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Phase Separation

Phase separation occurs when a glass melt separates into two or more liquids of slightly different chemistry (and therefore potentially different fired appearance and physical presence). The homogeneity of the fluidity of the melt can be disrupted late in the melting process or even in the cooling. It can develop as a product of the nature of the powder glaze layer 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 (excessive Al2O3 is implicated in devitrification, excessive Na2O3 will raise thermal expansion).

This phenomenon is also responsible for some of the most interesting glazes used in ceramics. A glaze without any visible phase separation can be seen on a sink or toilet, it can be considered a homogeneous glass. Silky surfaced dolomite matte glazes are an example of phase separation and happen because of the sudden melting of the MgO. If rutile is added to such glazes the variegation can be enhanced by selective crystal development. Rutile in a fluid iron containing glaze can create a phase separation that produces streaks of brilliant blue in a background of amber glass. In this case, the rutile appears to need a trigger (like the iron) to initiate the separation.

Micrograph of phase separation in a glaze

Micrograph of phase separation in a glaze

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

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.

Variegation and phase separation with about 5% rutile

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.

Frits melt so much more evenly and trouble free

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.

Out Bound Links

  • (Glossary) Crystalline glazes

    Crystals can form during cooling and solidification in many kinds of glazes and they can be microscopic or very large, widely scattered or completely covering. Matte glazes (e.g. high CaO) are often such because of a dense mesh of micro-crystals growing on the surface. Unwanted crystallization is ca...

  • (Glossary) Matte Glaze

    A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...

  • (Glossary) Reactive Glazes

    Variegated, or mottled, glazes are those that do not have a homogeneous solid color or character (i.e. like a ceramic sink or toilet bowl). They are often called 'reactive glazes'. They contain higher percentages of fluxes and additions intended to produce one or more variegation mechanisms. Variati...

  • (Recipes) G2571A - Cone 10 Silky Dolomite Matte Base Glaze

    A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies

    2003-12-18 - A standard Plainsman Clays cone 10R dolomite matte glaze used for many years. Colorants and opacifiers can be added to create a wide range of beautifu...

  • (Articles) High Gloss Glazes

    A transcript of a presentation at the 3rd Whitewares conference at Alfred University in the spring of 2000 by Richard Eppler.

In Bound Links

  • (Properties) Glaze Variegation

    In contrast to the typical homogeneous surfaces of sanitaryware and most table ware, reactive glazes exhibit discontinuities in texture, matteness, reflectivity, color, etc that are called variegation...

  • (Glossary) Dolomite Matte

    Dolomite matte glazes have traditionally been fired around cone 10 and have a pleasant-to-the-touch silky-feeling surface. The name has stuck because dolomite has been the most common source of the oxide needed for the effect: MgO. But other materials can also source it (e.g. talc, a frit). Thus the...

  • (Glossary) Melting Temperature

    Unlike crystalline minerals, glazes do not have a specific melting temperature, they soften over a range of temperatures. And after they have been melted they become increasingly fluid and homogeneous. The softening process is not a linear one, this is especially so because raw glaze powders are a m...

  • (Glossary) Rutile Glaze

    Many fluid glazes will do magic things (e.g. variegate) with the addition of rutile (usually less than 5%). The effects are often amplified when other colorants are present (especially iron). The classic rutile effect happens when a glaze melt runs in rivulet patterns. Employment of this effect is c...

By Tony Hansen

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