Make kiln posts using kaolin, kiln mortar using kyanite

Firebrick & kiln post/shelf clay - v1.0

Total:102.00

Notes

*An overlooked refractory material is just ordinary kaolin. It is available everywhere and by definition is pure clay mineral, without impurities that would lower its PCE. That means no fireclay can compare with high purity kaolin deposits for resistance to heat (some kaolins are not as pure and thus not as refractory).

Pioneer kaolin, by itself laminates and spalls on drying and firing and is not very plastic. But this grog and bentonite addition seem to solve that to a large extent. They produce a plastic material that seems to lubricate itself, extruding with exceptional ease. And drying very quickly with minimal warping.

Would this work as an inexpensive and convenient way to make kiln bricks? Shelves? Posts? We are testing to find additives that will minimize issues with using such a high kaolin content. It seems possible that cracks could propagate after repeated firings, we are working on versions of this recipe to prevent that.

Why did we not use kyanite? Kyanite has a lot of fines, it is not like a coarse grog at all. Without the coarse grog the kaolin will likely spall and laminate. While some sources indicate that refractories should have a certain minimum Al2O3 content it seems more logical to deliver the maximum Al2O3 while paying close attention to what is coming with it in materials (preferably only SiO2).

Pictures

L4543 thrown crucible

About 5lbs of clay easily throws to a 8x8" inch thin walled vessel. If feels smooth even though it contains 20% coarse grog. At leather hard it feels like soap!

L4543 kiln shelf

Although very soft, it easily stretched, with minimal rolling to make a test kiln shelf. It dried to 4.5mm thick in one day without cracking (between two sheets of Gyproc).

This body could work well for kiln posts

The plastic material is very slippery and should extrude well, especially when using a tapered die like this 3d printed one.

L4543 vs L4543B vs L4543A - Cone 10, 10R bars

L4543 extruded kiln posts fired at cone 10R

They look good, we will leave them in our kilns for a while to see how they perform on repeated firing cycles.

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Firebrick & kiln post clay v2.0

Total:99.50

Notes

*This has exceptional workability for extruding and rolling, even with this much coarse grog. However pattern cracking has developed compared to the L4543 mix (80:20 kaolin:grog recipe) so this recipe needs further adjustment, especially to reduce the firing shrinkage.

Made kiln shelves and larger 4, 5 and 6" long thin walled posts for firing at cone 10. Posts worked well but shelves warped considerably during firing at cone 10R.

Pictures

Kiln post extrusion die #3

This produces extrusion with an inside dimension of 37mm and outside of 45mm.

L4543 vs L4543B vs L4543A - Cone 10, 10R bars

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Refractory kiln post clay v4.0

Total:103.00

Notes

*In a previous mix we tried STKO grog but this Kyanite is much finer-grained, a pleasure to work with in the plastic state. Drying shrinkage is very low also.

The fired surface is smooth and completely crack-free.

At cone 11 oxidation the total shrinkage of this is less than 8%.

However, over time shelves are cracking and curling from the edges inward, this is likely because we have not been first-firing them at the highest temperature possible in our lab.

Pictures

L4543C thrown surface shows texture

Very good plasticity, works like a pottery clay body on the wheel.

L4543C kiln shelf and posts bisque fired

The shelf dried and fired very flat. Posts extruded very well even though the clay was very soft. It was possible to get a more precise cut because of the finer particle size compared to L4543B.

L4543C fired bars

Bisque-Fix, Kiln-Patch

Notes

*Traditional kiln patching or bisque fixing products are made by mixing refractory grog and sodium silicate. They have amazingly low drying and firing shrinkages if made using a grog having a particle size distribution that enables dense packing.

We have found that 48 kyanite is an ideal grog for this, its wide range of sizes produce a dense fired solid when bonded with sodium silicate.

A 30:70 sodium silicate and kyanite mix works best (a 33:66 mix produces a consistency that is a little too flabby and wet and 25:75 is too difficult to mix without special equipment). The kyanite stirs into the sodium silicate quite easily and wets all surfaces rapidly.

The material is not plastic but it does have plenty of cohesion and can be formed and pressed into recesses. It begins to harden on surfaces very quickly (it is thus important to wash tools and containers soon after mixing and wear latex gloves, it can be very difficult to wash off your hands).

The drying and firing shrinkage of this material is remarkable. 0.6% from wet to dry and 0.9% from dry to fired at cone 6!

Pictures

A 33:66 sodium silicate:kyanite mix

It is exceptionally easy to stir the kyanite into the sodium silicate, it is ready to form in 10 seconds.

A 30:70 mix produces a less flabby consistency and is still easy enough to mix.

Fired bar of L4608 kiln patch

The bar is very dense, looking like a vitrified stoneware.