• Materials for Ceramics

Kyanite

Alternate Names: Disthene, Cyanite

Notes

Kyanite is a super-duty refractory material, normally used in granular form. Kyanite grains have low thermal expansion so their presence in a material imparts resistance to thermal shock. For most products in which it is used the firing temperature will not be sufficient to change the crystal structure or physical properties of the kyanite grains or particles. It is widely employed in insulating brick, kiln furniture, refractory shapes, crucibles, 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 be 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.

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 heat up. 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 (when heated the long thin crystals irreversibly expand and interlock, holding the material tightly into place).

Kyanite 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 purest.

Related Information

Wedge kyanite in M390 for better drying, firing

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.

A little kyanite might improve your clay body

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 shapes of some pottery make them prone to fired warping

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.

M390 Cone 6 red with added kyanite, grog

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.

Bisque-fix/kiln-patch made using sodium silicate and kyanite

Traditional kiln patching or bisque fixing products made by mixing refractory grog and sodium silicate can have amazingly low drying and firing shrinkages: We measure ~0.5% from wet to dry and ~1% from dry to cone 6 for a 30:70 sodium silicate: 48 mesh kyanite blend (because the kyanite has a particle size distribution that enables dense packing and it expands when fired). Here I tried filling the large gap in a bisque mug handle that was cracked completely in two. The kyanite stirs easily into the sodium silicate, it wets all particle surfaces rapidly. While the mix is not plastic it does have plenty of cohesion and can be formed and pressed into recesses. It hardens on surfaces (even your hands) quickly. Notice that the non-porous nature of the kyanite gives poor glaze coverage here, I could have alleviated this by applying a thin layer of the clay over the repair (with CMC gum and possibly a little frit for fire bonding to the kyanite below). Given the density at which this compound fires at cone 6 no other additions seem necessary. One user even adds white glue to non-fritted versions of this to aid in adhesion.

Links

Data

Mechanisms

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk