|Monthly Tech-Tip |
Alternate Names: Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum
Description: Aluminum oxide
Calcined alumina is generally used in the manufacture of high-grade ceramic shapes, refractories and fused alumina abrasives. It can be compressed to produce a fired density of 3.8 or more (with the right particle size distribution). Amazingly, ceramic bodies containing 95% or more alumina are being employed to produce ceramic parts for a wide range of industries (fired to 1400C or more). Fabrication methods and glazing vary according to application.
Alumina has a very high melting temperature (about 2000C) and alumina ceramics can maintain up to 90% of their strength above 1100C. They are thus employed in many refractory materials (i.e. Calcium Aluminate Cements have PCEs above cone 35) and used to make parts that must withstand high temperatures.
While there are various degrees of calcining, the materials used in ceramics (alpha alumina) are made by calcining the powder at 1200-1300C to convert it to pure Al2O3 (when calcined near 2000C large hexagonal, elongated tablet-shaped crystals form as "Tabular Alumina"). The alpha form is the densest and most stable crystalline form. It is insoluble in water but is soluble in hydrofluoric acid and potassium bisulfate. Unground calcined aluminas are typically 100-300 mesh, but much finer grades (ground and reactive alumina) are produced by milling - one common grade is 98% minus 325 mesh (45 microns) and 90% minus 12 microns. Another reactive grade has 1-micron particles! In addition to heat treatment and particle size calcined aluminas also distinguish themselves by crystal size, soda content, and degree of thermal conversion to alpha phase. Soda content is a major factor in determining the final use (low soda materials are used for electronic applications, medium soda for electrical insulation and porcelains, high soda for glass, glaze, fibreglass and electrical porcelain).
Some exceptionally fine 'super ground' grades can be made into casting slurries of very high specific gravity, they cast well with very low shrinkage (this is amazing since alumina powder is not a plastic material). Deflocculation can be achieved using a low pH (3.5-4.5) positive anion mechanism employing hydrochloric or nitric acid, a high pH (11-12) cation mechanism with alkali hydroxide salt additions, or with the addition of standard alkali polyelectrolyte dispersants. With the addition of organic binders, alumina bodies can be cast and pressed into a wide variety of shapes requiring heat and abrasion resistance. Alumina parts are then sintered to permit discrete crystals to react with each other to form larger ones. Coorstek AD-94 is an example of a very high alumina content body (they publish a lot of physical data about the material).
Calcined alumina can be substituted for silica as a filler in porcelain bodies. Some claim that it reduces shrinkage, increases thixotropy, provides strength against warping during firing, benefits glaze fit, and adds fired strength (e.g. book "Clay Bodies" by Robert Tichane). However, these claims sound suspicious. Yes, it will reduce warping and fired shrinkage because the body will become more refractory, but that generally means lower fired strength (more feldspar will be needed to restore the degree of maturity). And there is no way that it will give better glaze fit than silica. That being said, alumina is expensive and so different than silica that it is only practical to design bodies around it, not substitute it for something else.
Although it might seem logical to calculate a chemically equivalent substitute of alumina and silica for part of the kaolin in a glaze recipe (i.e. to reduce glaze shrinkage in high kaolin recipes) this will not work unless the alumina is milled to micron sizes. Because of the high melting temperature of the raw alumina only micron-sized particles will dissolve the melt. The tile industry knows this - so they can add alumina to glazes to impart matteness (sometimes very low percentages will do the job, less than 5% for example). Pure calcined alumina powder can also stabilize glaze melts while maintaining most of the visual effect.
Unlike hydrated alumina, the calcined material has no loss in weight on firing. Thus it produces no gases of decomposition.
Fired alumina ceramic parts can be harder than tungsten carbide or zircon, two to four times as strong as electrical porcelain, and very resistant to abrasion. Alumina is thus used in grinding media, cutting tools, high-temperature bearings, and a wide variety of mechanical parts. Compared to zircon it has a high thermal conductivity and a higher thermal expansion.
Alumina (preferably in the calcined form) can be used in clay bodies as an aggregate and filler in place of quartz. This can increase the firing range, decrease quartz inversion firing problems, and increase hardness and whiteness in the fired body. However, again, the cost of alumina is prohibitive.
This production batch of G2934 cone 6 MgO matte glaze is firing almost glossy (upper left). Matte glaze chemistries are generally sensitive - big variations in surface character can result from small changes in firing or material chemistry/physics. This recipe relies both on high MgO and lots of Al2O3 (from high dolomite and kaolin in the recipe). A change in the frit has crossed a tipping point. But there is an amazing fix: A small addition of super fine calcined alumina (400 mesh). How small? only 1%! A small change in the frit turned it glossy so this small change has fixed it. Another factor is that greater additions of alumina (shown here are 1.5, 2.0, 2.5, and 3.0%) progressively matte it more (the slow-cool C6DHSC firing is the reason for the opacity). If the alumina was not dissolving we would expect cutlery marking and surface staining. But neither is happening, even with additions of up to 8%, achieving stoney matteness.
