpaulin allowed it to dry a little if needed.
With nicely damp clay, the adobe can be mixed in batches in 10 mins using a mortar mixer and a large mixing tube, like these from Amazon. I just bought a bit from Screwfix and used an old drill. The mixture will take slightly longer to dry though as it is slightly wetter.
A concrete mixer is no good – the sticky clay will just go round and round. It’s the cutting action of a mortar blade that effectively and quickly mixes the clay and sand together.
I found that I could mix a bucket of sand and half a bucket of clay with a drill and mortar bit in 10 mins with an up and down and round and round action. I pre soaked my very dry clay in buckets over night and then lifted clumps into a measuring bucket, before adding the sand. The drill got quite hot each time, but had time to cool as I laid the adobe sausages.
I used both methods in our build – having lots of friends around for the traditional method was a lot of fun and the help made it pretty quick too!