Digitalfire Ceramic Glossary



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Drying Shrinkage


Clays shrink when they dry. A typical plastic pottery clay, for example, shrinks about 6%. Highly plastic bodies can shrink up to about 7.5% before drying cracks become very difficult to avoid. Shrinkage can be reduced to 5% or below before bodies become too non plastic to form effectively. Bodies used in manufacturing and formed by machines can have lower drying shrinkages but organic binders are needed to achieve sufficient dry strength. Bodies containing extremely high percentages of aggregate can have near zero drying shrinkage.

The water content of bodies also determines their total shrinkages. At one extreme, a dust pressing body of near zero shrinkage may have as low at 7% water. Highly plastic bodies can have 25% or more water, these will not only have high shrinkage but will dry very slowly.

Different clays can shrink at drastically different rates and amounts. The amount of shrinkage is a product of the size of the particles, the distribution of sizes, the shapes, the amount of water present, the surface chemistry of the particles, the degree of electrolytic reaction between them and the water and the degree of mechanical densification occurring during machine forming.

Potters and industry know how to cope with the shrinkage of the bodies they use such that minimal drying cracks or breakage during handling occurs. Shrinkage can even be used as an ally, an example is slip casting where the shrinkage pulls the piece away from the mild so it can be extracted.

Different bodies exhibit differing drying shrinkage curves. The majority of shrinkage occurs earlier in the drying process. Thus, if a plastic clay is softer than usual the drying shrinkage will be greater. If different parts of a piece are allowed to dry such that variations in water content develop across the cross section (e.g. from drafts in the drying room or ware of varying wall thickness and air exposure) the stage is set for cracks to occur. At higher water contents there is still enough plasticity to absorb dimensional changes without cracking but as the densification process proceeds and the piece becomes increasingly rigid the variations in shrinkage across the ware will generate cracks to relieve the stress.

Pictures

Particle size drastically affects drying performance

These are DFAC drying performance tests of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.

High drying shrinkage of Plainsman A2 ball clay (DFAC disk)

This test shows the incredible dry shrinkage that a ball clay can have. Obviously if too much of this is employed in a body recipe one can expect it to put stress on the body during drying. Nevertheless, the dry strength of this material far exceeds that of a kaolin and when used judiciously it can really improve the working properties of a body giving the added benefit of extra dry strength.

Closeup of Halloysite particles

Electron micrograph showing Dragonite Halloysite needle structure. For use in making porcelains, Halloysite has physical properties similar to a kaolin. However it tends to be less plastic, so bodies employing it need more bentonite or other plasticizer added. Compared to a typical kaolin it also has a higher fired shrinkage due to the nature of the way its particles densify during firing. However, Dragonite and New Zealand Halloysites have proven to be the whitest firing materials available, they make excellent porcelains.

A bentonitic clay that takes a long time to dry

I finally gave up trying to dry the inner section of this DFAC test. During that test the inner part of the disk is shielded from the air flow or heat lamp. This sets up a shrinkage gradient that encourages cracking of the sample. But with some clays drying can be so slow that it can take a days. Serious cracking and high drying shrinkage almost always accompanies this phenomenon.

Turbo-charge plasticity using bentonite, hectorite, smectite.

These are porosity and fired shrinlage test bars, code numbered to have their data recorded in our group account at Insight-live.com. Plainsman P580 (top) has 35% ball clay and 17% American kaolin. H570 (below it) has 10% ball clay and 45% kaolin, so it burns whiter (but has a higher fired shrinkage). P700 (third down) has 50% Grolleg kaolin and no ball clay, it is the whitest and has even more fired shrinkage. Crysanthos porcelain (bottom, from China) also only employs kaolin, but at a much lower percentage, thus is has almost no plasticity (suitable for machine forming only). Do H570 and P700 sacrifice plasticity to be whiter? No, with added bentonite they have better plasticity than P580. Could that bottom one be super-charged? Yes, 3-4% VeeGum or Bentone (smectite, hectorite) would make it the most plastic of all of these (at a high cost of course).

Stonewares dry better than porcelains

The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.

A batch of fired clay test bars in the Plainsman Clays lab

A batch of fired test bars that have just been boiled and weighed, from these we get dry shrinkage, fired shrinkage and porosity. Each pile is a different mix, fired to various temperatures. Test runs are on the left, production runs on the right. Each bar is stamped with an ID and specimen number (the different specimens are the different temperatures) and the measurements have all be entered into our group account at insight-live.com. Now I have to take each pile and assess the results to make decisions on what to do next (documenting these in insight-live).

Do grog additions always produce better drying performance?

This DFAC drying performance test compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise there appears to be no improvement. While the grog addition reduces drying shrinkage by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Another issue is the jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.

Various grogs available in North America

Examples of various sized grogs from CE Minerals, Christy Minerals, Plainsman Clays. Grogs are added to clays, especially those used for sculpture, hand building and industrial products like brick and pipe (to improve drying properties). The grog reduces the drying shrinkage and individual particles terminate micro-cracks as they develop (larger grogs are more effective at the latter, smaller at the former). Grogs having a narrower range of particle sizes (vs. ones with a wide range of sizes) are often the most effective additions. Grogs having a thermal expansion close to that of the fired body, a low porosity, lighter color and minimal iron contamination are the most sought after (and the most expensive).

How to dry these mugs evenly to avoid cracks

It is important that during all stages of drying gradients (sections of different stiffnesses) do not develop in pieces. Thus I like to attach handles as soon after throwing as possible. An unavoidable gradient develops anyway because the rims need to be stiff enough to attach the handles without going out of shape too much. Now how can I stiffen these mugs for trimming and even them out at the same time? The first key is to put them on a plaster bat (as I have done here). Then I cover them with a fabric (arnel fabric works well because it flows). Then I put the whole thing into a large garbage plastic bag folded underneath to seal it. The plaster stiffens the bases and absorbs moisture in the air to stiffen the walls also. The next day every part of the piece is an even leather hard.

Same clay disk dried fast (heat gun) and slower (fan) for the DFAC test

The center portion was protected while the perimeter dried and shrank first (reshaping the central section). No cracks. But as the central area hardened it reached a point where it was stiff enough to impose forces that forced two cracks to start from the outer edge (opposite each other), these grew inward and found each other. Then the gap widened to dissipate more of the stresses (the width of this gap relates to the drying shrinkage of the clay). But the accelerated pace in the top disk left more stresses, they were relieved by the other hairline cracks from the outer edge, these happened at the very end.The lesson: The stage was set for cracking on both samples very early in the drying process. But the actual cracks occurred very late. Accelerating the process only created small extra edge cracks (on top disk).

Out Bound Links

  • (Glossary) Plasticity

    This term is used in reference to clays (or more o...


By Tony Hansen




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