Fine hairline cracks in bisque ware
 

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 another note - the pots are not being taken out of the kiln too early,
so it is not thermal shock either!
JM

Where are the cracks, and what shape pots are you making?

Where are the cracks, and what shape pots are you making?

Wow thanks for the quick response!

Well I have made both a teapot, a tagine, and a greecian urn. All
completely different shapes, but all have the same characteristic cracking.

The tagine has on both lid and base. Teapot has on the sides and bottom.
The urn has not been affected on the base, but appear on the sides and neck.
JM

"JM" wrote in message
...

Where are the cracks, and what shape pots are you making?

Wow thanks for the quick response!

Well I have made both a teapot, a tagine, and a greecian urn. All
completely different shapes, but all have the same characteristic
cracking.

The tagine has on both lid and base. Teapot has on the sides and bottom.
The urn has not been affected on the base, but appear on the sides and
neck.
JM

any chance your clay has been contaminated with another clay? when i do
glass and mix some from different manufacturers, different coe can cause
random cracks. or doesn't clay do that?

any chance your clay has been contaminated with another clay? when i do
glass and mix some from different manufacturers, different coe can cause
random cracks. or doesn't clay do that?
Charles,

Well, I cant say about the clay at college. It IS possible that the clay
has been contaminated there. To be honest the way they reclaim clay isn't
in the best controlled circumstances, and I have often found bits of metal
in the clay that I have been throwing (v dangerous - and nearly slashed my
hand open once). I do check the clay thoroughly for foreign objects now!

As for the teapot, I threw this approx 2 - 3 months ago, and has sat on the
shelf whilst other items got priority. I cannot remember whether I used my
own clay, or the colleges.

So I suppose that contamination may be the key, but I have not seen this in
other students' work.

I leave my mind open.....

Thanks for the response.

JM

Is there a chance that the pots got cold..... As in freezing cold??? that'd
do it.....
Hugs
Eddie
"JM" wrote in message
...
any chance your clay has been contaminated with another clay? when i do
glass and mix some from different manufacturers, different coe can cause
random cracks. or doesn't clay do that?
Charles,

Well, I cant say about the clay at college. It IS possible that the clay
has been contaminated there. To be honest the way they reclaim clay isn't
in the best controlled circumstances, and I have often found bits of metal
in the clay that I have been throwing (v dangerous - and nearly slashed my
hand open once). I do check the clay thoroughly for foreign objects now!

As for the teapot, I threw this approx 2 - 3 months ago, and has sat on
the shelf whilst other items got priority. I cannot remember whether I
used my own clay, or the colleges.

So I suppose that contamination may be the key, but I have not seen this
in other students' work.

I leave my mind open.....

Thanks for the response.

JM

I had a similar problem many years ago using a clay with a high
Staffordshire fireclay content. I was bisque firing at about 950oC and
getting very fine cracks in the ware which opened up in the stoneware
glaze firing. Turned out that at that temperature bisque the clay could
not withstand the shock of its own cooling. The answer was either to
bisque at 1000oC or soak the 950 bisque for about 35 minutes. I chose
the former and it cured the problem.

I notice you are using a UK ISP so this might have some relevance to
you.

What clay are you using and how high the bisque?

Steve
Bath
UK


In article , JM
writes
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



--
Steve Mills
Bath
UK

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




Subject: Fine hairline cracks in bisque ware
From: "JM"
Date: 11/24/2004 11:41 AM Pacific Standard Time
Message-id:

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










steve graber

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