Vitreous slips are found within a range that falls somewhere between a slip (i.e. a clay suspended in water) and a slop glaze. With their high clay content they typically make matt and satiny surfaced glazes and Frank and Janet Hamer in The Potter's Dictionary of Materials and Techniques suggest that such vitrified slips may be defined as a glaze that contains more than 50% of clay. Vitrifying a slip by adding fluxes to create a fired, glass-like state would appear to be a very straightforward operation. This seemingly simple idea, however, runs into a significant problem. A progression test, in which increasing amounts of flux are added to a clay, passes through an under-fired region characterised by blisters, craters, and pin-holes.
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