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Laminations

Laminations because of improper pugging of a clay body will cause separations and drying cracks in the ware.

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Details

Laminations are planes of weakly connected material in the plastic matrix of a clay body. Improper pugging or premixing is the most common cause. Laminations can be seen in a pugged slug by cutting a slice laterally and bending it. They are generally concentric to the center of shaft rotation.

While the soft clay is cut and layered thousands of times during the pugging process, pressures within the machine can still knit the layers back together. However, a number of factors can impede the knitting process (e.g. when the extrusion head does not compress the clay enough, when the blades in the mixing chamber are not properly set or are excessively worn, if there is inadequate pressure post-auger to knit together the core, when the machine is run beyond its capacity, when the vacuum chamber has inadequate vacuum).

Another factor is body formulation, if a mix contains too much sand or silt then it can be almost impossible to pug it without laminations. Or if the clay is being pugged with inadequate water. Particle surface area and shape also introduce complex dynamics into the pugging process. Pugmills are brute-force machines - they have very powerful motors and they inject a lot of energy into the clay to mix it. But they simply cannot wet all the particle surfaces, especially of plastic clays (especially ones containing highly plastic materials). Pugmills are expensive and they are designed for specific use-cases and when employed in another they produce a lower quality product. When knives, seals, bearings, casings, augers and the vacuum system are not maintained the machine is more prone to laminations in the extrusion. Operators also need experience to run the machine in a manner that minimizes the issue. Modifications made to machine can also be a factor.

Often, as long as the clay is used right away, as in a factory setting, it performs well. But on aging, as the water tries to find its way between all particle surfaces, it follows lamination fault-lines. Pottery clay manufactures are really affected by the phenomenon of age-induced lamination. But they can get away with less optimal pugging because they trust that potters will wedge the clay (the wedging process, when done well, is highly effective at completely removing laminations in an aged slug of clay). For this reason manufacturers often print warnings right on the box about this. Manufacturers that insert these slugs (or cuts of them) directly into production machines, without wedging, are the most vulnerable to this issue.

Laminations can also be caused by improper forming and working methods.The most obvious is when water or powder is trapped between the layers during wedging or kneading. Likewise, any form of throwing that uses layering between which water can be trapped can produce a lamination.

Laminations may not reveal themselves until firing. They can produce a phenomenon similar to bloating, but occurs in a body that is not over fired. If the surface of the clay is sealed by a melting glaze, this can disrupt the escape of gases of decomposition at the sites where they are generated. The result can be a buildup of pressure inside that exploits the weakness of a lamination. This matter will be complicated by the presence of larger particles in the matrix that must decompose and produce gases during firing, they produce larger volumes of gases at fewer sites.

Related Information

Laminations in unwedged clay


Two dried bricks, broken in half, revealing the stresses

The brick-halves on the left cracked in two during drying, the crack opened at the center. I dried six of them and all cracked in the same way. The one-inch-slices were cut laterally from an extruded slug of clay and sun-dried. The radial pattern of the laminations are clearly visible on the break. These laminations are "a weakness" formed-into this extruded and unwedged clay, they would, of course, extend to fired integrity, weakening the piece. The halves on the right are from a brick that I made by first wedging (kneading) the clay, then forming and cutting it to size. It was likewise sun-dried. But did not crack. I broke it (with difficulty), notice the break followed the stresses of the breaking process, not internal lines of weakness.

What happens when you dry and bisque a piece made of pure kaolin?


The way in which the walls of this bisque fired kaolin cup laminate reflect the plately and uniform nature of the kaolin particles. Because they are lining up during the wedging and throwing process, the strength to resist cracks is better along the circumference than perpendicular to it. The bonds are weak enough that it is very easy to break it apart by hand (even though it is bisque fired). The worst laminations were at the bottom where wall thickness was the most variable and therefore the most drying stresses occurred. However, if this kaolin were blended with feldspar and silica, this lamination tendency would completely disappear.

Laminations in a jiggered bowl


These are not cracks. They do not go through to the inside. During forming the clay was folded over itself rather than compressed against the mold.


Example of a lamination that has occurred in a fired stoneware body at cone 10 oxidation.

Another reason why clay should be wedged or kneaded


Left: A high-contrast photo of a cut across the cross section of an eight-month-old slug of Plainsman M370 pugged clay. Right: A cut of a just-produced material (which will exhibit the same pattern in eight more months). You can feel different stiffnesses as you drag your finger across this clay, these are a product of the aging process combined with the natural lamination that a pugmill produces. Clearly, the older material needs to be wedged before use in hand building or on the wheel.

Laminations in pugged clay: Wedge it or lose the ware


Laminations in pugged clay

These are cross-cuts from slugs of a production run of clay that was improperly pugged (inadequate vacuum). The problem becomes evident weeks or months after pugging. In this case the clay body contained 2% talc, our production must make doubly sure to monitor vacuum pressure at all times (or laminations will result). These are not a problem if clay is wedged well before use. If not the laminations "build in" failure points that initiate drying and firing cracks later. Even if pieces survive the drying and firing processes, weaknesses can persist making them more prone to failure-on-impact or stress. This being said, does that mean you do not need to wedge plastic clay bodies if they are not laminated? No. All clays laminate to some extent when pugged.

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Laminations: Will a studio pugmill solve the problem?

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Glossary Particle orientation
Ceramic clays have a flat particle shape. Various factors determine the extent to which they can bind face-to-face in pugged clay in the presence of particles of other materials.
By Tony Hansen
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