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Once-fire, once fired
The practice of applying slip glazes to dried ware and firing one operation. The once-fire process requires control of slip shrinkage, adherence, and melting properties in order to avoid problems with crawling and blistering. Once fire is popular in industry for everything from table ware to porcelain insulators. Do not underestimate the difficulty of getting a once-fire process working well.
Opacifier, Opacification
A glaze additive that transforms an otherwise transparent glaze into an opaque one. Common opacifiers are tin oxide and zircon compounds. Opacifiers typically work by simply not dissolving into the melt, the white suspended particles reflect the light. However another mechanism of opacity is crystallization, this can occur when a crystallizing ingredient is super saturated into the mix (e.g. TiO2) or when slow cooling a glaze to encourage crystallization of less saturated oxides that crystallize easily (e.g. boron blue).
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- (Materials)
Tin Oxide - SnO2
Stannic Oxide, Tin(IV) Oxide, Tin Dioxide, SnO2
- (Glossary)
Boron Blue
The blue haze in a transparent boron glaze that re... - (Glossary)
Opacity
Opacity is the property of being opacified, or the...
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- (Materials)
Titanium Dioxide - TiO2 - Anatase, Brookite
TiO2
- (Typecodes)
1: OPA - Opacifier
- (Project)
Ceramic Properties
A property in this context is a created physical p... - (Properties)
Glaze Variegation
In contrast to the typical homogeneous surfaces of... - (Properties)
Glaze Opacifier
Glaze opacity can be achieved using a number of di... - (Glossary)
Colorant
A material that transforms a glossy or white glaze... - (Glossary)
Transparent Glazes
A fully transparent glaze is simply one that does ...
Pictures
Flow tester demonstrates how zircon opacifys and stiffens a glaze melt
An example of what 5% tin oxide does in a transparent boron cone 6 glaze (G2884) on a dark firing clay body
Opacity
Opacity is the property of being opacified, or the degree to which a glaze is non-transparent. Non-colored glazes can be either transparent, opaque or somewhere in between. Opaque glazes are often just transparent glazes with additions of an opacifer (like tin oxide or zircon). Often, significant percentages of opacifier must be added to a transparent glaze to achieve opacity and opacifiers are expensive. Matt glazes, by definition, are never completely transparent but they can be partly translucent (to reveal underglaze decoration, for example).
Opacity can be 'designed in' and a result of crystallization that is occurring as the glaze melt cools, it can be the product of a simple addition of opacifier or it can be a combination of both, or it can be a glaze defect. Different glaze bases respond differently to opacification mechanisms and a good knowledge and testing regimen is needed to produce a good opaque glaze that is not overly expensive and does not exhibit some of the common problems associated with opacity (cutlery marking, poor glaze fluidity as associated issues like blistering and pinholing). Opaque glaze frits are available, the opacifier is smelted right in during the manufacturing process.
The opacity of a colored glaze determines its depth of color. Transparent glossy glazes normally have vibrant color, whereas opacification subdues the color by reducing its depth (see Zircon for more information). People testing opacification quickly learn how many shades of white there can be, white can be stained to a host of other colors depending on what else is in the glaze and body (yellowing due to iron presence is common, for example).
There are many mechanisms of opacity. These include the simple dispersion of refactory micro-particles (zircon or tin for example) which scatter the light, the develpment of crystalline phases in the glaze during cooling (from high CaO for example), the surface smoothness (mattes are more opaque because the surface is not flat and scatters light right at the surface), the development of multiple phases within the glaze matrix (islands of differing glass composition and structure which scatter light as it passes through) and degree of melting. If whiteness if needed, the opacification options are quite narrow, usually only tin and zircon additions are possible. But for colored glazes, opacifiers that yellow or variegate the glaze (like titanium) are options (but more difficult to implement).
Often opacity occurs when it is not wanted (boron blue for example). Titanium and rutile can produce a wide range of glaze effects, most of which are not transparent. CaO and ZnO like to crystallize and can do this to the point that the entire glaze surface is covered with micro crystals that are completely opaque.
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- (Glossary)
Boron Blue
The blue haze in a transparent boron glaze that re... - (Articles)
Identifying Glaze Mechanisms If you can look at a glaze recipe and pick out the materials add to produce the color, opacity and v...
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Pictures
Flow tester demonstrates how zircon opacifys and stiffens a glaze melt
Ovenware
Ovenware clay bodies have a lower thermal expansion than typical bodies so they can withstand more sudden changes in temperature without cracking. Flameware bodies are not the same, they can withstand an open flame and demand much more compromise in working properties, strength, glaze fit, etc.
While many potters make ware for use in the oven using standard clay bodies, ovenware manufacturers would object to calling this 'ovenware' since they dedicate considerable resources to producing low expansion bodies and matching glazes. Still, potters have found ways to get away with using standard bodies and glazes by making sure glazes fit well (no crazing), avoiding high-quartz and highly vitreous bodies, firing evenly to reduce built-in stresses, maintaining an even ware cross section, avoiding angular contours and larger sizes with broad flat bottoms and telling customers to be careful about subjecting ware to sudden temperature change.
