Leaking of Fired Ceramics

Details

This problem is often called "weeping", or just "leaking". To guarantee water tightness a clay needs to be vitrified to be dense, thus have a low porosity (typically less than 1%). If a glaze is not crazed this problem should never happen (since moisture cannot pass through glass). Many stonewares have porosities above 2%, thus water penetration will happen when glazes do not seal well. The SHAB test can be used to monitor body porosity.

The lower the firing temperature of a clay body, the more likely it will be to leak water. Terra cotta bodies have from 10-15% porosity when fired at cone 06-04. Feldspar can be added to stoneware and porcelain bodies to vitrify them down to about cone 5, below that low melting clays (usually red burning) can make dense bodies at a lower temperature (down to about cone 1). Some light-burning clay deposits contain natural feldspar, these can mature quite low also, cone 4 or even lower.

Well melted glazes that fit (do not craze or shiver) can seal any body against water penetration regardless of its porosity. However, if glaze thermal expansion is not well matched to the body, even if ware exits the kiln uncrated, over time glazes will craze and thus provide a channel for ware passage. Even if the glaze is well fitted, some porous bodies are subject to expansion when they absorb moisture (such as a piece having an unglazed foot that permits water to wick up into the walls). Glazes are most difficult to fit at lower temperatures because they don't bond as well with the body and need more flux to melt and the most useful fluxes have high thermal expansions.

Related Information

Silicone sealers prevent water absorbing in porous ceramic

The right side of this bisque-fired clay bar (which has about 15% porosity) has been surface-treated with a silicone sealant. It repels the water, the drop rolls around on the surface if I tilt the bar. The drop on the left side absorbed into the clay in seconds.

The high porosity of this clay enables sealing against water leakage

This body has high porosity, almost 25%. It is L4410P, a dolomite-based low-fire whiteware, Plainsman Clays makes this as a product named "Snow". But this high porosity has some advantages, one of them is that it soaks up silicone sealer well. The slip-cast piece on the left was sealed (you can see the surface sheen) and it is impermeable to water penetration (the glaze is not crazed so water cannot penetrate there either). The piece on the right soaks up water readily (on the lower unglazed portion). Sealing this specific body is doubly important because the dolomite particles within can rehydrate over time, especially in damp climates, causing pieces to crack. Even the foot rings of functional pieces should be sealed, not just to prevent hydration but also waterlogging.

Water can leak through a stoneware clay body

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.

By Tony Hansen
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