Glistering Phaeton on tue 29 dec 09
Dear All,
I decided to run the MC6G absorption tests on the cone 5 Laguna clay =3D
bodies
I throw with, and discovered the following after a 2-hour boil:
Two of them were 0% absorption (Electric Brown and Hagi Porcelain)
One was 1.3% absorption (B-mix w' Grog)
One was 4.17% absorption (Speckled Buff)!! Oy! And it - naturally - =3D
didn't
pass the microwave test either.
These were all fired on the same shelf in my electric kiln to Cone 6. =3D
My
firing schedule brings the temperature up to 2130=3DB0 and I hold it there =
=3D
until
I see cone 6 drop, usually around 1 hour 45 minutes. [I got to this =3D
ramp
through a lot of testing, trying to eliminate pinholes and other glaze
defects. All my glazes look good with this schedule, so I've stuck with
it.]
Here's my question...I know that cones measure heat AND time (heat
absorption), both of which are needed to mature the glaze, but does clay
vitrify only if you reach a certain temperature regardless of time? To =3D
wit,
even though the glazes melted with the long hold and cone 6 down, is it
possible that the Speckled Buff clay can't vitrify unless it hits an =3D
actual
cone 5 temperature?
Just wanted to find out for sure before I contact Jon at Laguna to see =3D
if
this 4.17% absorption is some strange anomaly.
[For the record, the test tiles were about 3-4" long and around 1/2" =3D
thick.
They ranged in dry weight from 120g to 145g.]
Thanks, and have a prosperous and happy New Year!
=3D97Adam
Los Angeles
"Down, down, I come; like glistering Phaeton, wanting the manage of =3D
unruly
jades."=3D20
-William Shakespeare, King Richard II
ivor & olive lewis on wed 30 dec 09
Dear Adam,
You ask << Here's my question...I know that cones measure heat AND time
(heat
absorption), both of which are needed to mature the glaze, but does clay
vitrify only if you reach a certain temperature regardless of time? >>
The given values for standardised cones are only true if they are heated at
the Manufacturers recommended ramp rate. You are effectively maintaining a
constant temperature that is the equivalent of Orton Cone 3 for almost two
hours. This may prevent glaze defects form happening but it is insufficient
for your highly absorbent samples to form sufficient glass phase to seal
communicating pathways between the refractory ingredients in the clay body.
Clay bodies have to achieve their "Deformation Eutectic". This happens when
the mass of the clay develops sufficient glassy fluid to give a specified
amount of bending. When the temperature is constant there is no movement of
heat into the mass. Time has been eliminated from the equation. So work is
not being done on those clay ingredients.
Best regards,
Ivor Lewis,
Redhill,
South Australia
Neon-Cat on wed 30 dec 09
Here's old old but very good extensive study on clay bodies and feldspars:
The Properties of Feldspars and Their Use in Whitewares
Joseph C Kyonka and Ralph L. Cook
Engineering Experiment Station
University of Illinois
(available online since 2007)
http://www.ideals.illinois.edu/bitstream/handle/2142/4264/engineeringexperv=
00000i00422.pdf?sequence=3D3
On page 12 (or page 16 of 42 of the document) it is written:
"Studies of binary systems of feldspars and clay have not revealed any
deformation eutectics; however, it has been established that as a
feldspar melts it takes the decomposition products of clay into
solution at a rate dependent upon the temperature and the surface
areas."
So, time and work have not been eliminated, Ivor. The clay body keeps
right on reacting whether or not temperature continues to increase.
The rate of the reaction is governed by the temperature and a longer
firing time ensures that ingredients do have time to react with one
another. There are times when a long and slow lower-firing might be
just the ticket. It all depends on the results wanted.
On page 19 (or page 23 of 42 of the document) it is written:
"It is noted that in all bodies the fired porosities obtained with the
laboratory kiln were lower than those obtained from the commercial
firing. This difference may be attributed to the difference in firing
rates, the commercial rate being almost twice as rapid as the
laboratory cycle."
This further points out how different firing schedules affect physical
(and chemical) changes in our finished products. All-in-all the
Illinois article is quite good if you've never had a chance to read
it. (Can't sleep? It works for that, too.) The basic premises have not
been challenged, just detailed, clarified, and elaborated upon down
the decades since its publication in 1954.
Ivor, you seem to view firing and fluxing as melt phenomenon; others
view these processes as chemical reactions. Just an observation...each
to his/her own.