Glaze fit is a major problem in designing an ovenware body since common glazes will craze. It is much easier to make a low expansion clay body than a glaze, thus it is normal to compromise the lowest possible expansion on the body in order to get a reliable glaze fit.
There are main two mechanisms for creating a low expansion body: By firing to form a crystalline matrix that has low expansion (e.g. Corningware) or by employing materials having particles of low expansion (e.g. mullite, pyrophyllite, petalite and kyanite) and formulating and firing in such a way that these particles are not altered. The former produces a more vitreous body and requires much more expertise and test equipment. As noted, the later is a bit of a 'crowbar' approach and is dependent on not firing to full maturity (otherwise mineral species can be dissolved by the feldspar in the body or simply altered in crystal form and the low expansion effect is lost). This can create a bit of a 'tug-of-war' in the body since the glass that glues all the particles into a matrix will likely have a higher expansion. Obviously, ovenware bodies should have much lower free quartz content, especially the larger particle sizes, since these have very high thermal expansion. This does not just mean avoiding only ground silica, ball clays also contribute alot of quartz.
Pottery ovenwares are typically made using a high percentage of spodumene (30%) along with some feldspar and pyrophyllite (about 10% of each) and a mix of ball clay and kaolin or stoneware clays. Glazes can be made using recipes with lots of MgO and bone ash and low KNaO.
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Overglaze
More correctly 'Onglaze'. Decorative liquids applied over the fired glaze surface. These include china paints, lusters, gold, and other metallics. Fired at lower temperatures (e.g. cone 018).
'Overglaze' can also refer to the process of painting metallic oxides or stain mixes over a raw glaze before firing. For example, this is done for standard low bisque stoneware and for majolica.
Oxidation
A firing where the atmosphere inside the kiln has sufficient supplies of oxygen to satisfy chemical reactions in the glaze and clay. Electric kilns are synonymous with oxidation firing, however they often have stagnant air flow and thus may fire to a more neutral atmosphere than intended or realized. Direct-connected kiln vents improve this situation. Oxidation glazes are brighter colored than reduction ones and iron is not a flux in oxidation kilns. Generally potters and hobbyists who use reduction fire at higher temperatures.
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Pictures
The same glaze in reduction (left) and oxidation at cone 10.
Alberta slip fired in reduction (left) is much darker than in oxidation at cone 10.
Oxide
An oxide is a molecule like K2O, Al2O3. They are the most basic form of matter that kiln temperatures can normally decompose materials into. Thus for calculation purposes we view fired glazes and ceramic materials as made of oxides. An oxide is a combination of oxygen and another element (designated "R"), there are only about 12-15 common oxides that we need to learn about. Each has specific effects on a fired glass. Glaze formulas compare relative oxide amounts. Oxides are divided into three categories that recognize their functions. There is a correlation between the amount of oxygen in each class and the contribution that class of oxide makes. Fluxes are designated RO, intermediates R2O3 and glass formers RO2.
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Oxide Interaction
In a glaze melt, oxides do not act alone, they interact. For example, while one material might not melt well by itself, when combined with others the mix as a whole can melt at a significantly lower temperature than any of the ingredients in the mix. Cornwall stone is an example, by itself it does not melt enough to even be a glaze at cone 10, but a glaze can be made using this material in combination with kaolins, silica, etc. Oxides that are even refractory by themselves can be powerful fluxes in combination with silica and alumina, a good example is the material calcium carbonate. By itself it is completely refractory and yet at cone 8-10 it is the principal flux in stoneware glazes. Thus, due to interaction, the function of the oxide is changed completely.
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Pictures
Cone 11 flow test of original cornwall stone, H&G substitute 2011 and L3617 calculated equivalent
Example of calcium carbonate (top) and dolomite (both mixed with 25% bentonite). They are fired to cone 9. Both bars are porous and refractory, even powdery.
Oxide System
In ceramic glaze calculation, a 'system' refers to a collection of glaze recipes that share a common set of oxides and material types (e.g. cone 10 dolomite mattes, cone 06 fritted boron glossies, cone 6 alumina matte, cone 8-10 crystalline) and preparation, application and firing methods. Also, a 'system' implies or states limits for each oxide beyond which unpredictable results are likely to occur. Ways of relating the oxide formulas of glazes to their physical fired results also imply confinement to a system.
For example, expansion calculations are relative within systems. For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of variations, the calculated expansions will give you an indication of which variations have higher and lower expansions. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system. Also, some oxides, like Li2O do not impose their expansions in a linear fashion, thus they compromise a system because they do not calculate as well. Another critical factor is melting: If a glaze is not completely melted the expansion calculation is invalid, the glaze is not within the system. A third factor: Crystalization: When a glass crystallizes its physical properties are different. A fourth factor: Non melting particles, like zircon, impose their expansions in a manner different than if they melt and participate in the glass chemistry.
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