Although not clearly visible on this photo, the unglazed body surface on both pieces has a sheen, like a glaze. And broken surfaces appear like glass, this can be seen in the broken chips on the right. Because it is angular that foot-ring is plucking all the way around. To prevent this on the other mug (left), the angle was rounded and it was fired on silica sand. This body is Plainsman 3D, wet-screened to 325 mesh with 10% feldspar. Because it is so fine-grained and contaminant-free it tolerates firing to the point of vitrification, reaching zero-porosity-density (most stonewares, and even whitewares, will blister if this is attempted). But this body does more, it can be fired cones higher and it not only resists bloating but continues to develop strength! Many potters do not realize how strong ware can be when it is fully vitrified like this. How high could this be fired? Until pieces warp during firing. These are not warping at all! If you want to make restaurant ware, knowing about super-durability is important.
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.
If you are a potter and want to make restaurant ware, read this. Many of the things you already think you know will mislead you in this type of venture.
Plainsman 3D, Mother Nature's Porcelain/Stoneware