|Monthly Tech-Tip |
A clay deposit that is near the site of erosion and alteration. They have more mechanical impurities and fewer chemical and mineralogical impurities. Primary clays have larger particles and less plasticity.
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Kaolinite is the closest thing we have to pure clay mineral. Clays that have been deposited at or near their site of alteration. They typically are contaminated by rocks and mineral particles that must be removed by processing. The cleanest and lowest iron clays are from this type of deposit. Kaolins are primary clays.
These mugs were fired in the same kiln load at cone 10R and have the same clear glaze, G1947U. The mug on the left is a Grolleg kaolin (52% Grolleg kaolin, 24% silica and 19% Mahavir feldspar, 5% bentonite). The one on the right is 15% M23 ball clay, 40% #6 Tile kaolin, 15% Nepheline, 25% silica, 3% bentonite. Clearly, the Grolleg porcelain fires so much whiter.
The whitest test bar here is a New-Zealand-kaolin-based cone 6 porcelain (NZK). NZK has low plasticity, so this body employs VeeGum to improve it. Immediately to the left of it are three North American-koalin-based bodies using standard bentonites. The bar to its right is a Grolleg-based body that uses a standard bentonite rather than a white burning one. All are plastic.
These are DFAC drying performance disks of a large-particle kaolin (Opticast) and a ball clay (Plainsman A2). The DFAC test displays a plastic clay's response to very uneven drying (these disks are dried with the center portion covered to set up a water content differential). These materials both feel super-smooth, in fact, the kaolin feels smoother. But the ultimate particles are ten to one hundred times smaller than kaolin, thus it shrinks much more. The ball clay also has much lower water permeability, being unable to channel water from the center protected portion fast enough. When the inner section finally dried the outer was already rigid so it split the disk in two and pulled all the edge cracks. Most ball clays shrink more and crack worse than this (cracks concentric to the center also appear). So why use ball clay? This kaolin is so lacking in plasticity it was barely possible to even make this disk. And it is so weak that it can easily break just by handling it. Still, it is useful to make casting bodies. But the ball clay, when used as a percentage of a body mix, can produce highly plastic bodies than can be dried without trouble if done evenly.
The most fundamental clay mineral. This mineral is found in nature in its purest form as kaolin. How
Clays form by the weathering of rock deposits over long periods. Primary clays are found near the site of alteration. Secondary clays are transported by water and laid down in layers.
|By Tony Hansen|
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