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Words beginning with O
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
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 not dissolving into the melt.Ovenware
Ovenware (and flameware) clay bodies have a lower thermal expansion so they can withstand more sudden changes in temperature. Ovenware bodies should thus have much lower free quartz content and employ low expansion minerals like mullite, pyrophyllite, petalite, kyanite and spodumene. While many potters make ware for use in the oven using standard clay bodies, ovenware manufacturers (like Corning) would object to calling this 'ovenware'. This is because they dedicate considerable resources to producing bodies and glazes that have a much lower thermal expansion and therefore are much more suitable. Potters are able to get away with using standard bodies and glazes by making sure the glaze fits well (no crazing), avoiding high-quartz and highly vitreous bodies, firing evenly to reduce built-in stresses, maintaining an even 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. Low expansion glazes typically employ lithia and high silica and alumina and avoid sodium and potassium. It is much easier to make a low expansion glaze at high temperatures where silica and alumina can be higher.
There are two mechanisms to creating a low expansion body: By firing to form crystalline minerals that have low expansion or by employing mineral particles and fluid glasses in the body recipe have low expansion. The former produces a more vitreous body and requires much more expertise and test equipment. 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 and the low expansion effect lost). The latter creates a bit of a 'tug-of-war' in the body since some particles (like quartz) want to expand on heating and contract on cooling whereas others (like kyanite, mullite) want to remain stable. Furthermore, the glass that glues all the particles together introduces a third expansion dynamic to the matrix.
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 which use it. Electric kilns are synonymous with oxidation firing, however they often have "stagnant" air flow and thus may fire to a more neutral atmosphere (direct-connected kiln vents improve this).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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