Chris Schafale on sat 20 feb 99
I recently may have hit on a useful variation of a technique to stop
s-cracks in bowls thrown off the hump. In the past I've had about
50% loss rates from off-the-hump items. Last week I tried again with
some small bowls, and this time, when they were leather hard and
before trimming them, I put them back on the wheel, right side up,
and used my fingers to compress the bottom, as I normally would when
throwing. (The Giffin Grip worked great to hold them while I was
doing this). They are bone-dry now, and no cracks so far.
Chris
> ----------------------------Original message----------------------------
> 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
>
>
Light One Candle Pottery
Fuquay-Varina, NC
candle@nuteknet.com
Liz Willoughby on mon 22 feb 99
I use a cone 10 porcelain claybody, and throw a lot of forms off the hump.
Mainly small mugs, tea-bowls, teapot lids, and small bowls. I rarely get
s/cracks, so thought I would give my two cents worth here on my method of
throwing, which works for me.
I use very little water, but just moisten the clay with wet fingers or a
sponge. I do not let any water sit in the bottom of the form while
throwing. I compress the clay 2 or 3 times going out to the wall from the
bottom and back again. But what I think helps most of all, is when I am
ready to take the pot off the hump, I use a bamboo skewer held at a 45
degree angle at the point where it is to be cut off, and as the wheel is
turning cut into the clay and then gradually move the bamboo skewer
horizonally. As I am doing this, (if the shape of the pot allows), I use my
left middle finger to compress the clay on the inside bottom of the pot.
There is very little clay left to cut off with the wire, between 1/2 to 1
inch. I set aside on a wooden bat, and later cover with plastic.
Bamboo skewers make wonderful tools for all kinds of things in the studio.
Another easy way of making a good "little" trimming tool, is to make use of
those cane handles that you might have around in the studio because the
brush has fallen out of them. Split the cane in two, and trim one end into
a point.
Meticulously Loose Liz
Liz Willoughby
R.R. #1
Grafton, Ontario
K0K 2G0
e-mail lizwill@phc.igs.net
Marcia Kindlmann on tue 23 feb 99
Subject: s/cracks
Here's what I've been doing to cut small bowls & cups off the hump:
instead of using the official cutoff tool from the catalogues (that
bevel-point wooden piece with a thread on the other end) I just do the
whole cutting off with one tool. I use a flattish wooden blade that's
angled at the cutting end (another standard catalog tool) and keep
pushing it toward the center, as the wheel continues to turn, until the
little cup or bowl is just beginning to tip and fall. Then catch it as
it's tipping, drop the tool, hold bowl with both hands cupping fingers
around outside & let the turning of the wheel twist off the tiny bit of
clay that's left.
Then place on ware board, getting it to sit level with one hand while
pushing down gently at center with other, inside bowl, just enough to
get it to stick in this upright position.
I think what helps avoid s-cracks in this way of cutting off is the way
the clay gets compressed at the base by the tool pushing all the way
toward center, and that this tool is not really thin and extremely
sharp, but has an angled edge that fattens rather quickly into a blade
that's almost 1/4 inch thick. Do these words construct a picture?
This method has worked well for me for both stoneware & porcelain, in
avoiding s-cracks, along with paying attention to what others have
noted also, to compress well while throwing and to not let water sit
in the bottom.
Marcia in Guilford CT
Olivia T Cavy on wed 24 feb 99
Hello all,
I use a different method of removing pots from the hump which I find
works well. My clay is not prone to S cracks, so I can't evaluate whether
this method helps avoid S cracks, but it does assist in getting the pots
off the hump without distortion and without overly thick bottoms. You can
see exactly where you are taking the pot off the hump.
I use a kitchen butter knife, although a metal kitchen spatula of the
type used to spread dip or cake frosting might be better. With the wheel
turning very slowly, I use the cutting edge of the knife to cut the hump,
slightly (very slightly) below where I want the bottom of the pot. I make
sure the knife is level, and gently press it inward until the pot is
removed from the hump, effectively cutting off the pot. When I've cut all
the way through, the pot is resting on the knife, and I stop the wheel
and use the other hand to help balance the pot to place it on another
surface for drying.
The advantage of using a wider bladed knife is that the pot is supported
after it is cut off the hump, and just needs a very light hand to keep it
on the knife until it is resting on a larger solid surface. Clearly you
need to use a knife which is relatively rigid-- you don't want your pot
spring boarding onto the floor!
Bonnie D. Hellman
Pittsburgh, PA
work email: bdh@firstcaptl.com or oliviatcavy@juno.com
home email: mou10man@sgi.net
On Tue, 23 Feb 1999 07:47:17 EST Marcia Kindlmann
writes:
>----------------------------Original
>message----------------------------
>Subject: s/cracks
>
>Here's what I've been doing to cut small bowls & cups off the hump:
>instead of using the official cutoff tool from the catalogues (that
>bevel-point wooden piece with a thread on the other end) I just do the
>whole cutting off with one tool. I use a flattish wooden blade that's
>angled at the cutting end (another standard catalog tool) and keep
>pushing it toward the center, as the wheel continues to turn, until
>the
>little cup or bowl is just beginning to tip and fall. Then catch it
>as
>it's tipping, drop the tool, hold bowl with both hands cupping
>fingers
>around outside & let the turning of the wheel twist off the tiny bit
>of
>clay that's left.
>
>Then place on ware board, getting it to sit level with one hand while
>pushing down gently at center with other, inside bowl, just enough to
>get it to stick in this upright position.
>
>I think what helps avoid s-cracks in this way of cutting off is the
>way
>the clay gets compressed at the base by the tool pushing all the way
>toward center, and that this tool is not really thin and extremely
>sharp, but has an angled edge that fattens rather quickly into a
>blade
>that's almost 1/4 inch thick. Do these words construct a picture?
>
>This method has worked well for me for both stoneware & porcelain, in
>avoiding s-cracks, along with paying attention to what others have
>noted also, to compress well while throwing and to not let water sit
>in the bottom.
>
>Marcia in Guilford CT
>
___________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
or call Juno at (800) 654-JUNO [654-5866]
| |
|
