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Tapper Clay
Tapper clay is used to seal the drain hole in melting vessels in the steel and related industries. It is refractory, low in potassium and sodium to prevent glass development (and therefore harden the plug excessively), and high in aggregate. The wet pugged clay comes into contact with molten metal, thus it needs to be able to liberate its water quickly without popping. Tapper clays are pugged very stiff to minimize the water content. They can be oil or water based.
Pictures
Example of a tapper clay fired to cone 8 and 10 oxidation and 10 reduction.
Target Formula, Limit Formula
The term 'target' typically refers to the chemistry profile of a balanced function glaze that melts well, is reasonably functional and does not have excessive amounts of any individual oxide that might lead to instability or reactivity (difficult to keep consistent). Different types of glazes for different temperatures have different targets. Target formulas typically show each oxide commonly used in the glaze type and the range of normal amounts for it. However the term 'target' can also be used in a more specific sense. For example, many special purpose glazes have oxide amounts that are well outside normal ranges and the key to their success is very tight control of the chemistry and process (the tile industry is a good example, the formulations of their fast fire glossy glazes would shock a technician making table ware!). Many trade secret formulations fit in this category. A common public domain example is crystalline glazes, they have almost no alumina, much higher than normal sodium and zinc. The term 'limit formulas' is often used, however this term suggests that glazes outside the ranges will not work and that glazes inside are somehow safe. However neither of these assumptions is normally true.
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Tenmoku
A reduction fired transparent glaze with about 10% iron oxide that fires to a variegated deep maroon to black and which breaks to iron-red crystallized areas where thin. Kaki glazes are closely related, however they contain more iron oxide so that crystallization occurs over the whole surface.
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Pictures
Alberta Slip used as a straight glaze at cone 10R, it looks much like a Tenmoku.
A cone 10 reduction tenmoku glaze with about 10% iron oxide.
Iron silicate crystals that often grow in tenmoku glazes when they are cooled sufficiently slowly.
Example of a tenmoku on a white stoneware and a Grolleg porcelain.
An example of what a tenmoku glaze looks like on a buff stoneware (left) and brown speckled stoneware (right).
Terra cotta
'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware, generally unglazed. Terracotta is normally used to make sculptures, tile, planters, garden and architectural ware. If ware is glazed the ware is often referred to as 'red earthenware' rather than the term 'terra cotta'.
Red clays have more flux impurities and fire to a harder stronger matrix than white burning materials at the same temperature. Still, terra cotta bodies fire to a porous matrix at cone 06-04 and do not have anywhere near the mechanical strength of vitrified stoneware bodies. Without significant additions of expensive frits it is impossible to vitrify a body at these temperatures. However many terra cotta clays do develop rapidly after cone 04 and turn from red to brown in the process. It is possible to produce fired ware that rivals stoneware in strength at cone 02-1, however few people do this because the clay is so volatile, slight overfiring will produce warping or bloating. In addition to cost one of the primary advantages of the terra cotta process in the warm red colors of the raw clay surface. In addition glazed low fired terra cotta remains red whereas at higher temperatures the glaze matures the surface and turns it brown.
Some terra cotta pieces may be glazed on the inside. Because terra cotta ware is weak and porous it is very important that the glaze and body thermal expansions match. The clay-glaze interface is not well developed (the glaze is not stuck on as well as stoneware) so a measure of resistance to chipping and crazing can only be achieved by a well melted glaze of low enough thermal expansion to resist crazing. In the past inexpensive lead compounds were used on terra cotta because they contributed exactly these properties plus they gave very bright and vibrant colors. Today boron glazes are employed. While safer to use they do not have the ideal set of properties that lead based compounds had.
'Majolica' refers to the use of a terra cotta clay with an opaque white glaze decorated with colored overglazes. Today red clays are used in this process because they provide maximum strength at low fire. In the past white low fire materials were not available.
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- (Materials)
Iron Oxide Red - Fe2O3 - Synthetic Hematite
Ferric Oxide, Red Iron Oxide, RIO, Iron(III) oxide, Fe2O3, Hematite
- (Glossary)
Bone China
True bone china is a special type of porcelain tha... - (Articles)
Monoporosa or Single Fired Wall Tiles A history, technical description of the process and body and glaze materials overview of the monopor... - (Glossary)
Earthenware
A clay fired at low temperatures (cone 010-02) whe...
Pictures
Example of a terra cotta clay fired at cone 04 and cone 02. Courtesy of Plainsman Clays.
Fired test bars of a terra cotta clay showing varying levels of maturity or vitrification, DFAC disk showing solubles on an iron stoneware
Theoretical Material
A material the way it would be if its crystal structure perfectly matches the unit-cell drawings you find in textbooks. In nature, materials are always contaminated to some degree. Calcium carbonate is never pure, feldspar never has an ideal 1:1:6 relationship between fluxes:alumina:silica, kaolin particles are never crystal-perfect, etc. Some materials are theoretical in physical properties but have no theoretical formula (e.g. ball clays) because they are a mix of many minerals and have a definition that can encompass a broad range of products. When learning ceramic chemistry students usually use theoretical materials (e.g. potash feldspar, kaolin). However when they begin working in the lab of a company they use the actual chemistry of real world materials.
