Many people would find the fired appearance of this cone 10 reduction red fireclay (Plainsman FireRed) compelling. But it is not at all suitable for functional ware. This crack grew wider over a period of a week (after firing) because the inside glaze is exerting forces it cannot resist. Notice that where there is a glaze cover on the upper outside section there is no cracking. But the stresses within are still there, waiting for an opportunity for release. It is inherently risky to glaze ware only on the inside if you are not able to determine the fit. This is especially so if the body is not vitreous and does not have the strength to resist the outward pressure. But even if it is vitreous, the internal stresses will weaken its ability to withstand bumps.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.