mel jacobson on mon 26 oct 09
i think we should make some clear distinctions here.
the work of hank, carol m. and others is about
holding the cooling cycle at very specific temps, for
very specific times.
hank has been very clear that holding at 1900F...that is
in `holding`, keeping the kiln at that temp will grow crystals.
i hold my kiln at 1900F for at least two hours.
it sure slows the cooling, but it is that specific temp that
i am trying to hold. i use one burner, adjust the damper
til the kiln stays at that temp, and then leave it alone. neutral atmospher=
e.
`i am thrilled with this system.`
we are sure that if you re/fire up from about 1800F to 2000F during the
cooling of red /copper/ you will make sure you get the cooling
right to achieve red. racing down through that temp will often give
you spotty results.
carol marians controls her cooling cycle. she stops at various stages
to allow the glazes to work. you must look carefully at her charts.
the changes come in the `controlled` cooling. it is not just slow
cooling.
we are clear that slowing the cooling process is good for
your pots and glazes.
john and ron have proven that beyond a shadow of doubt.
believe them.
but, there are some other things going on here beyond
slow cooling...or a steady slow drop in temp.
we don't think that the ancients re/lit their kilns. but, who
knows, they may have.
we know it took many days to cool a big hard brick kiln, and the
modern kilns just cool too rapidly.
these are all things to be considered. it is not a process for everyone,
and every glaze.
if you are thrilled with your work and firings, don't mess with this
system. if you are looking for some new answers, some new looks...
try it...`IT SURE DOES NOT HURT THE WORK`. but, it does change the
work. change is the key word here. we have never said...`much better`.
that would be an aesthetic judgement that we cannot make.
mel
from: minnetonka, mn
website: http://www.visi.com/~melpots/
clayart link: http://www.visi.com/~melpots/clayart.html
new book: http://www.21stcenturykilns.com
Chris Campbell on mon 26 oct 09
I don't glaze much but controlled
cooling has reduced the number of
cracks in my porcelain.
This next firing I am going to try
down firing to see if that
method would work even better.
I've been studying Carol's data
and trying to relate the profiles to
my needs.
I welcome any ideas on where to
begin my hold to minimize cooling
stress rather than maximize glaze crystals.
Thanks!
Chris Campbell - in North Carolina
Chris Campbell Pottery LLC
Designs in Colored Porcelain
9417 Koupela Drive
Raleigh NC 27615-2233
919-215-8644
Fax: 919-676-2062
www.ccpottery.com
http://neriagechris.blogspot.com/
ivor & olive lewis on wed 28 oct 09
Dear Mel,
<glazes.
John and Ron have proven that beyond a shadow of doubt. Believe them.>>
Although I can accept your comment,I would caution that this may only be
applicable in an Aesthetic sense and a Creative context.
I suggest that using variation of Cooling rate, extended Dwell Time at give=
n
Temperatures, Uniform Rates of Cooling that are slower than the natural
cooling rates of a kiln, to allow degrees of crystallisation or the total
precipitation of coloured crystals, may not always give a result that is
ideal or perfect in a material or utilitarian sense.
Preventing achievement of a uniform glassy state to create colour and visua=
l
textural variations may give aesthetic pleasure. But the result is a
De-Vitrified product. Without qualification, testing and judicious use,
these effects may be a liability.
In the Ceramics Technical article, "Predicting Glaze Phase Transformations"
(Ceram. Tech. No 13, 2001) six differing firing schedules are given. Study
of these diagrams will show that there are relationships between Isothermal
Phase Transformation, that is holding at a selected temperature for an
extended period of time and Continuous Cooling Phase Transformation. One
issue in need of further exploration would be the relationship between
temperature and degree of nucleation.
Best regards,
Ivor
Ivor Lewis,
Redhill,
South Australia
John Hesselberth on wed 28 oct 09
On Oct 28, 2009, at 12:26 AM, ivor & olive lewis wrote:
> Preventing achievement of a uniform glassy state to create colour
> and visual
> textural variations may give aesthetic pleasure. But the result is a
> De-Vitrified product. Without qualification, testing and judicious
> use,
> these effects may be a liability.
