Lili Krakowski on thu 29 jun 06
I have never had any dunting or shivering except on test tiles. Defining
"dunting" as the glaze lifting off in sharp "leaves" and defining
"Shivering" as the total cracking of the piece.
I know that crazing often is "delayed" A pot may craze weeks and months
after being fired. But what about dunting and shivering? Are they seen
immediately, and if later, how much later? Will a stressed glaze on a
reasonably vitrified pot start dunting when washed a number of times? Will
a pot shiver under same--delayed-- conditions?
Lili Krakowski
Be of good courage
Daniel Semler on fri 30 jun 06
Hi Lili,
I know of a case where a guy I know was making pieces for the
reception tables
at his wedding. They were slip case forms, not sure of the shape, but I
believe
they were some sort of low vase form. They had unloaded the kiln and
the pieces
were just sitting around and one 'exploded'. Not sure quite what they meant by
this but I believe it was a fairly catastrophic failure in the piece. Not
delayed by much I grant you, but a little.
Thanx
D
Snail Scott on fri 30 jun 06
At 06:16 PM 6/29/2006 -0400, Lili wrote:
>Will a stressed glaze on a
>reasonably vitrified pot start dunting when washed a number of times? Will
>a pot shiver under same--delayed-- conditions?
Often, dunting and shivering happen while the
work is still cooling in the kiln, but it can
happen later, too. I wouldn't consider washing
a major risk factor, but the various shocks of
normal handling can set off the latent stresses
in the clay. The longest time I can personally
recall between firing and dunting was about two
days, but I suspect a lesser problem might take
longer to manifest, and not get recognized as
dunting when it happens. Thump a bowl against a
table top during use, not too hard, and it cracks:
dunting or not? Maybe just the right unfortunate
impact on a weak or resonant spot, but maybe
delayed dunting. Hard to say.
Bought a nice mug a while back. Poured hot tea
into it: it split all the way down. Too much
thermal shock? Or delayed dunting, set off by a
'normal' stress?
Shivering:
I acquired a mug with a shivered glaze, abandoned
by its maker. I picked it up to use as a pencil
holder, figuring that the shivering was done. The
sharp handle edges were all pretty much done
shivering, true, but the softer edges on the lip
didn't start shedding chips 'til later, when
regular handling finally released those latent
stresses.
-Snail
Paul Herman on fri 30 jun 06
Hi Snail and Lili,
I have seen catastrophic dunting happen two weeks after the firing.
I'd been experimenting with a dark iron-rich clay, and my friend Mike
from AK put some glaze experiment made from portland cement on a
large vase. Early one morning I was rousted from my doze by a loud
"smash, tinkle", and thought the dog had knocked a pot over. It made
a noise like it had been dropped on the cement floor from a few feet up.
The vase had instantly turned into many small pieces and shards,
right on the counter top, with no help from the dog. It was pretty
spectacular. This clay is so pretty that I don't want to abandon it,
so I'm trying to get a recipe worked out that will hold together.
I've also seen pots that dunted from pouring hot water in them, like
a mug or teapot.
Good glaze fits to all, and please no dunting!
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Jun 30, 2006, at 7:43 AM, Snail Scott wrote:
> Often, dunting and shivering happen while the
> work is still cooling in the kiln, but it can
> happen later, too. I wouldn't consider washing
> a major risk factor, but the various shocks of
> normal handling can set off the latent stresses
> in the clay. The longest time I can personally
> recall between firing and dunting was about two
> days, but I suspect a lesser problem might take
> longer to manifest, and not get recognized as
> dunting when it happens.
Paul Herman on sat 1 jul 06
Greetings Ivor,
Would you give us a little more understanding of what "First
Principals" means? I'm always looking for new tricks.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Jul 1, 2006, at 1:06 AM, Ivor and Olive Lewis wrote:
> But since I stopped using recipe books, since I started working
> from "First Principles" these trials and tribulations have not
> afflicted the pots I make.
> Best regards,
> Ivor Lewis.
> Redhill,
> South Australia.
