Techno File: Glaze Fit and Fault Linda Bloomfield

Appears in the December 2023 issue of Ceramics Monthly.

It is critical for every potter who uses glazes to understand the importance of glaze fit to a clay body. If the fit is not close, either the glaze or the pot may crack. 

Defining the Terms 

Blistering Raised bubbles in a fired glaze caused by gas escaping during firing. 

Crawling A defect where glaze pulls away from the clay body, leaving bare patches. 

Crazing A network of fine cracks caused by a higher coefficient of expansion in the glaze than in the clay body. 

Cristobalite A crystalline form of silica, which forms in the clay body above 2012°F (1100°C) and contracts suddenly on cooling at 439°F (226°C). 

Dunting Cracking of a clay body caused by cold air entering the kiln during cooling. 

Pinholes Small holes in a glaze, caused by burst bubbles that have not healed over during firing. 

Shivering Glaze flaking off at rims and edges of handles, caused by poor glaze fit with the glaze contracting less than the clay body. 

Soak To maintain the top firing temperature to mature the glaze evenly throughout the kiln. 

Silica Phases 

As quartz is heated up, it changes in structure. The silica tetrahedra arrange themselves at slightly different angles to each other. The different forms are called phases. The silica phases include alpha-quartz up to 1063°F (up to 573°C), beta-quartz up to 1598°F (up to 870°C), tridymite up to 2678°F (up to 1470°C) and cristobalite, which eventually melts at 3110°F (1710°C) to form amorphous liquid silica. Tridymite and cristobalite each have their own alpha and beta phases. In the high-temperature beta phase, the bond angles between Si-O-Si are slightly straightened out. Once the silica has melted, the bonds are broken and the structure becomes disordered and is no longer crystalline. 

If molten glaze is cooled quickly, the silica will stay amorphous and disordered like in a liquid. However, the clay body may still contain some crystalline silica and this will undergo the reverse phase transitions on cooling. The crystalline silica in the clay body not chemically combined in kaolin or feldspar is known as free silica. The solid silica crystals change volume when they change from one phase to another at a specific temperature. Each silica phase has a slightly different volume, and potters need to be aware of the inversion temperatures as they can cause dunting in the kiln. 

Quartz and Cristobalite Inversions 

The two silica inversion temperatures important to potters, particularly when cooling the kiln, are the quartz inversion from beta to alpha at 1063°F (573°C) and the cristobalite inversion at 439°F (226°C) (6-1 and 6-2). The quartz inversion involves a 1% volume change and is a gradual change. The cristobalite inversion causes a sudden 3% volume change and can cause dunting if the kiln is opened at this stage. However, the cristobalite inversion can be useful in preventing crazing in glazed earthenware. This is known as the cristobalite squeeze as it contracts the clay body and compresses the glaze. Earthenware clay bodies can withstand this stress but not stoneware, particularly ovenware, which may be repeatedly heated to above 392°F (200°C), so cristobalite is not desirable in stoneware clay bodies. Fluxes in the clay body such as calcia and magnesia act as catalysts in the conversion of quartz to cristobalite. In stoneware and porcelain, more cristobalite forms the longer the ware is soaked or re-fired to high temperature, above 2012°F (1100°C). This may cause cracking when, for example, large plates are re-fired. The clay body suddenly changes in volume, but the glaze does not, causing stress and cracking of the ware. 

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.