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Bacteria, Mold on Clay

Over time mold can grow on pugged clay (stored in plastic bags). The amount and type of growth depends on temperature and available light, time and type of clay. The process is expected and natural. It might seem logical that porcelains, being made from higher purity materials and having a denser matrix and lacking larger particles, would grow less mold. But such is not the case. Often, the surface mold grows in small spots of intense color, dark enough that potters can mistake it for iron impurities. Patching patterns of discoloration (a slightly different shade of the clay itself) can penetrate into the clay from the surface. Inside the clay slug itself, aging can amplify the spiral pattern created by the pugmill, even creating zones of stiffer and softer clay, These are all manifestations of the aging process, it is thought these improve the plastic properties. A few moments of wedging will quickly bring the material back to its pristine production consistency.

Amazing mold (actually sprouting leaves) that has grown on pugged clay after 10 months of storage where there is sunlight.

Mold that has grown on pugged clay in a bag.

Mold has appeared on the surface of an eight-month-old slug of potters clay (Plainsman M370). It is part of the aging process and can appear on any clay, depending on the conditions of storage. On the right a thin slice of the material has been fired to cone 6, showing that these marks are not iron. Mixing these specks back into the interior of the slug can be done quickly by wedging, greatly diminishing worries about any health issues involved with them.

Another reason why clay should be wedged or kneaded

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.

Aged clay really needs to be wedged before use

Aged clay really needs to be wedged before use

This is a cut through an eight-month-old slug of pugged clay. The cut was done near the surface. The patchy coloration is a by-product of the aging process. If a slice of this was fired in a kiln, an even and homogeneous white surface would emerge, with no hint of what you see here. A few moments of wedging will mix the matrix and ready it for wheel throwing or hand forming.

Mold growing an a clay having added xantham gum

Mold growing an a clay having added xantham gum

This slug is about 6 months old. It contains 0.75% gum. The gum also destroys the workability of the clay.

Discoloration as porcelain ages

Discoloration as porcelain ages

This is a slug of Grolleg porcelain that is about 1 year old. When pugged it was perfect homogeneous white. These streaks and specks have all developed over time as it ages. A slice of this cross section fires pure white.

Out Bound Links

  • (Glossary) Wedging, kneading

    Wedging clay is similar to kneading bread dough. Clay tends to set up over time and the growth of mold discolors the surface and cross section and create non-homogeneous stiffness across the matrix. The process of wedging it loosens the overall stiffer clay and evens out the stiffness, returning it ...

By Tony Hansen

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