All Glossary

Primary Clay

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.

Related Information

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Ball clay and kaolin test bars side-by-side fired from cone 9-11 oxidation and 10 reduction.

Cone 10R Grolleg porcelain

vs American ball clay/kaolin porcelain

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Typical pottery porcelains are variations on the traditional triaxial body and commonly contain around 50% clay, either kaolin, ball clay or a mixture. Since the clay fraction is normally the largest source of iron and titanium impurities, its purity has a major influence on fired whiteness—especially in reduction.

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 porcelain (50% Grolleg kaolin, 25% silica and 20% Mahavir feldspar, 5% bentonite). The one on the right is 15% M23 ball clay, 40% #6 Tile kaolin, 15% Nepheline, 25% silica, 3% bentonite. The grey is the cumulative effect of the impurities in these clays.

Grolleg is considerably more expensive, but this is one place where the extra purity is plainly visible. Want to push whiteness even further? Use an ultra-low-iron New Zealand kaolin and replace ordinary bentonite with Veegum or another clean-burning plasticizer.

Guess which of these fired clays employs New Zealand kaolin

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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.

Kaolin and ball clay:

Why the uncracked disk is not the whole story

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Left: OptiKast, a coarse-particle kaolin intended for slip-casting. Right: Plainsman A2 ball clay. These DFAC test (drying performance) disks were dried with their centers covered, deliberately creating uneven drying. The exposed perimeter dries and stiffens first; the covered center dries and shrinks later. The resulting cracks reveal how each clay responds to that stress. OptiKast feels smoother, yet its individual clay particles are much coarser than those typical of ball clay. This kaolin has low drying shrinkage, so its disk remained intact. The ball clay shrinks more. As its center dried and tried to contract against the already stiff perimeter, stresses developed, splitting the disk and opening numerous cracks around the edge.

So why use ball clay? This particular kaolin has so little plasticity that even making this disk was difficult. And the dried disk is fragile. Yet it is very useful in casting bodies, where plasticity impedes permeability. Ball clays are highly plastic; high drying shrinkage comes with that. Adding some of this to poor-plasticity casting or forming bodies that can tolerate a darker fired color can transform them into ones that both form well and dry harder and stronger.

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