Hearth insulation

And now for the insulation. I'd decided to use Thermalite insulating blocks to give 4" of insulation under the firebrick hearth. I figured this would be a lot easier than pouring vermiculite or perlite concrete, and it was. It took about an half an hour to cut the blocks to rough shape (you can cut them with a handsaw, a bit like sawing up tough polystyrene foam), then about an hour to mortar them all in.

About as easy as it gets, and cheap - this lot cost me £12. You can see a couple of thermocouple wires here - there's already one mortared under the insulation, as I want to see how much heat gets through these things. The one arched across the top will go inside the centre firebrick in the hearth. There'll be others in the dome, in the oven space, and in the outer wall to see how much precious heat I'm losing.
The blocks suck up water like a sponge, so I'll need to get a cover over this pretty soon. Next comes the fun part - building the oven.

Hearth slab poured

I poured the hearth slab last weekend, but was too worn out after doing it to post on the blog... I took the forms off today and was pretty pleased with it.

Amazingly, it's exactly level. That or I need a new spirit level. The plastic 'skirt' method I used to hold the timber ring in place has made a strange sort of edge to the hearth slab, but it worked!

It's done a good job of holding all the concrete in, and everything is well bonded together on very uneven walls. This gives me a nice flat, level slab to start building the oven on, a bit like a blank page. The little hole near the front of the slab is where the thermocouple wires come up from the front panel - there'll be six thermocouples by the time I'm finished.

Making the hearth form

Well, it took a while to figure out how to do it, but I've finally got the hearth formwork finished and level. I settled on making a ring of 12 3"x2" waste timber offcuts, cut with 15 degree ends to make a circle. I held the timber together with staples (pretty weak, wobbled all over the place), then clamped a ratchet strap around it. When tightened this was really strong and rigid, much to my surprise.

When I placed this on the stand and levelled it there were some pretty large gaps (>4" in parts), so the easy way to fix that was to make a black plastic 'skirt' inside the ring, and then trowel on a sloped edge of concrete. The black plastic stops the concrete falling out of the gaps. When this has set I'll pour the rest of the hearth - hopefully with no leaks.

The poured hearth slab will end up being about 5" deep in the middle (over the three block piers, and tied to them with the bent rebar), and at the 6" near the edge it will slope up to the edge of the form where there's still space for about 2" of pour.