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• Materials for Ceramics

Calcined Alumina

Alternate Names: Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum

Description: Aluminum oxide

Oxide	Analysis	Formula	Tolerance
Al2O3	100.00%	1.00
Oxide Weight		102.00
Formula Weight		102.00

Notes

Calcined alumina (referred to here simply as alumina) is a high-purity, calcined form of Al₂O₃ used extensively in advanced ceramics, refractories, abrasives, and specialty glaze and body formulations. Its principal industrial use is in the manufacture of high-grade ceramic shapes, fused alumina abrasives, and refractory products. With the correct particle-size distribution, alumina powders can be compacted to produce fired densities of 3.8 g/cm³ or higher. Remarkably, ceramic bodies containing 95% or more alumina are widely used to manufacture technical ceramic parts for demanding industrial applications, typically fired above 1400°C. Forming methods, binders, and firing schedules vary according to the intended application.

Alumina has an exceptionally high melting temperature (about 2050°C), and dense alumina ceramics can retain up to 90% of their room-temperature strength even above 1100°C. Because of this, alumina is widely employed in refractory systems and high-temperature components. Calcium aluminate cements, for example, achieve pyrometric cone equivalents above cone 35 because of their high alumina content.

Ceramic-grade calcined aluminas are normally alpha-alumina, produced by calcining alumina hydroxide at approximately 1200–1300°C to convert transitional aluminas into the stable alpha phase. Alpha-alumina is the densest and most thermodynamically stable crystalline form of Al₂O₃. At much higher calcining temperatures, near 1900–2000°C, large dense crystals develop, producing tabular alumina, a highly refractory aggregate material.

Calcined aluminas differ widely in particle size, crystal size, soda content, and degree of thermal conversion. Powdered grades are commonly supplied in 100–300 mesh sizes, while milled grades range from standard 325-mesh material down to reactive aluminas with median particle sizes near 1 micron. A common technical grade may be 98% passing 325 mesh (45 microns), with 90% finer than 12 microns. These differences strongly affect sintering behavior, packing density, and final fired properties.

Soda content is a major factor in determining end use: low-soda grades are preferred for electronic ceramics, medium-soda grades for electrical insulation and porcelains, and higher-soda grades for glass, glaze, and fiberglass applications.

Exceptionally fine “super-ground” aluminas can be deflocculated to produce casting slurries of very high specific gravity and surprisingly low drying shrinkage, despite alumina having no inherent plasticity. Deflocculation may be achieved by low-pH dispersion (using hydrochloric or nitric acid), high-pH systems (using alkali hydroxides), or conventional polyelectrolyte dispersants. With suitable organic binders, alumina bodies can be slip cast, dry pressed, or extrusion formed into complex shapes requiring heat resistance, wear resistance, and electrical insulation. During sintering, adjacent alumina crystals bond and grow to produce dense, strong ceramic structures. CoorsTek AD-94 is a well-known example of a 94% alumina industrial ceramic body for which extensive physical property data is published.

In traditional ceramics, calcined alumina is sometimes added to porcelain bodies as a refractory filler or aggregate. It can reduce drying and firing shrinkage, improve whiteness, reduce quartz inversion effects, and increase refractoriness. However, because alumina is much more refractory than silica, additions often lower body maturity unless feldspar or other fluxes are increased to compensate. For this reason, alumina is usually more effective when a body is designed around it rather than used as a direct silica substitute.

In glaze technology, calcined alumina behaves very differently from kaolin-derived alumina. Because raw alumina has such a high melting temperature, only very fine particles (typically micron-sized) dissolve readily in glaze melts. Coarser grades often remain partially undissolved, contributing to matteness or melt stabilization rather than acting as a chemical alumina source. This is why chemically equivalent substitutions of kaolin with alumina plus silica often fail unless extremely fine alumina is used. The tile industry exploits this effect by adding very small percentages of micronized alumina to control gloss and stabilize melts.

