Digitalfire Ceramic Glossary



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COE, Co-efficient of Thermal Expansion


A measure of the reversible volume or length change of a ceramic material with temperature. The more the expansion during heating the more contraction must occur while cooling it back down. Expansion values are very small and recorded in scientific notation (e.g. 6.5 x 10-7 which is 0.00000065). Typically the power-of-ten is dropped, so a number might simply be: 6.5. Higher numbers indicate higher expansion.

It might not seem like an expansion of 0.00000065 in/in/degree C is much but in a brittle material like ceramic it is. The phenomena of thermal expansion usually first comes to the attention of a potter, for example, as a crazed glaze (having a crack pattern that is a product of the glaze being stretched onto the ware). This is actually a serious problem, crazing impacts food safety and ware strength. The opposite problem, shivering, is where a glaze is under compression; this is even more serious since sharp flakes of glaze can pop off ware into food and drink. Thermal expansion is also the basic phenomena behind why ceramic ware often cracks when suddenly heated or cooled. Flameware bodies have an almost zero thermal expansion and can even withstand a flame directly on the surface. Bodies high in free quartz particles (by virtue of the percentage of silica in the recipe and not being too vitreous) have the highest expansion. Vitreous porcelains (where the feldspar glass has dissolved much of the silica) and bodies made from low expansion minerals (like phyrophyillite, mullite) have the lowest expansions.

When heated from room temperature to 2000F fused silica (non crystalline) exhibits an expansion of almost zero whereas quartz mineral, having the same chemistry, has the amazingly high expansion of 1.5%! By comparison fused alumina (at 1400C) is 0.9% and stabilized zircon 0.8% (also very high). It is possible to fit a glaze to an alumina body by formulating it to have a very high thermal expansion. Likewise, it is possible, although much more difficult, to fit a glaze to an ovenware body by formulating it to have an extremely low expansion.

Very few people have access to equipment that can measure this property, but when potters and technicians know about it they see it exhibited in their ware and test specimens. They can adjust glaze and body recipes such that their expansions increase or decrease. With glazes, thermal expansion is a product of their chemistry. With bodies it is a product of the mineralogy of the materials and the degree to which vitrification occurs.

Pictures

Shivering is not just a glaze problem with Terra Cotta

Low fire terra cotta mugs have cracked. Why? The white glaze is under compression, its thermal expansion is too low (that is why it is also shivering off the rim). As the piece is cooling the kiln the thick layer of white glaze first solidifies. As cooling proceeds the body shrinks (thermally) at a faster rate than the glaze. The puts the glaze under compression and stretches the body. As some point (e.g. last stages of kiln cooling, a thermal stress during use) the body cracks to relieve the stress (notice how the white glaze is pushing the cracks apart). Neither the body or glaze are at fault, in this case they are simply made by different manufacturers and are thermal expansion incompatible. One solution would be to mix it with a white glaze that is crazing (the opposite problem). Or you could add some nepheline syenite to the glaze to increase its thermal expansion (maybe 10% by dry weight).

Substituting alumina in a clay body dramatically lowers thermal expansion

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, alumina has the lowest. 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%!

A dunting crack

Example of a dunting crack in a flat deep cone 6 porcelain bowl. The bowl has a wide bottom that heat-sinks to the shelf, so during firing there is a temperature gradient between the walls and the base. That difference in temperature translates to stress because it means that different parts of the piece are experiencing different thermal contractions as it cools in the kiln.

Crazing after a year of use. What is the problem?

A cone 10R grey stoneware mug that has begun to craze on the inside. The greyer coloration around the craze lines indicates that water is soaking into the slightly porous body. This mug has lost the ring it had when it was new (it is only about a year old). It could be refired to be as good as new but would soon return to this condition. The only real solution is to reformulate this glaze to reduce its thermal expansion.

No crazing out of the kiln. So it is good. Right? Wrong!

The side of this white porcelain test mug is glazed with varying thicknesses of VC71 (a popular silky matte), then fired to cone 6. Out of the kiln there was no crazing, and it felt silky and wonderful. But a 300F/icewater test was done and then it was felt-pen marked and cleaned with acetone. This is what happened! This level of crazing is bad, the dense pattern indicates a very poor fit. Then why was it not crazed coming out of the kiln? The glaze is apparently elastic enough to handle the gradual cooling in the kiln. But what the kiln did not do, time certainly will. This recipe has 40% feldspar (a big high-expansion KNaO contributor), that much in a cone 6 glaze it a red flag to crazing. Coupled with that was low Al2O3 and SiO2, another tip-off.

