[wlively]

The easiest way to explain thermal mass is to picture a sponge. The refractory material (brick, ect) is like a sponge. The more mass, the more heat it will hold, the thicker the sponge the more water it will hold. The insulation layer has nothing to do with heat mass, it just reduces the rate at which the heat leaves, like a layer of plastic around a sponge drastically reduces the amount of water from leaking or evaporating. Also like a sponge the higher the mass, the more heat (ie. water) it will take to saturate (fill/soak) it. That is why high mass ovens take longer to heat.