Soapstone - Where is the best place to include it in the RMH

I am new to writing on the forum. I have been planning my rocket mass heater/stove/water heater build, for about 3 months. I purchased a generous amount of soapstone tiles, and pieces from a broken slab. I want to utilize the fantastic idea in the https://permies.com/t/21815/rocket-stoves/castable-burn-chamber. I wanted to use the soapstone as the liner for the bottom of the feed tube, and also wanted to line the entire burn chamber with soapstone, which I intend to cast right into the unit. I wanted to put a large piece of soapstone on the top of the barrel 3 inches larger than the diameter. I wanted to put chunks of soapstone in the bench/battery, I'm not sure I want to use cob as the other material with the soapstone. I will put soapstone tiles on top of the bench/battery. I also purchased 50 firebricks, although I'm not sure I need them. I would appreciate opinions. Thanks Diane

Diane Monteil : This is where I ask if you have been to cobcottage.com to get your PDF Copy $15.oo of Evans' and Jackson's Great Book 'Rocket Mass Heaters', There is STILL
No other book in any language that contains more Rocket Stove, Pocket Rocket, And Rocket Mass Heater family information ! ( And i don't make a dime ! )

The fire brick Contains and insulates the fire, allowing for a more rapid creation of the Extremely high working Temperatures that creates the clean burn of the Rocket Mass Heater
R.M.H., The Castable Core as presented in the two videos, will work and will deliver nearly as Hot Exhaust Gases through the combined Feed tube, Burn Tunnel. and base of the Heat
Riser! There the next Castable section the Heat riser will deliver the hot exhaust gases to the barrel ( where the 'magic' occurs that makes an under $300.oo R.M.H.
perform like a $30,000 Masonry Wood Stove !)

It is from this point on that the Cob comes into its own. Out of simple Cob you create the leakproof shell and sealed system That delivers the hot exhaust gases to the horizontal pipes
that deliver the exhaust gases and their Heat Energy to your Thermal Mass !

From the base of your R.M.H. which if not made out of cob requires the laying up of brick to channel your hot exhaust gases ( Both work, which is in your skill set ?! ) And through
the Thermal Mass. The cob carefully shaped around your horizontal pipes serves as a near perfect heat transfer medium, and at the same time protects the stove pipe against
collapse and also makes a second seal against the penetration of the exhaust gases, again the only other possible material to create the horizontal piping through the thermal mass is
laid up brick, which will not transfer the heat energy as effectively, sending more of it up the chimney !

Cob is a wonderful and dirt cheap -pun intended- to accept and transfer Heat Energy through itself to the surface of the Thermal bench and your bum ! It does however transfer and
store that not as well as denser material like rock, the denser the better !

Here we are going to talk about the Utility of your Soapstone not its Beauty, because of the mechanical characteristics of this rock it contracts and expands much less as it cools and
heats up, and resists heat stress much better than, and at higher temperatures than Iron or Steel. Because of these characteristics everywhere is a good place for the soapstone !

I personally would make a Sketch up of my Thermal bench using the Soapstone as 'dressing stone' on the sides and possible back of your thermal bench burying lots of other common
stone deep in your cob, and save any left over Soapstone for trading materials rather than burying it out of sight !

This will give you a good start, Read 'The Book' and come back here often ! Big AL ! As Always your comments/questions are Solicited and are welcome ! A. L.
heat

Diane, I'm envious of your soapstone stash! You will not want to put any solid material in the core casting as the refractory mix will shrink a lot as it dries while the stone will not. That will break the core to pieces in the drying stage. Additionally, one of the main reasons to cast the core is to create a highly insulated burn chamber, as opposed to one with mass. Paul, Ernie, and Erica noted in a past podcast that in their experience with an insulated burn chamber they realized 80% better performance over a fire brick burn chamber. That performance improvement is exactly why I prefer the cast core over a heavy fire brick build. So, yeah, keep your excellent soap stone away from the heart of the burn and use it downstream of the barrel to store the heat.

Matt Walker : Good catch ! Thank you! I was not thinking about your castable cores shrinkage, I am glad that you were so prompt to respond, I will try to keep that in mind
in the Future. For the Craft ! Big Al

Thank you for the quick responses.
Big Al, I am greatly appreciative for all of the knowledge you so enthusiastically share with the forum, and specifically myself. Yes Al, I do have a copy of the book, I can't imagine trying to figure out the measurements and mathematics with some guidance. I did order the full set of DVD's and I am very impatiently awaiting for them to arrive, I hope that it does not give me more questions, than answers. I mostly have just been using the summer to collect all of the things I think I will need for the build.
Matt, you are my hero!!! I got my streak of passion for rocket mass heaters from watching you. I did want to ask you a question about the riser. I realize you cast the riser out of the same material, that being about 14 parts of fire clay and perlite, to one part refractory cement, but have you experimented with sodium silicate (aka waterglass). I have seen the finished refractory products made from perlite and sodium silicate and remain very impressed with the refractory strength. I was considering trying a riser with the sodium silicate , and like you forming it into a round shape, like that of a stove pipe. I do have the materials to make the sodium silicate myself here at home, so I can experiment a bit first. I don't know if adding the sodium silicate to your formulation would enhance or devalue its abilities. I'll keep you posted.

With my heartfelt appreciation,
Diane