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crack on INSIDE of bottom of pot



 
 
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  #1  
Old June 1st 04, 12:18 AM
dkat
external usenet poster
 
Posts: n/a
Default crack on INSIDE of bottom of pot

I have had this happen now to at least 4 pieces in the last couple of months
and I have never had it happen to me before (in a few decades). Has anyone
ever had cracks on the inside of a pot on the bottom? Someone asked if I
had let water accumulate in the bottom and that could have happened to the 2
tall cylinders I did but not the little dish. I was using recycled clay
and in fact have recently been trying to throw 'dry'. The crack is the
shape of a backward 'C' (actually looks more like an ear) with a small crack
coming off of the top of the curve. I don't remember what the other cracks
looked like. The outside is perfectly fine and this is the only flaw in the
piece (the walls are uniform in thickness both on the sides and the bottom,
there is very little if any foot, the thickness of the piece is about 3/8ths
at most). Actually it is uniform everywhere except for the inside of the
cracked area and this seems to have something of a rise to it that was not
there after the bisque fire. Almost like a lump. The dish bottom is about
2.5". The crack is about 1".


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  #2  
Old June 1st 04, 08:04 AM
wayneinkeywest
external usenet poster
 
Posts: n/a
Default


"dkat" wrote in message
et...
I have had this happen now to at least 4 pieces in the last couple of

months
and I have never had it happen to me before (in a few decades). Has

anyone
ever had cracks on the inside of a pot on the bottom? Someone asked if I
had let water accumulate in the bottom and that could have happened to the

2
tall cylinders I did but not the little dish. I was using recycled clay
and in fact have recently been trying to throw 'dry'. The crack is the
shape of a backward 'C' (actually looks more like an ear) with a small

crack
coming off of the top of the curve. I don't remember what the other

cracks
looked like. The outside is perfectly fine and this is the only flaw in

the
piece (the walls are uniform in thickness both on the sides and the

bottom,
there is very little if any foot, the thickness of the piece is about

3/8ths
at most). Actually it is uniform everywhere except for the inside of the
cracked area and this seems to have something of a rise to it that was not
there after the bisque fire. Almost like a lump. The dish bottom is

about
2.5". The crack is about 1".

DKat,

Most common causes of bottom cracking a

--excessive water left on the bottom while throwing,

--insufficient compression while throwing

--unequal drying, which causes stresses in the
claybody (and may not be noticed until the expansion
caused by the heat of firing.)

Cracks (can/will) usually show up on the inside for
each of these reasons.

If your entire bottom is cracked through, I'd venture a
guess that you dropped it g

From what you described, I would think that it is a
stress related crack. Have you done any testing, such
as throwing the identical pieces, one after the other,
using recycled clay for one, and new for the other?
That should rule out the clay body if neither or both crack.
If only the recycled cracks, I would check your recycling method.

If you have a "lump" where it cracked, and this is a bisque firing,
take a hammer to the piece and see if there is an air pocket or
evidence of bloating indicating over-firing. You might want to fire those
pieces on their rims next time, to even out the heat, or place the piece on
some wadding to keep the bottom off the shelf.

Hope that helps,
--
Wayne Seidl
Key West, Florida, USA
North America, Terra
Latitude 81.45W, Longitude 24.33N
Elevation 3.1 feet (1m)


  #3  
Old June 4th 04, 04:23 AM
sandi
external usenet poster
 
Posts: n/a
Default

"wayneinkeywest" wrote in message ...
"dkat" wrote in message
et...
I have had this happen now to at least 4 pieces in the last couple of

months
and I have never had it happen to me before (in a few decades). Has

anyone
ever had cracks on the inside of a pot on the bottom? Someone asked if I
had let water accumulate in the bottom and that could have happened to the

2
tall cylinders I did but not the little dish. I was using recycled clay
and in fact have recently been trying to throw 'dry'. The crack is the
shape of a backward 'C' (actually looks more like an ear) with a small

crack
coming off of the top of the curve. I don't remember what the other

cracks
looked like. The outside is perfectly fine and this is the only flaw in

the
piece (the walls are uniform in thickness both on the sides and the

bottom,
there is very little if any foot, the thickness of the piece is about

3/8ths
at most). Actually it is uniform everywhere except for the inside of the
cracked area and this seems to have something of a rise to it that was not
there after the bisque fire. Almost like a lump. The dish bottom is

about
2.5". The crack is about 1".

