Michael Wendt on thu 15 apr 04
Andrew,
Actually, the stiffening I am referring to is separate and distinct from
the real observation that dryer clay is stiffer. When I start wire wedging,
I spray the canvas surface of my plywood wedging table with water
so that it actually adds moisture and also assists the bond formation
between the layers.
Not every clay I tested became stiffer when wedged this way. None of
the Seattle Pottery Supply clays got stiffer. They all showed half the
stiffness
after wedging that the cores tested from the undisturbed pug showed.
SIDE NOTE:
I now use lead shot as my weight medium and have found that the rate
weight is added to the plunger on the clay tester affects the total amount
the plunger travels. Two cores taken side by side show different apparent
stiffnessess when the weight is added slowly versus quickly, with the
slow addition giving much less travel than the fast weight addition.
I suspect this is due to the differences in the coefficients of static
versus
kinetic friction between the non clay components since my clay body is 38%
Helmer Kaolin and 62% Feldspars and Silica.
Finding the correct rate to add the weight for testing
clay softness in a production line will be the challenge.
To make a repeatable test, we will need to all add the same amount of
weight at the same rate and then observe the total travel of the plunger.
I had hoped we could hold plunger travel to a common figure but it is too
hard to hit and takes too long so I propose we select one weight standard as
a common starting point and one rate of weight addition and then compare the
total travel.
Using this standard means the tests can be done quickly and without the
operator standing there. Add the known weight of beads to the funnel and
wait until the plunger stops. That is the final value. A small travel
indicates stiff clay. A large value indicates soft clay.
Comments?
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Andrew wrote:
Hi,
I m coming into this a little bit late, and with the renaming of the topic
tracking back has been a little difficult.
Anyway as I read the posts it seems to have been suggested that after
prolonged wedging a clay body loses some plasticity. Correct?
If this is what has been observed could it not simply be that moisture is
being lost? Certainly handing damp clay does take water from the clay
body, and with reducing moisture the workability falls.
The increase in plasticity after mechanical input, be it from wedging or
pug, has been well established.
Wondering if I've misunderstood these last few posts ....
Andrew
mailtoandrew@FSMAIL.NET on fri 16 apr 04
Thanks for the clarification Michael. As I noted I was having a little
difficult tracking back so confusion on my part, not difficult!, had crept
in.
Your observations are interesting, and led to a bit of head scratching.
And whilst I can not claim any insight perhaps the following may be worthy
of consideration
Despite your precautions to avoid moisture loss have you tested this? If
nothing else it would confirm the condition is constant and this factor
can be eliminated.
Have you tried wedging just clay rather than a clay body? Is the same
phenomenon found? Perhaps it is lessened, or magnified?
What are the differences between the bodies, or clays, that show the
greatest and least change?
Mechanical work into clay does have an effect, and generally includes an
increase in plasticity and green strength. This change against input is
subject to the law of diminishing returns, and hence a plateau is reached
rather than a negative result at the extreme. But what mechanism explains
your observations? Perhaps particle alignment, or due to deagglomeration a
change in packing? Dont know but its got me thinking ...
About your side note ... what is this device youre building? And why? You
mention its use on a production line ...
Regards,
Andrew
Michael Wendt on fri 16 apr 04
Andrew,
The device I am referring to is a softness tester I have proposed
that crushes a 1" diameter by 1" tall cylinder of clay cored by
means of a Monoject Syringe with the end cut off.
The plunger is machined to exactly 1 square in of surface area so that by
the time the plug has been crushed down 30%, the entire face is supported
by the clay.
The main idea was to create a repeatable softness test for a clay mixing
line that would also be available to potters so they could say to a
manufacturer that they like their clay to have softness value 8.5 psi.
Unfortunately, it turns out to be very hard to hit the 30% figure exactly by
adding weight.
It is far easier to simply add the same amount of weight at the same rate
each time and observe the degree of crushing. Soft clays crush down
(actually, they extrude out from under the plunger in all directions
unimpeded) much more than stiff clays under the same amount of weight.
See the tester on my web site.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Andrew wrote in part:
Thanks for the clarification Michael. As I noted I was having a little
difficult tracking back so confusion on my part, not difficult!, had crept
in.
Your observations are interesting, and led to a bit of head scratching.
And whilst I can not claim any insight perhaps the following may be worthy
of consideration
Despite your precautions to avoid moisture loss have you tested this? If
nothing else it would confirm the condition is constant and this factor
can be eliminated.
Have you tried wedging just clay rather than a clay body? Is the same
phenomenon found? Perhaps it is lessened, or magnified?
What are the differences between the bodies, or clays, that show the
greatest and least change?
Mechanical work into clay does have an effect, and generally includes an
increase in plasticity and green strength. This change against input is
subject to the law of diminishing returns, and hence a plateau is reached
rather than a negative result at the extreme. But what mechanism explains
your observations? Perhaps particle alignment, or due to deagglomeration a
change in packing? Dont know but its got me thinking ...
About your side note ... what is this device youre building? And why? You
mention its use on a production line ...
Regards,
Andrew
mailtoandrew@FSMAIL.NET on sat 17 apr 04
Thanks for the reply to my question Michael.
Whilst building ones own analyser is certainly laudable are you not
reinventing the wheel? Such devices already exist.
Regards,
Andrew
Michael Wendt on mon 19 apr 04
Andrew,
The idea was to design and build a device that was cheap,
easy to build and gave results that anyone anywhere could
compare and reproduce. This device can be made for $20.00
and is very sensitive so the results are consistent and accurate.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Thanks for the reply to my question Michael.
Whilst building ones own analyser is certainly laudable are you not
reinventing the wheel? Such devices already exist.
Regards,
Andrew
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