Around southern Vancouver Island, very few people have heard of natural building, and even fewer of wood-fired masonry heaters – never mind traditional clay ones. So I often find myself pondering building issues in a cultural vacuum; and trying to explain things without common vernacular. Explaining something that the listener has never seen or heard of can get pretty old – unless you are curious. So long as you are curious… I am happy to ramble.
When I set about building any kind of dwelling or hearth, I consider the pros and cons of many approaches. What is the desired effect? How does a material/technique achieve this effect? How does this affect other processes? What are the lateral, synergistic benefits? What are the undesirable side-effects or repercussions?
For example, the BC Building Code stipulate that a masonry fireplace be built with techniques and materials that prevent it from burning the house down, or poisoning occupants with CO, or collapsing in an earthquake. These are all very sensible expectations; worthy of being met one way or another.
But to my mind, the hearth had best offer more than this. I want it to not only be standing after a major earthquake, but still be fully functional. This way, it will become a place for family, friends and neighbours to gather, take safe respite, warm our bodies, dry clothes and herbs, cook food, and heat water… all the things it is meant to do. In a worse case scenario, this could be happening in a winter rainstorm, on top of 2 feet of snow, with after-shocks, howling winds and blown out windows. Under such circumstances, the masonry heater may be getting fired constantly (rather than the usual 1 or 2 hours a day). So, when I am choosing materials and techniques for my heaters I ponder: What is the highest temperature this material is going to reach in its lifetime?
As my love for clay-brick-cob heaters has grown over the past few decades, I have been fortunate to connect with some very cool and wise ‘old-timer’ artisans. And they all echo the same thing: Once below 550c, any clay brick will do. thus, when deciding what to use where, I round that number down to 500c, and then picture what will happen when the heater is firing 24-7 in a windowless winter deepfreeze… literally.
(This page is currently under construction) (Below are some orienting points to be fleshed out…)
- common wisdom says that once the flue gasses are 4 or 5 feet downstream from the firebox, the likelihood of materials reaching 500c is negligible; so this is where I transition from double wall to double skin. With double wall the refractory core (firebrick and specialized castables) is built physically separate from the facing (outer brick/stone/cob work)
- Where hotter than 500c, and/or occasionally cycling above 250c, i switch to firebrick or clay flue liner core.
- With firebox i go double wall with ceramic blanket.
- Exhaust mainfold i use firebrick lined with splits of IFB, glued with super 3000 (sodium silicate and talc putty).
- The whole heater is covered in cob plaster with a high content of finely chopped straw.. This offers tremendous tensile strength and acts as a diaphragm and envelope.
- For hotter areas of the heater body I substitute 4.5 oz fiberglass mesh embedded in the outer 1/4″ of browncoat.
Thinner wall sections I also embed the fibermesh on the inside channels, and include about 25% S-3000 in the mud.
All in all this is a hybrid technique between
– cob mass,
– old school single-skin clay-brick and clay-sand mortar,
– ‘double skin’ with fibrous parge coats, and
– modern double wall…
with 2 or 3 stages of transition from one end to the other.
The trick is knowing where to expect thermal expansion and let things move around within a shell (double wall with padding)… and where to snug things up tight…
I have more to say about the clay-based techniques, and how when and why i incorporate higher tech materials into the build. But for now just ant to offer a quick thanks to some people who have influenced me: Eckhardt Bichel who has explored and restored countless 400 + year old clay-brick-cob heaters in Weimar, Germany. And thanks to his student and my first teacher Holger Laerad, for humbly carrying and sharing the knowledge across the water. Thanks to Trev, Matt, Peter, all the folk who contribute to proboards forums… And big thanks to Norbert, Alex, Max, and all the folks at mha-net.org…
more later 🙂