>
>
Ivor and Olive Lewis on sat 1 jul 06
I suppose Frank Hamer and Harry Fraser are the authorities on this topic =
and I see not reason to doubt what they write.
Most people will suffer from the effects of poor technique or mismatch =
of glaze to clay from time to time. I have had bowls sitting as two =
pieces when the kiln was opened. Classic Dunt where one side of the pot =
cooled faster than the other as temperature declined through the "Quartz =
Inversion"
I have had mugs that shattered into lightning tracery fractures when the =
interior glaze was a mismatch to the outer glaze and the differential =
thermal stress tore the clay part. The distortions of the final forms =
were amazing
I have had flasks that, long after they had been removed form the kiln, =
shattered with the sound of a rifle being discharged due to excessive =
changes in glaze thickness .=20
But since I stopped using recipe books, since I started working from =
"First Principles" these trials and tribulations have not afflicted the =
pots I make.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on mon 3 jul 06
Dear Paul Herman,=20
Since I replied to you in private and sent an attachment which would not =
pass to clayart, here is an abridged answer for the group.
By "First Principles" I mean that basic conventions and concepts of the =
subject under discussion are used to formulate workable answers. These =
are the fundamental ideas upon which our knowledge base is founded.
A simple analogy would be in the realms of Physics where all ideas can =
be related or founded on our knowledge of Mass, Time and Distance. =
These are fundamentals, our "First Principles". All other properties, =
those of electricity, light, sound, magnetism, force, motion, heat, =
density, viscosity and so on are derivatives and can be related directly =
back these three fundamentals. The same applies to Ceramics. We have =
fundamental ideas like the behaviour of Clay/water systems or glaze =
systems where fused silica is the foundation of a glaze. By design, =
Modifiers are used in both systems to alter properties and give products =
we desire.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on mon 3 jul 06
Hi Lili,
In word - yes - although I have the feeling it will happen sooner than
delayed crazing - which can be due to water absorption in the clay.
Pouring hot water into a pot, which has liner glaze with too low an
expansion, will shorten the time it takes. The inside glaze, which is
already too "big" for the clay - heats up a fraction of a second and
expands before the clay catches up - and the tea pot cracks. This can take
months - perhaps even years - depending on how often the pot is used and
how big the degree of clay/glaze mismatch. It is always the same reason -
liner glaze has too low an expansion rate for the clay.
I remember reading - somewhere - In the Hamer book I think - some English
potters feel much safer if their glazes craze once in a while - because a
crazed glaze will not crack pots.
You can still have uncrazed glazes that do not break pots but you need to
be on guard - especially when starting with a new batch of clay. Always a
good idea to do a few simple tests.
RR
>I know that crazing often is "delayed" A pot may craze weeks and months
>after being fired. But what about dunting and shivering? Are they seen
>immediately, and if later, how much later? Will a stressed glaze on a
>reasonably vitrified pot start dunting when washed a number of times? Will
>a pot shiver under same--delayed-- conditions?
>
>
>Lili Krakowski
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on mon 3 jul 06
Hi Paul,
Reduced high fire iron bodies are particularly subject to the problem -
which usually is cristobalite.
High iron bodies - in reduction need a lot of care - iron helps make
cristobalite - KNaO prevents it.
Peter Sohngen wrote the article which explains it all - see Studio Potter -
vol 28 #1.
RR
>I have seen catastrophic dunting happen two weeks after the firing.
>I'd been experimenting with a dark iron-rich clay, and my friend Mike
>from AK put some glaze experiment made from portland cement on a
>large vase. Early one morning I was rousted from my doze by a loud
>"smash, tinkle", and thought the dog had knocked a pot over. It made
>a noise like it had been dropped on the cement floor from a few feet up.
>
>The vase had instantly turned into many small pieces and shards,
>right on the counter top, with no help from the dog. It was pretty
>spectacular. This clay is so pretty that I don't want to abandon it,
>so I'm trying to get a recipe worked out that will hold together.
>
>I've also seen pots that dunted from pouring hot water in them, like
>a mug or teapot.
>
>Good glaze fits to all, and please no dunting!
>
>Paul Herman
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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