Digitalfire Ceramic Glossary



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Calculated Thermal Expansion


Insight-live and desktop Insight calculate the thermal expansion of a glaze from its oxide chemistry. The number it reports is based on the contributing expansion factors and amounts of each oxide in the formula. These numbers 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.

Results are determined by the set of expansion numbers (different values are available from different sources) and the method of additive calculation method chosen (based on formula or mole%). Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. For example, suppose you have a dolomite, whiting, feldspar, kaolin, silica glaze. If you vary the amounts of these materials and these new recipes all melt properly, the comparative calculated expansions are a good indicator of which ones will craze more or less. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system - the direction or magnitude of change of the calculated thermal expansion may not be as expected. Also, some oxides, like Li2O or B2O3 do not impose their expansions in a linear fashion, thus they do not calculate as well.

Another factor relating to the degree to which calculated thermal expansion matches a lab-measured one is the homogeneity of the material. Frits, for example, compared to raw materials, have glass particles of the same chemistry, thus every particle is going to do something predicable during melting. Raw materials, on the other hand, have particles of possibly a dozen different minerals, each having it's own complex melting behavior that is a product of it's mineralogy, chemistry, particle size and shape. In addition, these particles interact in complex ways.

As noted, thermal expansion calculations assume a glass, where all oxides have freedom of movement and can impose their proportionate expansion on the whole. Thus, if a glaze is not completely melted the expansion calculation is invalid. Likewise, a clay bodies do not melt like glazes, they undergo complex crystallization and complex phase changes while cooling in the kiln. A glass and crystal of the same chemistry usually have wildly different physical properties. Consider SiO2: Its percentage may be equal in two bodies, but one may have most of the SiO2 in quartz grains and the other might have it as a molecular component of feldspar and kaolin. These will, of course, have vastly different thermal expansions. Clay bodies are loaded with crystallization, glass formation and also unchanged mineral and man-made particles. The mix and even stability the mix of these, in turn, relates to other factors that affect the progressive stages of decomposition and the play-out of interactions that break and build molecular bonds: e.g. variations in particle size distribution, particle mineralogy and shape, firing speed, atmosphere and duration of firing.

Another factor is non melting particles suspended in the glass melt. An example is zircon: its particles impose their expansions differently than if they melt and participate in the glass chemistry.

Do you have expansion data for a certain clay body? Are you trying to match that to calculated expansions for glazes? In view of the above it is not going to work, if it does it is purely an accident. Measured thermal expansions are a non-linear line on a graph across 1000 degrees crow-bared into one number. Dilatometer measurements are applicable when they are done by the same team of people on both bodies and glazes and rationalized based on a history of learning to interpret them and observing relationships with real world fired results of those bodies and glazes. Are these lab-measured numbers useless then? No. They enable you to line up a group of clay bodies from lowest to highest thermal expansion. If you know how your glaze fits on one of the bodies then you are in a position to predict what it will do on another. For example, if your glaze is crazing on body A and body B has a higher thermal expansion, then your glaze should fit that body better.

How does one make practical use of calculated thermal expansion numbers? Remember, they are relative, not absolute. So you use them in that way. If a glaze is crazing that means its thermal expansion is too high. Adjust the formula to bring the calculated expansion down and fire a test and subject it to thermal stress (using the 300F-into-icewater test, for example). If it still crazes move it downward further. If it does not craze then stress test it from ice-water to boiling water to check for shivering. With experience you will learn the amount of change needed and will need to do fewer calculate-test cycles. As a general guide, suppose a glaze crazes badly out of the kiln and the expansion is 7.0. Try to move it down to 6.5. If it crazes only after thermal stress testing then drop it less. Of course, adjusting a glaze to control its thermal expansion will have side effects (e.g. change in the degree of gloss or melt fluidity). Often resourcefulness and plenty of testing are needed to succeed.

Pictures

A down side of high feldspar glazes: Crazing!

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to return pieces. The fix: A transparent base that fits your ware. Add colorants and opacifiers to that. Another fix: substitute some of the KNaO for a lower expansion flux (like MgO, SrO, CaO, Li2O) and add as much SiO2 and Al2O3 as the glaze will take (using glaze chemistry software).

The unexpected reason for this crazing can be seen in the chemistry

This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion than CaO does.

Adding silica will fix crazing, right? Not here.

G2926B (center and right) is a clear cone 6 glaze created by simply adding 10% silica to Perkins Studio clear, a glaze that had a slight tendency delay-craze on common porcelains we use. Amazingly it tolerated that silica addition very well and continued to fire to an ultra gloss crystal clear. That change eliminated the crazing issues. The cup on the right is a typical porcelain that fits most glazes (because it has 24% silica and near-zero porosity). The center one only has 17% silica and zero porosity (that is why it is crazing this glaze). I added 5% more silica to the glaze, it took that in stride, continuing to produce an ultra smooth glossy. It is on cup on the left. But it is still crazing just as much! That silica addition only reduces the calculated expansion from 6.0 to 5.9, clearly not enough for this more severe thermal expansion mismatch. Substituting low expansion MgO for other fluxes will compromise the gloss, so clearly the solution is to use the porcelain on the right.

Lights go on with side-by-side fired samples and chemistry

10 grams balls of these three glazes were fired to cone 6 on porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as pure as the frit. Notice the calculated thermal expansion: That greater melting came at a cost, the thermal expansion is alot higher so 2 and 3 glaze will be more likely to craze than G2926B (number 1).

Do you know the purpose of these common Ferro frits?

I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 has a very-low expansion, it is great for crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).

Why is this crystalline glaze not crazed? Even in the pool at the bottom?

Because this is Plainsman Crystal Ice, it contains 40% silica (quartz). It also does not vitrify, so as much of the quartz remains undissolved as possible. This produces a body with a much high thermal expansion so it can put more of a squeeze on the high-expansion glazes used in the crystal glazing process (it is very common for such glazes to be crazed, it is accepted as part of the process).

Do your functional glazes do this? Fix them. Now.

These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.

Out Bound Links

In Bound Links

  • (Glossary) Glaze Compression

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

  • (Glossary) SiB:Al Ratio

    Conceptually we consider fired glazes as being com...

  • (Glossary) Si:Al Ratio

    Conceptually we consider fired glazes as being com...

  • (Glossary) Material Expansion

    Ceramic materials exhibit thermal expansions just ...

  • (Troubles) Glaze Shivering
    Ask the right questions to analyse the real cause ...
  • (Troubles) Glaze Crazing
    Ask the right questions to analyse the real cause ...
  • (Oxides) MgO - Magnesium Oxide, Magnesia

By Tony Hansen




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