Ivor and Olive Lewis on sun 19 oct 08
Subscribers to Pottery Making Illustrated may recall the 2000 Fall
Issue which had two feature articles. One, by Jeff Zamek told how to
prevent S-Cracks, the other by Ivor Lewis considered the movement
inside a pug of clay as it was thrown into a cylinder.
I agree with Vince Pitelka that consolidation of the base is a
necessary task. Several passes where fingertips move from the inner
wall to the centre causing clay to flow towards the axis of rotation
are necessary. The sensation that you are causing a knob of clay to
advance inwards is true, but then it is diverted downwards and exudes
outwards. The final effect is that there is no change in thickness of
the base. You still have a flat inner base. But flock structures
change through shear and tension to a series of thin overlapping
sheets. Attenuated flocks are now interleaved and form a structure
that resists the tensile forces of shrinkage.
Clay Flocks, after pugging, wedging and kneading will most likely have
irregular polyhedral forms. Coning shears and thins flocks as clay
extrudes through your
hands. Clay at the base of the pug remains relatively immobile during
the centring and coning phases. So it will retain its coarser flock
texture.
The top illustration on page 17 indicates the outer skin of clay of
the cone is moved but the inner core at the base remains immobile.
Compare this to the picture of Jeff Zamek at the top of page 20. Not
an "S"crack, but a straight fracture following the pattern of the
marker clay I used.
Working the bottom clay as described militates against Base Cracks. If
clay is repeatedly swept from the centre of rotation to the periphery
the base thins and eventually you will break through to the wheel head
or the surface of your throwing bat.
Cavitation is an interesting concept. But I think the speed of
throwing clay is not high enough, though the pressures of coning can
create a conical cavity at the axis of rotation at the bottom of the
clay.
Burnishing and polishing are interesting processes that involve
mobilising the surface atoms or molecules of an object without
removing any surface material.
How many crystals of Kaolin are needed to cover the head of a pin ?
Interesting topic !
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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