Antoinette Badenhorst on mon 11 aug 08
Judy, for the sake of other potters that might be interested in this information, I repeat part of my
e-mail to you on Clayart:
There are 2 kinds of "s" cracks across the bottoms of pots, but I am sure you know
that very well after 30 years of potting. I do want to give you info from Harry Frazer's book about
the "s" crack. "Ceramic their faults and remedies" though ( and that is the
reason why I asked you if you had an injury of something that you had to adjust
your BODY lately)
He suggests the pressure of clay in the bottom( which I am
sure you do), but then he also suggests "if the crack is wider at the base of
the piece ( than on the inside, upper surface) throwing the clay down more
firmly onto the wheel head before throwing may help."
Another thing that I 've learned from personal experience is that some clays
needs more compressing after the trimming process than others. Also some
trimming tools are more unforgiving on clay than others.
All the best.
--
Antoinette Badenhorst
www.clayandcanvas.com
www.studiopottery.co.uk
Mike Gordon on mon 11 aug 08
Antoinette wrote..... from Harry Frazer....
He suggests the pressure of clay in the bottom( which I am
sure you do), but then he also suggests "if the crack is wider at the
base of
the piece ( than on the inside, upper surface) throwing the clay down
more
firmly onto the wheel head before throwing may help."
Did he mention what to do if you throw off the mound? We used to call
them surface cracks since they didn't go all the way through the bottom
of the pot. I get them from time to time, I used to compress the bottom
after trimming, I guess I will have to get back to that habit. Mike
Gordon
Antoinette Badenhorst on mon 11 aug 08
Unfortunately not Mike, but he do talk about "orientating the surface layers" and then he talks about the "less orientated areas underneath" I guess throwing off the hump will fall into that category.
What I found when I throw off the hump ( I am not too good with that since I do that maybe once a year) is that I leave the bottoms too thick. That is also a cause for cracks in the bottom. Even walls and bottoms are so important, specifically as one goes bigger.
--
Antoinette Badenhorst
www.clayandcanvas.com
www.studiopottery.co.uk
-------------- Original message ----------------------
From: Mike Gordon
> Antoinette wrote..... from Harry Frazer....
> He suggests the pressure of clay in the bottom( which I am
> sure you do), but then he also suggests "if the crack is wider at the
> base of
> the piece ( than on the inside, upper surface) throwing the clay down
> more
> firmly onto the wheel head before throwing may help."
>
> Did he mention what to do if you throw off the mound? We used to call
> them surface cracks since they didn't go all the way through the bottom
> of the pot. I get them from time to time, I used to compress the bottom
> after trimming, I guess I will have to get back to that habit. Mike
> Gordon
Ivor and Olive Lewis on tue 12 aug 08
My suggestion would be to read that portion of Frank Hamer's
Dictionary concerned with Cracks. About nine pages of text and
illustrations.
One fact I did find out from looking at cut sections of pots that had
been thrown with coloured markers in strategic places is that working
the bottom does unexpected things. The usual description of
"compressing", that is applying a considerable downward pressure which
squeezes clay between a tool, finger or thumb and the wheelhead or bat
would not account for flow patterns that are created as the clay moves
towards or away from the centre of a rotating pot.
When clay is swept from the periphery to the centre of rotation the
combined effect of pressure, rotation and lateral motion causes
shearing in the clay as it circulates.This destroys the original flow
patterns that develop when clay is kneaded, pugged or wedged. Rather
than "orientating the surface layers" the patterns I observed seemed
to suggest their total disruption. This was documented in an early
PMI.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lee Love on tue 12 aug 08
On Tue, Aug 12, 2008 at 7:18 AM, Antoinette Badenhorst
wrote:
> What I found when I throw off the hump ( I am not too good with that since I do that maybe once a year) is
>that I leave the bottoms too thick. That is also a cause for cracks in the bottom. Even walls and bottoms are
>so important, specifically as one goes bigger.
You can compress things off the hump just before you trim them. Put
them on the wheel and use a rib on the inside to compress.
Clay does make a huge difference. Mashiko clay is very sandy
and never "S" cracks. It also has a large degree of particle size
variation.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
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