iandol on sun 2 nov 03
Dear Friends,
Some time ago there was a long thread concerning the manufacture and use =
of plaster wedging and kneading surfaces.
I am seeking information about how these affect the working properties =
of clay which is prepared on this particular absorbent surface.
Have users of plaster surfaces noticed any improvement or deterioration =
in the degree of plasticity of their clay.
Accepting that removal of water from the clay will be one of the =
processes involved, is there an opinion as to whether the change differs =
from clay which might have lost water due to evaporation rather than =
contact with plaster.
Anecdotal information derived from your observations would be =
appreciated.
Thank you and best regards,
Ivor Lewis. Redhill, South Australia
Susan Setley on sun 2 nov 03
In a message dated 11/2/03 5:09:31 AM, iandol@TELL.NET.AU writes:
<< Dear Friends,
Some time ago there was a long thread concerning the manufacture and use of
plaster wedging and kneading surfaces.
I am seeking information about how these affect the working properties of
clay which is prepared on this particular absorbent surface.
Have users of plaster surfaces noticed any improvement or deterioration in
the degree of plasticity of their clay. >>
Where I throw our last shipment of buff stoneware was ... incredibly soggy. I
don't usually use it but I wanted it for some planters with a specific glaze
in mind. Even though I got some of the last bags -- it was mush.
We have a small wedging table in the studio, which I would use because it was
a little lower than the work tables and ergonomic. I just found out that it
has plaster under the canvas.
I wedged some clay, laid it out like a pancake for a while, flipped it, and
finally rewedged it. It was markedly improved. It dried the clay out more
evenly than laying it in a bisque bowl did. I wish we had a larger table like that.
The studio manager made it easily by building a low table with a raised wood
frame. he caulked the seams and poured in plaster, and leveled it. That was
all it needed, except for a canvas covering.
Milla Miller on mon 3 nov 03
Dear Ivor,
I've wedged many tons of clay , both new and recycled, on plaster and at 59
am no worse the wear from it [other than the usual age and hereditary stuff
]and clear lungs by xray and spiral ct, no tb nor asthma and found after many
other surfaces ,that it worked beautifully for ME.I loved the way my clay dried
and, yes, I felt the plasticity was inhanced and that was my experience ,not
scientific, just intuitive.I still prefer plaster bats and plaster wedging
boards.I am careful and do not use a canvas over the plaster , as I think it is
ok for the wedging ,but I believe the canvas may contribute dust.I leave the
board with either very wet clay drying or wiped clean at the end of a session.
My 2c worth,
Margaret
Allyson May on mon 3 nov 03
I built a plaster wedging bench for my studio and use it to =
wedge/prepare all of my clay prior to handbuilding or throwing. The =
wedging table is very close to my wheel so any lumps of clay or failed =
pots are just tossed up on the table. When I finish throwing I just =
wedge up the extra bits which have usually dried to throwing consistency =
again and throw one or two more pieces out of the scraps. I usually =
make large plates or platters from this clay because it is nice and soft =
and easy to flatten. I also reclaim clay on a large plaster slab. I =
have not noticed any loss of plasticity but I don't know that there has =
been any improvement of plasticity either. The plaster wedging and =
reclaiming surfaces were what I was introduced to in my first pottery =
experiences. I continued to use them because they are just an easy and =
inexpensive way for me to get things done. =20
Peace,
Allyson May
Bloomington, IN
Susan Setley on mon 3 nov 03
In a message dated 11/3/03 8:33:09 AM, AMay4@MSN.COM writes:
<< I built a plaster wedging bench for my studio and use it to wedge/prepare
all of my clay prior to handbuilding or throwing. The wedging table is very
close to my wheel so any lumps of clay or failed pots are just tossed up on the
table. When I finish throwing I just wedge up the extra bits which have
usually dried to throwing consistency again and throw one or two more pieces out
of the scraps. I usually make large plates or platters from this clay because
it is nice and soft and easy to flatten. I also reclaim clay on a large
plaster slab. I have not noticed any loss of plasticity but I don't know that
there has been any improvement of plasticity either. The plaster wedging and
reclaiming surfaces were what I was introduced to in my first pottery
experiences. I continued to use them because they are just an easy and inexpensive way
for me to get things done. >>
I have seen great improvement, especially in the porcelain we use (which may
not be "porcelain" to purists, since it isn't translucent) if it is not just
rewedged but reclaimed, and then allowed to rest for several months. It just a
joy to throw, and me, as a very middle level student potter -- I produced some
of the best pieces I've ever made, including a horsehair Raku that just made
me -- and others -- gawk. :)
Eleanor on mon 3 nov 03
Dear Ivor Lewis,
I'm not sure this answers your actual question:
I can't tell how plasticity is affected because I've never wedged on
a non-absorbent surface and thus cannot compare.
