Enhancing the thermal mass of cob for cheap

I have heard that water stores 4 times the heat cob does pound for pound. I have made 2 experimental thermal masses in my house utilizing recycled glass bottles and water. I have no way to test them or anything to compare them to but anecdotally they work pretty well, even in thier unfinished state. I think thermal mass could be added to a rocket mass heater or trombe wall provided the temps don't get too hot or too cold.

I used empty beer bottles and mortar to make an insulated slab over a vapor barrier. Next I tapped 1 gallon jugs full of water into the mortar. I used a stiff mortar to build up a perimeter against the wooden form. I plan to fill the interior with either sand or clay dirt. I will cob and plaster to finish.

Hi Wills,
I think you have hit it when you say "as long as it doesn't get too hot or too cold".

My worry would be them getting too hot, starting to boil, the steam builds up... and you either have a leak or a mini explosion.

Could you leave the top without a cover and open to the elements so they don't pressurize?

Glass will crack or break if the temperature gets to hot.

I do not have experience with how hot a RMH gets though I feel they are too hot for glass to work as thermal mass.

Actually I feel that RMH will melt the glass though I have no experience.

I have placed these bottles in places where they won't freeze or boil. Antifreeze or alcohol could be added if freezing was a worry. There are plenty of sections of my rocket mass heater that don't get too hot. Obviously it would not be good to put these water bottles near the combustion chamber or the flue pipes. On the mass part of the bench temperatures don't go much higher than 100 f, at least on mine.

First of all, are there records of the heat levels achieved inside RMH systems and where.
I imagine it has been measured and recorded somewhere.
Then compare that figure with the melting point of glass bottles, otherwise we are just guessing.

Well water boils at 212 degrees f. This would ne more immediate danger than the glass melting because steam pressure in a tightly sealed bottle could cause the glass to rupture. This is a good reason not to cap the bottles too tightly! Glass melts at 1400 degrees. Of course I am not proposing putting these bottles near the hottest parts of the stove. My stove exhausts around 200 degrees and the non flue mass part (on many rmhs this is the back rest) has never gotten above 100. So there are a few places this technique could be used in a rmh safely.

Offhand, my concern is that the differing thermal expansion of the various materials will create gaps that impair thermal transfer. As I understand it, the contiguous block of thermally conductive materials that expand/contract at the same rate is part of what makes RMH's work well. My 2c.

I think a loop of copper pipe embeded in the cob bench and connected to an old water heater could be simple and effective.
~40 gallons of water in an open topped thermal convection loop should be very robust and safe.

Douglas- I have seen issues related to differing material properties in a rmh, especially in the hotter areas. My lime finish cracked a little due to expansion and contraction rates between the cob and the lime finish. However what is the disadvantage of slow heat transfer within the thermal mass? The heat dosent dissappear or anything. Isn't the whole point of a thermal mass to slowly transfer heat to the room? If I wanted to quickly transfer heat wouldn't I just get a regular steel woodstove?
William- I added pex radiant tubing to my cob bed. It works pretty well at storing excess heat from my cookstove. I considered adding pex pipe to these masses but the radius required would be too tight. I didn't want to have any fittings inside the cob in case of a leak. I may use this method of enhancing thermal mass with radiant pex tubing in my guest room bed.

My thoughts on water and glass are less about the "explosive stuff" happening, but the fallout from that "failure". A buried glass bottle with a plastic cap isn't likely to explode and send the entire bench into the next room as shrapnel, but "pop and hiss" as it vents the steam, then piss all that water out into the room. So you have a chance of 14 gallons on the floor, more likely 4-8 if it did happen, then you get to worry about the remainder and when it leaks next. Then, if you decide to "fix it" you are digging for broken glass. fun. (not.)

If you don't "fix it" then maybe there's air pockets rather than buried mass, so your gains are diminished, or lost in the case of a total failure (a not-so-unlikely freeze-up scenario). A quick calculation on: 1/2 water/1/2 air vs. granite vs. clay is pretty much equal. (within <10%)

Obviously, as in your bed example, water excels at being easily moved around through pipes, and a good heat transfer medium. The nice thing about pipes is that they can be drained or blown out to winterize, and by themselves don't amount to a large volume of water (though could be connected to very large storage). You could also mix your heating methods, if you incorporated solar hot water collectors, or had an electric or gas heater as emergency backup.

Kenneth- I live in a mild climate in a house with huge amounts of thermal mass. It's never been below 50 inside even when I was away and lit no fires. You must live in a very cold climate if the interior of your house gets below 32! Did you insulate your interior pipes? Glassware can be frozen without cracking as well. I do it all the time with chicken broth. All this being said, I may still add some propylene glycol antifreeze to the bottles before I cover them with cob.
As far as a steam explosion, how dang hot does the thermal mass on the rmh you built get? I can burn mine 6 hours straight and the bench dosent get much above 100 degrees. In fact I find temps over 100 uncomfortable to sit on! I really doubt loosely capped bottles could build up enough pressure to spray hot steam everywhere. I think you underestimate how much heat cob and water can absorb. All this being said, worst case scenario I got some water on my hands. Cob can dry. I have mixed over 100 tons of cob over the years. Rebuilding a little bench dosent faze me!