A joyous New Year to you all!
Marian
Neon-Cat
David Finkelnburg on wed 30 dec 09
Adam,
While the ratio of flux to alumina in the glass phase of the body will
be constant, the amount of silica dissolved into the glass phase is a
demonstrated function of peak firing temperature. Soak time is important
only where the ware or stacking is so dense and thick that a significant
temperature difference exists between the pyrometer and the ware. So,
depending on your ware and stacking, you MAY find denser test tiles if you
fire faster to a higher peak temperature at the same cone. It's something
for you to test when you are not under other constraints.
Good potting,
Dave Finkelnburg
http://www.mattanddavesclays.com
Glistering Phaeton on wed 30 dec 09
Dear Ivor,
Thanks for the feedback! You raise an interesting point regarding the
development of glassy fluid in the clay at a given temp, which is kind =3D
of
where I was thinking the answer to my question would be. But Ron =3D
replied
off-list and said that he thought that although there isn't as much flux =
=3D
in
clay as in glaze, there is enough to make soak time a consideration. He
also pointed out that the other clays (all rated to the same cone by =3D
Laguna)
all vitrified with that firing schedule.
Lou Turner also pointed out (off-list as well) that I overlooked the
(glaringly obvious) fact that Laguna has rated this clay as 3% =3D
absorption
=3DB11%, which puts me only slightly above their range. I will be testing =
=3D
this
clay again, wedging before making the tile (Ron suggested that in a 2008
post) to see if I can get the absorption down.=3D20
I don't have the time to try a number of different temperature =3D
approaches to
cone 6 to see if they made any appreciable difference to the absorption
number, so I think that I'll just save this clay for the =3D
non-microwave-bound
pots. I've got three low-absorption clays as it is, and I'm loath to =3D
have
to trash any more pots than I already do!
If I ever get experimental again with firing schedules though, I'll put =3D
this
clay through the ringer!
Thanks again,
=3D97Adam
Los Angeles
-----Original Message-----
From: Clayart [mailto:Clayart@LSV.CERAMICS.ORG] On Behalf Of ivor & =3D
olive
lewis
Sent: Tuesday, December 29, 2009 10:29 PM
To: Clayart@LSV.CERAMICS.ORG
Subject: Cone Number: Glaze Maturity vs Clay Vitrification
Dear Adam,
You ask << Here's my question...I know that cones measure heat AND time
(heat
absorption), both of which are needed to mature the glaze, but does clay
vitrify only if you reach a certain temperature regardless of time? >>
The given values for standardised cones are only true if they are heated =
=3D
at
the Manufacturers recommended ramp rate. You are effectively =3D
maintaining a
constant temperature that is the equivalent of Orton Cone 3 for almost =3D
two
hours. This may prevent glaze defects form happening but it is =3D
insufficient
for your highly absorbent samples to form sufficient glass phase to seal
communicating pathways between the refractory ingredients in the clay =3D
body.
Clay bodies have to achieve their "Deformation Eutectic". This happens =3D
when
the mass of the clay develops sufficient glassy fluid to give a =3D
specified
amount of bending. When the temperature is constant there is no movement =
=3D
of
heat into the mass. Time has been eliminated from the equation. So work =3D
is
not being done on those clay ingredients.
Best regards,
Ivor Lewis,
Redhill,
South Australia
Glistering Phaeton on thu 31 dec 09
Dear Dave, Marian, et al.,
Thank you for your replies! Given that the responses seem to have given me
both possible answers, more testing is definitely in order. The first test
I'll be able to run is Ron's "wedge first" test at the same firing schedule
to see if that changes the absorption percentage. In terms of changing the
ramp, I won't be able to do that until I fire a load of glazes that don't
exhibit any of the pinholing at the higher-temp-shorter-hold rate. It may
be a while before I have a chance to do that (maybe a load of pots with
liner glaze only and slips and stains on the outside?), but when I do, I
will post back to the list with the results.
Even when the questions don't produce definitive answers, the responses mak=
e
for great and informative reading! Thank you all again.
