These two pieces were in the same firing, the right one exited broken and cracked like this. They contain the same percentage of iron (it is supplied by red fireclay). The glaze is the same. The walls on both of these are completely black-cored. Multiple other pieces of this same size and shape were made of each of these two clays and the result was the same on all of them. The difference: The body on the left has 10% added feldspar. That feldspar is combining with the iron oxide, reduced to black FeO, to vitrify the body - that left mug is incredibly strong. The one on the right is fragile, a light tap with a hammer and it exploded into a dozen pieces. Why? The FeO, for lack of a vitrified matrix, has increased the thermal expansion of the body to the point pieces are failing spontaneously from glaze compression.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.