RMH mass material... pea gravel OK?

Sorry if this has already been answered somewhere.  I'm just leaning the search function and didn't find what I was looking for...

I have easy and cheap access to many forms of washed gravel, stone, etc. nearby and can haul close to a half ton at a time (maybe a half yard).  I'm thinking for an average size RMH storage bench I could probably get enough rock for under thirty bucks.  What is a good size?  I understand that more dense packed rock would be best.  But that would suggest that sand would be the best mass and I haven't seen anyone using sand for fill.

Any thoughts?

Tim

Hi Tim.  Welcome to the Wild World of Rocket Mass Heaters!

Your question has been a common one over the years and has had many takes on its performance.
Generally speaking rocks, pea gravel and sand, no matter how densely packed are inherently surrounded by tiny air pockets which act as insulation.
What your looking for is a solid monolithic mass free from voids or air spaces as much as possible.
Its not to say that you can't use a loose fill (for a quick and/or portable installation) that allows air to percolate up through the pebbles, but again, its performance at retaining heat is not nearly as good as a solid mass.
So you could still use the rocks, stone, gravel you have access to, but it would be better to fill all the voids in with a clay mortar. Go heavy on the rocks and just enough clay/cob to join it all together.

The cool RMH in the Fisher Price home has a pebble based mass system.  As does the new RMH in the Red Cabin.  I believe I heard that they're less efficient but the convenience and undoability of them make it worth considering.  

Hi Tim;
Gerry and Mike both had it correct.
Sand or pea gravel have both been used successfully.  It is a quick solution using materials on hand.
It all comes down to longevity.  
A solid mass with no air pockets will absorb more of your heat and share that heat with your room for a much longer period of time.
So depending on your location and how much heat you might want or need overnight either one will work.
I always use large solid rock bedded in clay and the whole mass contained with bricks.  
A large solid mass keeps my plastic greenhouse at 50F with no fire for 8 +  hrs in northern Montana...