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.
| 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. |
| Glossary |
Porcelain
How do you make porcelain? There is a surprisingly simple logic to formulating them and to adjusting their working, drying, glazing and firing properties for different purposes. |
| Materials |
Tile #6 Kaolin
The most plastic kaolin commonly available in North America, most suitable for making plastic porcelains. |
| Materials |
Grolleg Kaolin
A white burning kaolin from the UK, commonly used in porcelain bodies and as a glaze suspender. Sticky when wet, low plasticity. |
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