Dawn Isaacs on mon 25 nov 02
Acouple of students from my evening class have been rolling out clay onto
hump molds and then having to leave them to dry till the following week=92s
evening class.
We have discovered that by the time next week comes around they have
usually cracked.
I assumed that they were drying out too quickly, due to central heating and
so I suggested they lightly wrap them in plastic bags but leave end open so
the drying process will be more gradual. This they did but even so, the
same thing has happened. How do I solve this problem because if they
completely wrap =91em up they will not be dry enough to be fired by followin=
g
week?Would be grateful for your suggestions. Thanks very much and thanks
for advice about slips and engobes =96 very helpful.
L. P. Skeen on mon 25 nov 02
Dawn,
Prolly they are sticking to the plaster at some point, and this causes
stress during the drying process because the clay is shrinking. If it's
stuck, it can't shrink evenly, thus the cracks. That's my guess. Best
would be that they make arrangements to come in at some point the day after
class and remove their work from the plaster and leave it out to dry.
They're going to need to sponge down the edges anyway to remove the sharp
edges.
L
----- Original Message -----
From: "Dawn Isaacs"
To:
Sent: Monday, November 25, 2002 3:00 PM
Subject: plaster hump molds
Acouple of students from my evening class have been rolling out clay onto
hump molds and then having to leave them to dry till the following week's
evening class.
We have discovered that by the time next week comes around they have
usually cracked.
I assumed that they were drying out too quickly, due to central heating and
so I suggested they lightly wrap them in plastic bags but leave end open so
the drying process will be more gradual. This they did but even so, the
same thing has happened. How do I solve this problem because if they
completely wrap 'em up they will not be dry enough to be fired by following
week?Would be grateful for your suggestions. Thanks very much and thanks
for advice about slips and engobes - very helpful.
____________________________________________________________________________
__
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.
John Jensen on mon 25 nov 02
I'd guess these slabs are cracking because of stresses related to
shrinking over a rigid mold. In my limited experience the best way to
deal with this is to watch over the situation and remove the clay from
the mold as soon as it is firm enough to hold its shape. If you leave
it on the mold it is going to crack.
John Jensen, Mudbug Pottery
mudbug@toadhouse.com, www.toadhouse.com
Lily Krakowski on mon 25 nov 02
I probably am just ignorant, but I have never heard of actually roling the
clay once it is on the mold. I always have roled my clay out, and then
placed it on the mold. If the mold is an uneven shape a gentle push here
and there may be needed....
Also if you proceed your way try dampening the mold?
Dawn Isaacs writes:
> Acouple of students from my evening class have been rolling out clay onto
> hump molds and then having to leave them to dry till the following week’s
> evening class.
> We have discovered that by the time next week comes around they have
> usually cracked.
> I assumed that they were drying out too quickly, due to central heating and
> so I suggested they lightly wrap them in plastic bags but leave end open so
> the drying process will be more gradual. This they did but even so, the
> same thing has happened. How do I solve this problem because if they
> completely wrap ‘em up they will not be dry enough to be fired by following
> week?Would be grateful for your suggestions. Thanks very much and thanks
> for advice about slips and engobes – very helpful.
>
> ______________________________________________________________________________
> 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....
Alistair Gillies on mon 25 nov 02
Hi,
You do not say what the hump mold is made of. If it is of an absorbent
material [for example, plaster or bisque clay] then whatever you do the
mold will absorb the water from the clay during the week. This would
mean that the clay would shrink onto the mold and probably crack.
If the evening class lasts for more than an hour try drying a
plaster or bisque hump mold - put it onto a source of moderate heat for
a day, or a week!
If you put the slab onto the mold as soon as the class starts the mold
should absorb enough water from the clay for the shape to hold before
the class finishes.
I suspect that the mold is too wet to rapidly dry the clay but that it
is gradually absorbing the water before the next class.
All the best
Alistair Gillies
Shropshire, England
>> Acouple of students from my evening class have been rolling out clay onto
>> hump molds and then having to leave them to dry till the following week’s
>> evening class.
>> We have discovered that by the time next week comes around they have
>> usually cracked.
Cate Loveland on mon 25 nov 02
Clay left to dry on a hump mold will crack because clay shrinks as it drie,
10% more or less. Drying slowly won't evade the issue. It needs to come off
the mold as soon as it will hold its shape and then left to dry slowly with
special care to keep the edges covered with plastic so they won't dry out
more quickly, shrink more, and cause problems.
You might try lining the mold with strips of newspaper and putting the clay
on top of that. It might allow the clay to move up as it dries and shrinks.
I've had good luck with that in a slump mold (my aluminum wok top) but haven'
t tried it with a plaster hump mold. you will have to smooth out the marks
from the paper strips on the inside, but the extra compression would also
help keep it from cracking.
Good luck!
