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Words beginning with T
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.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.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. A good casting slip is the product of maintaining the specific gravity, viscosity and thixotropy. It should have the required specific gravity, be tuned to the needed viscosity yet gel after a set time to prevent sedimentation.
Plastic clay is 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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