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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).
Materials |
Kyanite
Kyanite is a granular material used in the manufacture of ceramics and abrasives. It is notable for low thermal expansion and one-way expansion on heating. |
Glossary |
Bisque
Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it. |
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