Karen Shapiro on mon 26 aug 02
Hi folks,
I'm hoping to get some help here. I'm having a big problem with cracking. I work with slabs, am very good at construction, but can't seem to succeed with a certain shape. It's a large (14") slab bottom, either round or squared off, with a wall around it. (picture a flat tin or jar lid, which is what it looks like). The wall is from 1-1/2" high to around 3" high, and is attached to the flat base. I have done this piece successfully in the past without problems, now it cracks at the joint (not the join to the base, but the join of the strip). I've tried drying slowly, firing slowly, firing base down, firing base up. Am ready to pull out my hair! Any suggestions?
thanks,
Karen in Gualala
---------------------------------
Do You Yahoo!?
Yahoo! Finance - Get real-time stock quotes
Donn Buchfinck on mon 26 aug 02
In a message dated 8/26/2002 9:12:26 AM Pacific Standard Time,
rakukaren@YAHOO.COM writes:
> Karen
first of all, questions need to be answered. what kind of clay are you using,
do you mix it yourself or do you buy commercial.
actually it comes down to that, if you mix your own, a mistake has been made.
if you buy commercial then, well, er, I don't want to besmirtch the producers
of commercial clays, but at times mistakes are made there also.
my advice is to take your clay and wedge into it some 60mesh sand, a good
amount.
or if you have a reclaim bucket and it is all wet, pour some sand into the
wet mixture before you but it out to dry.
say a 5 gaooln bucket holds about 50lbs of wet clay, realy wet, well then
dump in 5 lbs of 60 mesh sand. use the 60 mesh, then you get the larger
particle size and not the fine stuff that takes away from plasticity.
This should fix the problem
Donn Buchfinck
San Francisco
Snail Scott on mon 26 aug 02
At 09:47 AM 8/26/02 -0700, you wrote:
>The wall is from 1-1/2" high to around 3" high, and is attached to the
flat base. I have done this piece successfully in the past without
problems, now it cracks at the joint (not the join to the base, but the
join of the strip).
Do you overlap the clay, or just butt the ends together?
A long shallow-bevelled overlap can really help get rid
of the weak spot at the joint. If I'm unclear, look at
how wood is overlapped when carpenters lay a trim strip
like a baseboard and need to join two pieces. (They do
it that way for different reasons; I only mention it to
describe the geometry of the angled overlap.)
-Snail
Marshall Kregel on mon 26 aug 02
Karen says: "I have done this piece successfully in the past without
problems, now it cracks at the joint (not the join to the base, but the join
of the strip)."
Karen, Then you have to tell us what has changed. Have you changed clay?
Kiln? Kiln shelves?
Do you roll the slab but extrude the walls? Are the walls much thicker than
the base? Thinner? Are you firing the piece across two kiln shelves instead
of one whole shelf?
Do you use sand under the slab so the piece slides as it heats or cools? Are
the walls of the piece too close to the walls of the kiln? Maybe on just one
side?
If you are adding colored slips to the walls of the piece while leather
hard, then it is possible that minute cracks are forming in the wall before
the piece even makes it to bisque. Cracks that may be hard to see but are
already there.
Are you using the exact same clay for both base and walls? A change in the
shrinkage coefficient between 2 different types of clay would cause cracks.
Furthermore, if the base and walls are not done from the same batch, at the
same time, and attached with identical water density, you can have cracks
form either in the wall joint, or along the base, especially in broad forms
such as this.
The reason I ask is because there are a lot of coefficients working here.
Even more than a broadly thrown and shallow bowl.
Heating and cooling dynamics have sharp contrasts when the piece of work is
large, shallow, of multiple joints, and with parts having different
thicknesses.
I know some excellent potters in the Dallas area that do beautiful works in
sculpture, thrown, thrown and altered, but...develop cracks in their walls
or slabs when doing the type of construction you are talking about. I am
anxious to hear what other people might have to say in this thread.
Marshall
Dallas
Karen Shapiro on mon 26 aug 02
Hi Marshall,
No changes ... same clay, same technique, same hands! And, I do use the same clay and thickness for both parts. The only change is that it's hotter now than in the winter -- which is why I tented it with plastic to slow dry the piece. My electric kiln has half-shelves, so all large pieces fire on 2 shelves (will look for one-piece shelf next time I'm at the supply joint). I use vermiculite under my larger pieces for "wiggle room".