DKat,

Most common causes of bottom cracking a

--excessive water left on the bottom while throwing,

--insufficient compression while throwing

--unequal drying, which causes stresses in the
claybody (and may not be noticed until the expansion
caused by the heat of firing.)

Cracks (can/will) usually show up on the inside for
each of these reasons.

If your entire bottom is cracked through, I'd venture a
guess that you dropped it g

From what you described, I would think that it is a
stress related crack. Have you done any testing, such
as throwing the identical pieces, one after the other,
using recycled clay for one, and new for the other?
That should rule out the clay body if neither or both crack.
If only the recycled cracks, I would check your recycling method.

If you have a "lump" where it cracked, and this is a bisque firing,
take a hammer to the piece and see if there is an air pocket or
evidence of bloating indicating over-firing. You might want to fire those
pieces on their rims next time, to even out the heat, or place the piece on
some wadding to keep the bottom off the shelf.

Hope that helps,


DKat,
Have you swithced clay bodies? I have found that some
clays are more prone to cracked bottoms. Standard #112 is one. I use
it though because I love the effects I get with certain glazes. I
haven't had near as many cracks with it since I started being extra
careful to compress the bottom really well while making the piece and
then also when trimming it. If it has no foot ring I still invert it
and go over the bottom well with a hard rubber rib or a wooden rib.
It seems to help to keep if from getting cracks.
Sandi
  #4  
Old June 9th 04, 07:23 PM
D Kat
external usenet poster
 
Posts: n/a
Default

112 is the clay body in question and these pieces did not have feet. How is
it that you compress with trimming and how dry are they when you do this
(I'm having a hard time imaging this since you are pressing down on air if
you are pushing with a rib when the piece is upside down)?

I have always been careful about compressing with pieces I throw off the
hump and I am always careful to compress 'down/backandforth' when opening
any piece. I guess then that with this clay body need I compress 'in' on
the bottom as I would with a piece off the hump? I think I can get into the
habit of doing that. They were just such odd cracks (not S cracks, not in
the center and on the inside of the pot rather than the outside), I dry
slowly in a damp closet and I was throwing dry so I did not have a clue what
the issue was. Thank you for the info and time (both posters). It is
appreciated.


"sandi" wrote in message
om...
"wayneinkeywest" wrote in message

...
"dkat" wrote in message
et...
I have had this happen now to at least 4 pieces in the last couple of

months
and I have never had it happen to me before (in a few decades). Has

anyone
ever had cracks on the inside of a pot on the bottom? Someone asked

if I
had let water accumulate in the bottom and that could have happened to

the
2
tall cylinders I did but not the little dish. I was using recycled

clay
and in fact have recently been trying to throw 'dry'. The crack is

the
shape of a backward 'C' (actually looks more like an ear) with a small

crack
coming off of the top of the curve. I don't remember what the other

cracks
looked like. The outside is perfectly fine and this is the only flaw

in
the
piece (the walls are uniform in thickness both on the sides and the

bottom,
there is very little if any foot, the thickness of the piece is about

3/8ths
at most). Actually it is uniform everywhere except for the inside of

the
cracked area and this seems to have something of a rise to it that was

not
there after the bisque fire. Almost like a lump. The dish bottom is

about
2.5". The crack is about 1".

DKat,

Most common causes of bottom cracking a

--excessive water left on the bottom while throwing,

--insufficient compression while throwing

--unequal drying, which causes stresses in the
claybody (and may not be noticed until the expansion
caused by the heat of firing.)

Cracks (can/will) usually show up on the inside for
each of these reasons.

If your entire bottom is cracked through, I'd venture a
guess that you dropped it g

From what you described, I would think that it is a
stress related crack. Have you done any testing, such
as throwing the identical pieces, one after the other,
using recycled clay for one, and new for the other?
That should rule out the clay body if neither or both crack.
If only the recycled cracks, I would check your recycling method.

If you have a "lump" where it cracked, and this is a bisque firing,
take a hammer to the piece and see if there is an air pocket or
evidence of bloating indicating over-firing. You might want to fire

those
pieces on their rims next time, to even out the heat, or place the piece

on
some wadding to keep the bottom off the shelf.

