mel jacobson on wed 17 feb 99
it was joy having ron with us........but, he got after me about s cracks.
seems that changing clay is not the answer.....or, well, not the entire
answer.
\his theory is that when one hump throws (with any clay body), the tiny
piece on the top gets
squeezed and forms the potential for the crack.
only with good knowledge of hump throwing, and perfect drying habits
will you avoid the cracks.
it seems that i was taught the correct way to throw while in japan, and use
an inside rib of wood that compresses almost perfectly.
therefore i never get s cracks....or, only when i use white, very fine
bodies......my sandy, grog laden bodies just seem to take care of
themselves when i hump throw.
so, listen to ron about s cracks.
and slow cooling of kilns......and long bisque firings....and then insight
calc.
but, the bottom line for me is, make pots, lots of pots, and do decent glazes
on them...........i fear for people that will make concept pots with perfect
formulas....just computer images, with perfect glazes from the computer.
and we will not be able to put hot soup in them.
ron roy is a real potter, a very good potter, that makes wonderful pots...and
i agree with tony, we need a great many more ron roy pots.
ron wants us all to learn, get better at glaze and clay.
i will do my best, but it is like speaking japanese, it takes time.
mel/mn
p.s.
thanks again ron, you are a gracious man.
http://www.pclink.com/melpots
minnetonka, minnesota/ u.s.a.
stephen baxter on thu 18 feb 99
Hi:
My experience with s-cracks is that the bottom of the pot has been kept too
wet. The theory goes as follows. If the outside of the pot shrinks, which
it will do naturally if not kept in a humidified environment, you have a
cylinder of clay which holds the bottom. Think of a darning hoop as the
cylinder and the bottom as the cloth held by the hoop. As the bottom dries
it can't pull the cylinder in towards the center so the only place for the
clay to go is out toward the sides - hense an s-crack! So to eliminate
s-cracks constantly mop your bottom when you are throwing and don't let the
outside of the pot dry too quickly. You can even wrap the outside bottom
with plastic and let the pot dry from the inside. Also if your bottom is
too thick or you have a hump in the center this will make s-cracks worse,
for the same reason as above- the bottom will shrink slower than the
outside. If you want to prove the theory try this: cover the top of a pot
with plastic and put it on a non-absorbent surface in a warm dry place I
guarantee you will have an s-crack in the bottom!
Steve baxter
----------
> From: mel jacobson
> To: CLAYART@LSV.UKY.EDU
> Subject: s/cracks/ron roy
> Date: Wednesday, February 17, 1999 9:17 AM
>
> ----------------------------Original message----------------------------
> it was joy having ron with us........but, he got after me about s cracks.
> seems that changing clay is not the answer.....or, well, not the entire
> answer.
> \his theory is that when one hump throws (with any clay body), the tiny
> piece on the top gets
> squeezed and forms the potential for the crack.
>
> only with good knowledge of hump throwing, and perfect drying habits
> will you avoid the cracks.
>
> it seems that i was taught the correct way to throw while in japan, and
use
> an inside rib of wood that compresses almost perfectly.
> therefore i never get s cracks....or, only when i use white, very fine
> bodies......my sandy, grog laden bodies just seem to take care of
> themselves when i hump throw.
> so, listen to ron about s cracks.
> and slow cooling of kilns......and long bisque firings....and then
insight
> calc.
>
> but, the bottom line for me is, make pots, lots of pots, and do decent
glazes
> on them...........i fear for people that will make concept pots with
perfect
> formulas....just computer images, with perfect glazes from the computer.
> and we will not be able to put hot soup in them.
>
> ron roy is a real potter, a very good potter, that makes wonderful
pots...and
> i agree with tony, we need a great many more ron roy pots.
>
> ron wants us all to learn, get better at glaze and clay.
> i will do my best, but it is like speaking japanese, it takes time.
> mel/mn
> p.s.
> thanks again ron, you are a gracious man.
>
> http://www.pclink.com/melpots
> minnetonka, minnesota/ u.s.a.
Ron Roy on fri 19 feb 99
Right on Steve - what you describe is part of the problem - let em dry
evenly and your chances of avoiding s-cracks is much better. Getting the
ware upside down as soon as possible helps to even out the drying.
There are two other factors - when you throw the sides the clay platelets
get arranged differently than the bottom clay so shrinkage is different -
compression of the bottom clay helps this by arranging the platelets as
well.
The cracks are s shaped because the clay is twisted during the forming of
that ball on top of the hump - keep the clay well lubricated during the
forming of that ball and try not to twist it.
Many potters have to work with clay that is not ideally suited for this
type of forming - one of the most common ways of preventing this cracking
is to re compress the bottom clay by paddling it when the ware is partly
dry.
RR
>----------------------------Original message----------------------------
>My experience with s-cracks is that the bottom of the pot has been kept too
>wet. The theory goes as follows. If the outside of the pot shrinks, which
>it will do naturally if not kept in a humidified environment, you have a
>cylinder of clay which holds the bottom. Think of a darning hoop as the
>cylinder and the bottom as the cloth held by the hoop. As the bottom dries
>it can't pull the cylinder in towards the center so the only place for the
>clay to go is out toward the sides - hense an s-crack! So to eliminate
>s-cracks constantly mop your bottom when you are throwing and don't let the
>outside of the pot dry too quickly. You can even wrap the outside bottom
>with plastic and let the pot dry from the inside. Also if your bottom is
>too thick or you have a hump in the center this will make s-cracks worse,
>for the same reason as above- the bottom will shrink slower than the
>outside. If you want to prove the theory try this: cover the top of a pot
>with plastic and put it on a non-absorbent surface in a warm dry place I
>guarantee you will have an s-crack in the bottom!
>Steve baxter
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Ron Roy on fri 19 feb 99
This is for everyone and especially for Mel.
If you want to understand "gracious" spend some time with Mel - he is a master.
RR
>ron wants us all to learn, get better at glaze and clay.
>i will do my best, but it is like speaking japanese, it takes time.
>mel/mn
>p.s.
> thanks again ron, you are a gracious man.
>
>http://www.pclink.com/melpots
>minnetonka, minnesota/ u.s.a.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Burt Cohen on sun 21 feb 99
This is all good stuff but, I wanted to add other important factors in
S cracks. The first is the nature of the clay material. If your clay is
montmorillonitic in nature, the likelihood for s-cracking is greatly
increased due to the unwillingness of the clay material to allow
moisture to pass through the body of the pot. Potters who persistently
have difficulties with clays bottom cracking and uneven drying
characteristics may be dealing with a montmorillonite. Being new to
clayart, I am slowly getting a sense that there are a lot more North
Americans. By in large we are using blended clay bodies. Some on these
clays may not be kaolinitic. Second-the design of a given thrown shape
can contribute greatly to its tendency to crack. This is more difficult
to put into rules. I've seen many pots which break all the rules- and do
not crack because of thickness or rapidity of drying. I do know that
certain shapes in a given clay will consistently crack...-Burt
Cohen-Northfire Pottery-in the Sunny Okanagan Valley BC Canada
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