iandol on mon 28 apr 03
Dear Friends,
The idea of Compression in this instance is a bit misleading. In the =
usual event of throwing on a bat or the wheel head the solid surface =
beneath the clay acts as an anvil against which the clay can be forged =
as your finger or thumb moves too and fro across the base of the pot. =
This causes the clay to circulate, mix up and eliminate flow structures =
generated in kneading and coning. Once these structures are disoriented =
there are fewer weak planes which can fracture under tension.
Now go on the Hump. No firm surface against which the clay can be =
worked. But if before commencing you can get a circumferential pressure =
below the mass of clay which is to be worked you will multiply the flow =
structures. Doing this again after penetrating the clay increases the =
disorder and doing again prior to wiring off again disrupts the =
patterns. This process reduces the severity or frequency of base cracks.
Best regards,
Ivor Lewis.
iandol on thu 1 may 03
Dear Chris Morgan,
You make an interesting proposition. I also have access to Lawrence and =
West and find their values relating to differential shrinkage somewhat =
confused and confusing. In addition, they present no concrete evidence =
or facts in their book by which we might validate their suppositions.
To get some sort of a handle on this you have to consider the aspect =
ratios of the particles in question and the volumetric relationship =
between the solid and liquid fractions of the material under =
observation. If you make the following assumptions: That there is an =
aspect ratio of ten to one for width to thickness for all of the =
particles; That there is a uniform film of liquid water surrounding each =
of and separating all of the particles: that particles form a regular =
array to make a flat sheet; That the sheets bed parallel to each other, =
then the shrinkage ratio of the orientation model they propose would be =
ten to one for thickness to length per unit of measure. My measurements =
do not support this conclusion.
To look at this problem another way. Start with a cube of clay. Flatten =
it by any means you wish. Cut the resulting sheet into uniform cubes. =
Measure each in three dimensions, precisely recording the orientation of =
the applied pressure. Measure the edge lengths of these cubes when they =
are dry. Calculate the differences for each of the three dimensions and =
see if you get a ratio which is nearer to one to one or ten to one. Now =
this an experiment anyone who subscribes to this list can do and it =
requires no expert scientific or mathematical skills beyond the =
abilities to observe, measure and subtract one number form another. So =
anyone can post their results for the clays that they use.=20
If you check the archives you will find that I have repeatedly asked for =
SEM images of plastic clay which has been worked on the potter's wheel =
to be provided to substantiate claims about particle orientation due to =
the application of pressure. When these are forthcoming I will accept =
the evidence. Until that time, I prefer to work with my own =
Metallurgical Model for the structure and behaviour of plastic clay. I =
do this on the basis that the assumptions I make are supported by =
observable phenomena.
Now, why should coarse sand or grog prevent "S" cracks. Who has a good =
handle on that one?
Best regards,
Ivor Lewis. Redhill, South Australia
John Hesselberth on thu 1 may 03
On Wednesday, April 30, 2003, at 10:35 PM, iandol wrote:
> Now, why should coarse sand or grog prevent "S" cracks. Who has a good
> handle on that one?
Hello Ivor,
Well, here is one theory. Any discontinuity in a solid will provide a
point for stress concentration when you have things like differential
shrinkage occurring . If you have a zillion via grog/sand, each will
be small and the load will be spread. If you have a homogeneous body
the same level of total stress will be generated given the same
differential shrinkage, but it will be concentrated in fewer--maybe a
single--place and can more readily result in a rupture.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on tue 6 may 03
Because cracking is related to shrinkage and even drying and grog helps
this. The reason the cracks are S shaped when pots are thrown off the hump
is due to the twisting of the clay as the top clay is centered.
This results in shearing of the aligned particles and that is why
compression sometimes cures the fault.
Careful, even drying is the most profitable way to prevent these types of
cracks and some recompression in the area affected.
Some clays can take being thrown off the hump quite well - others are
difficult. All that is needed is some understanding of why it is happening
and some creative problem solving.
See the sections in the Potters Dictionary of Materials and Techniques by
Hamer - 10 pages on cracks and why they happen.
RR
>Now, why should coarse sand or grog prevent "S" cracks. Who has a good
>handle on that one?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
clifton wood on wed 7 may 03
Ron - i read your message with interest.
The reason the cracks are S shaped when pots are thrown off the hump is due to the
twisting of the clay as the top clay is centered.
now, don't laugh too hard... remember, i'm a village idiot when it comes to clay...
but
since i have a reversing wheel, and can center in either direction...
would s cracks be lessened if i center in one direction & throw in the other?
thanks.
sabra wood
rochester ny
Ron Roy on sat 10 may 03
You did make me smile - but I think your idea demonstrates that you are not
without some brain in this area.
You will have to try it to be sure but -
I think not because - the cracking is helped by the shearing of the clay
during the twisting. That disarrangement is permanently recorded in the
clay - recompression is an attempt to rearrange that clay and realign the
particles. If you twisted the other way I don't think you would accomplish
that - in fact I think you might make it worse.
The other factor here is (see Hamer on shrinkage and warping and S cracks)
the bottoms of pots don't go through the same particle arrangement that the
walls do - due to throwing. If the particles are arranged differently in
the walls and the bottom then shrinkage is affected differently.
Let me sight a phenomenon I see constantly.
I consult for clay companies - I receive data on absorbency and shrinkage
of all batches of clay produced by Tuckers. The particles of some clays -
due to the action of the pug mill - are disarranged (lets say they resemble
the bottom part of pots) - I can see this clearly because the shrinkage is
higher than I would expect. Our lab person always wedges at least one of
the samples from a string of batches. Those wedged samples ALWAYS have less
shrinkage than the others.
To extrapolate a bit further - during centering and throwing particle
arrangement is going on - but because the bottom clay is attached to the
wheelhead it is not much affected - that is why recompressing bottoms tends
to help this problem.
Some clays are more prone to cracking than others - sometimes it's too much
shrinkage - some times not enough - and sometimes all that is needed to
overcome is even drying and some extra compression - or particle
arrangement.
The moral of all this - don't disarrange - and if you arrange one part and
not the other - and get cracking - arrange the other part. Every one should
be testing their clay to see how much shrinkage is going on - and check the
absorbency as well. It only takes a few minutes to make a couple of bars
from each shipment of clay - if you have to problem solve you will have
excellent data to start from.
RR
>Ron - i read your message with interest.
> The reason the cracks are S shaped when pots are thrown off the
>hump is due to the
>twisting of the clay as the top clay is centered.
>
>now, don't laugh too hard... remember, i'm a village idiot when it comes
>to clay...
>but
>
>since i have a reversing wheel, and can center in either direction...
>
>would s cracks be lessened if i center in one direction & throw in the other?
>
>thanks.
>
>sabra wood
>rochester ny
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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