-Adam
Los Angeles
-----Original Message-----
From: David Finkelnburg [mailto:dfinkelnburg@gmail.com]
Sent: Wednesday, December 30, 2009 10:01 PM
To: clayart
Cc: GlisteringPhaeton@GMAIL.COM
Subject: Re: Cone Number: Glaze Maturity vs Clay Vitrification
Adam,
While the ratio of flux to alumina in the glass phase of the body will
be constant, the amount of silica dissolved into the glass phase is a
demonstrated function of peak firing temperature. Soak time is important
only where the ware or stacking is so dense and thick that a significant
temperature difference exists between the pyrometer and the ware. So,
depending on your ware and stacking, you MAY find denser test tiles if you
fire faster to a higher peak temperature at the same cone. It's something
for you to test when you are not under other constraints.
Good potting,
Dave Finkelnburg
http://www.mattanddavesclays.com
ivor & olive lewis on thu 7 jan 10
Dear Marian,
Your midnight reading matter is entertaining and anyone who is interested i=
n
the formulation of clay bodies ought to download a hard copy. It covers a
lot of ground. The authors do not make it clear if the inclusion of
marginal quantities of Group 2 Alkali Earth compounds in a clay body alter=
s
the fusion point or changes the viscosity of fluids derived from molten
felspar.
To remind everyone, Marion (Neon Cat) suggested that I review
:
The Properties of Feldspars and Their Use in Whitewares
Joseph C Kyonka and Ralph L. Cook
Engineering Experiment Station
University of Illinois
(available online since 2007)
http://www.ideals.illinois.edu/bitstream/handle/2142/4264/engineeringexperv=
00000i00422.pdf?sequence=3D3
Well worth the effort. But do not take every word as chiselled in stone. Tr=
y
to keep an open and critical mind and compare what its authors have said
with the notes of Kingery, Bowen and Uhlmann in "Introduction to Ceramics",
especially Ch 10.
I was pleased to see that Kyonka and Cook confirmed the thought I had about
mixtures of Potassium and Sodium Felspars represented by rocks such as
Nepheline Syenite.
Without doubt formulating a stoneware of porcelain body is a balancing act.
The Art is to find ratio of raw materials that give an impervious fabric
without developing a degree of pyroplasticity that would register as a
deformation eutectic or precipitating Cristobalite. No one wishes to have
clays which slump or dunt.
Regards,
Ivor
Neon-Cat on thu 7 jan 10
Ivor and all, if there is one thing I hope I've done on clayart it is
to show that there is a wealth of educational literature from
reputable sources on the web for those wishing to delve more deeply
into clay science and ceramic materials, increase their knowledge and
understanding, and find practical aids for their studio practice.
Recent and past events on clayart show me that clay science is one
topic to avoid in 2010 on list. From here on out you'll have to
contend with the goofy new artist or the zany list cat. Who knows, it
might also be entertaining...
Marian
Neon-Cat
On Wed, Jan 6, 2010 at 9:41 PM, ivor & olive lewis
wrote:
> Dear Marian,
>
> Your midnight reading matter is entertaining
ivor & olive lewis on fri 8 jan 10
Yes Marian, I found the Kyonka-Cook paper very interesting. Some of the dat=
a
given is pertinent to the question of corrosion of Aluminium Alloy Pug Mill
components. Last year (or was it 2008) I tested the basicity of water that
had been used to wash felspars and nepheline syenite samples from my stock
pile. I found that there was a shift away from pH 7 towards pH 11. So I am
not surprised persistent processing of porcelain produces corrosion product=
s
in aluminium barrels.
I have attached a couple of images of models made to think about the nature
of clay. You will recognise the theoretical structure. On the T face I hav=
e
indicated the residual coulomb charge of the bridging Oxygen atoms with
orange beads. On the O face which is composed of what are generally termed
"Hydroxyl units" in the literature, I have indicated a residual coulomb
charge with a blue bead . Given that in aqueous solutions Hydroxyl units
carry a negative charge, what is the nature of the charge at these location=
s
on the O layer. I would suggest they are Positive charges. What would your
opinion be ?
Just be careful with that Artist Label. Tends to stir up emotions.
Best regards,
\_ Ivor
----- Original Message -----rom: Neon-Cat
To: ivor & olive lewis
Cc: Sent: Friday, January 08, 2010 8:58 AM
Subject: Re: Cone Number: Glaze Maturity vs Clay Vitrification
Ivor and all, if there is one thing I hope I've done on clayart it is
to show that there is a wealth of educational literature from
reputable sources on the web for those wishing to delve more deeply
into clay science and ceramic materials, increase their knowledge and
understanding, and find practical aids for their studio practice.
