Vince Pitelka on sun 5 dec 04
> Not sure why clay dries out in plastic bags. The clay I receive from my
> supplier in those very thick plastic bags, probably at least 4 mils, will
> start to harden after a few months. When I get the 2 ton order I take
> each
> 50 pound box and put it into another plastic covering bag and it still
> hardens.
> When I wedge out say ten 5 pound balls and put them in plastic bags,
> probably 2 mils, they too will start to dry out on a few weeks. I even
> went
> so far as to blow air in the bags, sort of like a balloon, and let them
> sit
> for a month and the air stays in. Not too scientific but it is a test..
Bruce -
As you know, evaporating water creates vapor pressure, and it will excape
from anything except a hermetically-sealed vessel. But that only accounts
for long-term drying. All my intermediate and advanced students mix their
own claybodies, but we also mix stock claybodies for workshops and local
clients, and we bag them in 25 pound amounts. If the bagged clay sits
around for six months it starts to stiffen up.
Are you sure that what you are experiencing is clay drying out? When clay
sits for a period of time, the platelets settle together, with less
thickness of water layers between them. There is still just as much water
in the clay, but it tends to concentrate in larger voids, with more contact
and friction between the platelets. Thus, the clay seems much stiffer. As
soon as you drop the bag on the floor a few times, the clay becomes mobile
again, as the water is redistributed in layers between the platelets. Have
you tried that?
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Bruce Freund on sun 5 dec 04
Not sure why clay dries out in plastic bags. The clay I receive from my
supplier in those very thick plastic bags, probably at least 4 mils, will
start to harden after a few months. When I get the 2 ton order I take each
50 pound box and put it into another plastic covering bag and it still
hardens.
When I wedge out say ten 5 pound balls and put them in plastic bags,
probably 2 mils, they too will start to dry out on a few weeks. I even went
so far as to blow air in the bags, sort of like a balloon, and let them sit
for a month and the air stays in. Not too scientific but it is a test..
I live in south Florida where the humidity is always very high .
Would like to hear some opinions.
--
Best of the Best,
Bruce Freund
Michael Wendt on mon 6 dec 04
Bruce,
When we first started doing clay for sale, I too noticed the clay dried
out. I asked our bag supplier why and he told me that water vapor can pass
through low density polyethylene. Mylar is so impermeable that even helium
stays in, but no customers are willing to pay for Mylar bags so that leaves
us with poly.
Try this:
get some garbage bags, put a few pugs in along with a pan of water and seal.
That might help the moisture loss but the quick hardening you are seeing is
more likely the "setting" effect common in most clay bodies. Left
undisturbed, they seem to harden, but after wedging, are in fact soft.
My clay softness tester showed that this effect starts right after your
wedge and becomes more pronounced with additional time elapsed. It is
probably why clay is plastic and useful... apply force- clay moves freely...
stop applying force-clay freezes in new form.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
William Sheppard on mon 6 dec 04
Bruce,
I wrap my 25 pound clay loafs in a piece of water soaked bed sheet and
put it back into the plastic bag. Every few weeks I just pour a little
water over the sheet wrapped clay and it stays nice and soft.
I usually do this to a couple of boxes of clay when I get a new
shipment and then to more boxes as I use up the clay I have had wrapped up.
I experience the same humidity you have, here in St. Petersburg,
Florida, and this method works great for me.
William A. Sheppard
http://www.patyoungceramicarts.com
Ivor and Olive Lewis on tue 7 dec 04
Dear Michael Wendt,
I really like your turn of phrase <force-clay freezes in new form. >> I think this may be truer than you
realise.
We should not forget that some clay bodies depend upon the addition of
plasticisers that seem to cause thixotropy.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Cindy on wed 8 dec 04
I've been thinking about this--
I used to live in Florida. Yes, it is humid, but it's also hot, hot, hot!
Now I live in South Dakota. It is as dry as Florida is humid. Sometimes it's
even as hot. But I've always stored my clay in a damp, cool basement.
I think the plastic bags can handle cool, dry air better than hot, humid
air. Perhaps, if possible, a small window AC unit in the clay storage area
would help. Otherwise, maybe find a cooler place.
Best of luck,
Cindy in SD
Ivor and Olive Lewis on thu 9 dec 04
Let us not forget that many plastic materials are biodegradable so the
effects of bacteria and their acidic metabolites within the bag may
cause plastic to degrade and become penetrable by water vapour.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
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