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London Clay for Fire Bricks

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Here in suburban London I'm trying to carve out a little piece of paradise, but being in the city natural materials are a bit harder to come by. Sure I can get anything I want via the internet shipped from China, but my goal is to see what I can source as locally and naturally as possible.

Everyone here in London uses charcoal barbeques, and before I even visit Rocket Mass Heaters, this is a good little project to get used to the technology by building a small outdoor stove to cook food when the British weather allows (barbeques are either spontaneous or often rained off here!)

Anyway, I processed a lot of the locally available clay here, known as London clay, last summer. It was commonly used to fire common bricks known as London Commons. From what I can pick up online, Fire Clay has a silica content of 40-80% whereas common Brick clay 40 - 65%.

I'll probably bite the bullet and just give it a go anyway, making insulative bricks following Dr Winiarski's techniques in this video

He uses a recipe of 500g dry sawdust, 900g grams of moist clay, and 1200g of water. But I thought I'd just see if anyone has tried something similar already and had any experience. I've read a fair bit about this and it seems that some people do use steel and common bricks for the combustion chamber and never have any problems, but maybe they're infrequently using a stove like me. I'm sure a well used Rocket Mass Heater running under a heavier load would definitely need to higher temperature clay. So I may get away with it in my situation.

I can buy some fire clay from the UK, but obviously local would be better.

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Location: Upstate NY, zone 5
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I'm pretty sure medieval London Ware potters used local clay to build their kilns, which would have reached similar temperatures to a RMH sustained for many hours. Even without rigorous lab tests, I suspect I would be comfortable using it. If you can find a potter willing to fire some test bricks, you should be able to get a good sense of its capabilities. In particular, mark a dried brick with some exact dimension and see how much it shrinks in firing.
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Well, use your london clay. No prob. With sawdust if you want. I don't know about this. But i'm prety sure you can find old crumbling bricks for nothing. Not far from you. And you could make grog out of these, to make teh mineral charge in your clay. That's how they would have made firebricks at the time.
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