• Materials for Ceramics

Alumina Hydrate

Alternate Names: Alumina Hydroxide, Aluminum Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite

Notes

Hydrated alumina is more correctly termed "aluminum hydroxide" (the water is not locked into the crystal structure, as with other hydrates, but exists as hydroxide radicals that yield as water on heating). There are differing water contents depending on the type of alumina hydrate, but the main refined article of commerce in the market is known as alpha aluminum trihydroxide or ATH (LOI of about 34%). If you are unsure about the material you have do a simple LOI test (by firing a sample of powder to 300C and noting the percentage weight loss).

Since it’s decomposition occurs about 220C as a glaze material it poses no bubble clouding issues in the melt. That being said, considerable amounts of water are generated and these could affect the density of the glaze lay-down or its adherence to the body or another glaze (or underglaze) layer.

Hydrated aluminas are fine granular white powders that have good flow properties. As with any form of alumina, this material has a very high melting temperature. Notwithstanding this, it disassociates enough in many glaze types to be useful as a source of Al₂O₃ to the melt (if the particle size the is really fine). The hydrated version of alumina stays in suspension better in glaze slurries and has better adhesive qualities than the calcine. Also, using hydrated alumina in glazes and glasses can promote a fining by coalescing finely dispersed gas bubbles. Small additions of fine alumina hydrate added to a glaze can also enhance the color of Cr-Al pinks. Larger additions of fine material can impart matteness if the glaze is able to take it into solution (sourcing alumina from kaolin, feldspar and frits is obviously more practical since these decompose readily in glaze melts).

Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.

We are not sure of the CAS#, it seems to have a number of them.

Production of this material poses considerable environmental threats (view the red sludge pond in Stade, Germany on Google maps for an introduction to this topic).

Related Information

Corundum On Feldspar

2, 5, 10 and 15% alumina hydrate added to Ravenscrag Slip

Pure Ravenscrag Slip is glaze-like by itself (thus tolerating the alumina addition while still melting as a glaze). It was applied on a buff stoneware which was then fired at cone 10R (by Kat Valenzuela). This same test was done using equal additions of calcined alumina. The results suggest that the hydrated version is decomposing to yield some of its Al₂O₃, as an oxide, to the glaze melt. By 15% it is matting and producing a silky surface. However crazing also starts at 10%. The more Al₂O₃ added the lower the glaze expansion should be, so why is this happening? It appears that the disassociation is not complete, raw material remains to impose its high expansion.

2, 5, 10 and 15% calcined alumina added to Ravenscrag Slip

The Ravenscag:Alumina mix was applied to a buff stoneware fired at cone 10R (by Kat Valenzuela). Matting begins at only 5% producing a very dry surface by 15%. This "psuedo matte" surface is simply a product of the refractory nature of the alumina as a material, it does not disassociate in the melt to yield its Al₂O₃ as an oxide (as would a feldspar, frit or clay). The same test using alumina hydrate demonstrates that it disassociates somewhat better (although not completely).

An original container bag of Alumina Hydrate

Also often labelled as alumina hydroxide.

Original Container Bag of Alumina Hydroxide

Also known as hydrated alumina.

Calcining aluminum hydroxide to 1200F. Guess how much weight it loses?

32.5%! I started out with 100 grams in this calcining bowl, now there is only 67.5.

Why is hydrated alumina better than calcined for kiln wadding?

A popular recipe for kiln wadding is 50:50 EPK and hydrated alumina. These bottom two SHAB test bars are the hydrate and calcine versions fired to cone 10 oxidation (the former fire-shrinks 7.5%, the latter 3%). Both produce a workable plasticity with about 20% water and both have a drying shrinkage of about 5%. The top two LDW test samples show the hydrated version has an LOI of 24.5% while the calcined one has 7.5%. Although not as plastic as many other kaolins, EPK is certainly among the stickiest, this makes it well suited for this task (since low drying shrinkage and adherence in the plastic state are important, that-being-said, some people use a dab of white glue to hold the plastic tabs on through drying). However the choice of which alumina is more important. On one hand, the more refractory calcined version seems like it would be better. But that is trumped by a key advantage of the hydrated one: It has a significant firing shrinkage coupled with much higher porosity (25% vs 15%), that helps with release from the vitreous foot rings or bases.

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Mechanisms

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