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Fine hairline cracks in bisque ware



 
 
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  #11  
Old November 25th 04, 11:16 AM
JM
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What clay are you using and how high the bisque?

Steve
Bath
UK

Hi Steve,


I dont know what stoneware clay they use at the college, but it is dark grey
in colour and fires to a v light grey. They bisque fire to 1100oC (which I
think is too high - personally).

The clay I use at home, is Potclays 1143M Draycott White and fire to 900oC -
I have not had any problems in the past firing at this temp.

Mmmm - I could try firing to 1000oc I suppose, and see what happens! But it
doesnt explain why the college are getting the same results - esp when they
fire at 1100oC.

Food for thought.
Thanks


Ads
  #12  
Old November 25th 04, 11:18 AM
JM
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sounds like a bad batch of clay? maybe your supplier forgot to add a key
ingrediant? or maybe the stuff was too new to use?

steve

Perhaps...cant say for the college though, as I dont know what clay they
use. Probably bog standard school clay.
JM


  #13  
Old November 25th 04, 02:31 PM
Bob Masta
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On Wed, 24 Nov 2004 19:41:26 GMT, "JM" wrote:

Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled as
the pots.

Recently my ware has developed fine hairline cracking during bisque firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted all
known possibilities.

JM



Just a thought, but try skipping the hot air gun on a few
pieces. I'd guess that drying the surface much faster
than the interior must be building up some stress.

If this turns out to be the problem, then the next question
is how other folks get away with it. Different bodies?
Or maybe some technique difference with the hot air gun?
It might even be that a torch would work better than the
gun by drying only a very thin layer very quickly, not
penetrating quite so deeply.

Just a thought....



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
  #14  
Old November 25th 04, 11:07 PM
Eddie Daughton
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Default

When you use the hot air gun do you keep it + the pot moving? Gun up and
down pot round + round? so that no area gets too hot? Just a thought.....
Hugs
Eddie
"Bob Masta" wrote in message
...
On Wed, 24 Nov 2004 19:41:26 GMT, "JM" wrote:

Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled
as
the pots.

Recently my ware has developed fine hairline cracking during bisque
firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the
items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage
for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused
by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition
I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted
all
known possibilities.

JM



Just a thought, but try skipping the hot air gun on a few
pieces. I'd guess that drying the surface much faster
than the interior must be building up some stress.

If this turns out to be the problem, then the next question
is how other folks get away with it. Different bodies?
Or maybe some technique difference with the hot air gun?
It might even be that a torch would work better than the
gun by drying only a very thin layer very quickly, not
penetrating quite so deeply.

Just a thought....



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com



  #15  
Old November 26th 04, 07:43 AM
Bosco
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I don't do pottery but I do own my own ceramic shop. You might consider the
kiln. If an element is not working or if the kiln is not firing hot enough,
it could be under-fired bisque. Try using a witness cone test firing. Good
luck.
"JM" wrote in message
...

Is there a chance that the pots got cold..... As in freezing cold???
that'd
do it.....
Hugs
Eddie


Thanks Eddie,
I really appreciate all your inputs everyone...
I dont think that the conditions at college would get that bad - I think
Health and Safety might have something to say about that. Plus, my studio
at home is constantly heated - ok its not needed during the summer months,
but as soon as it starts getting colder I put the heating on in there. It
never gets under 10oC.
Any other suggestions?
JM



  #16  
Old November 27th 04, 02:13 AM
Roz Lacey
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The name of the clay would be on the bag. Sounds like Raku - modelling
variety to me. I hate the stuff. I have seen more disasters using that
than with any other. It's OK for small, fairly thick items, bisque fired,
ready for a Raku Party. When you go into thinnish walled teapots, bottoms
crack off, appendages crack, and I have seen tiles go into flaking mode. If
it is not Raku, the clay suppliers will be only too happy for you to send
them a lump, so that they can do a test firing to suss out the problem and
refund if it is faulty.
"Eddie Daughton" wrote in message
...
When you use the hot air gun do you keep it + the pot moving? Gun up and
down pot round + round? so that no area gets too hot? Just a thought.....
Hugs
Eddie
"Bob Masta" wrote in message
...
On Wed, 24 Nov 2004 19:41:26 GMT, "JM" wrote:

Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled
as
the pots.

Recently my ware has developed fine hairline cracking during bisque
firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the
items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage
for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused
by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition
I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted
all
known possibilities.

JM



Just a thought, but try skipping the hot air gun on a few
pieces. I'd guess that drying the surface much faster
than the interior must be building up some stress.

If this turns out to be the problem, then the next question
is how other folks get away with it. Different bodies?
Or maybe some technique difference with the hot air gun?
It might even be that a torch would work better than the
gun by drying only a very thin layer very quickly, not
penetrating quite so deeply.

Just a thought....



