Digitalfire Ceramic Glossary

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Refractory, as a noun, refers to a material that does not melt at normal kiln temperatures (of the industry being referenced). The term also refers to the capacity a material to withstand heat without deforming or melting. Kiln shelves and firebricks are refractory. Many natural clays and minerals are also refractory. Highly refined materials like alumina oxide and zirconia oxide are super refractory, but common quartz particles likewise melt well beyond normal kiln temperatures. Some materials are refractory when fired alone, but when mixed with others they become fluxes (e.g. calcium carbonante, dolomite). When refractory materials are fired, the individual particles do not melt but they do fuse together at points of contact.This type of bonding (where little or no glass formation is occurring) is called sintering. 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. 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 that will support the addition of grog and low price. However, ordinary kaolin is far more pure 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, many common ceramic oxides exceed that easily. 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 5,400F (3,000C).


How do metal oxides compare in their degrees of melting?

Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.

Firing shrinkage variation between various clays

Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and diring 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.

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.

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

It is made from 96.5% calcined alumina and 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 mug like this, demonstrating how refractory alumina is. You can make larger shelves, big enough for small electric kilns, however, since you likely do not have a furnace to fire these as high as they should be fired to sinter them properly (for hot strength), remember to support larger spans in the center to prevent sagging. Also note that alumina does not have nearly the thermal shock resistance that cordierite has (which, by the way, you can also make yourself if you can fire to 1350C).

Cone 10R heavily grogged vitreous body that actually throws

This is a grog clay with 25% Christy Minerals STKO22S grog (20 mesh one size). This piece is about 8 inches tall fired at cone 10R. This body is a Redart, Ball clay base that totally vitrifies to a chocolate brown. But with the added refractory grog it is fairly stable in the kiln and is much more vitreous than other grog bodies. Because it is such a plastic smooth base and because the grog is only one size, this is actually throwable. And it is very resistant to splitting during hand building.

Each stain has its own personality for coloring the body

All of 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). 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).

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.

Can you throw zircopax on the potters wheel? Yes!

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), a have amazing resistance to thermal shock. I could pour molten metal into them and they will not crack. I can heat one area red hot and it will not crack. I can throw the red hot piece into water and it will not crack!

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).

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 what vitrified means). But it has a high fired shrinkage, very poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder, all that cements it together is sintered bonds between closely packed particles. Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, zero thermal expansion and maintain its strength and hardness to very high temperatures.

My first zircopax kiln shelf passed with flying colors

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 any kiln that I have will generate and far in excess of that. It is made from a plastic body having the recipe 80% Zircopax Plus, 16.5% 60-80 Molochite grog and 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, so care is needed. First-firing at cone 4 yielded a firing shrinkage of 1%). Notice that cone on the shelf: It is not stuck so no kiln wash is needed! Zircopax is super refractory! It is held together by sinter bonding, so the higher the temperature you can fire to the stronger it will be.

Out Bound Links

  • (Glossary) Sinter, sintering

    The term "sintered" refers to the particle-to-part...

  • (Materials) Calcined Alumina - Al2O3 - Aluminum oxide

    Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum

  • (Materials) Steatite


  • (Materials) Cordierite - 2MgO.2Al2O3.5SiO2 - Crystalline magnesium aluminosilicate
  • (Glossary) Non Oxide Ceramics

    Fired ceramic that contains no oxygen in the cryst...

  • (Materials) Zircon - ZrO2.SiO2

    Zirconium Silicate

In Bound Links

  • (Typecodes) 1: REF - Refractory
  • (Glossary) Flux

    On the theoretical chemistry level, a flux is an o...

  • (Glossary) Fireclay

    A refractory naturally occurring secondary clay. F...

  • (Glossary) Ceramic

    A man-made solid produced by the fusion of non-met...

  • (Glossary) Cordierite Ceramics

    Cordierite ceramics are well known for their low t...

  • (Glossary) Firebrick

    A brick capable of withstanding high temperatures ...

By Tony Hansen

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