Common Glaze Faults and How to Correct Them Linda Bloomfield

It is critical for every potter who uses glazes to understand glaze faults and why they occur. This knowledge can help potters avoid issues in the first place, or correct them and prevent them from happening again.

In today's post, an excerpt from the Ceramics Monthly archive, Linda Bloomfield explains the causes of the most common glaze faults, and gives tips on preventing them. –Jennifer Poellot Harnetty, editor

PS. Dive deeper into glaze fit and faults in Linda’s article in the December 2023 issue of Ceramics Monthly!

Glaze Faults and How to Correct Them

For many potters, the first time they turn to information like this will be when they need to resolve a glaze fault. Some faults, including crazing, cracking, and shivering, are caused simply because the glaze does not fit the clay body used. It is important to understand how to modify the glaze in order to make it fit. Other problems such as pinholes and blisters may be caused either by under- or over-firing.

Crazing occurs after the pot has cooled from the kiln, if the glaze is too small for the clay body (1, 2). A network of fine cracks appears some time after the pot is removed from the kiln, accompanied by a series of pinging sounds. In earthenware, the cracks can cause the pot to leak. In all glazed ware, crazing greatly weakens the pot. Crazing can be corrected by adding materials with low expansion such as boron or reducing any high expansion materials such as feldspar. This list (see list above) shows glaze oxides in order of decreasing expansion. Low expansion materials such as borate frit, silica, clay, or talc can be added a few percent at a time to a series of glaze tests. It is better to change one material at a time when trying to correct crazing. Also, it is simpler to alter the quantity of a material already in the glaze than to add a new component.

Shivering is the opposite of crazing, when the glaze is too big for the clay body, and is a more serious fault. In earthenware and stoneware, slivers of glaze spontaneously chip off rims and the edges of handles. This can be dangerous to the user of functional ware. In porcelain, the glaze is more firmly bonded to the clay body and stresses between glaze and clay body can cause the pot to crack. Shivering can be corrected by adding high expansion materials to the glaze, such as alkaline frit or feldspar, which contain sodium and potassium.

Crawling occurs when the glaze cracks on drying and melts in the kiln to form beads with bare patches in between. It points to the glaze having its surface tension too high, e.g. in a similar way, water (high surface tension) will bead on a greasy surface, whereas soapy water (low surface tension) will coat it smoothly.

Crawling can be corrected by calcining materials with high drying shrinkage such as china clay and zinc oxide and reducing materials with high surface tension such as zirconium and tin oxide. The list above shows the oxides in order of decreasing surface tension, with high surface tension at the top and low surface tension at the bottom. The materials at the top of the list should be reduced if there are crawling problems. It may also help if the glaze is thinned with water, as thickly applied glaze can crack on drying. Dusty or greasy bisque ware can also cause crawling. If this is the case, crawling can be prevented by sponging the bisque ware and leaving it to dry overnight before glazing.

Pinholes can occur in underfired or viscous glazes, where gases have escaped during firing but have not healed over (3). A soak at the top temperature for half an hour can help to smooth out the glaze. Pits left from grog removed during throwing can also act as sites for pinholes. Sponging the clay surface after throwing will prevent this. Some glaze materials such as zinc oxide can cause pinholes.

Blisters sometimes occur in overfired glazes. They can cause sharp edges, which render functional ware unsafe to use. Fluxes such as sodium oxide and borate frits become volatile above 2192°F (1200°C). Some glaze materials such as bone ash also give off gases. If the glaze is viscous, the gas bubbles may be trapped. Blisters can be ground down and refired.

Excerpted from Science for Potters by Linda Bloomfield. Published by The American Ceramic Society and available for purchase in the Ceramic Arts Network Shop at mycan.ceramicartsnetwork.org/s/shop.