Glaze Fit and Faults

Why do pieces crack after glaze firings? What kinds of defects occur from firings?

Understand the chemistry of glazes- and prevent these issues!

Glazes are constituted of three main elements: a glass former, a stabilizer, and flux. The glass former is always silica, giving the glossy look to the piece after it melts and coats the piece. The stabilizer is often alumina, which acts to slow the dripping of the silica as it melts and runs down the pot. Finally, the flux is a variable compound added to lower the overall melting point of the mixture and add other desired effects. Combinations of an infinite number of compounds in varying amounts accounts for the vast variation seen in high-fire glazes.

Silica Phases

As it heats up, silica goes through a series of chemical transformations. Silica in 3D

structure shares oxygens between silicon items by twos, having the molecular formula (SiO2)n. These molecules of silica are linked together, with specific bond angles between the molecules governed by temperature, resulting in different levels of rigidity and volume taken up. The bond angle between silicon and oxygen atoms in the 3D structure of a piece will slightly straighten as more heat is added, going through a series of alpha, beta, tridymite, and cristobalite at increasing temperatures. This activity of the silica also works in the inverse direction when cooling, and often has more intense consequences when cooled too quickly. The changing of these bond angles in silica causes shrinking and expanding, which when not considered beforehand, will likely break the piece.

Two especially important silica inversion temperatures are beta to alpha conversion at 1063°F, and the cristobalite inversion at 439°F. The quartz inversion is a gradual 1% volume change, whereas the cristobalite is a sudden 3% volume expansion. The sudden changing of volume of the free silica in the glaze, as opposed to the structured silica in the clay body, causes cracking to occur on the piece, where the clay and glaze are changing sizes at different rates. Specific glaze issues arise from the principles of free silica laid out here.

Fluxes

Both clay and glazes contain fluxes- compounds which in a mixture reduce the melting temperature. When compounds are combined, specific proportions of each will results in varying melting temperatures depending on the strength of the bonds present . In glazes and clays, 10% alumina and 90% silica is the eutectic point- the lowest overall melting point of a mixture. This can be seen on the graph to the right, where mullite is the source of alumina. Adding alkali metals, including lithium, sodium and potassium, to glaze mixtures lowers the melting point, with the metals acting as a flux to increase the hardness of the glaze. Addition of fluxes can alter the shrinkage rates of glazes, so when formulating a glaze, be sure that the shrinkage rate of constituent elements is roughly equivalent to the shrinkage rates of the clay (check this using test tiles of any new batch made!). Glazes with high concentrations of flux have a glossy finish, and run further down pieces, as the melting of the glaze occurs sooner at the eutectic point.

Glaze Faults

Crazing

Crazing occurs from a glaze body shrinking more than the clay body, causing the glaze exterior to be too small. This appears as thin cracks soon after firing, and reduces the structural integrity of the piece. Crazing can be avoided by adding low expansion materials to the glaze formula, such as silica or boron oxide.

Shivering

Shivering is the inverse of crazing, when the glaze is too large for the clay body. In stone and earthenware, slivers of glaze will spontaneous fall from the piece, while in porcelain (where the clay-glaze bond is stronger), the piece will often crack. The addition of high expansion materials to the glaze formula, like sodium and potassium oxides, will help alleviate this issue.

Crawling

Crawling refers to the tendencies of glazes with high surface tensions to adhere to themselves, and appear to ‘crawl’ down a piece as individual beads, leaving the clay near it bare. This effect is similar to the phenomena of water beading on a greasy surface, whereas soapy water will coat it smoothly due to their differing surface tensions. This issue can arise from user error, where a dusty bisque-ware piece won’t properly adhere to the glaze, or from high surface tension materials like aluminum and magnesium oxide having too high concentration in the piece. Reducing the concentration of these high surface tension materials, or adding more water to prevent thick application, should fix these issues.

Pinholes

Pinholes occur when gas escapes rapidly from the melting glaze, often in under-fired or especially viscous glazes containing sodium, which escapes rapidly in a melting glaze. This can be fixed by refiring the piece, however stoneware and certain types of glazes can often crack or become to runny after repeated firings; refire sparingly!

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Overall, it is important to keep in mind which glazes you are using with your bisque-ware's clay composition. Iron oxides in a clay body will react with oxidizing agents in a glaze, and give wildly different effects to pieces- some may give gorgeous deep reds with stoneware, and appear a muted brown with porcelain. When a clay body shrinks at a different rate than the glaze from differing proportions of flux, glazing faults appear, and can ruin your final piece. Keeping in mind the relative shrinkage rates and alkali metals present in glazes will assist in avoiding these faults, as well as giving you the power to accurately predict your final product.