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.