I'm anxious for responses too -- one friend of mine suggested a "mortar/tenon" type join, either cut-in squares or triangles. I'll be trying that along with much longer beveling (I already bevel all joins).
Wish me luck and thanks for the input.
Karen
---------------------------------
Do You Yahoo!?
Yahoo! Finance - Get real-time stock quotes
iandol on tue 27 aug 02
Dear Donn Buchfinck=20
You say "..use the 60 mesh, then you get the larger particle size and =
not the fine stuff that takes away from plasticity. This should fix the =
problem.."
Would you please explain what is going on here. Why does the level of =
plasticity fall if you use sand which has got a smaller mesh size but =
not a larger mesh size?
Best regards,
Ivor Lewis. Redhill, South Australia
Lily Krakowski on tue 27 aug 02
Frans Wildenhain used to blame a lot of bad things happening to good pots on
drafts. So if in this heat you have fans on--and possibly the AIR is drier
than in winter--if you have dehumidifiers etc.
But rather than play Sherlock Holmes; I paint trouble spots with wax. Just
liquid wax. I have found plastic tents wonderful, but they hold moisture,
and then dribble. Invariably on the most delicate part of my sculpture! I
prefer cloches made of styrofoam insulation board, gleaned from construction
sites,(cheap as always) and held together with duct tape. They seem to not
get all drippy on the inside.
HOWEVER; covering trouble spots with wax really seems to help. Hope it work
for you.
PS I do not think the Two Shelf problem is it, unless the joint is right
over that crack. In which case a "current" might do it. I also might
suggest that you mix up a small batch of the mix used for making shelf
supports--make it into tiles, and set your pots on those. I have found this
mini-shelf helps with the "big pot straddling two half shelves"problem.
Karen Shapiro writes:
> Hi Marshall,
>
> No changes ... same clay, same technique, same hands! And, I do use the same clay and thickness for both parts. The only change is that it's hotter now than in the winter -- which is why I tented it with plastic to slow dry the piece. My electric kiln has half-shelves, so all large pieces fire on 2 shelves (will look for one-piece shelf next time I'm at the supply joint). I use vermiculite under my larger pieces for "wiggle room".
>
> I'm anxious for responses too -- one friend of mine suggested a "mortar/tenon" type join, either cut-in squares or triangles. I'll be trying that along with much longer beveling (I already bevel all joins).
>
> Wish me luck and thanks for the input.
>
> Karen
>
>
>
> ---------------------------------
> Do You Yahoo!?
> Yahoo! Finance - Get real-time stock quotes
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
william schran on tue 27 aug 02
Karen - You write about slab joint cracking "with the wall around
it". I assume this means you are wrapping the slab wall around the
base. Given shrinkage and movement issues, you should attach the slab
wall on top of the base, not around the edge. This way all will
shrink together and be less prone to cracking/separation.
Bill
>Hi folks,
>
>I'm hoping to get some help here. I'm having a big problem with
>cracking. I work with slabs, am very good at construction, but can't
>seem to succeed with a certain shape. It's a large (14") slab
>bottom, either round or squared off, with a wall around it. (picture
>a flat tin or jar lid, which is what it looks like). The wall is
>from 1-1/2" high to around 3" high, and is attached to the flat
>base. I have done this piece successfully in the past without
>problems, now it cracks at the joint (not the join to the base, but
>the join of the strip). I've tried drying slowly, firing slowly,
>firing base down, firing base up. Am ready to pull out my hair! Any
>suggestions?
>
>thanks,
>
>Karen in Gualala
>
>
>
>---------------------------------
>Do You Yahoo!?
>Yahoo! Finance - Get real-time stock quotes
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
bob huskey on wed 28 aug 02
> I'm hoping to get some help here. I'm having a big problem with
cracking. > Karen in Gualala
Karen, try putting the slabs together with defloculated slip of the same
body as the slabs are made of.
And, try the Mountain Horse brush from Oriental Art Supply
http://www.ningyeh.com/oas/
Bob Huskey - Tallahassee,Fl.
| |
|