These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!
This homemade kiln shelf (left) for our test kiln was fired at cone 10. This is a third the weight (and thickness) of the cordierite one on the right. However it does not have the thermal shock resistance of cordierite, uneven heatup can crack it. It is made from a body I slurry up consisting of 96.25% calcined alumina and 3.75% Veegum. It rolls out nicely and dries flat between pieces of plasterboard, taking about three days (if you try this and the body is not plastic then your alumina is not fine enough or you did not blender mix the slurry well enough). Alumina produces a lighter shelf than Zircopax and shrinks much less than refractory bodies we have tried (e.g. L4543), I cut the slab only 1/4" larger and it has fired to the same size.
It is made from 96.5% calcined alumina (plus 3.5% Veegum to provide plasticity for forming). At cone 6, with no prior firing to a higher temperature, a 5mm thick slice can support a piece like this. Of course, prefiring to a higher temperature for extra hot strength is a better idea. A caution: This is just typical sintered alumina, it does not have nearly the thermal shock resistance that fully crystallized tabular alumina has.
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).
I mixed a cone 6 porcelain body and a cone 6 clear glaze 50:50 and added 10% Mason 6666 black stain. The material was plastic enough to slurry, dewater and wedge like a clay, so I dried a slab and broke it up into small pieces. I then melted them at cone 6 in a zircopax crucible (I make these by mixing alumina or zircopax with veegum and throwing them on the wheel). Because this black material does not completely melt it is easy to break the crucible away from it. As you can see no zircon sticks to the black. I then break this up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. As it turned out, this mix produced specks that fused too much, so a lower percentage of glaze is needed. I can thus fine tune the recipe and particle size to theoretically duplicate the appearance of reduction speckle.
This is due to its inability to withstand thermal gradients across its width. Typical sintered alumina is refractory, but it is not thermal shock resistant like tabular alumina. The inner part of the shelf was being protected from the rising heat because of this heavy, slow-to-rise calcimine vessel on top of it. The moment of the crack was so dramatic that, in spite of the weight on top of it, the shelf blew apart leaving 4 pieces with an inch-gap separating them.
Typically the G2934 cone 6 MgO matte recipe fires with a surface that is too matte for functional ware (with cutlery marking and staining problems). This is intentional - it enables users to blend in a glossy base transparent to tune the degree of matteness. However, we have seen variation in the Ferro Frit 3124, serious enough that a recent production batch of glaze came out glossy (upper left in this picture)! This happened despite a C6DHSC slow cool firing. Shown here is a trial with additions of 4% calcined alumina (upper right) and 6 and 8% (bottom). All of these were too matte (1.5% turned up to be good). Although the slow-cool C6DHSC firing is the likely reason for the opacity here, opacity disruption still turned out to be a factor for stain additions (muting the colors slightly) even in faster cool firings. This is a testament to the critical chemistry balance that produces this matte surface. And the need to have adjustment options when inevitable variation occurs. Of course, it is important to use ultra-fine alumina (e.g. 400 mesh) to assure it will dissolve in the melt.
The glaze is G2934 cone 6 matte base. Because it was not firing matte enough additions of 2, 4, 6 and 8% super fine calcined alumina were tested. Each addition made it progressively more matte. But with the mattness comes increasing susceptibility to cutlery marking and staining. To test the latter we marked each using a felt pen and then cleaned off the black ink using Acetone. The only one with noticeable staining is the 8% addition (the 6% addition has a slight stain also). The testing also showed no obvious cutlery marked on any of them. The results are reassuring since only 2% or less alumina is needed to achieve the degree of matteness desired so no danger of either problem is indicated. In addition, the integrity of the fired glass suggests that the alumina is dissolving in the melt - that means it is likely contributing to increased surface hardness and durability.
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.
In ceramics, potters make crucibles to melt frits, stains and other materials. Crucibles are made from refractory materials that are stable against the material being melted in them.
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
Alteo PDF describing a wide range of alumina products for refractories - Very educational
Coorstek Ad-94 94% Alumina body
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
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.
An overview of the hazards of calcined and hydrated alumina materials in the ceramic glazes and clay bodies
|Bulk Density g/cc (Packed)||1.0-1.3|
|Density, loose packed (lbs/cu fut)||0.7-1.0|
|Index of Refraction||1.765|
|Frit Softening Point||2040C|
|Density (Specific Gravity)||3.75-3.90|
|Surface Area (m2/gm)||0.5-25|
|Body Thermal Expansion||A very low expansion material.|
|By Tony Hansen|
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