rocket mass heater riser - perlite and clay

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


Here is the part that I wrote


Perlite is a type of sand that is heated to about 1800 degrees and then sorta “pops” like popcorn.  

The riser is made by greasing up the outside of a duct and the inside of a larger duct and filling the space between the two ducts with a mix of clay and perlite.  The mix is heavy on perlite and light on clay. The recipe is generally to make a ball of the mix about three inches in diameter and pinch the ball from opposite sides.  If you cannot make a ball, add more clay.  If the ball smooshes, add more perlite.  If the ball breaks/pops/explodes it is just right ( https://youtu.be/GkHOwmKyL7A?t=55 ).  When the clay dries, remove the duct.  As the riser is used the clay near the fire will become ceramic.

I used perlite and clay for a lot of riser projects and later found melted glass in the burn tunnel.  This is expected.  The idea is that the perlite that is closest to the fire will melt out - and maybe some of the perlite a bit further into the riser might melt and sit in a perlite shaped “bubble”.  And the perlite closer to the outside of the riser will be unphased.

Years ago I looked it up and found several sources reporting that perlite melts at 1400 degrees F.   Sounds about right for my experience.  So this is what I repeated for years and years.   BUT WAIT!  More recent online data suggests that perlite melts at 1600 to 2300 degrees F.  

Experience suggests that this type of riser will last about three years.  Eventually, the riser just sorta turns to dust.  And it was never a really great performer anyway.  If I were to try this again, I think I would wrap it in high temp wool.

Total cost $15 to $30.


And then Mud adds

In his very good video tutorial https://www.youtube.com/watch?v=AqvTdbRSlwk Matt Walker reports that his perlite and clay riser was still in perfect shape when he pulled it out to try something else after 8 years of hard use. Thomas Rubino (photos below) reports some erosion on his after 2 years but still completely functional. He has since replaced it with a more rockety 5 Minute Riser (see above).  

Here is a video I made from 2009 featuring this technique




And another from 2010




Please load us up with lots of info, pics, videos, etc.!

Thomas Rubino's perlite and clay riser (new)

Thomas Rubino's perlite and clay riser (after 2 years). He has since replaced it with a 5 Minute Riser made from the new less toxic Morgan Superwool he carries on his website https://dragontechrmh.com/morgan-superwool-plus-non-ceramic-fiber-blanket/

Screen grabs from the two videos in the top post: