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.

Bargain firebricks

Firebricks are really pricey in the UK compared with the US - anywhere from £1 to £3.50 each. I needed cheap firebricks, and it didn't get a lot better than these ones that I got from ebay yesterday. £10 for 34 of them, fresh from a demolished fireplace. That's about 30p each - cheaper than the reclaimed commons from the local reclaim yard. There's just about enough of them to make a 31" herringbone hearth when they're laid on their sides.

They're a bit tatty on the edges, but I can live with that. I'll be cutting them to make a nice circle anyhow, so I'll just stick the best ones in the middle and put the grotty ones round the edges. I have no idea what type of firebrick they are - yellow, with some red in them, heavy, and bigger than regular commons. And a big letter D stamped on them - wonder what that means?

I felt I should do some real work too, so I mixed three barrows of concrete, bent up some rebar into right angles, and filled the cores of my support blocks, using a couple of bars in each.

It all looks pretty ugly inside at the moment. The rebar will tie the cores to the hearth slab, which will be the next thing to be poured. First I need to figure out the best way to hold the concrete in over those very uneven stones.

Experiment 2: testing Thermalite blocks for hearth insulation

I'll need to insulate under my oven hearth to keep the heat in. The most popular ways to do this seem to use concrete made with perlite or vermiculite. The traditional way to do it is to use sand, but this isn't as good an insulator. Visiting the builders merchant today I noticed some Thermalite insulating concrete blocks. According to the website (http://www.hanson.co.uk/67/blocks/thermalite/products/turbo.html) they're very environmentally friendly as they're 80% recycled. They're made from pulverised fuel ash waste, and have a low thermal conductivity of 0.11 W (m.K). Vermiculite concrete has a thermal conductivity of 0.094 W (m.K) according to this website: http://www.william-sinclair.co.uk/industri.nsf/Concretes?OpenPage - pretty close. I like these blocks as they'll be a lot quicker to use than pouring fancy concrete, plus the vermiculite/perlite seems pretty pricey. These blocks are cheap - about 60p each. I wondered what they'd be like when heated? Let's have another experiment...
That's one of the blocks in the background, which got the hot end of my blowtorch for ten minutes. The spot I heated glowed red after a few minutes, then a satisfying yellow colour after ten minutes. I turned off the blowtorch after ten minutes and started my highly scientific test. The block was still cold on the back, so I tested to see how close I could put my thumb to the spot that I'd heated without screaming in pain. Told you this was scientific.
I could hold my thumb comfortably at about 4cm from the spot that was glowing yellow - this was about twenty seconds after I'd stopped the blow torch. The block was paler where it had been heated, although still pretty tough. I scraped a hole in the block, and the discolouration was only for the first mm or so.
I think these look promising - as insulation they'll never need to take this sort of direct heat, so my test was way over the top. They're clearly good insulators - I think I'll use these under the heath. I'm impressed that they're a recycled product themselves, which fits nicely into the ethos of my oven build.

Experiment 1: testing arch bricks

I was given a small pile of solid red bricks a couple of days ago (thanks again freecycle!) that have the corner chopped off in a 45 degree angle. They probably have a proper name, if you know your bricks. They look like they'd be useful for building the inner arch of the doorway, so long as they can take the heat from the fire. I was burning a load of garden waste in the incinerator anyway, so I stuck a few of the bricks in a tower in the middle of the fire to test them.

The fire burned for about 3 hours (a lot of stuff to get rid of), then burned down overnight. The embers were still red and glowing after 6 hours, so those bricks got pretty hot. 24h later they were still too hot too touch, and were almost too hot to hold with my thick welding gloves on. The bricks survived fine - didn't seem any worse for wear after their cooking. That's a cooked one on the left - the one on the right is uncooked. I think they'll be fine for my doorway arch - the chopped off bit should allow the inner door to fit nice and tightly. And they're free!

Base finished

Well, it took the best side of day to finish it, but the base is completed! That's used up a huge pile of rubble for the walls on the back, and most of my free rockery stone on the front.

I had some help from a very enthusiastic three year old with the last few lumps of rubble (thanks Thomas!), but still had a fair bit left. No matter, simply bung it in the spaces around the concrete support pillars - I can use it to help support the hearth when I pour it.

I'll fill the last foot or so with more rubble once the mortar's set on the top stones. I'm pretty pleased with that for a first effort - the uneven levels on the walls will be taken care of when I pour the (level!) hearth. If anyone else asks me how I'm going to get the pizzas to fit in that little brick hole, I'm going to hit them.