• Materials for Ceramics

Kyanite

Alternate Names: Disthene, Cyanite

Notes

Kyanite is a refractory material, normally used in granular form (or granular/powder mix). The grains have relatively low thermal expansion so their presence in a material imparts resistance to thermal shock. Kyanite is widely employed in insulating brick, kiln furniture, refractory shapes, crucibles, etc. and 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 has properties that make it very different from most ceramic raw materials:
-It expands significantly during its first firing (typically about 4–8%, depending on particle size and firing conditions). This unusual irreversible expansion is highly valuable in refractory and thermal-shock-resistant applications because it can offset normal body shrinkage.
-Its particles resist sintering together during firing. Because expansion tends to separate particles rather than pull them into contact, kyanite does not readily develop self-bonding strength. For this reason, other materials must supply the bonding phase: kaolin is commonly used in lower-temperature systems, while mullite-forming clays, alumina, or other refractory binders are preferred at higher temperatures.
-It converts readily to mullite and silica during firing. At sufficient temperature and soak time, kyanite decomposes to form mullite crystals, making it a convenient mullite precursor in many refractory compositions. This conversion can occur in a single high-temperature firing or progressively over multiple firings at lower temperatures. Some industrial applications deliberately calcine kyanite first to complete this transformation before use.

Kyanite’s elongated, angular particles contribute additional advantages in ceramic bodies. They tend to interlock mechanically, creating a crystal framework that helps resist shrinkage even in bodies fired to relatively low temperatures. This is why potters often use it in thermal-shock-resistant bodies such as raku, sometimes at additions up to 30%.

Because of this particle geometry, surprisingly high percentages can be added without severely reducing workability. In smooth plastic throwing bodies, for example, additions of 10–15% coarse kyanite (such as 48 mesh) can increase green strength, improve drying behavior, and reduce drying shrinkage while still preserving acceptable plasticity.

Kyanite is available in grain sizes down to 325 mesh. Because the long thin crystals irreversibly expand and interlock on initial heatup, they hold the material tightly into place. The density is enhanced further when granule/powder forms of the material are used. This expansion phenomenon has proven invaluable in cements, crack fillers, ramming mixes, and mortars. And even in stoneware bodies to counteract the shrinkage during firing.

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

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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

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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

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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

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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

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Traditional kiln patch or bisque fix products are made by mixing kyanite (with other powders) and sodium silicate. They leverage a special property of kyanite particles: They expand on firing, 5% or more. A 30:70 sodium silicate:kyanite mix shrinks only ~0.5% from wet to dry and ~1% from dry to cone 6. While 48 mesh kyanite has a particle size distribution (from fines to coarse) tuned to maximize density to take advantage of the expansion, that is still amazing.

To test, I tried filling the large gap in a bisque mug handle that was cracked completely in two. The kyanite mixes easily with the sodium silicate to form a cohesive material that can be formed (it hardens on surfaces, even your hands, quickly). Glaze coverage over it is poor, thus commercial products would contain other ingredients to make it more "ceramic" in nature (it should be able to tolerate being mixed with materials used to make porcelains and stonewares, in sufficient concentration its expansion will counter their shrinkage).

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