Sunday, 16 September 2007

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.

Saturday, 8 September 2007

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 ( 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: - 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!

Sunday, 2 September 2007

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.