The one thing I learned from my old clay oven is that insulation underneath the firing base is critical if you want the oven to get hot quickly and stay hot for a long time. Most of the suppliers like www.kilnlinings.co.uk recommend Calcium Silicate boards, but these are quite expensive. So after some research I discovered “Vermicrete”. this is basically a concrete made with Vermiculite instead of sand. Vermiculite is a heat proof insulation material typically used in chimney backfills and is cheap and easy to find in your local builders yard.
We created a 4″ layer of Vermicrete made up of 9 parts Vermiculite to 1 part Portland Cement – a part is a spade full! This needs to be mixed in something like a gorilla tub* with a bit of water using a spade. A cement mixer will destroy the Vermiculite down to a powder, making it a useless insulator.
Vermicrete is a very light concrete and the rule of thumb for insulation is the lighter the better! Rado Hand reckons a one inch layer of Vermicrete has the same insulation properties as 20″ of concrete.
I made a quick frame out of some 4″x1″ planks to contain it. The layer dried overnight and was ready for building on the next day. My old base was 48″ square so I needed two bags of Vermiculite and 1 bag of Portland cement.
The next job is a layer of bricks that will be used to build the fires on and cook the pizzas. To make life easier in our confined space we build a couple of layers of the back and sides before starting.
We mixed up a load of heat resistant mortar as these bricks are going to get hot and laid the bricks out, using a spirit level to get them nice and level. You don’t want to put mortar between the bricks – but you can brush in some kiln dried sand at the end if there are gaps..