Ilene Mahler on mon 25 nov 02
I have been having the same problem and using wood molds and metal with
cheese clothe leave them for about 6-8 hours then cover them over night open
for a couple hours and cover again and still cracking Last year I just cover
once over night and no stress cracks at the rim in..Ilene in Conn
----- Original Message -----
From: John Jensen
To:
Sent: Monday, November 25, 2002 4:12 PM
Subject: Re: plaster hump molds
> I'd guess these slabs are cracking because of stresses related to
> shrinking over a rigid mold. In my limited experience the best way to
> deal with this is to watch over the situation and remove the clay from
> the mold as soon as it is firm enough to hold its shape. If you leave
> it on the mold it is going to crack.
>
> John Jensen, Mudbug Pottery
> mudbug@toadhouse.com, www.toadhouse.com
>
>
____________________________________________________________________________
__
> 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.
Karen Ami on mon 25 nov 02
In my classes, which are a limiting 3 hours a week, we use both hump and
slump molds. The best solution I have come up with is a blow dryer to harden
the clay enough to get it out of the mold. This way the student can speed up
waiting-to-dry time and work on the finish detaisl of the piece. If you use a
blow dryer (any cheapo hand held blow dryer works) just be careful to get the
piece too dry in one place. Hope this helps!
Karen Ami
MudhouseStudio
Chicago
Saultman on mon 25 nov 02
Hi Dawn,
The problem is that the clay is contracting as it shrinks.
The best solution is to create slump type plaster molds.
Concave forms.
Then as they shrink and contract they will not fight against the form and
they will not crack. You will not have to worry about as much plastic wrap
either although I suggest using plastic about 2 inches along the edges to
prevent warping.
When it comes to clay, slumping is better than humping!
Dan Saultman
Detroit
on 11/25/02 3:00 PM, Dawn Isaacs at dawnie.isaacs@NTLWORLD.COM wrote:
> Acouple of students from my evening class have been rolling out clay onto
> hump molds and then having to leave them to dry till the following week=92s
> evening class.
> We have discovered that by the time next week comes around they have
> usually cracked.
> I assumed that they were drying out too quickly, due to central heating a=
nd
> so I suggested they lightly wrap them in plastic bags but leave end open =
so
> the drying process will be more gradual. This they did but even so, the
> same thing has happened. How do I solve this problem because if they
> completely wrap =91em up they will not be dry enough to be fired by followi=
ng
> week?Would be grateful for your suggestions. Thanks very much and thanks
> for advice about slips and engobes =96 very helpful.
Marcia Selsor on mon 25 nov 02
Dear dawn,
If you look at my article on bird baths in this issue of PMI, I apply
Linda Blossom's method of removing slabs from hump molds (she does this
with sinks). It involves making a lip and sandwiching the lip between
two pieces of plywood. Both fit around the mold and can lift the slab
right off the mold within an hour of forming it on the hump.The bottom
piece must be in place prior to placing the slab onto the hump mold.
Marcia Selsor
>
> Dawn Isaacs writes:
>
>> Acouple of students from my evening class have been rolling out clay onto
>> hump molds and then having to leave them to dry till the following week’s
>> evening class.
>> We have discovered that by the time next week comes around they have
>> usually cracked.
>> I assumed that they were drying out too quickly, due to central
>> heating and
>> so I suggested they lightly wrap them in plastic bags but leave end
>> open so
>> the drying process will be more gradual. This they did but even so, the
>> same thing has happened. How do I solve this problem because if they
>> completely wrap ‘em up they will not be dry enough to be fired by
>> following
>> week?Would be grateful for your suggestions. Thanks very much and thanks
>> for advice about slips and engobes – very helpful.
>>
>> ______________________________________________________________________________
>> 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....
>
> ______________________________________________________________________________
> 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.
>
>
--
Tuscany in 2003
http://home.attbi.com/~m.selsor/Tuscany2003.html
claybair on mon 25 nov 02
Dang...
Someone already found my fabulous way to cut labor.
I slab roll some clay, cut out some circular pieces with a biscuit cutter,
place on my convex bisque mold, roll the clay with a textured bisque roller,
add 3 feet and I have a quick soap dish.
I remove the piece immediately which prevents cracking and speeds
production.
Note, the mold must be pretty dry to prevent sticking. I occasionally dry it
carefully with a torch though I don't think I would use a torch on a plaster
mold.
Lili- Before I got my slab roller I would place a wedged ball of clay on the
mold and got similar results. The edges might be a bit ragged but they could
be trimmed or sponged. I like the ragged look as it suits my soap dishes.
Gayle Bair
Bainbridge Island, WA
http://claybair.com
-----Original Message-----
From: No title defined [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Lily Krakowski
Sent: Monday, November 25, 2002 2:17 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: plaster hump molds
I probably am just ignorant, but I have never heard of actually roling the
clay once it is on the mold. I always have roled my clay out, and then
placed it on the mold. If the mold is an uneven shape a gentle push here
and there may be needed....
Also if you proceed your way try dampening the mold?