Hope that helps,


DKat,
Have you swithced clay bodies? I have found that some
clays are more prone to cracked bottoms. Standard #112 is one. I use
it though because I love the effects I get with certain glazes. I
haven't had near as many cracks with it since I started being extra
careful to compress the bottom really well while making the piece and
then also when trimming it. If it has no foot ring I still invert it
and go over the bottom well with a hard rubber rib or a wooden rib.
It seems to help to keep if from getting cracks.
Sandi



  #5  
Old June 10th 04, 03:52 AM
sandi
external usenet poster
 
Posts: n/a
Default

"D Kat" wrote in message ...
112 is the clay body in question and these pieces did not have feet. How is
it that you compress with trimming and how dry are they when you do this
(I'm having a hard time imaging this since you are pressing down on air if
you are pushing with a rib when the piece is upside down)?

I have always been careful about compressing with pieces I throw off the
hump and I am always careful to compress 'down/backandforth' when opening
any piece. I guess then that with this clay body need I compress 'in' on
the bottom as I would with a piece off the hump? I think I can get into the
habit of doing that. They were just such odd cracks (not S cracks, not in
the center and on the inside of the pot rather than the outside), I dry
slowly in a damp closet and I was throwing dry so I did not have a clue what
the issue was. Thank you for the info and time (both posters). It is
appreciated.


"sandi" wrote in message
om...
"wayneinkeywest" wrote in message

...
"dkat" wrote in message
et...
I have had this happen now to at least 4 pieces in the last couple of

months
and I have never had it happen to me before (in a few decades). Has

anyone
ever had cracks on the inside of a pot on the bottom? Someone asked

if I
had let water accumulate in the bottom and that could have happened to

the
2
tall cylinders I did but not the little dish. I was using recycled

clay
and in fact have recently been trying to throw 'dry'. The crack is

the
shape of a backward 'C' (actually looks more like an ear) with a small

crack
coming off of the top of the curve. I don't remember what the other

cracks
looked like. The outside is perfectly fine and this is the only flaw

in
the
piece (the walls are uniform in thickness both on the sides and the

bottom,
there is very little if any foot, the thickness of the piece is about

3/8ths
at most). Actually it is uniform everywhere except for the inside of

the
cracked area and this seems to have something of a rise to it that was

not
there after the bisque fire. Almost like a lump. The dish bottom is

about
2.5". The crack is about 1".

DKat,

Most common causes of bottom cracking a

--excessive water left on the bottom while throwing,

--insufficient compression while throwing

--unequal drying, which causes stresses in the
claybody (and may not be noticed until the expansion
caused by the heat of firing.)

Cracks (can/will) usually show up on the inside for
each of these reasons.

If your entire bottom is cracked through, I'd venture a
guess that you dropped it g

From what you described, I would think that it is a
stress related crack. Have you done any testing, such
as throwing the identical pieces, one after the other,
using recycled clay for one, and new for the other?
That should rule out the clay body if neither or both crack.
If only the recycled cracks, I would check your recycling method.

If you have a "lump" where it cracked, and this is a bisque firing,
take a hammer to the piece and see if there is an air pocket or
evidence of bloating indicating over-firing. You might want to fire

those
pieces on their rims next time, to even out the heat, or place the piece

on
some wadding to keep the bottom off the shelf.

Hope that helps,


DKat,
Have you swithced clay bodies? I have found that some
clays are more prone to cracked bottoms. Standard #112 is one. I use
it though because I love the effects I get with certain glazes. I
haven't had near as many cracks with it since I started being extra
careful to compress the bottom really well while making the piece and
then also when trimming it. If it has no foot ring I still invert it
and go over the bottom well with a hard rubber rib or a wooden rib.
It seems to help to keep if from getting cracks.
Sandi

To compress the bottom while trimming a bowl...After I trim the
footring and shape the sides of the bowl the way I want it, I take a
wooden rib and go inside the footring on the bottom of the bowl and
just go back and forth over the area inside the footring. If it is a
very small area I use the rounded curved end of the wooden tool. You
know the one that is indispensable for cutting away the excess clay at
the bottom of the pot before running the cutoff wire under it. Your
pot should be almost leather hard to do the compression on the bottom.
The rib makes a nice smooth bottom. It almost looks burnished.
Sandi
  #6  
Old June 10th 04, 04:09 AM
dkat
external usenet poster
 