Hi Ivor,
I would say the result is precipitated particles encased in a glassy
phase.
I like to think of it this way: Rapid cooling may result in a super-
saturated solution which, in fact, is not particularly good glass. It
contains excess materials which really don't want to be there. Slow
cooling allows some or most of those excess materials to crystalize
out improving the aesthetics and, probably, the "goodness" of the
remaining glass phase as well. In any case, that is my working
hypothesis.
Now for some facts. I have never seen slow cooling result in more
leaching in the standard leaching test than a similar fast-cooled
sample. Within experimental error they have been the same. I don't
have a huge amount of data, but enough to convince me there will be
very little, if any, difference in stability. More data to prove a
different case is always welcome.
Regards,
John
Lee Love on wed 28 oct 09
On Tue, Oct 27, 2009 at 11:26 PM, ivor & olive lewis
wrote:
> Dear Mel,
>
>
>
> <> glazes.
>
> John and Ron have proven that beyond a shadow of doubt. Believe them.>>
>
> Although I can accept your comment,I would caution that this may only be
> applicable in an Aesthetic sense and a Creative context.
Slow cooling is good for certain effects and only if you don't have a
high silica clay body because of cristobalite formation.
Down firing in electric kilns isn't so much an innovation
as it is a way to make up for the deficiencies of a low mass electric
kiln. In kilns that take a week or longer to cool, you don't have to
worry about it.
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
Larry Kruzan on wed 28 oct 09
<<<De-Vitrified product. Without qualification, testing and judicious use,
these effects may be a liability.>>>>>>>>>>>
Hi Ivor,
I curious about this part of your statement. Are you contending that the
"excessive" crystal growth leads to a surface that is more porous than a
surface with no crystal growth? Or are you saying that the surface that had
been fired to a temperature adequate for a proper melt would return to a
pre-vitrified state as a result of slow cooling?
Larry Kruzan
Lost Creek Pottery
www.lostcreekpottery.com
ivor & olive lewis on thu 29 oct 09
Accepted Lee,
But there are several ways of exploiting modern thermal insulation and
electronic control circuitry which lead to a versatility that would be
impossible to achieve with a dense high thermal mass kiln. This would
include simulation of the style of kiln you describe.
Recall that the cooling rate of former kilns was not uniform. The rate of
cooling diminishes from minute to minute, being greatest when the fire is
extinguished and least prior to opening the kiln.
Best regards,
Ivor Lewis,
Redhill,
South Australia
----- Original Message -----
From: Lee Love
To: ivor & olive lewis
Cc: Clayart@lsv.ceramics.org
Sent: Thursday, October 29, 2009 2:17 AM
Subject: Re: down firing vs controlled cooling
Down firing in electric kilns isn't so much an innovation
as it is a way to make up for the deficiencies of a low mass electric
kiln. In kilns that take a week or longer to cool, you don't have to
worry about it.
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=F3g ar chul an tI=97tIr dlainn trina ch=E9ile"=97that is, =
"The
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
ivor & olive lewis on fri 30 oct 09
No arguments with your observations John. Selecting the time to recommence
cooling after an Isothermal event will govern the proportions of vitreous
and crystalline phases. Example E in the CT article shows how to make this
happen. I believe there are examples of Zinc Crystalline glazed pots coming
out of China which have surfaces totally covered in crystalline material.
The idea of Supersaturation and the suppression of crystallisation due to
rapid cooling seems to be contrary to the Precepts you and Ron have defined
to ensure getting stable glazes. I am not saying that supersaturation canno=
t
happen. This seems to be the idea behind adding Titanium Dioxide to Zinc
Crystal batches to ensure growth of Willemite crystals. Insoluble TiO2
fragments act as seed on which ZnSiO3 crystals can grow.
