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Alumina Hydrate

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

Oxide Analysis Formula
Al2O3 65.39% 1.00
LOI 34.61%n/a
Oxide Weight 102.00
Formula Weight 155.99


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?

Fired kiln wadding test bars

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.


Alumina Manufacturing Process
Alumina Hydrate/Hydroxide at Wikipedia
Huber Alumina Hydrate page
Materials Tabular Alumina
Materials Alumina
Materials Pechiney Alumina
Materials Calcined Alumina
Hazards Alumina Toxicology
An overview of the hazards of calcined and hydrated alumina materials in the ceramic glazes and clay bodies
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Alumina
Alumina products
Oxides Al2O3 - Aluminum Oxide, Alumina
Minerals Bauxite
The main ore from which alumina oxide products are refined, contains diaspore, gibbsite and boehmite
Minerals Gibbsite
An ore of aluminum.
Temperatures Alumina Hydrate Decomposition (200-450)


Bulk Density g/cc (Packed) 1.4-1.7
Density, loose packed (lbs/cu fut) 1.0-1.4
Hardness (Moh) 2.5-3.5
Index of Refraction 1.57
Frit Softening Point 3000C D
Density (Specific Gravity) 2.42
Surface Area (m2/gm) 0.1-0.15


Glaze OpacifierAlumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.
By Tony Hansen
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