Hmmm... not sure I can agree with the suggestion that using a porous thermal mass
inside the cooker is better than one that is highly conductive (of heat). Consider, for example, that the thermal mass is entirely contained inside the cooker and that any energy that it absorbs or releases is within the cooker as well. As the temperature in the cooker goes up (spike in charcoal temperature), you want the thermal mass to 'react' quickly, by absorbing that heat. If the spike in temperature ends and the cooker starts to cool, then you want the thermal mass to release the energy back into the chamber quickly as well. Porous materials such as clay, sand, or concrete are noted for use where their heat insulation capabilities are important ...like an adobe house that is cool during the day in spite of the rising temperature of the hot sun outside the house. In that case, you are trying to prevent the temperature differential on opposite sides of the adobe from conducting through the material. With a BBQ, something like a ceramic or clay BBQ body would help maintain a consistent temperature inside the cooker by preventing sun/wind/rain from conducting heat into/out of the cooker, but a highly conductive mass (say, iron) that is inside the cooker stabilizes the temperature inside the cooker by easily absorbing spikes in energy/heat and easily giving out energy/heat when the inside of the cooker cools - a stabilizing element. This is why The Big Green Egg (and similar) are made from clay, but no manufacturer of BBQs suggests using sand or clay or bricks inside the cooker ...instead preferring water, both because it conducts heat more easily and also has that 212 F state conversion (liquid to vapor) constant that makes the BBQ more forgiving of too-open vents.
Here's some thermal conductivity numbers, followed by an experiment that someone with a remote thermometer should try (source:
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html):
Air 0.024
Insulating Brick 0.15
Dry Sand 0.15 - 0.25
Water 0.58
Building Brick 0.60 - 1.0
Granite 1.7 - 4.0
Iron 80
An experiment (hopefully in a somewhat controlled environment, e.g. same outside temperature and no wind or rain):
1. Empty bowl (the 'Air' test) - run BBQ at say 250 F and stabilize for 15 minutes, then open vents fully and monitor temperature every 30 seconds (or 1 minute).
2. Bowl of water (weighed), steaming/simmering (water plus state change test) - run BBQ at say 250 F so it is stable for at least 15 minutes (water must be steaming/changing liquid to vapor), then open vents fully and monitor temperature as above.
3. Repeat with same weight in dry sand as water, same procedure as in #1
4. Repeat with same weight in iron, same procedure as in #1
Not perfectly scientific, but the range in thermal conductivities is wide enough to at least illustrate a pattern. The best temperature damper will show the slowest response to opening the vents. Maybe some weekend, I will try this?
Brian