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Fine hairline cracks in bisque ware



 
 
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  #21  
Old November 29th 04, 10:48 AM
JM
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Steve Mills....I am having trouble replying to your e-mail you sent me. My
post keeps coming back!
JM


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  #22  
Old November 29th 04, 02:20 PM
Bob Masta
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On Mon, 29 Nov 2004 09:21:35 GMT, "JM" wrote:

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


"Bob Masta" wrote in message
...


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.


Actually, the stress I was thinking about is between the
interior and exterior of the clay itself, so constant
turning won't help. The idea is that the skin dries out
quickly to a certain depth, but the interior is still wet.
Then as the interior does dry later, the shrinkage
causes stress build-up because the exterior is already
dry and can't shrink to match it. Or another way to
think about this is that the rapid exterior drying may be
causing rapid exterior shrinkage, which causes invisible
cracks because the interior is not shrinking to match.
The cranks only show up later.

At any rate, if you skip the hot air gun on a few
pieces and let them dry naturally (however long
that takes) and the problem goes away, then
you know where to look.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
  #23  
Old November 29th 04, 08:44 PM
W_D_GREAT_DIVIDER
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Do I understand you correctly that you are getting the same issue (hairline
cracking) with 2 different clays, fired at 2 different temperatures, with
different shaped items? Is this something that is consistently happening or
just sporadic in nature?


"JM" wrote in message
...

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




  #24  
Old November 30th 04, 12:58 PM
JM
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Do I understand you correctly that you are getting the same issue
(hairline
cracking) with 2 different clays, fired at 2 different temperatures, with
different shaped items? Is this something that is consistently happening
or just sporadic in nature?


Hi,
I know it's baffling! This is why I posted to the site..This is only
something I have had happen recently...I have never come across this problem
before.
The teapot is the only thing I fired at home. This item had been sitting on
my shelves for a good couple of months. I cannot, for the life of me,
remember whether that had been an piece I had brought home from college to
finish off at home. So it IS possible it came from the same clay.
Other than that, the only thing in common, is that I used a hot gun on these
items...but this has never created a problem in the past.
JM



  #25  
Old November 30th 04, 01:04 PM
JM
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HI Bob,
I do use the gun on the interior also..I dont just use on the outside. But
I dont hold it close; so it has more the same effect as a warm cupboard
would have. My ex pottery tutor used to heat his stuff up soooo much it
used to positively steam and he had no probs!
Hope this helps to clarify.
Thanks
JM


  #26  
Old November 30th 04, 01:58 PM
Bob Masta
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On Tue, 30 Nov 2004 12:04:29 GMT, "JM" wrote:

HI Bob,
I do use the gun on the interior also..I dont just use on the outside. But
I dont hold it close; so it has more the same effect as a warm cupboard
would have. My ex pottery tutor used to heat his stuff up soooo much it
used to positively steam and he had no probs!
Hope this helps to clarify.
Thanks
JM


By interior I mean within the clay itself, below the surface,
not the inner surface of a pot. You are shrinking the skin
but not the guts. Cracks are the expected outcome.
I don't think it's safe to assume too much from the fact
that others get away with this, since technique and temperature
may make a great difference. You are assuming that since
you are using low temperatures, you should be less likely
to have problems. But that means that to get any given
amount of surface drying you are probably penetrating
deeper than a hot, rapid technique. That may be working
against you.

Just try a few pieces with normal drying, and see if the
problem goes away.

Best regards,



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
  #27  
Old November 30th 04, 07:41 PM
Eddie Daughton
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Maybe when you is doing it you is doing it more..... When i do dry pots on
the wheel using a hot air gun (small one, not the one that does soldering) i
only dry them a bit so's they'll retain shape when i pushes them around a
bit, ceratainly not enuff for steam..... Plus i then lets 'em dry naturally
the rest of the way....
Hugs
Eddie
"Ken" wrote in message
...
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






  #28  
Old December 1st 04, 10:24 AM
JM
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Maybe when you is doing it you is doing it more..... When i do dry pots on
the wheel using a hot air gun (small one, not the one that does soldering)
i only dry them a bit so's they'll retain shape when i pushes them around
a bit, ceratainly not enuff for steam..... Plus i then lets 'em dry
naturally the rest of the way....
Hugs
Eddie


Reply:-
That's exactly how I use it...certainly not to steaming point (as my tutor
does). I get my ware to the point of holding its shape, then let dry out
normally.
Jm


  #29  
Old December 1st 04, 10:40 AM
JM
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Another note:

I returned last night to find that the bowl I had left at the college (in
the wet cupboard) was as wet as when I had thrown it a week ago...certainly
too wet for turning. Therefore I have brought it home to harden a little
here. I am using this bowl as an experiment by not using the gun on it;
just to see if it makes any difference.

Judging by certain comments, it would appear that using gentle heat on ware
may be more harmful than a quick hot blast. But the warm cupboard used at
college dries ware out slowly - perhaps this works better as it is not
localised drying? We have to wait and see!

Thanks for everyones input - I'll let you know of my findings.

It would be nice to know how others speed up the drying process - I have
even been told of placing ware on top of a firing kiln to speed things up.
I know some of you are lucky enough to live in warmer climate, and so have
no probs drying out.

JM


  #30  
Old December 1st 04, 12:13 PM
Deborah M Riel
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In article ,
JM wrote:

It would be nice to know how others speed up the drying process - I have
even been told of placing ware on top of a firing kiln to speed things up.
I know some of you are lucky enough to live in warmer climate, and so have
no probs drying out.

JM


Where I do my work, we have a dry box--a closet with heat lamps and
shelves in it. If you leave your pieces in there for just a short
while they'll firm up a bit. I try *not* to use it very often. My
preferred method of drying is to put the piece on a board and wrap in
plastic (like the kind from a drycleaner). In a day or so, I might
loosen up the plastic a bit, or if I'm working at home, I'll take the
plastic off for a little while, then wrap it back up again.

I've found that drying slowly is much better for the piece than drying
quickly, and that certain clay bodies (like porcelain) develop drying
cracks easier than other clay bodies.

Deb R.
 




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