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Alumina Hydrate

Formula: Al2O3.3H2O
Alternate Names: Alum Hydrate, Aluminum Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite

OxideAnalysisFormula
Al2O365.39%1.000
LOI34.61
Oxide Weight102.00
Formula Weight155.99
If this formula is not unified correctly please contact us.
DENS - Density (Specific Gravity) 2.42
XREF - Index of Refraction 1.57
HMOH - Hardness (Moh) 2.5-3.5
DNLP - Density, loose packed (lbs/cu fut) 1.0-1.4
GSPT - Frit Softening Point 3000C D
SAMG - Surface Area (m2/gm) 0.1-0.15
BDGC - Bulk Density g/cc (Packed) 1.4-1.7

Hydrated alumina (or aluminum hydroxide) contains significant water in its crystal structure. 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 alumina hydrate you have in stock do a simple LOI test (by firing a sample of powder to 300C and noting the percentage weight loss).

Since the decomposition occurs about 220C it poses no threat to creating bubbles in already-melting glazes. 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 (however the finer the particle size the better). The hydrated version of alumina stays in suspension better in glaze slurries and has better adhesive qualities. Also, using hydrated alumina in glazes and glasses can promote a fining operation 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.


Mechanisms

Corundum On Feldspar

Corundum On Feldspar

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

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

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

An original container bag of Alumina Hydrate

Also often labelled as alumina hydroxide.

Original Container Bag of Alumina Hydroxide

Original Container Bag of Alumina Hydroxide

Also known as hydrated alumina.

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

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.

Out Bound Links

In Bound Links


By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Alumina Hydrate" descrip="" searchkey="Alum Hydrate, Aluminum Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite" loi="0.00" casnumber="21645-51-2"> <oxides> <oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="65.390" tolerance=""/> </oxides> <volatiles> <volatile symbol="LOI" name="Loss on Ignition" percent="34.610" tolerance=""/> </volatiles> </material>


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