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Glass vs. Crystalline


In ceramic technology the term 'glass' is contrasted with the crystalline state, it is seen as a "super-cooled liquid". When crystalline materials solidify the molecules have opportunity to orient themselves in the preferred pattern during freezing whereas in a glass the random orientation of molecules is frozen into the solid. The most familiar glass in ceramic is of course glazes. However all non-refractory bodies also contain glass as a product of the melting of feldspar particles (and other minerals) between the grains of refractory particles (like quartz, alumina minerals). This micro-glass cements the mass of non-melted particles together and imparts more and more density to the matrix as it is fired higher.

In ceramics, glass is the preferred structure and it naturally forms because cooling cycles are rapid. In nature, cooling cycles are slow and rock crystallizes by virtue of the long periods of time it has to cool. This crystallization has produced much of the tremendous range of natural minerals we find in nature.

"Ceramics for the Potter - University of Toronto Press" 1952 called it "silica and two or more bases, which are combined under heat to form a molten solution. On cooling, the solution becomes so viscous that the molecules cannot move about freely enough to form crystals before the state of rigidity is reached. If glass were allowed to cool slowly, it would be as crystallized and as opaque as granite - it is the fast cooling, with the viscosity, that makes glass transparent. Glass is, in short, a solid solution."
In 1945 the American Society for Testing Materials suggested the following definition if glass: "Glass is an inorganic product of fusion which has cooled to a rigid condition without crystallizing."

In 1962 the British Standards Institution adopted the same phraseology. Later more complex methods of producing this state led to revisions such as: "Glass is a non-crystalline solid" and....."glass is an x-ray amorphous material which exhibits the glass transition.."

Which one contains more SiO2?

Which one contains more SiO2?

These cone 04 glazes both have 50% Gerstley Borate. The other 50% in the one on the left is PV Clay, a very low melting plastic feldspar. On the right, the other 50% is silica and kaolin, both very refractory materials. Yet the glaze on the right is melting far better. How is that possible? Likely because the silica and kaolin are supplying Al2O3 and SiO2, exactly the oxides that Gerstley Borate needs to form a good glass.

Out Bound Links

  • (URLs) Glass on Wikipedia

    http://en.wikipedia.org/wiki/Glass

  • (URLs) Crystal on Wikipedia

    http://en.wikipedia.org/wiki/Crystal

  • (Glossary) Mineralogy

    In contrast to man-made materials (like frits), ceramic minerals have a highly ordered atomic structure and a specific range of crystalline manifestations. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry when recipes are mixtures of ...

In Bound Links

  • (Glossary) Crystallization

    When ceramic melts are cooled they prefer to solidify as an organized molecular structure. Given sufficient time and sympathetic chemistry, they will form a crystalline structure. But if cooling is faster they solidify as a glass. Crystals can grow in cooling glaze melts if one or more of the fol...

  • (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...

  • (Materials) Volcanic Ash

    Pumicite

  • (Project) Ceramic Minerals Overview

    The materials we use are powders and we assess their physical presence on that level. However these powders are generally composed of microscopic mineral particles (except for frits of course). In man...

  • (Properties) Glaze Crystallization

    Crystal glazes are normally evaluated on the recipe level, people simply have a recipe that they know works and add colors to it. However there is a great benefit to knowing why crystals grow the way ...

  • (Glossary) Glaze

    A glaze is a glass that, on its most basic level, has been tuned to melt to the desired degree at the target temperature, have a thermal expansion compatible with the body to which it is attached (a typical soda-lime bottle glass, for example, would craze badly on typical clay bodies). In ceramics, ...

  • (Glossary) Ceramic

    A man-made solid produced by the fusion of non-metallic mineral substances in a kiln. The term 'ceramic industry' or 'pottery industry' are subjective terms that can mean different things in different circles. In recent years the field of non-oxide ceramics has become popular, thus the term 'cerami...

  • (Glossary) Amorphous

    Without a regular structure. Amorphous minerals do not have a repeating crystalline matrix. Glass, for example, is amorphous because it is cooled in the kiln quickly enough that no crystals have an opportunity to form.

  • (Glossary) Decomposition

    Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view). "Elements" are one kind of atom which cannot be broken down any further (except b...


By Tony Hansen




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