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. The main causes are over firing ware to the point that body fluxes begin to bridge the gap between shelf and foot and clays that contain soluble salts that migrate to the surface on drying and melt and glue ware to the shelf.
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 (a fault called Plucking). If the shelf had been better dusted with alumina powder or sand this may not have happened.
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 this problem is fairly serious, making it very difficult to get a good foot ring. The other, which has only slight plucking is also a little over vitreous (having too much feldspar). While the one on the right could likely be fired with no plucking at all using kiln wash powder on the shelves, the other will likely pluck even if the shelves are coated.
Plucking in a cone 10R stoneware body having soluble salts
The solubles salts form the brown coloration on the clay surface. While the actual salt layer is very thin, it is enough to glue parts of the base to the kiln shelf (if the latter does not have adequate kiln wash or sand). This is even more important when the glaze line is close to the foot.
Out Bound Links