rocket mass heater riser - clay brick wrapped with high temp wool

Mud and I are working on a book.  This is the part about clay brick + wool risers.  We strong encourage everybody to share their experiences.  We hope that this thread will grow to ten pages of information about clay brick + wool risers.



Note that it must be true clay brick - not anything that looks like a brick that might be made with portland cement.  Bricks with portland cement will not survive these high temperatures.  

There are different qualities to clay brick.  Depending on the clay and how the brick was fired, you could have a clay brick that would last longer than most firebricks.  But odds are that most firebrick will outlast most clay brick.  

The bottom line is that a riser built with clay brick and wrapped in high temp wool will operate the same as a riser built with firebrick and wrapped in high temp wool.  The only detail left is which will last longer?  Some will go through 20 winters and look like they are ready for 20 more, and some will crumble at the end of the first winter.

Total cost is about $35 to $80.

Mud says  

Clay brick are not generally made to the same tight tolerances as firebrick, so they can be more challenging to drystack. Clay brick will generally need to be laid up in a clay or refractory cement mortar.

Watch your local classified ads (Craigslist, Facebook Marketplace, Thrifty Nickle) for "used chimney brick". Most modern brick is either decorative veneer brick (with holes in it for less weight) or some sort of patio paver, either of which has portland cement in it. Older clay chimney brick is thermally strong but is solid (heavy) and doesn't hold up well to being part of a patio so is often given away super cheap or free or sent away as fill. Do be aware that any repairs to brickwork in the last hundred years was probably done (improperly) with portland cement mortar, which is stronger than the brick itself so is hard to clean off. Older lime mortar will scrape off easily. Test a few before you take them home and find yourself with a big pile you can't do anything with.

paul wheaton wrote:
The bottom line is that a riser built with clay brick and wrapped in high temp wool will operate the same as a riser built with firebrick and wrapped in high temp wool.  The only detail left is which will last longer?  Some will go through 20 winters and look like they are ready for 20 more, and some will crumble at the end of the first winter.

Total cost is about $35 to $80.

Mud says  

Clay brick are not generally made to the same tight tolerances as firebrick, so they can be more challenging to drystack. Clay brick will generally need to be laid up in a clay or refractory cement mortar.

I've acquired literally tons of old unused chimney brick. I dry stacked some for a chimney on a rocket stove. It functions but the lack of tolerances is clear. Would clay mortar be like thicker clay slip or is there more to it?

Coydon Wallham wrote: Would clay mortar be like thicker clay slip or is there more to it?

Yes indeed, use a high quality mason's sand to make the fire clay mortar. A good mix to start with is one part fire-clay to about 4 parts sand.