1 hour ago, rancho_gordo said:The pot will stay much hotter than a metal pot after the fire is turned off. It will keep simmering much longer than it would with a metal pot. I would think that the clay is what's different.
If what you're saying is that stoneware is a better insulator than metal, you're absolutely right.
But if you're saying a clay pot holds more heat than a metal pot, we need to be clear about the two pots being compared. A material's ability to store heat is called its specific heat; it's expressed as the amount of energy (usually a calorie, Kcal or Joule) necessary to raise a given weight (usually a gram or Kg) by a degree of temperature (usually Celsius). Water's specific heat is 1.00 cal/gram per degree C. Clay varies, but usually falls between 0.20 and 0.33. By comparison, Aluminum is 0.22, and cast iron is 0.11. So you are also correct that clay will store more heat per fixed unit of weight.
Note that we then have to adjust these numbers for density, because wall thicknesses of pots may (or may not) be similar, but the weights of such pots can be much different. Kaoline clay has a density of 961 Kg/m3. Aluminum's density is nearly three times greater at 2712, and cast iron's is 6800-7800 or 6-7x greater. You can do the math, but when you do, you discover that the heat-holding ability of a clay pot of the same size and thickness as a metal pot, isn't much different.
Then you have to account for how efficiently a material moves heat--its conductivity. And them you need to consider how easily it exchanges its stored heat with its surroundings--its thermal diffusivity adjusted for specific heat. Clay has terrible conductivity (about 1 W/mK), whereas aluminum's is 200x greater, iron's is 55x, and copper's is 400x! So if the idea is to take heat from an external source and apply it to food in the most efficient way possible, clay's not a good choice.
After the heat is shut off, clay will contain the disproportionately large volume of heat stored in your beans' water quite well. But you could do the same thing (actually better) if you put a metal pot in an old-time hay box oven. https://en.wikipedia.org/wiki/Haybox
I actually like clay pots. At least in ovens.
