The mug on the left is made from a whiteware body (Plainsman M370), the one on the right is a highly vitreous translucent one (Plainsman Polar Ice). Both have been over-fired slightly. The Polar Ice mug has stuck to the shelf somewhat, taking chips out of the base on the outer perimeter of the bare porcelain (a fault called Plucking). If the shelf had been better dusted with alumina powder (or a kiln wash based on it) rather than a cheap kiln wash made from a kaolin:silica mix there would have been no problem.
A high-melting powder applied to kiln shelves to prevent ware from sticking to the shelf during firing. It is also used as a parting agent for to keep elements of fired ware from sticking to each other.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
A firing issue in ceramics where the foot rings of vitreous ware stick to the kiln shelf. Removing them leaves sharp fragments glued to the shelf.