This clay normally dries well, but not this time. Strangely, this crack is not at the handle join, it is penetrating into the mug wall. Actually, this is not a crack, it is a split. Excessive slip around the join was not removed, that is bad when a body has larger particles, they permit water left on the surface to penetrate inward and begin a split. An aggravating factor was that the handle was allowed to dry faster than the mug itself, pulling at this join and opening the split even more.
Particle Size Distribution
Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.