These broken shards are from the vertical walls of similar cylindrical vessels. But they are not the same clay body. Both of the vessels were fired in the same cone 10R kiln. Both contain about the same amount iron. Both were glazed inside and out. But the body on the left is more vitreous and finer. The one on the right contains grog and is more porous. It is able to vent gases of decomposition better.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.