|Monthly Tech-Tip |
Alternate Names: Disthene, Cyanite
Kyanite is a super-duty refractory material with very high resistance to thermal shock. It is widely employed in insulating brick, kiln furniture, refractory shapes, etc. and in body formulations used in porcelain, tile bodies, and casting mixes. The material reduces fired shrinkage, increases mechanical strength and thermal shock resistance and allows products to be made with thinner walls, better resistance to dunting, deformation and chipping. Kyanite's long jagged particles tend to form an interlocking no-shrink crystal matrix (like a felt) in bodies fired to low heat (potters use it up to 30% in raku bodies). Because of the particle shape, considerable amounts of kyanite can be added to bodies without adversely affecting their plasticity. For example, 10-15% 48 mesh kyanite can actually increase the plastic strength of smooth throwing bodies, making it possible to throw larger sized vessels. The above mentioned benefits will accrue and, in addition, drying properties will be better. However the dry and fired surface will been a little rougher.
Its large grain sizes make an excellent high temperature grog. It is volume stable and has excellent hot load strength. Because of its expansion characteristics it makes an excellent crack filler.
It is found in large deposits in India, Africa and the USA. American kyanite occurs in association with quartz, from which it must be mechanically separated by grinding. Indian kyanite is processed from surface boulders, and can be calcined in lump form and graded in coarser sizes. American kyanite is said to be the most consistent and Indian the most pure.
Kyanite is available in varying grain sizes down to 325 mesh and in calcined-to-mullite form. The decomposed mullite form is volume-stable with temperature increase, while the raw form of kyanite displays definite expansion during heatup. Depending on grain size, this phenomenon permits the use of kyanite in clay formulations to counteract the shrinkage of the clay body during firing (see also Spodumene). Kyanite has proven invaluable in cements, ramming mixes, and mortars for this purpose.
Plainsman M390 with 12.5% 48 mesh kyanite wedged in. This was added to improve the drying properties while maintaining the plasticity. However, the throwing also improved! It was easier to pull up into a tall cylinder. The surface texture is only moderately disrupted by a slight graininess.
Plainsman M340 with 11% added 48 mesh kyanite. The kyanite was added to improve the plastic strength to stand up when throwing large shapes. It has done this. Its grainy texture (in an otherwise smooth body) is only slightly noticeable while throwing, but it lifts better. The kyanite was simply wedged into the clay using a slice-and-wedge technique, the stiffness was affected only slightly. An added benefit will be a reduction in the thermal expansion (and thus thermal utility) of the fired clay (of course there is a chance that glazes will need to be adjusted to deal with crazing).
The goblet on the left is bending, not just because the clay is somewhat unstable at the temperature being fired, but because this shape is also inherently unstable. Where extreme shapes are prone to warping, ware must be made from clays that do not vitrify (that introduces issues of strength and functionality). In this case, the clay recipe is based on a terra cotta material that matures at a very low temperature. The problem was dealt with by employing a recipe of 60:40 clay:200# kyanite.
Samples fired to cone 6. Lower left: Plainsman M390. Upper left: M390 plus 12.5% 48 mesh kyanite (no visible effect on fired color or character). Upper right: M390 plus 12.5% Christie STKO 22S 40 mesh grog (strangely it fires a darker color and appears more vitreous and there is no soluble salts circle). Both grogs were wedged into the clay and did not stiffen it or affect plasticity much (in fact, both were easier to pull up during throwing). The percentage water content went from the 21% to 19% in both.
Kyanite at Wikipedia
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.
Materials that melt at high temperatures. These are normally used for kiln bricks, furniture, etc. or for ceramics that must withstand high temperatures during service.
Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
|Temperatures||Kyanite decomposes to Mullite and Silica (2750F-)|
|Frit Softening Point||CONE 37|
|Density (Specific Gravity)||3.00|
|Body Thermal Expansion||Kyanite is a super-duty refractory material with very high resistance to thermal shock.|