J. B. Clauson on thu 26 dec 02
I have always dried my slabs between layers of newspaper between two pieces
of plywood. The stack is weighted. I turn it as often as I can (every day,
if possible) and by "turning" I mean top to bottom replacing the damp
newspaper with dry as I go. When the clay is a consistent leather hardness,
I do my carving. I have done several bas relief fish and and inlaid scene
(different colored clays finished with china paints) with no cracking. One
this to note, when I turn the slabs, I do not pick them up. I remove the
top piece of plywood, remove the damp newspaper, lay and smooth a dry layer
of newspaper, replace the plywood, pick up two pieces of plywood with the
clay slab between, flip it over, remove the the new top layer of newspaper,
lay and smooth a dry layer of newspaper and carry on from there turning each
slab between two solid pieces of plywood in order to prevent stressing the
clay while it is drying. It's a lot of work, but I have very little
cracking - actually, the only slabs that cracked (so far, fingers crossed,
prayers of thanksgiving to all deities) were terra cotta texture tests that
I over-fired.
Jan C.
Snail Scott on thu 26 dec 02
At 12:28 PM 12/26/02 -0500, Gail wrote:
>When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
>experienced frequent and frustrating cracking problems...when I throw
porcelain platters of that size, I
>seldom have any problems...Is this an engineering phenomenon, i.e.
something to do with the unraised rim?...
I do think it has something to to with the rims (or
lack thereof). Slightly curved forms, or even flat
ones with a raised lip, have a 'direction' that
they will tend to move in, as the stresses of drying
and firing occur. Instead of the edge becoming too
small for the inner diameter and cracking, the entire
piece often becomes slightly more concave to accomodate
the smaller edge circumference without rupture. Truly
flat shapes, whether thrown or rolled or otherwise
fabricated, do not have this 'safety-release' for the
stresses of unequal shrinkage, and need lots of extra
time and care to avoid cracking.
-Snail
Gail Dapogny on thu 26 dec 02
When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
experienced frequent and frustrating cracking problems, usually in the
center. These occur sometimes toward the end of drying (no matter how
slow), but occasionally in the glaze firing instead, and now and then in
the bisque firing! These are stoneware disks, incidentally.
Yes, I always compressed carefully, and yes, I dried them slowly. Now,
having explained that---- when I throw porcelain platters of that size, I
seldom have any problems. Is this an engineering phenomenon, i.e.
something to do with the unraised rim? Would a slight foot thrown later
on the back (or the whole thing thrown upside down complete with foot)
help?
There is one other variable: when I throw the disks, I carve them at the
leatherhard stage. Is the carving pressure somehow the straw that breaks
the camel's back? If so, how to get around it?
I throw them because I am comfortable throwing. Would rolling them out be
a better solution?
---Gail
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
Valice Raffi on thu 26 dec 02
Gail,
I've carved 16" disks that I've rolled (by hand) and haven't had problems
with cracking. I used a grogged sculpture clay fired to ^6.
Valice
in Sacramento where the State budget problems have ended my years of
teaching in the prisons (they have just cancelled the contracts for about
150 artists).
>When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
>experienced frequent and frustrating cracking problems,
>
>I throw them because I am comfortable throwing. Would rolling them out be
>a better solution?
Dan Saultman on thu 26 dec 02
Hi Gail,
I do 18" plaques and have had to go through a lot of testing to solve the kinds of
problems you are experiencing.
1. I chose a clay that had more grog
2. I place my plaques on a single sheet of newspaper to absorb moisture from the
bottom of the surface while air drying from the top.
3. After a day or two I move the discs to a drying rack so that both sides get air
circulation. (discard the newspaper)
4. To avoid warping I use rings of drycleaners plastic sheeting?the rings being
about 2.5 inches wide (18" dia.) and place the ring on top while the discs dry.
5. When I fire them I sprinkle white silica sand on the kiln shelf to minimize
drag as the discs shrink. (You may need to use a granular alumina hydrate if your
temp will melt the sand)
6. I place curved strips (trimmings) of clay around the discs on the kiln shelves
when firing to serve as a heat buffer to protect the disc's edges.
I use a slab roller for the slabs, place them on round wooden bats and trim them
roughly at that time. Later when the clay jas firmed up I place the 18" wooden
bat on my potters wheel and finish trimming them.