Unlike hydrated alumina, calcined alumina has no chemically bound water and therefore produces no decomposition gases during firing. It can itself be produced by calcining alumina hydroxide.

Dense fired alumina ceramics can exceed the hardness of tungsten carbide and approach that of some zirconia systems while maintaining excellent abrasion resistance, high thermal conductivity, and chemical durability. These properties make alumina suitable for grinding media, cutting tools, kiln furniture, electrical insulators, bearings, and many engineered wear components.

Compared with zircon-based ceramics, alumina has higher thermal conductivity but also higher thermal expansion, factors that must be considered when thermal shock resistance is important.

Related Information

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

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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

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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 alumina mini proof-of-concept home-made kiln shelf (5 mm thick)

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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.

Making your own hexagonal shelves using calcined alumina

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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.

Substituting alumina in a clay body dramatically lowered thermal expansion

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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%!

Original Container Bag of Calcined Alumina

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Making your own crucibles to make your own speckle

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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, dry and break into small pieces. I then melted them at cone 6 in a Zircopax crucible (I make these by mixing alumina or zircopax with 3-4% veegum and throwing them on the wheel). This material does not completely melt so it is easy to break the crucible away (it does not stick to the zircon). I then break the black up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. If specks fuse too much I can lower percentage of glaze (and vice versa). Of course, the particles are glass, jagged and sharp-edged so care is needed in handling them.

An alumina kiln shelf that has cracked during firing

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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.

Adding calcined alumina to a ceramic glaze until it stains

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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 (Al₂O₃ is the key to matteness and this material is a pure source, if kaolin were added it would also source SiO₂ and require much more). 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.

What if G2934 fires too glossy, how can you increase matteness?

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Typically the G2934 cone 6 magnesia 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 power of calcined alumina to matte a magnesia glaze

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This production batch of G2934 cone 6 magnesia 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 stony matteness.

Alumina parts are ceramics on steroids!

Created by Gemini using a prompt to create a typical alumina part

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In terms of hardness, wear resistance, and high-temperature stability, alumina ceramic is far superior to even the strongest mullite porcelain. Such porcelains are mixes of kaolin, feldspar and silica. Alumina parts are just micron-sized calcined alumina powder fired to an incredible cone 30 or more, often held there for days! The powder is mixed with binders and formed by pressing or injection molding. Precision "green machining" is also used (while parts are chalky). With super fine particle size, high purity, dense packing and prolonged firing, surfaces can be very white and so smooth they are glossy (e.g. spark plugs are not glazed). While parts can even be translucent they are not vitrified, no glass is developed during firing. Rather, they are sintered - the fine particles fuse into a material approaching diamond hardness.

Links

Materials	Mullite A highly refractory low-expansion alumino-silicate crystal formed naturally only rarely but produced industrially and within fired kaolin bodies.
Materials	Zircon
Materials	Alumina
Materials	Tabular Alumina
Materials	Alumina Hydrate
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
Typecodes	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.
Oxides	Al2O3 - Aluminum Oxide, Alumina
URLs	http://www.coorstek.com/materials/ceramics/alumina/ad94.asp Coorstek Ad-94 94% Alumina body
URLs	https://www.alteo-alumina.com/wp-content/uploads/2019/07/ALTEO_2018_Refractory_brochure_web.pdf Alteo PDF describing a wide range of alumina products for refractories - Very educational
Glossary	Calcination 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.
Glossary	Refractory 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.
Glossary	Crucible 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.

Data

Bulk Density g/cc (Packed)	1.0-1.3
Density, loose packed (lbs/cu fut)	0.7-1.0
Hardness (Moh)	9.0
Index of Refraction	1.765
Frit Softening Point	2040C
Density (Specific Gravity)	3.75-3.90
Surface Area (m2/gm)	0.5-25

Mechanisms

Body Thermal Expansion	A very low expansion material.

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