iandol on sat 23 mar 02
Not a lot seems to be known about the Mechanics of Pots and Plates in =
the context of the Physical Forces which act on their structures.
Plates, platters and chargers are notorious for their ability to sag in =
the centre. I would note Robin Hopper as being among the few who have =
made a positive response to this particular problem. He draws attention =
to the relationship between the area enclosed by the footring and the =
total diameter, emphasising that the footring should not be made too =
wide.
I would add to this that there is a need to consider the Mass of Clay as =
well. It is my opinion, and something I do to prevent this curse, that =
the underside of the plate within the footring should be turned to a =
shallow concave contour. I also try to get an imperceptible but slight =
concave contour on the upper surface as well. This can only be achieved =
if more clay than might be anticipated is used so that the initial =
thickness of the clay under the Well or Dish is much greater than =
considered necessary. I recall that the Poster of the initial inquiry =
said they had applied a lot of pressure across the base of the plate. =
This may be counter productive if done beyond necessity, for the action =
may reduce the thickness to a degree which would not allow the =
pyroplastic clay to support its own weight (Force of Gravity on Mass) as =
the Deformation Eutectic Temperature is approached. Using a tapered =
contour which thickens towards the rim, or footring, ensures this force =
is reduced at the centre. This should delay the onset of Pyroplastic =
Flow
My suggestion would be to have the initial throwing make the thickness =
of clay at the centre 20 millimetre or three quarters of an inch thick =
for anything in the twenty inch/500mm diameter range or greater. So at =
least six and a half kilograms of clay would be needed even though the =
final fired weight might be less than four and a half kilos.
Those solutions which consider turning additional rings or projections =
may work but they do not address the fundamental cause of the problem, a =
combintinon of Mass, Leverage and Pyroplastic Flow.
Best regards to all.
Ivor Lewis. Redhill, South Australia.
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