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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 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.
"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 and x-ray amorphous material which exhibits the glass transition.."
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Glass-Ceramic Glazes
Glass-ceramic glazes are made by the controlled devitrification (crystallization) of a frit composition to create a homogeneous structure in a glassy matrix. As a result these glazes have improved hardness. Glass ceramic coatings are common on tiles because of their superior abrasion resistance compared to glazes. Crystalline frits are available specifically for this use.
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- (Tests)
HMA - Heating Microscope Analysis for Frits
Glaze
A thin glassy layer formed on the surface of fired ceramic. Glazes are a finely ground mixture of mineral and man-made powders tuned to melt and flow at a specific temperature. Many clays will melt well at higher temperatures and thus qualify as 'slip' glazes. Glazes are normally mixed with water, suspenders, and hardeners to make them harden on drying and produce a suitable consistency for application by painting, dipping, or spraying.
Glazes are often classified (e.g. unleaded, raw, fritted) to designate type within a specific industry or type of ceramic ware.
Glaze Bubbles
During melting gases from decomposition of organic and carbonate materials in the glaze are generated, this forms entrained bubbles. With increasing temperature these bubbles will dissipate (if the glaze is fluid enough and adequate time is available). Fritted glazes generate far fewer bubbles (these come from the clay used to suspend it or any binders present). Simple air pockets in the glaze layer (after drying) will also create airbubbles, thus industry puts emphasis on dense laydown. The photo shown here is demonstrating bubbles caused by materials in the glaze itself, however bodies also generate bubbles and these must come up through the glaze and can cause real problems with glaze defects.
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Pictures
Example of how bubbles dissipate in a glaze with increasing temperature. This is a gerstley borate based recipe.
Close-up of bubbles (upper half) that become evident in a double thickness of a cone 10 celadon glaze that is lacking in flux
Glaze Compression
Every solid has a thermal expansion, that is, an amount by which is expands and contracts on heating. If the thermal expansion of a glaze does not match the body it is on, then the glaze either cracks (when it is under contraction) or chips off when under compression.
The compression occurs while the piece is cooling in the kiln. When the glaze solidifies it acquires its solid characteristics. This might happen at 1500F, for example. As the piece continues to cool in the kiln it contracts. If the body is contracting more than the glaze then the glaze is being put under compression. Some compression is actually desirable and strengthens the glaze-body combination. However too much compression puts the piece under internal stresses seeking an opportunity to relieve themselves. When the difference is severe the piece will not survive cooling in the kiln without fracturing. When individual shards of such pieces are dropped onto a cement floor, for example, they will literally explode into hundreds of tiny pieces. In less severe mismatches, glazes will flake off areas where they wrap around contours (e.g. the lips of mugs), this is known as shivering. This can be serious if it occurs while a piece is being used and someone ingests a micro-flake of glaze (having razor sharp edges).
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Glaze fit, glaze body fit
The relationship between the thermal expansion of body and glaze. Ideally a glaze should have an expansion that is slightly lower than the body so that contraction during cooling puts the glaze under compression and thus prevents crazing.
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Dialometric thermal expansion chart
Glossy Glaze
'Gloss' refers to how shiny and light-reflective a glaze is. Glazes high in glass former (SiO2, B2O3) are glossy. Those high in Al2O3 tend to be matte. Fluid glazes can crystallize to a matte surface if cooled slowly or a glossy surface if cooled quickly. The SiO2:Al2O3 ratio is taken as a general indicator of glaze gloss, ratios of more than 8:1 are likely to be glossy. In some industries, gloss is a more of a product of firing than chemistry. For example, a glaze may normally fire matte (by having a chemistry that crystallizes heavily on cooling, for example), but when super-cooled it will fire glossy.
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Matte Glaze
A glaze that is not glossy. Of course, unmelted gl... - (Glossary)
Semi-Matte Glaze
It is difficult to draw a line between what is mat...
Grog
A granular material made from crushed brick, refractory rock, or other pre-fired ceramic product. It is added to bodies to reduce drying and firing shrinkage and thermal expansion, increase stability during firing, and to add texture.
Pictures
Example of a grogged cone 10 reduction vitreous and non vitreous iron speckled sculpture clay.
Examples of various sized grogs from CE Minerals, Christy Minerals, Plainsman Clays
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