Dawn Isaacs writes:
> Acouple of students from my evening class have been rolling out clay onto
> hump molds and then having to leave them to dry till the following week’s
> evening class.
> We have discovered that by the time next week comes around they have
> usually cracked.
> I assumed that they were drying out too quickly, due to central heating
and
> so I suggested they lightly wrap them in plastic bags but leave end open
so
> the drying process will be more gradual. This they did but even so, the
> same thing has happened. How do I solve this problem because if they
> completely wrap ‘em up they will not be dry enough to be fired by
following
> week?Would be grateful for your suggestions. Thanks very much and thanks
> for advice about slips and engobes – very helpful.
>
>
____________________________________________________________________________
__
> 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....
____________________________________________________________________________
__
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.
Snail Scott on mon 25 nov 02
At 03:00 PM 11/25/02 -0500, you wrote:
>...rolling out clay onto
>hump molds and...next week comes around they have
>usually cracked.
>I suggested they lightly wrap them in plastic bags...
>same thing has happened.
Are they cracking because they're shrinking too
much for the mold? This can happen very early on
in the drying process. I'd have the students do
their molds early in the class session. They need
to get the clay off the molds before they leave
class that day. (If the clay isn't stiff enough
to remove by the end of class, they can hasten the
drying a bit with a hairdryer.) Shedded foam or
wadded-up newspaper can help support the form
after removal. Then wrap the edges pretty well
with a strip of plastic and wrap the whole thing
in newspaper, to give fast even drying.
-Snail
Ababi on tue 26 nov 02
There is another option. If you can remove it but it is too soft, you can put a small
napkin on top of the mold and put the ware back. It will keep the shape and give a
small space between your work and the mold.
Ababi
---------- Original Message ----------
>Clay left to dry on a hump mold will crack because clay shrinks as it drie,
>10% more or less. Drying slowly won't evade the issue. It needs to come off
>the mold as soon as it will hold its shape and then left to dry slowly with
>special care to keep the edges covered with plastic so they won't dry out
>more quickly, shrink more, and cause problems.
>You might try lining the mold with strips of newspaper and putting the clay
>on top of that. It might allow the clay to move up as it dries and shrinks.
>I've had good luck with that in a slump mold (my aluminum wok top) but haven'
>t tried it with a plaster hump mold. you will have to smooth out the marks
>from the paper strips on the inside, but the extra compression would also
>help keep it from cracking.
>Good luck!
>______________________________________________________________________________
>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.
Ababi on tue 26 nov 02
Lisa you gave the best answer!
There are some rules here:
Don't make the slab all the way down on the mold, leave place to your hands incase
you need power to push it up (and away).
Your hump has to be dry and the plaster "airy" I mean not too much plaster in the
water plaster mixture
I do it this way:
Let's say half bucket of water. I add to it plaster slowly, until the pile of plaster raises a
bit above the water.
In an old PMI there is a nice way to make light weight ( less plaster- hump molds.)
NEVER fasten the drying, the best take to the sun, to have even drying.
Watch the works, as been written before.
You can use claybodies with lower shrinkage and they will not crack.
See
http://members4.clubphoto.com/ababi306910/912566/
slide 56-60
http://members4.clubphoto.com/ababi306910/186015/
and most of my plates
Ababi Sharon
Glaze addict
Kibbutz Shoval Israel
ababisha@shoval.org.il
http://members4.clubphoto.com/ababi306910/
http://www.milkywayceramics.com/cgallery/asharon.htm
---------- Original Message ----------
>Dawn,
>Prolly they are sticking to the plaster at some point, and this causes
>stress during the drying process because the clay is shrinking. If it's
>stuck, it can't shrink evenly, thus the cracks. That's my guess. Best
>would be that they make arrangements to come in at some point the day after
>class and remove their work from the plaster and leave it out to dry.
>They're going to need to sponge down the edges anyway to remove the sharp
>edges.
>L
>----- Original Message -----
>From: "Dawn Isaacs"
>To:
>Sent: Monday, November 25, 2002 3:00 PM
>Subject: plaster hump molds
>Acouple of students from my evening class have been rolling out clay onto
>hump molds and then having to leave them to dry till the following week's
>evening class.
>We have discovered that by the time next week comes around they have
>usually cracked.
>I assumed that they were drying out too quickly, due to central heating and
>so I suggested they lightly wrap them in plastic bags but leave end open so
>the drying process will be more gradual. This they did but even so, the
>same thing has happened. How do I solve this problem because if they
>completely wrap 'em up they will not be dry enough to be fired by following
>week?Would be grateful for your suggestions. Thanks very much and thanks
>for advice about slips and engobes - very helpful.
>____________________________________________________________________________
>__
>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.
>______________________________________________________________________________
>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.
Karin Hurt on tue 26 nov 02
at school we use a Benz-O-Matic torch and dry it a little bit and then take
it off, we've had little cracking.
Karin in Arizona
www.lauhingbearpottery.com
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