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Thermal shock
Stresses imposed on a ceramic by the volume changes associated with sudden shifts in temperature. Ceramic materials with good thermal shock resistance are able to withstand sudden temperature changes without cracking. Cracking usually occurs when one part of an item is a different temperature than another part and therefore expanding or contracting at a different rate.
Fired ceramic does not withstand thermal shock nearly as well as other materials like steel, plastic, wood, etc. Ceramic is hard and resistant to abrasion but it is brittle and propagates cracks much more readily.
Thermocouple, pyrometer
A simple probe made from two kinds of wire (i.e. platinum, rhodium) welded together. This probe is wired to a sensitive electronic meter that displays a reading of the voltage it generates when heated.
However the world of high temperature measurement and thermocouples is a complex one. There are many kinds of thermocouples. Some generate a nice smooth voltage increase that bears a direct relationship to temperature increase, others require complex software to make the translation. There are also different manufacturing processes, calibration techniques, response to different atmospheres, abilities to measure different temperature ranges, different types decay in their accuracy in different ways, variations in frequency of need for recalibration, etc.
Maintaining accurate pyrometers can be expensive and typical inexpensive type K devices used in potters kilns are not accurate at higher temperatures (most potters won't pay for the platinum/ 10% platinum-rhodium (type S) thermocouples and control systems that really should be used, and the more expensive plated switches and contacts). However the type K are more resistant to oxidation than types E, J, and T at temperatures over 500C.
Thixotropy
Thixotropy refers to the way a slurry's viscosity changes with time and motion. In technical terms it refers to a decrease in the viscosity of a slurry when it is subjected to shearing following by a gradual recovery to the viscous state. For example, a good casting slip remains fluid after being agitated and while in use, but when allowed to stand without disturbance it gels, thus preventing settling of the materials. The tendency to gel is inherent in many ceramic minerals when they are suspended in water (generally because they contain solubles), but this tendency can be imparted to any slurry. It is the product of choice of deflocculant type and of maintaining the specific gravity and viscosity such that the slurry is not deflocculated to its most fluid state. Ceramic slurries are sometimes flocculated, or made much more viscous and less fluid for practical reasons (decorating slips stay put on vertical surfaces when flocculated).
Plastic clay is also sometimes called thixotropic. This usually refers to material that is very elastic, can be pulled and twisted like taffy, and does not set until left still for a time.
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Throwing rings, throwing spiral
A technique of throwing where the final stage is to put a quick spiral in the piece. This is often done to give it a hand-made look (difficult to reproduce in a machine) and to add more interest to the profile.
Pictures
Tenmoku mug with throwing rings.
Tranlucency
Translucent glazes are neither opaque or transparent. A good example is a matte glaze that contains no opacifier. Glazes that are opacified will of course have varying degrees of translucency according to the amount of opacifier present. A colored glaze can be transparent, translucent or opaque and the color quality with vary accordingly.
Porcelains can be translucent, for example bone china. The translucency is a product of the degree of melting (the more melted the more translucent it will be) and, or course, the wall thickness. Low TiO2 clays are used to make translucent porcelains.
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- (Glossary)
Transparent Glazes
A fully transparent glaze is simply one that does ... - (Glossary)
Bone China
True bone china is a special type of porcelain tha... - (Glossary)
Opacity
Opacity is the property of being opacified, or the...
Transparent Glazes
A fully transparent glaze is simply one that does not have opacity. There are degrees of transparency, if a glaze is matte it will show the color of underlying body and decoration, but these will be muted somewhat. Completely transparent glazes can be difficult to achieve, entrained bubbles and associated surface imperfections often disrupt them. Bubbles can come from the body, the glaze materials or the manner of glaze lay down. In addition, the most brilliant transparents are high in Na2O and K2O, but these oxides contribute to a high thermal expansion and crazing. Another problem with transparents is the growth of crystals during cooling, this is because transparents tend to be more fluid and be high in boron, both of which can breed crystal growth. Transparent glazes provide depth for coloring and can produce vibrant results, but variations in glaze thickness will produce variations in coloration. Transparents can amplify the coloring effect of iron in an underlying body. For example, at cone 6, a porcelain or white stoneware may appear yellowish under a transparent. One method to deal with this is add 0.05 to 0.1% blue stain. Transparents will flux the body surface and make it more mature. While this contributes to the glaze:clay interface, in low fire red bodies, for example, it can darken the color to the point that it is brown instead of red. For this reason, terra cotta bodies are fired well below the red-to-brown transformation point.
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- (Glossary)
Colorant
A material that transforms a glossy or white glaze... - (Articles)
Concentrate on One Good Glaze It is better to understand and have control of one good base glaze than be at the mercy of dozens of... - (Glossary)
Opacifier, Opacification
A glaze additive that transforms an otherwise tran... - (Glossary)
Crazing
Small hairline cracks in glazed surfaces that usua... - (Glossary)
Boron Blue
The blue haze in a transparent boron glaze that re... - (Glossary)
Devitrification, Crystallization
Crystals can grow in cooling glaze melts if one or...
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Pictures
A cone 6 transparent glaze (center) with 0.1 Mason 6336 blue added. One the right, no blue is present. On the left is a cone 10 reduction transparent (inside of mug).
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