Location: Grand Valley of Colorado's Western Slope
posted 5 years ago
They look like concrete, are they? If so, they could be mass at some point down the line, but probably not the fire box or riser, where the temperatures get so high. One concrete material advertised as heat resistant can be exposed to intermittent temperatures up to 1000F, and constant heat of 300F. Rocket stoves have sustained temperatures much higher than 300F.
One of the problems with concrete is the temperature differential in between one part of the chunk of concrete and another.
Those look a lot like a couple I have from my mother's lake cottage storage heaters. They have iron slag (perhaps that is magnetite), are very heavy and impervious to weather - they have been in upstate New York weather for 10-15 years. I wish I had a pile of them.