I.Lewis on wed 11 aug 99
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Here goes, getting myself out of step with the rest of the World.
I find it difficult to believe that adding grog or sand will cure this =
problem.
I know there is a strong belief that adding these to a soft clay will help =
when
a thrower is unable to attain the required height but I am not convinced. =
Quick
sand is a mixture of sand and very fine colloidal clay. Mixing a few buckets=
of
sand does not make a morass of quaking quicksand any safer to walk on. It =
still
remains =22Quick Sand=22
Neither Frank Hamer nor Robert Fournier have anything to say about squatting
clay as far as I can ascertain. Michael Cardew in Pioneer Pottery suggests
tempering with another clay, possibly Kaolin.
Can someone please explain in simple terms why adding a non plastic material
increases plastic strength and provide experimental evidence which supports =
this
view? Have any of you ever made sand castles and been frustrated because =
they
collapsed? Wet sand has almost no shear strength. Colloidal clay provides =
that
shear strength. Breaking step, I say reducing none plastics can increase the
strength of a throwing clay. Adding sand, grog, pitchers, molochite or any
similar materials will never remove the cause of thixotropicity. Give =
=22Tooth=22,
yes. Let your fingers grip and sense the clay, yes. But increase the value =
of
modulus of shear, never.
Some one mentioned =22Dry Throwing=22 as a solution. The reference for that =
is =22Dry
Throwing=22 by James Fox, to be found in Ceramics Monthly Vol 21, no 8 =
(October
1973) pp.22-6, reported by Leon Nigrosh in his book =22Claywork=22 1975. I =
have been
taken to task already for saying publicly that clays which have sufficient =
water
to create a paste plastic enough for throwing require no further lubrication
beyond that needed for centring and opening.
I=92m still thinking about plasticity. I think I can explain it but the
mathematics of the physics gets me a bit confused. When I get some numbers =
to
apply to clay possibly something will work out, one way or the other.
Regards to all,
Ivor, in South Oz, putting his left foot forward.
Bill Campbell on thu 12 aug 99
the clay may not be thixo it may be just the opposite.
if yoou understand a deflocculation curve and how to alter it you can
probably get where you want to be with a small addition of a deflocculant or
a reflocculant.
i can explain how to make this correction and the nature of the
deflocculation curve for you,however it is too involved to write it all out
at this time. if you want to talk about it phone me 814 398 2148 bill campbell
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