Pinholing and blistering are related surface defects caused by gas trying to escape through a glaze that has already begun to melt and seal the surface. They differ mainly in whether the resulting bubble breaks before the glaze sets.
¶ Source of the gas
The gas involved usually comes from one of two places: the clay body itself, as organic matter burns out and chemically-bound water leaves the clay structure during the early stages of firing, or the glaze materials, many of which include carbonates that release CO₂ as they decompose on the way to melting. Problems show up when that gas is still trying to escape after the glaze has already become viscous enough to trap it.
¶ Pinholing vs. blistering
Pinholing occurs when an escaping bubble breaks through the glaze surface and the melt does not have enough time or fluidity left to flow back and heal the resulting crater before the kiln cools, leaving a small pit. Blistering occurs when the bubble does not break at all, and cools as an intact glassy dome sitting proud of the surface. Blisters tend to be larger and, if they are ever knocked or popped after firing, leave a sharp-edged crater that is a real hazard on functional ware.
¶ Firing-schedule levers
Because the underlying issue is timing (gas needs to finish escaping before the glaze seals over), the usual fixes are firing-schedule adjustments: a slower climb through the temperature range where the body is decomposing and off-gassing, and an adequate hold (soak) at peak temperature to give the glaze time to heal over any bubbles that did form before the kiln starts cooling. See kiln types and cone charts for more on how firing schedules are actually controlled.
Some raw materials are more prone to contributing gas than others. Carbonates, including whiting (calcium carbonate) and dolomite, release CO₂ as they decompose on the way to melting, and a recipe leaning heavily on them has more gas to clear than one that gets the same oxides from a source that does not off-gas. A slow, controlled cool, sometimes with a brief hold partway down (a reduction-cooling or soak-cooling schedule), can also help a glaze that sealed slightly too early re-open enough to release trapped gas and heal back over, rather than freezing in whatever bubbles were present when the kiln started cooling.
¶ References
- Hansen, Tony. "Pinholing." Digitalfire Reference Library. digitalfire.com
- Hansen, Tony. "Glaze Blisters." Digitalfire Reference Library. digitalfire.com
- Photo by Encik Tekateki, via Wikimedia Commons, licensed CC BY-SA 4.0 (resized and re-compressed from the original).