Simple dilatometric curve produced by a dilatometer

Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.

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.

How much silica can some glazes accept?

G2922B is a cone 6 clear glaze that started as a well-known recipe "Perkins Studio Clear". We substituted Gerstley Borate with a frit (while maintaining the chemistry) and then noted that the glaze was highly fluid. Since I wanted to keep its thermal expansion as low as possible, I added 10% silica. 2926B shows that it is very well tolerated. Then I added 5% more (2926D) and 10% more (2926E which is still very glossy). That means that E represents a full 20% silica addition! SiO2 has no real downsides in any well melted glossy glaze, it hardens, stabilizes and lowers expansion.

Same body, glaze, thickness, firing. Same thermal shock. Only the tile crazed?

Why did the glaze on the tile craze? The tile is double the thickness of the walls of the mug. Thus, when quenched in ice water, a greater gradient occurs between the hot interior of the clay and the cooling surface.

Dilatometer curve of vitreous porcelain (red) vs. stoneware body

The 500-600C zone is the alpha-beta inversion of quartz. Notice the vitreous body experiences a bigger expansion change there. But in the 100-270C cristobalite inversion region the stoneware undergoes a much more rapid change (especially in the 100-200C zone). This information affects how ware would be refired in production to avoid cracking (slowing down in these two zones). In addition, that stoneware would not be a good choice for an ovenware body. Photo courtesy of AF

The high thermal expansion of a low-fire talc body

Talc is employed in low fire bodies to raise their thermal expansion (to put the squeeze on glazes to prevent crazing). These dilatometer curves make it very clear just how effective that strategy is! The talc body was fired at cone 04, the stoneware at cone 6. The former is porous and completely non-vitreous, the latter is semi vitreous.

Thermal expansions for body, engobe and glaze in ceramic tile

The upper line is the body. The center line is the engobe. The lower line is the glaze. The ceramic tile industry is very conscious, not only of glaze-fit but also engobe-fit. Engobes (slips) are employed to cover brown or red burning bodies so they glaze like a porcelain. Typically technicians tune the formulation of the engobe to have an expansion between the body and glaze. The body is highest so that during cooling, as it contracts, it puts a squeeze on the engobe. The engobe thus never finds itself under tension. The glaze has the lowest expansion, it is under a state of compression by the engobe, and slightly more by the body. This equilibrium enables the tile to wear for many years without crazing or shivering. Chart courtesy of Mohamed Abdelmagid.

Out Bound Links

In Bound Links

  • (Troubles) Glaze Shivering
    Ask the right questions to analyse the real cause ...
  • (Glossary) Cristobalite Inversion

    Cristobalite is a crystalline form of silica (SiO2...

  • (Tests) COLE - Co-efficient of Linear Expansion
  • (Glossary) Corning Ware, Pyroceramics, Pyrex

    While this is a trade-name of a specific type of c...

  • (Project) Ceramic Properties

    A property in this context is a created physical p...

  • (Properties) Body Thermal Expansion

    Materials in this class are added to bodies to red...

  • (Tests) BWIW - Boiling Water:Ice Water Glaze Fit Test
  • (Troubles) Glaze Crazing
    Ask the right questions to analyse the real cause ...
  • (Glossary) Ovenware

    Ovenware clay bodies have a lower thermal expansio...

  • (Glossary) Oxide System

    In ceramic glaze calculation, a 'system' refers to...

  • (Glossary) Calculated Thermal Expansion

    Insight-live and desktop Insight calculate the the...

  • (Glossary) Glaze Compression

    Every solid has a thermal expansion, that is, an a...

  • (Glossary) Cordierite Ceramics

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

  • (Glossary) Material Expansion

    Ceramic materials exhibit thermal expansions just ...

  • (Videos) Lesson 3 - Dealing With Crazing - D. Desktop Insight

    This video explains what crazing is, how it is rel...

  • (Tests) IWCT - 300F:Ice Water Crazing Test
  • (Oxides) MgO - Magnesium Oxide, Magnesia
  • (Materials) Grog - Crushed Brick, Body Agregate
  • (Glossary) Food Safe

    In recent years potters and small manufacturers ha...

  • (Glossary) Dishwasher Safe

    Dishwasher safe ceramic ware is generally ware tha...


By Tony Hansen




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