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Plucking refers to the chipping away of small fragments of the base of a ceramic vessel because the piece sticks to the kiln shelf during firing. The stuck piece either pops off during kiln cooling (due to the difference in thermal expansion with the shelf) or it needs to be broken off. There are several causes. Over-firing ware to the point that body fluxes begin to bridge the gap between shelf and foot. Clays that contain soluble salts that migrate to the surface on drying and melt and glue ware to the shelf. The use of a low quality kiln wash that does not offer a powdery, non-stick surface to melted glass. Porcelains that are over-mature (having too much feldspar or other flux).

The problem can almost always be solved using quality kiln wash.

Plucking on a vitreous porcelain at cone 6

Plucking on a vitreous porcelain at cone 6

The mug on the left is made from a whiteware body (Plainsman M370), the one on the right is a highly vitreous translucent one (Plainsman Polar Ice). Both have been over-fired slightly. The Polar Ice mug has stuck to the shelf somewhat, taking chips out of the base on the outer perimeter of the bare porcelain (a fault called Plucking). If the shelf had been better dusted with alumina powder (or a kiln wash based on it) rather than a cheap kiln wash made from a kaolin:silica mix there would have been no problem.

The foot ring on the left is plucking, the right one is not. Why?

The foot ring on the left is plucking, the right one is not. Why?

These are translucent porcelains, they are vitreous. The firing is to cone 10. The one on the left is a cone 6 body, and, while it survives to cone 10 it does warp. But more important, it is much more vitreous (more melted). The plucking problem makes it quite difficult to get a good foot ring. The other, which has only slight plucking, is also quite vitreous (high in feldspar). The plucking problem on both can be solved by simply using a better kiln wash. What is better? More refractory, and therefore having a powdery, non-stick surface. Spend more money on your kiln wash, base it on calcined alumina or zircon.

Plucking in a cone 10R stoneware body having soluble salts

Plucking in a cone 10R stoneware body having soluble salts

The soluble salts have formed the brown coloration on the bare clay foot ring. While the actual salts layer is very thin, it is glassy and enough to glue parts of the base to the kiln shelf (the latter did not have adequate kiln wash or sand). The glaze line is close to the foot and this complicates the problem. There are a couple of solutions. Sand the foot ring at the dry stage to remove the soluble salt layer. Use a more refractory kiln wash that offers a powdery, non-stick surface.

Even with good kiln wash plucking can sometimes occur

Even with good kiln wash plucking can sometimes occur

These were left by ware made from a cone 6 highly vitreous translucent porcelain. For that type of clay it would be advisable to set ware on a thin layer of silica sand or granular alumina.

Out Bound Links

  • (Glossary) Soluble Salts

    In ceramics, certain compounds (e.g. calcium or magnesium sulphate) in clays and glazes can dissolve into the water of the plastic material, then on drying these are left on the surface as the water evaporates. For more information, see the topic Efflorescence.

  • (Glossary) Vitrification

    Vitrification is the solidification of a melt into a glass rather than a crystalline structure (crystallization). Glass, clay bodies and glazes vitrify, but in ceramics use of the term focuses most on clay bodies. Vitrification is a process. Bodies do not have specific vitrification points. As cl...

  • (Glossary) Kiln Wash

    A refractory powder that can be mixed with water (and gum solution) and painted on kiln shelves to prevent ware and accidental glaze drips from sticking. Porcelain clays, for example, melt enough during firing that they tend to stick onto the kiln shelf. But with a good wash there are not problems. ...

In Bound Links

  • (Glossary) Sulfates, Sulphates

    Sodium, potassium, magnesium sulfates can be found in many clays. These are soluble and migrate to the surface during drying. Soluble salts can be iron stained, but often are white. Depending on the temperature of the firing and the nature of the salt, they can leave scum on the surface (often calle...

By Tony Hansen

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