The unglazed stoneware mug on the right has 2.5% porosity at cone 6 (according to an SHAB test). It is stoneware and it is strong and durable. But the fact that a porosity can be measured demonstrates that water should be able to pass through it (as shown in this 24-hour test on a piece of paper). The left one has 1% porosity. While theoretically water still should be able to pass, bodies with this level of vitrification normally do not leak. The point: A non-crazing glaze is needed for water tightness for the body on the right. This is not a problem, bodies having far more porosity than this are routinely used in industry, if the inside glaze fits they don’t leak.
Leaking of Fired Ceramics