Posts: n/a
Default


"sandi" wrote in message To compress the bottom
while trimming a bowl...After I trim the
footring and shape the sides of the bowl the way I want it, I take a
wooden rib and go inside the footring on the bottom of the bowl and
just go back and forth over the area inside the footring. If it is a
very small area I use the rounded curved end of the wooden tool. You
know the one that is indispensable for cutting away the excess clay at
the bottom of the pot before running the cutoff wire under it. Your
pot should be almost leather hard to do the compression on the bottom.
The rib makes a nice smooth bottom. It almost looks burnished.
Sandi


Thanks. I will without question do this from now on with 112. If a crack
shows up after the bisque, I can easily deal with it but to think you have a
sound piece, put in the glazing and firing to then lose it is somehow insult
to injury.


  #7  
Old June 10th 04, 12:41 PM
wayneinkeywest
external usenet poster
 
Posts: n/a
Default

Regarding 112 Standard...
This appeared today on the Clayart listserve, and bears reading.
I am copying it and pasting it he

(Begin copied message)
Hi to all 112 users,
To continue my saga of 112 problems let me tell you the latest.

I did send clay samples to Ron Roy on my own without Standard sending them.
Ron was gracious to test it for me and to take the time to call and discuss
the findings with me. His dialometer tests showed a marked increase in
cristobolite in the cone 6 body. This is not supposed to be there and
causes cracking and shivering of glazes. I have had an abnormal amount of
pieces splitting in two after several days of coming out of the glaze
firing, some shivering of glaze off the edges of pieces and on any place the
glaze may break of over relief and have had a lot of loss due to cracking of
green ware at the bone dry stage (may be an unrelated problem, not sure) and
sharp metallic looking growths growing out of the glazed fired ware (
definite fireclay problem). Any way Ron was worried about using this clay
for dinnerware sets because of the chance of glaze shivering off and being
ingested by customers. Boy did this open my eyes.

I shot off a letter to Standard and copied to Ron and to my clay supplier.
I did not get a direct response from Standard but did through my supplier
who was in contact with Standard. It is from this conversation that I
learned Standard is sending new clay samples to Ron Roy for testing and they
are addressing the issue. From what I have gathered there was a definite
problem with the fire clay that they obtained from Christy Refractories that
may be the culprit. Since I do not have first hand knowledge of this I
cannot say for sure. Hopefully Ron will post his results to the list after
his contact with Standard clay.

Ron, I hope I have not put you on the chopping block with this post but
wanted those having the problem to know someone was following up on it. To
all that I have had conversations and emails with having problems with the
112 clay, I hope this helps to know that you are not crazy and are not doing
anything wrong with the way you make pots. (This was what I thought I was
doing).

I have been reading up on fireclay in "Out of the Earth Into the Fire" by
Mimi Obstler and see several references to excessive amounts of cristobalite
causing glaze fit and fired strength problems. Of course it also states
that minerals are highly vulnerable to change and should be closely
monitored. It looks as though quality control practices leave a little to
be desired if this problem as been continuing for a good while as noted by
all the problems listed with this clay body. What puzzles me is I am sure
the fireclay supplier must sell to more than just Standard Clay so why is
the problem not showing up in more clay bodies?

I do want to make it clear that Julie at Standard Clay has been checking
with me regularly to monitor my problem and had not just left me hanging. I
do appreciate that fact and am sure that this is something that can be
worked out.

My supplier has been ready and willing to assist me in solving this problem
also and I would like to publicly commend Tammy with Mid South Ceramic
Supply in Nashville. She always had time to discuss problems and offer help
where needed.
(END copied message)

Thought that you wold be interested. It may not be anything you are doing
or NOT doing. It may just be the clay!
--
Wayne Seidl
Key West, Florida, USA
North America, Terra
Latitude 81.45W, Longitude 24.33N
Elevation 3.1 feet (1m)


Thanks. I will without question do this from now on with 112. If a crack
shows up after the bisque, I can easily deal with it but to think you have

a
sound piece, put in the glazing and firing to then lose it is somehow

insult
to injury.




