All Glossary

Phase Separation

Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.

Key phrases linking here: phase separation, phase separate, phase changes - Learn more

Details

Phase separation produces discontinuities at the micro-scale, the glass melt separates into droplets of two or more liquids of slightly different chemistry (and therefore potentially different fired characteristics). The homogeneity of the fluidity of the melt, as a whole, can be disrupted by the sum of phase changes occurring late in the melting process (or even in the cooling). It can develop as a product of the nature of the powdered batch or the way particles decompose and interact (or fail to interact). Oxides that influence color and other surface characteristics can preferentially gather in one of the phases producing variegation.

Phase separation is a problem in glazes that need to be transparent and crystal clear. The internal surfaces created by the discontinuities scatter light and impede transparency. The glass industry employs judicious additions of Al₂O₃ and Na₂O₃ to suppress phase separation (they understand excessive Al₂O₃ to be implicated in devitrification, although in ceramics this seems counter intuitive; excessive Na₂O₃ will raise thermal expansion).

This phenomenon is also responsible for some of the most interesting glazes used in ceramics (called "reactive glazes"). A glaze without any visible phase separation can be seen on a sink or toilet, it can be considered a relatively homogeneous glass. Silky surfaced dolomite matte glazes are an example of phase separation and are thought to happen because of the sudden melting of the MgO. Rutile or titanium additions increase the degree of discontinuity and can introduce crystal development. In fluid-melt glazes, especially those containing iron, rutile can create a phase separation phenomenon that produces streaks of brilliant blue in a background of amber glass.

Related Information

Micrograph of phase separation in a glaze

Variegation and phase separation with about 5% rutile

The glaze is a dolomite matte fired to cone 10R. High fire reduction is among the best processes to exploit the variegating magic of rutile.

A glaze whose visual effect is partly a product of phase separation

Example of a rutile-iron stained glaze. Rather than crystallizing to form the visual effect, the rutile is forming a phase separation that produces the streaking blue in the amber background glass.

Frits melt so much more evenly and trouble free

These two specimens are the same terra cotta clay fired at the same temperature (cone 03) in the same kiln. The chemistry of the glazes is similar but the materials that supply that chemistry are different. The one on the left mixes 30% frit with five other materials, the one on the right mixes 90%+ frit with one other material. Ulexite is the main source of boron (the melter) in #1, it decomposes during firing expelling 30% of its weight as gases (mostly CO2). These create the bubbles. Each of its six materials has its own melting characteristics. While they interact during melting they do not mix to create a homogeneous glass, it contains phases (discontinuities) that mar the fired surface. In the fritted glaze all the particles soften and melt in unison and produce no gas. Notice that it has also interacted with the body, fluxing and darkening it and forming a better interface. And it has passed (and healed) most of the bubbles from the body.

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