Pinholing and Blistering

Pinholes are tiny, sharp-edged craters that appear in the fired glaze surface. Blisters are larger versions of the same problem. Both defects are caused by gases escaping from the clay body or the glaze itself as it matures. If the glaze melts and seals the surface before all the gas has escaped, the bubble is trapped. When it eventually bursts during cooling, it leaves a sharp crater.

Common Causes:

• Firing too fast: The kiln moves through the critical maturation range too quickly, not allowing gases (carbon, sulfur, chemically bound water) to burn out.

• Dirty ware: Dust, oil from hands, or sponge residue on the bisque can create gas pockets.

• Over-application: A glaze layer that is too thick will seal over before the body beneath has fully outgassed.

How to Correct It:

• Adjust the Firing Schedule: The most effective fix is to add a “hold” or “soak” at the end of the firing (e.g., 20–30 minutes at peak temperature). This allows the glaze to smooth out and the bubbles to heal. Alternatively, slow the ramp rate during the last 100°C.

• Improve Application: Ensure bisque ware is meticulously clean before glazing. Apply glaze in thinner, more controlled layers.

Crawling

Crawling appears as bare patches of clay where the glaze has withdrawn and balled up, leaving exposed ceramic beneath. It looks as though the glaze is trying to run away from certain areas of the pot. This defect is almost always caused by high surface tension in the glaze combined with a compromised bond between the glaze and the bisque.

Common Causes:

• Dust or Oil: The most common cause. If the bisque ware is dusty, or if there are oils from handling (lotion, skin oils) on the surface, the glaze cannot adhere properly. As the glaze dries and shrinks, it pulls away from these contaminated spots.

• Glaze Application on Greenware: Applying glaze to bone-dry greenware (raw clay) is risky. As the glaze dries, it shrinks and often causes the underlying raw clay to flake off, taking the glaze with it.

• Excessive Glaze Shrinkage: The glaze itself may contain too much clay or material that shrinks excessively upon drying.

• Applying Glaze to Dusty Bisque: Similar to oil, heavy dust prevents a strong bond.

How to Correct It:

• Meticulous Surface Preparation: Wipe bisque ware with a damp sponge (or denatured alcohol) just before glazing to remove all dust. Handle ware only by the foot after cleaning.

• Glaze Only Bisque: Avoid glazing greenware unless using specialized raw-glazing techniques.

• Add a Binder: If the glaze is prone to crawling (perhaps it’s very matte and high in clay), adding a small amount of gum arabic or a commercial “anti-crawling” agent to the liquid glaze can improve adhesion and reduce shrinkage.

Shivering

Shivering is the exact opposite of crazing, and it is far more dangerous. While crazing means the glaze is under tension, shivering means the glaze is under compression. The clay body shrinks more than the glaze during cooling. This compresses the glaze until it has nowhere to go but off the clay body. Shivering usually occurs on sharp edges, rims, and handles, where the glaze literally flakes or pops off, often taking chips of clay with it.

Common Causes:

• Excessive Compression: This is caused by a severe mismatch in thermal expansion. The glaze is too low in silica (low expansion) and too high in fluxes (like talc or magnesium oxide), while the clay body has a very high silica content.

• Over-ground Clay: Clay bodies formulated with excessive amounts of flint (silica) are prone to shivering.

• Under-firing: While less common, a severely under-fired body may not have developed the proper structure to hold the glaze.

How to Correct It:

• Adjust the Glaze Chemistry: You must increase the thermal expansion of the glaze. This means adding more silica (flint) or increasing high-expansion fluxes like soda or potash while reducing low-expansion fluxes like talc or zinc.

• Change the Clay Body: Switch to a clay body with lower silica content or one that matures at a different temperature.

• Avoid Sharp Edges: While a design fix, rounding sharp corners on forms (like the rim of a bowl) can sometimes reduce the stress concentration that causes shivering.

Crazing appears as a network of fine, hairline cracks in the glaze surface. It looks like a shattered glass effect. This defect is not a surface contamination issue; it is a structural, fit issue between the clay body and the glaze. Crazing occurs when the glaze is under tension. During the cooling phase of the firing, the glaze shrinks more than the clay body it is attached to. Since the clay won’t shrink further, the glaze is forced to crack.

Common Causes:

• Mismatched Thermal Expansion: The primary cause. The silica-to-flux ratio in the glaze is incorrect for the specific clay body being used. The glaze has a higher coefficient of expansion (and thus contraction) than the clay.

• Under-firing: If the clay body is not fired to maturity, it may be porous and weak. Over time, moisture can absorb into the ware through the crazing, causing the piece to dunt (crack structurally) or fail.

How to Correct It:

• Adjust the Glaze Chemistry (The best fix): You need to lower the glaze’s thermal expansion. This is usually done by adjusting the silica-to-alkali ratio. In practical terms, you would add more silica (flint) or increase the clay content (like EPK) while reducing fluxes like soda or potash.

• Change the Clay Body: Switch to a clay body that matures at a higher temperature or has a higher silica content, which increases its own expansion rate to match the glaze.

• Fire Higher: Ensure your clay body reaches its proper maturity (cone) to achieve maximum density.