  #8  
Old June 11th 04, 04:46 AM
Horace James
external usenet poster
 
Posts: n/a
Default

"wayneinkeywest" wrote in
:

Regarding 112 Standard...
This appeared today on the Clayart listserve, and bears reading.
I am copying it and pasting it he


There is only one reason why clay shivers and falls off a body.

The glaze has not been adjusted to fit the clay body. That is the body
shrinks more than the glaze does and tension is set up in the glaze and it
falls off.

All you need to do is to adjust the glaze shrinkage by manipulating the
glaze formula.I suggest that you go to the Insite web pages. They have a
great number or articles and this problem is well covered there. If you
don't know how to do this google Insite

There will be nothing wrong with the fire clay, only that an adjustment
hasn't been made to the glaze formula to take into account what the formula
of the fire clay is. All clay is silica and alumina in different
proportions and what it is called or retailed as is immaterial.

All you need is a basic understanding of glaze chemistry.

If you like post up your glaze recipe and I'll run it through Matrix for
you

Regards

John W
  #9  
Old June 11th 04, 10:23 AM
wayneinkeywest
external usenet poster
 
Posts: n/a
Default

John:
I'm simply passing the information on for the cracking issue, not glaze
shivering. I do not use 112, personally. Just thought it might be
an issue with the clay, and not a compression problem, so when that info
presented itself...

Thanks, though for offering to run it through Matrix. I use GlazeMaster,
myself :)

Best Regards,

Wayne Seidl
Key West, Florida, USA
North America, Terra
Latitude 81.45W, Longitude 24.33N
Elevation 3.1 feet (1m)

"Horace James" wrote in message
. ..
"wayneinkeywest" wrote in
:

Regarding 112 Standard...
This appeared today on the Clayart listserve, and bears reading.
I am copying it and pasting it he


There is only one reason why clay shivers and falls off a body.

The glaze has not been adjusted to fit the clay body. That is the body
shrinks more than the glaze does and tension is set up in the glaze and it
falls off.

All you need to do is to adjust the glaze shrinkage by manipulating the
glaze formula.I suggest that you go to the Insite web pages. They have a
great number or articles and this problem is well covered there. If you
don't know how to do this google Insite

There will be nothing wrong with the fire clay, only that an adjustment
hasn't been made to the glaze formula to take into account what the

formula
of the fire clay is. All clay is silica and alumina in different
proportions and what it is called or retailed as is immaterial.

All you need is a basic understanding of glaze chemistry.

If you like post up your glaze recipe and I'll run it through Matrix for
you

Regards

John W



  #10  
Old June 21st 04, 08:05 PM
D Kat
external usenet poster
 
Posts: n/a
Default

I just now found this post (my server at home only holds things for one
day). We actually had someone in the studio who had chunks of their pot
just drop off. Again, these are things happening that none of use have seen
before and they match what you have posted. Thank you very much for
sharing.


"wayneinkeywest" wrote in message
...
John:
I'm simply passing the information on for the cracking issue, not glaze
shivering. I do not use 112, personally. Just thought it might be
an issue with the clay, and not a compression problem, so when that info
presented itself...

Thanks, though for offering to run it through Matrix. I use GlazeMaster,
myself :)

Best Regards,

Wayne Seidl
Key West, Florida, USA
North America, Terra
Latitude 81.45W, Longitude 24.33N
Elevation 3.1 feet (1m)

"Horace James" wrote in message
. ..
"wayneinkeywest" wrote in
:

Regarding 112 Standard...
This appeared today on the Clayart listserve, and bears reading.
I am copying it and pasting it he


There is only one reason why clay shivers and falls off a body.

The glaze has not been adjusted to fit the clay body. That is the body
shrinks more than the glaze does and tension is set up in the glaze and

it
falls off.

All you need to do is to adjust the glaze shrinkage by manipulating the
glaze formula.I suggest that you go to the Insite web pages. They have a
great number or articles and this problem is well covered there. If you
don't know how to do this google Insite

There will be nothing wrong with the fire clay, only that an adjustment
hasn't been made to the glaze formula to take into account what the

formula
of the fire clay is. All clay is silica and alumina in different
proportions and what it is called or retailed as is immaterial.

All you need is a basic understanding of glaze chemistry.

If you like post up your glaze recipe and I'll run it through Matrix for
you

Regards

John W





 




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