Laminations because of improper pugging of a clay body will cause separations and drying cracks in the ware.
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.
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.
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.
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.
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.
These are cross-cuts from slugs of a production run of clay that was improperly pugged (inadequate vacuum). The problem is not often evident at time of extrusion but exhibits after weeks or months of storage. This clay body does not contain significant fine-grained material, but 2% talc is added to aid vitrification. The production crew claims that this talc makes it doubly important to monitor vacuum at all times (or laminations will result). These are not actually a problem if you wedge the clay well. But if you do not (e.g. inserting them into a hand-extruder, pressing them in a mold) then they "build in" failure points that will initiate drying and firing cracks later. Even if they survive the drying and firing processes, weaknesses will persist makings pieces 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, even if not visible. Because clays have such incredible particle surface area, even the best pugmills cannot wet the surfaces of all of them.
This company was plagued with drying cracks in their solid porcelain pieces. After some time they discovered that the deaired plastic material received from their suppliers had laminations (revealed in a cross section cut of the slug). Since they were not wedging, but simply inserting the clay into their hand extruders and presses, these laminations produced built-in weaknesses, the stresses of drying later exploited these. The obvious fix seemed to be to buy a vacuum pugmill to remix the clay. But that did not work. Why? Commercial pugmills commonly have multiple shafts, hundreds of blades, large powerful motors, separate mixing and vacuum chambers, shredders, high-compression heads, etc. Small studio pugmills have none of these features. They are still great for recycling and mixing clay that will later be wedged. But for the machine-forming purposes of this company, this pugmill actually made the laminations worse!
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.
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.
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