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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.

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The term "refractory" refers to a material that does not melt at normal kiln temperatures or the capacity of a material to withstand heat without deforming or melting (in the industry being referenced). In ceramics, the first refractories encountered are normally kiln shelves and firebricks. Many natural clays and minerals are also refractory. Highly refined alumina oxide and zirconia oxide are super-refractories. Even common quartz particles melt well beyond normal traditional ceramics kiln temperatures. Some materials are highly refractory when fired alone, but when mixed with others they become fluxes (e.g. calcium carbonate, dolomite, talc). When refractory materials are fired, the individual particles do not melt but they do fuse together at points of contact in a type of bonding called sintering (where little or no glass formation is occurring). The fusion of particles can take place at relatively low temperatures to give the product adequate service strength. But as a material is fired much higher, particles increasingly pack themselves together and very high fired strength can be achieved. Typical clay bodies contain both refractory particles (that form the skeleton) and particles that melt (to fill in the spaces between).

While many metallic coloring oxides melt very actively, chrome and rutile, for example, are very refractory (for example, even when mixed 50% with a high borax frit they do not flow at cone 6). Stains are smelted mixes of metallic colors and stabilizers and are intended to be refractory enough to suspend in a glaze melt without dissolving into it.

Fireclays are often referred to when discussing refractories (pictures of fired test bars of a number are shown here). These clays are stable at high temperatures because they have low levels of common fluxing oxides (like K₂O, Na₂O, CaO, MgO), often they are simply coarsely ground ball clays. These materials are popular because they combine serviceable refractory character, high plasticity, support for the addition of grog and low price. Interestingly, ordinary kaolin is far purer and therefore more refractory (although not as plastic).

High-tech, highly processed refractory materials of many kinds are used to make parts needed by a wide range of manufacturing industries. Different refractories offer different properties (e.g. low dielectric strength, high tensile or compressive strength, low thermal expansion, low or high thermal conductivity, low or high density, etc). Although certain metal alloys exist that can handle more than 2000C, there are common ceramic oxides that exceed that easily. Alumina and zirconia are the "gold standards" of ceramic oxides, technologies have been and are being developed to make them into countless refractory products. Non-oxide ceramics go even further. For example, Russia's Tomsk State University is developing a ceramic whose multiple layers (based on hafnium carbide, zirconium diboride and zirconium oxide) can survive temperatures over 3,000C!

If you need to build a kiln or oven, it is good to be aware of the range of refractory products available (likely well beyond what your supplier stocks). For an example, check the link to the Morgan Advanced Materials product data book (other companies will have similar guides). They make fibers, boards, bricks, castables, blankets, felts, papers, blocks and modules made from various materials.

Amazing work is being done milling and blending refractory materials to particle size distributions that pack to high density. This produces minimal shrinkage on firing and higher sintered strength. Advancing machining techniques have been developed to produce parts of tight dimensional tolerance. And injection molding can now be done to produce parts of unbelievable surface, strength and thermal properties.

Related Information

Firing shrinkage variation between various clays

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and drying shrinkages vary widely. Materials normally acting as fluxes (like dolomite, talc, calcium carbonate) are refractory here because they are fired in the absence of materials they react normally with.

Pine Lake fireclay lab test bars

Fired to cone 10R (top) and 7,8,9,10 oxidation (from bottom to top). A refractory material.

IMCO 400 Fireclay fired bars

Cone 10R top, 11 oxidation and downward below that. This material, although called a "fireclay", is more fine grained and much more vitreous than what would normally be considered a refractory fireclay. Is it similar to Lincoln Fireclay (from California).

Calcium carbonate and dolomite are refractory when used pure

Examples of calcium carbonate (top) and dolomite (both mixed with 25% bentonite to make them plastic enough to make a test bars). They are fired to cone 9. Both bars are porous and refractory, even powdery. However, put either of these in a mix with other ceramic minerals and they interact strongly to become fluxes.

The difference between vitrified and sintered

The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature (because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely). That is what vitrified means. But it has a high fired shrinkage, poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder (VeeGum), all that cements zircon ceramics together is sinter-bonds between closely packed particles (there is some glass development from the Veegum here). Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, low thermal expansion and maintains its strength and hardness at very high temperatures.

The difference in fired character between kaolin and ball clay at cone 10R

The top one is EP Kaolin, the bottom one is Old Hickory M23 Ball Clay (these materials are typical of their respective types). These materials have low alkali contents (especially the kaolin), this lack of flux means they are theoretically highly refractory mixes of SiO₂ and Al₂O₃. It is interesting that, although the kaolin has a much larger ultimate particle size, it is shrinking much more (23% total vs. 14%). This is even more unexpected since, given that it has a lower drying shrinkage, and should be more refractory. Further, the kaolin has a porosity of 0.5% vs. the ball clay's 1.5%. The kaolin should theoretically have a much higher porosity? What is more, both of these values are unexpectedly low. This can partly be explained by the particle packing achieved because of the fine particle size. Despite these observations, their refractory nature is ultimately proven by the fact that both of these can be fired much higher and they will only slowly densify toward zero porosity.