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com





  #17  
Old November 27th 04, 05:44 PM
Ken
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Posts: n/a
Default

I would suggest you allow your ware to dry normally( Slow) Or change your
clay body formulation . Your rapid drying has not cracked pots but rather
created stress lines that turn to cracks upon bisque firing . Glaze firing
rather than curing problem only applifies it as there is difference of
expansion during firing that further stresses cracks in ware . Try same clay
body dried slower or another formulation that isnt as stress sensitive to
rapid drying .
Good Luck
Ken
"JM" wrote in message
...
Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled

as
the pots.

Recently my ware has developed fine hairline cracking during bisque

firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the

items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage

for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused

by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition

I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted

all
known possibilities.

JM




  #18  
Old November 29th 04, 10:21 AM
JM
external usenet poster
 
Posts: n/a
Default

Yeh, I constantly turn the pot so that the temp is even. I also keep the
gun away some distance, so the heat is not fierce.
My last tutor used to bring a large gas bottle and a blow torch to class,
and we used to use that. Those who were not used to using the blow torch
method would find that, if you were not careful, small chips would shoot off
the ware! It was quicker, but I found my little gun perfectly adequate, and
in my mind, a lot gentler on the ware.
JM

"Eddie Daughton" wrote in message
...
When you use the hot air gun do you keep it + the pot moving? Gun up and
down pot round + round? so that no area gets too hot? Just a thought.....
Hugs
Eddie
"Bob Masta" wrote in message
...
On Wed, 24 Nov 2004 19:41:26 GMT, "JM" wrote:

Hi all,
I have a problem which has left myself, and my pottery teacher as puzzled
as
the pots.

Recently my ware has developed fine hairline cracking during bisque
firing.

All ware is thoroughly dried before firing (when pots are both fired at
college and at home) - so I don't damp is not the problem. One of the
items
has been on the drying shelf for about 2 months!

I have been using a hot-air gun to get some pieces to leatherhard stage
for
turning, but have not held it too close - I know a potter who even uses a
blow torch. So I am not even sure that this is the problem creater.

Thinking that when glazing the glaze might seep in and seal the fine
cracking, but the second firing being hotter opens them up even more.

The cracks are not localised and run in all directions, so are not caused
by
lack of compression.

If I can't get to the bottom of this irritating and frustrating condition
I
could always start a line in jigsaw pots.

Please come up with suggestions of possible causes - as I have exausted
all
known possibilities.

JM



Just a thought, but try skipping the hot air gun on a few
pieces. I'd guess that drying the surface much faster
than the interior must be building up some stress.

If this turns out to be the problem, then the next question
is how other folks get away with it. Different bodies?
Or maybe some technique difference with the hot air gun?
It might even be that a torch would work better than the
gun by drying only a very thin layer very quickly, not
penetrating quite so deeply.

Just a thought....



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com





  #19  
Old November 29th 04, 10:23 AM
JM
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Posts: n/a
Default

As I said before, I dont know what type of clay the college uses, but my
clay is 1143M Draycott white stoneware clay. Its about the whitest firing
stoneware clay that Potclays sell.

JM


  #20  
Old November 29th 04, 10:42 AM
JM
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Posts: n/a
Default


I would suggest you allow your ware to dry normally( Slow) Or change your
clay body formulation . Your rapid drying has not cracked pots but rather
created stress lines that turn to cracks upon bisque firing . Glaze firing
rather than curing problem only applifies it as there is difference of
expansion during firing that further stresses cracks in ware . Try same
clay
body dried slower or another formulation that isnt as stress sensitive to
rapid drying .
Good Luck
Ken


Hi Ken,
I will try this, but the reason I use the hot gun is because the class is
only 2hr duration.

If when leaving the items in the college wet cupboards, you return to find
the cupboards were too wet and your hard work has been reduced to a soggy
mess, or they are too dry and has completely dried out leaving it too hard
to continue work on the following week.

So, because of this, what I always aim to do is complete whatever I start
that session - hence using the hot gun.

Also, I sometimes take the project home to finish off there. I hate leaving
items at the college as they are on constant display to all, often being
moved around, and sometimes we return to find broken pieces. The classrooms
are in constant use (by schools also), and we cannot always rely on the
college staff firing greenware on a regular basis. Sometimes items are left
for a couple of weeks in the kiln room waiting to be fired. I feel sorry
for those who dont have the ability to finish their items at home, like I
can.

However, I have done a test bowl and left it to dry there. I will see if
the results are any different.

If its sooo bad there then why do I attend the college? - Well I like the
company, its great to share ideas, and gain inspiration from others. Being
stuck in my own studio can be lonely, and can easily get stuck in a rutt of
making the same things over and over again. Its great to mix, have a laugh
in the process, try new ideas once in a while, and if you do get a problem -
well a problem shared, is a problem halved! But in this case, everyone is
baffled!

Perhaps too fast drying is the key here, but my ex-tutor used a blow torch
which was far more powerful (on the very same clay) and we had no problems
then.

I'll let you know how the bowl turns out!

Keep the ideas flowing!

JM


 




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