In clay classes where there were plaster benches I noticed that the
plaster was no longer level--it had been worn down from use. Where
did that plaster go? Into the clay?
I wedge on a large, smooth-surface concrete paving stone. It is
absorbent. If my clay is too soft, some of the water is absorbed,
rendering the clay "throwable". If the clay is a little too firm, I
spray the stone a bit and the clay gets a little softer--doesn't work
with very firm clay.
I have tried various pre-packaged moist clays from various companies
and wedged them as above. Each clay performed differently on the
wheel, ranging from totally unworkable (for me) to extremely
compliant. My current clay is Tuckers ^6 smooth white. It practically
throws itself. I would prefer a brown clay for (my) aesthetic
reasons, but with Tuckers ^6 brown (mid-cal I think) I couldn't get
it to move.
I think that's what you mean by plasticity. So in the end, I
attribute plasticity or lack of it, to the clay formula, not the
wedging surface.
You have always been helpful to me; I hope this is helpful to you.
Eleanor Kohler
Centerport, NY
John Rodgers on mon 3 nov 03
I have been using a plaster table for wedging and reclaiming clay scrap
or a long time. I have used both a canvas covered table and also worked
directly on the plaster. My preference is to work directly on the
plaster, although more care is required so as not to damage the plaster
and maintain a smooth surface. As for getting plaster in the clay, it
has never been a problem. Just never happens. I also prefer and use
plaster bats. I have chosen to use the Pure and Simple batmaking system
for making my bats. Works slick as a charm. What I really like about
plaster bats is the self releasing characteristic. When I am finished
with a piece, I do not bother with cutting it off. I simply set it
aside, and after a bit enough water is pulled from the clay as to
release the clay from the bat. I like to throw with a thicker bottom,
and sides that are thicker at the bottom than the top. The plaster helps
speed water removal and drying that is a problem when using a plastic bat.
For reclaiming clay scrap I throw the dried scraps into a plastic bag,
then crush it all up with a big hammer. Next the scrap is shaken into a
bucket of water to slake and it is covered. After a week, sufficient
water is usually added to make a slurry, by beatting the mix with a big
heavy duty paint mixer. The the slurry is poured out over the weding
table. My slurry is not a wet slurry but a drier slurry, so it does not
run all over. I spread it around, and let it begine to dry. As it
solidifies into a sheet, I turn it over and press it down. After a bit
-- I check it regularly -- it dries to a consistency that allows me to
begin wedging.
My $0.02!
John Rodgers
Chelsea, AL
Allyson May wrote:
>I built a plaster wedging bench for my studio and use it to wedge/prepare all of my clay prior to handbuilding or throwing. The wedging table is very close to my wheel so any lumps of clay or failed pots are just tossed up on the table. When I finish throwing I just wedge up the extra bits which have usually dried to throwing consistency again and throw one or two more pieces out of the scraps. I usually make large plates or platters from this clay because it is nice and soft and easy to flatten. I also reclaim clay on a large plaster slab. I have not noticed any loss of plasticity but I don't know that there has been any improvement of plasticity either. The plaster wedging and reclaiming surfaces were what I was introduced to in my first pottery experiences. I continued to use them because they are just an easy and inexpensive way for me to get things done.
>Peace,
>Allyson May
>Bloomington, IN
>
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iandol on tue 4 nov 03
Dear Eleanor,
Thank you for your observations.
The wear on the plaster surface is interesting. I do not recall this =
being mentioned by anyone else. I wonder if it is a common occurrence?
Your other observation about water being adsorbed back into clay from a =
dampened concrete slab is interesting since it supports my supposition =
that this might happen when clay is worked on plaster slabs.
Your help is appreciated as it provides additional insights into the =
science of our most precious material.
Best regards,
Ivor Lewis. Redhill, South Australia
iandol on tue 4 nov 03
Dear John Rodgers,=20
Thanks for those notes.
