Pumice griddle bricks

Just an idea, I did a bit of searching around the forums and the web and did not find any examples of these griddle bricks used as an insulating material in construction of an rmh. Apparently pumice melts between 1600 and 1800 F so it can withstand decent temperatures. I am not sure however that these figures would mean the same for these typical bricks used for cleaning flat-top grills. They can be ordered from most restaurant suppliers and online you can get a case of 12 for about 15 bucks. The big ones are 3 1/2 x 4 x 8 inches but one appealing feature is that they can be easily cut, ground or otherwise shaped into any dimension. From this they could be packed together very tightly without the necessity of a slip or mortar in between them. I have read in other threads that the R value of pumice is poorer than rock wool, perlite/slip or vermiculite but those comparisons seemed to be to the loose-fill pumice one might find at a nursery. There might be an advantage in the tight uniform structure of this particular material.

They could be easily employed in insulating the feed tube and the burn chamber, outside of the fire brick interior but I am not so sure about the heat riser. Perhaps with a more heat tolerant material on the interior of the core.
Anyway, I just thought they might be useful in this sort of application and this is where I come to find out whether or not my ideas would be a waste of money and time! Thanks!

Hi Aaron:  Never heard of anybody trying pumice stone.  1600-1800 F isn't quite enough. definitely a no go on a riser and way to close for me to use in a burn tunnel.  The price is sure appealing but I don't believe it would last.  Regular insulated fire brick is also easily shaped and cut and is generally good to well over 2300 F or more . Quite a bit more expensive though.
The pumice stone would work between an inner core and an outer shell but I believe they would melt anywhere inside a core.  

How about adding a refractory hot face to the surface? I know allot of forges are built with low temp insulating material covered with a refractory hot face. I have one of those stones and have thought the same thing you have. Very easy to shape and light weight. Seems like there has to be some use for them other then scrubbing all the flavor off the grill.

I bought one of these to check it out today.
When exposed to a propane torch it doesn't melt, rather small pieces pop off of it reminiscent of popcorn popping, quickly leaving a depression where exposed to flame.
I'll try a lower intensity heat some other time, but I suspect the result would be near the same.

Very cool Alley! My only experience in using them so far is cleaning flattops. I have yet to research how one actually takes readings inside an rmh, I would imagine using sensors capable of measuring those temps. I can also imagine that the installation of those sensors would be a hassle, though necessary, but as well having a potentially compromising effect on the integrity of the rmh by drilling holes, etc, maybe not. I am ignorant of the temperature measuring processes of rocket mass heaters besides the method of using a material that is rated up to a specific temperature, conducting burns over a period of time and  then dismantling the structure and observing how well that particular material held up. Still, any conclusions one might reach from that method would seem to be more in the ballpark range rather than an exact number. I am interested in knowing the rate at which the temperature drops as the distance increases from the interior of the burn chamber. This would depend on several factors, mainly the thickness and type of firebrick and the temperature of the fire within the burn chamber. But this data would be useful in determining which materials could safely be used in insulating outside of the feed tube and the burn chamber. It seems that knowing the gradient value of temperature in relation to the distance from the heat source would open up possibilities for more cost effective materials to be used in the overall construction of an rmh. I have noticed that a lot of the materials in constructing an efficient and clean burning rmh must be high quality (and consequently not cheap) in order to have a well working, long term setup.