In an example of an isothermal TTT diagram for an unspecified glass
composition Michel Barsoum has cooling rates approaching 1000 deg K per min
as being needed to suppress crystallisation.
I also believe there is an implicit assumption that attaches to all glaze
recipes, regardless of their provenance. We assume that, at the cone values
given for the maturation of glazes, all ingredients will fully melt or
dissolved in the glassy melt. People who attempt to maintain their pyromete=
r
reading for half an hour or longer once their Cones tip may be doubting thi=
s
assumption.
Good to hear from you. Hope things are going well.
Best regards,
Ivor Lewis,
Redhill,
South Australia
Original Message -----
From: John Hesselberth
To: ivor & olive lewis
Cc: Clayart@LSV.CERAMICS.ORG
Sent: Wednesday, October 28, 2009 10:59 PM
Subject: Re: down firing vs controlled cooling
On Oct 28, 2009, at 12:26 AM, ivor & olive lewis wrote:
> Preventing achievement of a uniform glassy state to create colour
> and visual
> textural variations may give aesthetic pleasure. But the result is a
> De-Vitrified product. Without qualification, testing and judicious
> use,
> these effects may be a liability.
Hi Ivor,
I would say the result is precipitated particles encased in a glassy
phase.
I like to think of it this way: Rapid cooling may result in a super-
saturated solution which, in fact, is not particularly good glass. It
contains excess materials which really don't want to be there. Slow
cooling allows some or most of those excess materials to crystalize
out improving the aesthetics and, probably, the "goodness" of the
remaining glass phase as well. In any case, that is my working
hypothesis.
Now for some facts. I have never seen slow cooling result in more
leaching in the standard leaching test than a similar fast-cooled
sample. Within experimental error they have been the same. I don't
have a huge amount of data, but enough to convince me there will be
very little, if any, difference in stability. More data to prove a
different case is always welcome.
Regards,
John
Ron Roy on sat 7 nov 09
Lee's comment shows a misconception - cristobalite is produced starting
around 1100C (2000F) and continues till the melting temperature of silica
1700C - unless there is sufficient sodium, potassium or Lithium to melt it
as it forms.
Slow cooling down to 1100C would produce more cristobalite - a significant
problem for wood firers because the heat source (coals) is still giving off
significant heat. This calls for proper formulation of clay bodies for soli=
=3D
d
fuel kilns if cristobalite is to be avoided or controlled.
There is no point in slow cooling glazes above 1100C because no crystals ar=
=3D
e
going to form till lower temperatures.
Some one asked - how come no cristobalite in glazes - because there is
always enough flux to melt it - well almost always - I did find a little in
an underfired glaze once.
Anyone interested in finding out if they have any cristobalite in a wood
fired clay body? Let me know and I'll tell you how to make a sample for me
to measure - it's easy.
RR
On Wed, Oct 28, 2009 at 10:47 AM, Lee Love wrote:
> On Tue, Oct 27, 2009 at 11:26 PM, ivor & olive lewis
> wrote:
> > Dear Mel,
> >
> >
> >
> > <=3D
nd
> > glazes.
> >
> > John and Ron have proven that beyond a shadow of doubt. Believe them.>>
>
> >
> > Although I can accept your comment,I would caution that this may only b=
=3D
e
> > applicable in an Aesthetic sense and a Creative context.
>
> Slow cooling is good for certain effects and only if you don't have a
> high silica clay body because of cristobalite formation.
>
> Down firing in electric kilns isn't so much an innovation
> as it is a way to make up for the deficiencies of a low mass electric
> kiln. In kilns that take a week or longer to cool, you don't have to
> worry about it.
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>
> "Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97t=
hat is, =3D
"The
> land of eternal youth is behind the house, a beautiful land fluent
> within itself." -- John O'Donohue
>
--=3D20
Ron Roy
15084 Little Lake Road
Brighton, Ontario, Canada
K0K 1H0
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