Do you wedge on a Plaster surface. The information I seek relates to =
that process. I wish to know if people notice any difference in =
comparison to kneading on wood or concrete or slate. I wish to confirm =
or dispute that contact with Calcium Sulphate during kneading and =
wedging leads to a an improvement in the plastic quality of a clay. I =
suppose in colloquial terms, does a clay wedged on Plaster increase in =
"Fatness" or does it become "Short"
Best regards,
Ivor Lewis. Redhill, South Australia
iandol on tue 4 nov 03
Dear Margaret Miller=20
Thank you your observations.
It is this perception that although the clay is getting drier, =
plasticity is improving which intrigues me. Working with the theory =
given in Yvonne Hutchinson Cuffs (p 18) book about "Ion Exchange" has =
prompted some ideas which might provide a scientific explanation..
I am wondering if water which is absorbed by plaster from wet clay =
dissolves micro amounts of calcium sulphate. If it does Calcium sulphate =
could be transferred back to the clay. Being in solution it would =
displace any Sodium which might have contaminated the clay. If it does =
work like that I can understand why plasticity improves.
Might be intuitive but it was a good 2 Cs worth. Again, thanks!
Best regards,
Ivor Lewis. Redhill, South Australia=20
Logan Oplinger on thu 6 nov 03
On Tue, 4 Nov 2003 17:32:58 +1030, iandol wrote:
>Dear Margaret Miller
>
>Thank you your observations.
>
>It is this perception that although the clay is getting drier, plasticity
is improving which intrigues me. Working with the theory given in Yvonne
Hutchinson Cuffs (p 18) book about "Ion Exchange" has prompted some ideas
which might provide a scientific explanation..
>
>I am wondering if water which is absorbed by plaster from wet clay
dissolves micro amounts of calcium sulphate. If it does Calcium sulphate
could be transferred back to the clay. Being in solution it would displace
any Sodium which might have contaminated the clay. If it does work like
that I can understand why plasticity improves.
>
>Might be intuitive but it was a good 2 Cs worth. Again, thanks!
>
Hello Ivor,
I had an idea this was where you were going with your original post, after
reflecting back to the topic of calcium sulphate contaminating clay from
municipal or ground water supplies.
I suspect that plaster wedging tables may contribute small amounts of
plaster micro particles to the clay from the abrasion of the plaster
surface, especially if the clay contains grog. These particles could then
contribute Ca++ & SO4-- ions to the clay. If a clay body already contains
a proportionately higher percentage of Ca++ ions to Na+ ions, then this may
not be a concern, unless the clay has a reserve capacity to absorbe more
Ca++ ions. Also, it may be that if a clay body has enough Na+ ions
present, then the amount of time a ball of clay is wedged on the plaster
may not be enough to allow for enough Ca++ ions to be absorbed to alter the
plasticity.
I do have another concern, that of the migration of soluble salts from the
clay body into the plaster causing the plaster pores to to become clogged
with deposited salts, and reducing the absorption of the plaster (similar
to floculants/defloculants added to casting slips). I have seen one or two
instances where plaster wedging surfaces take on a kind of patina after
some years of service, and wonder if this may be happening, especially if
some clay bodies may have been adjusted for plasticity by the addition of
soluble salts. Also, if the water supply contributes a high amount of
soluble salts, then these may find their way into the plaster from the clay
body. Here we have a very high contribution of Ca++ ions as a bicarbonate
(HCO3-) in our water from our limestone aquifer.
All of this is very intriguing and I wish I had the time and resources to
properly investigate.
Logan Oplinger
Another Tropical Island
iandol on fri 7 nov 03
Dear Logan Oplinger,=20
In the theory proposed by Y H-C, Calcium ions will displace Sodium ions =
from clay though she does not say that the cations must form soluble =
salts with competing anions. If this it true then ion exchange will =
decrease the degree of deflocculation and a flabby clay will regain its =
plasticity when monovalent ions are replaced by polyvalent ions. This is =
why Epsom salts are so useful as a glaze flocculant.
Sorry, can't help with the idea of caused of deterioration of Plaster =
wedging tables. I only use plaster slabs for dewatering. Use wood for =
wedging and kneading.
Best regards,
Ivor Lewis. Redhill, South Australia.
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