Why pure stain powders make poor inks, slips, underglazes and overglazes

On the left are pure blue stain brush strokes, on the right are green ones (both painted over a glaze). Clearly, the green is refractory, stiffening the glaze enough to trap bubbles and sit on the surface as a dry, unmelted layer. The blue is the opposite, melting and bleeding profusely into the glaze. Under the glaze, these problems would be magnified (the blue bleeding more, the green causing crawling and blistering). Stains are not ceramic, they are ceramic additives. Stains are not safe for direct food contact. Stains are expensive. Stains don't suspend in water, paint poorly and dry as a lose powder. These stains each need to be added, as a minor percentage, to a ceramic painting medium (one with CMC gum and a mix of ceramic materials tailored to melt to the desired degree and have a compatible chemistry for develop the color (as per manufacturer guidelines).

How do metal oxides compare in their degrees of melting?

These metal oxides have been mixed with 50% Ferro frit 3134 and fired to cone 6 oxidation. Chrome and rutile have not melted, copper and cobalt are extremely active melters, frothing and boiling. Cobalt and copper have crystallized during cooling. Manganese has formed an iridescent glass.

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

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.

Each Mason stain has its own personality for coloring the body

These Mason stains make the porcelain more refractory, but some more so (e.g. 6385, 6226). Some do not develop the intended color (e.g. 6006 pink, it is a glaze stain only). Some need a higher concentration (e.g. 6121, 6385). Some need a lower concentration (e.g. 6134). Some do not impart a homogeneous color (e.g. 6385). The data sheets from the stain manufacturer normally make it clear which of their stains are suitable in bodies. But it is up to you to test concentrations needed to get the desired color and what adjustments to the porcelain are needed to compensate its degree of vitrification in response to the effects of the stain.

Can you make things from zircopax? Yes.

Only 3% Veegum will plasticize Zircopax (zirconium silicate) enough that you can form anything you want. It is even more responsive to plasticizers than calcined alumina is and it dries very dense and shrinkage is quite low. Zircon is very refractory (has a very high melting temperature) and has low thermal expansion, so it is useful for making many things (the low thermal expansion however does not necessarily mean it can withstand thermal shock well). Of course you will have to have a kiln capable of much higher temperatures than are typical for pottery or porcelain to sinter it well.

Throw Zircopax on the potters wheel. Just add VeeGum.

These crucibles are thrown from a mixture of 97% Zircopax (zirconium silicate) and 3% Veegum T. The consistency of the material is good for rolling and making tiles but is not quite plastic enough to throw very thin (so I would try 4% Veegum next time). It takes alot of time to dewater on a plaster bat. But, these are like nothing I could make from any other material. They are incredibly refractory (fired to cone 10 they look like bisqued porcelain). However I have had mixed results for thermal shock resistance.

My first zircopax kiln shelf

It is 5 mm thick (compared to the 17mm of the cordierite one). It weighs 650 grams (vs. 1700 grams). It will perform at any temperature that my test kiln can do, and far in excess of that. It is made from a body I slurry up (80% Zircopax Plus, 16.5% 60-80 Molochite grog, 3.5% Veegum T). The body is plastic and easy to roll and had 4.2% drying shrinkage at 15.3% water. The shelf warped slightly during drying (I should have dried it between sheets of plasterboard). Firing at cone 4 yielded a shrinkage of 1%. Notice that cone on the shelf: It has not stuck even though no kiln wash was used! Zircopax is super refractory! This is sinter bonded, so the higher the temperature you can fire the stronger it will be. Although it would be very hard to make full 18 or 22-inch shelves for larger kilns, smaller ones designed to "network" would enable a tighter load of ware with a much lower shelf-to-ware weight ratio (especially using my own lightweight posts). Like alumina, this does not have the thermal shock resistance of cordierite, uneven heating can crack these.

An electric kiln half shelf that costs $500!

This Advancer Nitride-bonded Silicon Carbide shelf is 26 inches wide (by 1/4 inch thick) weighs 9 lbs. These are incredible durable and strong. However there are cautions to their use. They can act as an electrical conductor so must not contact elements and should not be used in kilns with unpinned elements protruding from grooves. They must be stored in a dry place to prevent moisture penetration (which can cause a steam explosion during heatup). The company has a recommend drying schedule if shelves do absorb moisture (the application of kiln wash is not considered a prolonged exposure and is OK).

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