|Monthly Tech-Tip |
Alternate Names: Alumina Hydroxide, Aluminum Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite
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 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 (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.
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.
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).
Also often labelled as alumina hydroxide.
Also known as hydrated alumina.
32.5%! I started out with 100 grams in this calcining bowl, now there is only 67.5.
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 at Wikipedia
Huber Alumina Hydrate page
|Temperatures||Alumina Hydrate Decomposition (200C-450C)|
An overview of the hazards of calcined and hydrated alumina materials in the ceramic glazes and clay bodies
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.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
|Bulk Density g/cc (Packed)||1.4-1.7|
|Density, loose packed (lbs/cu fut)||1.0-1.4|
|Index of Refraction||1.57|
|Frit Softening Point||3000C D|
|Density (Specific Gravity)||2.42|
|Surface Area (m2/gm)||0.1-0.15|
|Glaze Opacifier||Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.|