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Alternate Names: Isinglass, Muscovite, Biotite, Phlogopite
Stable, virtually inert except to hydroflouric and concentrated sulfuric acid. Impervious to water and atmosphere. Unaffected by exposure to UV light, water or extended low and high temperatures. Free of asbestos and other non-asbestos fibrous materials. It is non-flammable, non-toxic and non-hazardous. Mica is invaluable in the electrical and electronics industries because of its unique combination of physical, chemical and thermal properties, low power loss factor, dielectric constant and dielectric strength. Also used in insulation, paint, metalurgical, and polymers.
Mica flake is used in low fire clay bodies to add a sparkle effect. Typical grades are too fine, drilling mud grades are more suitable. Typically its use is limited to below cone 02 since the particles can melt above that (test your material to find out). Water washed micas are superior for this purpose. While this technique has been employed by various potters it is actually an old process. For more information about its history, see the book "All That Glitters" (available at www.axner.com) and micaclay.com.
Data sheets for various mica products quote a wide range of decomposition and melting temperatures (as high as 1800C for melting and 1500C for decomposition). A typical melting point of muscovite is around 1250-1300C. However there are many different kinds of mica. Thus it is impossible to give a formula and difficult to give a general chemistry (micas are never employed in ceramics for their chemistry anyway, the mineralogy of the material is what is important).
An example of how a small addition of mica affects the fired appearance of a terra cotta clay. The effect is still working at cone 03 (left) but is more commonly employed at cone 06 (right). Notice that it is still visible even under the glaze. This body is popular on the west coast, it was designed by D'Arcy Margesson. Standard grades of mica are too fine for the effect, this is likely Custer LCM Drilling Mud Mica.
Muscovite Star Mica
|Materials||Georgia Industrial Minerals Mica|
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Mica at Wikipedia