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Today we pushed on, built a brick support around the base of the firebrick burner, – above, and then backfilled all the gaps with perlite and a clay mix to thoroughly insulate the fire itself, which we want to burn as hot as possible. The clay mix is a little hit and miss, but is roughly 1 part clay to 4 parts sand. For areas around the burner we’ve stuck to using the clean clay bought from a pottery supplier. For everything else, it will be the locally dug clay, at around 1 sand: 3 or 4 clay. No straw in the bits immediately adjacent to the pipework, apparently, though its a bit late for that in one or two spots.

 

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The next step was to surround the firebrick riser with a metal cylinder (an old water tank in this case) which was again filled with perlite insulation and capped with clay. Over the top of this goes the 50 gallon drum, leaving a 2 inch gap around between it and the inner riser. This is then connected to the horizontal flue by means of a variety of bits of firebrick and more cob.

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Having got all this together, and dropped on the top part of the insulated flue, it was time for the moment of truth. Rocket Mass Heaters aren’t known for easy lighting, as until they get warm, there’s a tendency for smoke to blow back up the fuel feed. Other than a bit of initial smoke, it went really well, especially considering that it is trying to dry out a load of clay mortar along its way. The barrel got nice and hot, while the insulated flue stayed cool, and the exhaust really was a little smoke, and mostly warm steam, just like I’ve seen in countless videos. I don’t know why, but I’m still surprised it actually works..

In this short video you can hear the distinct ‘rockety’ sound that gives these stoves their name.

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