Hope this gives you some things to try.
Dan saultman/Detroit
Gail Dapogny wrote:
> When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
> experienced frequent and frustrating cracking problems, usually in the
> center. These occur sometimes toward the end of drying (no matter how
> slow), but occasionally in the glaze firing instead, and now and then in
> the bisque firing! These are stoneware disks, incidentally.
Michael Wendt on thu 26 dec 02
Gail,
To see if the cause is the carving, leave one uncarved and see if it cracks.
I suspect you are seeing the case hardening effect that plagues me... if I
cave at the wrong time, the dry outer skin gives way to the softer, wetter
inside clay which then cracks. If the clay is a little drier, it doesn't
crack,
weird,
huh?
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Avenue
Lewiston, Idaho 83501
1-208-746-3724
wendtpottery.com
Dave Finkelnburg on thu 26 dec 02
Gail,
I am sorry to hear about the disks cracking. :-(
In my limited experience the cause is usually irregular drying in that
the rim dried faster than the center. If you "dissect" these cracked disks,
how does the disk thickness vary across the disk? If it's not uniform, then
that can be the cause by itself, because the thicker portion tends to dry
slower. OR, if the thickness is uniform but the rim dries faster, that's a
problem. Wrapping the outer three inches in plastic to slow that part's
drying may help.
IF the ware is dry enough to crack from the pressure of carving,
certainly that can cause failure. It's very hard to tell without seeing
what you are doing.
Hope these ideas help,
Dave Finkelnburg in Idaho
----- Original Message -----
From: "Gail Dapogny"
> When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
> experienced frequent and frustrating cracking problems, usually in the
> center. These occur sometimes toward the end of drying (no matter how
> slow), but occasionally in the glaze firing instead, and now and then in
> the bisque firing! These are stoneware disks, incidentally.
>
> Yes, I always compressed carefully, and yes, I dried them slowly. Now,
> having explained that---- when I throw porcelain platters of that size, I
> seldom have any problems. Is this an engineering phenomenon, i.e.
> something to do with the unraised rim? Would a slight foot thrown later
> on the back (or the whole thing thrown upside down complete with foot)
> help?
>
> There is one other variable: when I throw the disks, I carve them at the
> leatherhard stage. Is the carving pressure somehow the straw that breaks
> the camel's back? If so, how to get around it?
>
> I throw them because I am comfortable throwing. Would rolling them out be
> a better solution?
claybair on thu 26 dec 02
Hi Gail,
A few questions & comments:
How thick are your disks?
If too thin perhaps the carving causes sections to dry faster.
Do you compress both sides?
Your platters are compressed on both sides which could be why they do not
crack.
How much pressure is placed on the disk when you are carving?
When carving I support the disk/slab on a piece of very dense foam and try
to use the least amount of pressure possible.
How deeply do you carve?
I have to be pretty careful not to get too rambunctious and remember that I
am not carving a wood block!
What surface do you have the disk on when drying?
Depending on what surface you use it could be drying more on one side than
another.
Do you flip the disk over when drying?
This could even out the drying surface differences.
Can you tell which side the cracks start? Which side has the larger gap?
This could tell you where the problem is originating.
I do a lot of carving & have been using Laguna's ^6 B-mix with sand with
success. It seems pretty forgiving. I have made 3 converts to B-mix in the
past 6 months.
Recently I bought a slab roller one of my first pieces was a highly carved
tray with a slight rim. Two corners of the rim did crack but that was due to
it my cavalier drying technique & the slab being a bit too thin. It was a
very nice piece.
Didn't get a pic.... it was during my holiday madness crunch time. Since I
have been using slabs any incidence of cracking has been almost nil.
Gayle Bair- no connection to Laguna outside of being a devotee of B-mix
Bainbridge Island, WA
http://claybair.com
-----Original Message-----
From: Gail Dapogny
When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
experienced frequent and frustrating cracking problems, usually in the
center. These occur sometimes toward the end of drying (no matter how
slow), but occasionally in the glaze firing instead, and now and then in
the bisque firing! These are stoneware disks, incidentally.
