This stoneware mug was glazed inside and halfway down the outside with pure silica. At some point during heatup the outside layer, not shrinking like the piece, simply fell down. And was sintered enough to hang together and remain intact through the rest of the firing (on the inside, the shrinking forced the silica to flake off into a pile at the bottom). Cone 10 has sintered the silica enough that it will not slake in water but it is fragile and soft and must be handled carefully.
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
Silica, sold as a white powder, is pure quartz mineral. Quartz is pure SiO2 silicon dioxide. It is the most abundant mineral on earth and most used in ceramics.