Yes, I always compressed carefully, and yes, I dried them slowly. Now,
having explained that---- when I throw porcelain platters of that size, I
seldom have any problems. Is this an engineering phenomenon, i.e.
something to do with the unraised rim? Would a slight foot thrown later
on the back (or the whole thing thrown upside down complete with foot)
help?
There is one other variable: when I throw the disks, I carve them at the
leatherhard stage. Is the carving pressure somehow the straw that breaks
the camel's back? If so, how to get around it?
I throw them because I am comfortable throwing. Would rolling them out be
a better solution?
---Gail
Earl Brunner on fri 27 dec 02
Don't know if slabbing would be better than throwing, you would probably
have to experiment on that. I do know that when I throw anything with a
large flat bottom (whether it has a rim or not) I have found that if I
cover the edges and let the middle dry faster than the outside that I
seem to lose less. I don't know if this rationale is correct or not,
but I always figured that things shrink as they dry, when the outside
gets dryer first then the middle is shrinking OUT and has to give
somewhere, so you get a crack. By having the middle dryer first things
keep shrinking IN.
The rationale might be flawed, but it seems to work for me for what ever
the reason.
Gail Dapogny wrote:
> When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
> experienced frequent and frustrating cracking problems, usually in the
> center. These occur sometimes toward the end of drying (no matter how
> slow), but occasionally in the glaze firing instead, and now and then in
> the bisque firing! These are stoneware disks, incidentally.
>
> Yes, I always compressed carefully, and yes, I dried them slowly. Now,
> having explained that---- when I throw porcelain platters of that size, I
> seldom have any problems. Is this an engineering phenomenon, i.e.
> something to do with the unraised rim? Would a slight foot thrown later
> on the back (or the whole thing thrown upside down complete with foot)
> help?
>
> There is one other variable: when I throw the disks, I carve them at the
> leatherhard stage. Is the carving pressure somehow the straw that breaks
> the camel's back? If so, how to get around it?
>
> I throw them because I am comfortable throwing. Would rolling them out be
> a better solution?
>
> ---Gail
>
>
>
> Gail Dapogny
> 1154 Olden Road
> Ann Arbor, MI 48103-3005
> (734) 665-9816
> gdapogny@umich.edu
> http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
> registered with Silverhawk)
>
> ______________________________________________________________________________
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>
John Kimpton Dellow on sat 28 dec 02
I have had experience making terra cotta saucers up to 24" in
diameter.
I dry on a slatted pallet made from the wooden slats used to make
garden lattice.
Also place these as high as possible and sometimes lay fly screen
over the pallet.
This gives good circulation of air ,and being close to the
ceiling less likelihood
of draughts .
John Dellow "the flower pot man"
Home Page http://www.welcome.to/jkdellow
http://digitalfire.com/education/people/dellow/
Ron Roy on sun 29 dec 02
Hi Gail,
As others have pointed out - drying evenly could be the key here.
I would look up the explanation of "shrinking" in the Hamer book. If
paricle arrangement is more towards the outside of the disk then shrinkage
may be different because of that. When I make plates I always recompress
them from the outside in - making sure the clay at the center is well
compressed - I also leave a raised part there - because if you go past the
center with a rib it digs in. I scrape off that thick spot when the is
leather hard - just before trimming.
I think getting them upside down as soon as you can will help them to dry
out more evenly - if - as others have said - the outside dries before the
inside - the inside cannot shrink (held firm by the outside) and there fore
has to crack.
RR
>When I have thrown flat disks -- 16 - 18 inches in diameter -- I have
>experienced frequent and frustrating cracking problems, usually in the
>center. These occur sometimes toward the end of drying (no matter how
>slow), but occasionally in the glaze firing instead, and now and then in
>the bisque firing! These are stoneware disks, incidentally.
>
>Yes, I always compressed carefully, and yes, I dried them slowly. Now,
>having explained that---- when I throw porcelain platters of that size, I
>seldom have any problems. Is this an engineering phenomenon, i.e.
>something to do with the unraised rim? Would a slight foot thrown later
>on the back (or the whole thing thrown upside down complete with foot)
>help?
>
>There is one other variable: when I throw the disks, I carve them at the
>leatherhard stage. Is the carving pressure somehow the straw that breaks
>the camel's back? If so, how to get around it?
>
>I throw them because I am comfortable throwing. Would rolling them out be
>a better solution?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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