Recent and past events on clayart show me that clay science is one
topic to avoid in 2010 on list. From here on out you'll have to
contend with the goofy new artist or the zany list cat. Who knows, it
might also be entertaining...
Marian
Neon-Cat
Neon-Cat on sat 9 jan 10
Ivor I was serious about clay science being one topic for me to avoid
in 2010 on list. I will say that I find your kaolin models, however
elegantly crafted, to be confusing. Put on your mineralogist's cap
when thinking about kaolinite structure and bonding. I=3D92ll have to give
a little bit of a description of a standard accepted model of
kaolinite crystal structure =3D96 I can=3D92t answer your question about th=
e
nature of charge any other way.
Ideal minerals are electrically neutral. They are defined that way =3D96
it=3D92s a fact of life. Linus Pauling formulated a set of rules we know
as Pauling=3D92s rules of crystal configuration. You cannot have your
residual coulomb charges anywhere. OK?
Kaolinite has the formula Al2Si2O5(OH)4. This tells us that there are
2 aluminum atoms in octahedral occupation; two silicon atoms with 5
oxygen atoms in tetrahedral occupation; four coordinating anions of
OH. In Pauling=3D92s first rule it is seen that each cation in a crystal
structure is surrounded by anions. The number of anions that can be
packed around and coordinate a cation is called the coordination
number (abbreviated CN). The CN for a Si4+ ion in coordination with
oxygen anion is four. There are standard reference tables for these
values.
Pauling=3D92s second rule informs us that the negative charge of an anion
must be neutralized by positive charge from surrounding cations for a
stable structure (kaolinite is stable). In the tetrahedral layer of
kaolinite with =3DBD-unit cell of Si2O5 each of the two structural Si4+
has four bonds that radiate out in tetrahedral coordination. Each has
a bond strength of +1 (strength =3D3D valence/coordination number). Each
silicon atom in the unit cell coordinates with three basal oxygen and
they share one of these oxygen atoms so we have 5 basal oxygen atoms
in coordination with two silicon atoms as the formula of kaolin tells
us. This is typical of the phyllosilicate minerals (clay minerals)
were the silicon-to-oxygen ration must be 2/5 for a basic Si unit
defined by Si2O5 and a change per Si unit of -1. The sharing of these
three basal oxygen atoms is what gives us our familiar sheet
structure. The S-O bonds are 51% ionic and 49% covalent. In
mineralogy, bonds that are 50% or greater are said to have ionic
character and are termed ionic bonds. The charge on the apical oxygen
will be satisfied when it is used in the aluminum hydroxide layer
(octahedral layer).
In the octahedral layer of a kaolinite mineral the Al3+ is in
octahedral coordination (CN=3D3D6) and is surrounded by six O2- ions. To
satisfy charge it uses two (apical) oxygen anions from the tetrahedral
layer per Al and four hydrogen ions (protons, H+) to balance the
remaining four oxygen anions that coordinate with the Al atom. Each
Al-O bond has a strength of +0.5 (valence/coordination number or
3+/6). Here in the octahedral layer anions are also shared between
cations to satisfy charge. We also see that coordination extends
between the octahedral and tetrahedral layers making am integrated
cohesive unit. These 1:1 layers are so strongly coordinated they are
unlikely to be pulled apart without considerable expenditure of energy
and this helps to account for the longevity of kaolinites in the
environment. The Al-O bonds are 37% covalent and 63% ionic. The H-O
bonds are polar covalent types and are approximately 61% covalent and
39% ionic. (The percent ionic character of a metal-oxygen bond can be
calculated using the appropriate formula when electronegativity values
are known or values can be found in reference tables.)
Now to get from single kaolinite crystals to a whole stack of them (a
larger platy crystal) the hydrogen bond comes into play. Hydrogen
bonding may be defined as the intermolecular attraction (van der Waals
attraction) that results when a hydrogen atom comes between two small,
highly electronegative atoms. It is the strongest intermolecular
attractive force especially when it involves oxygen, nitrogen, or
fluorine. In our example of kaolinite the hydrogen bond forms between
two oxygen anions. The octahedral sheet of one completed 1:1 kaolin
crystal is connected to the tetrahedral sheet of another 1:1
assemblage through hydrogen bonds between the basal Si oxygen atoms of
one and the Al-bound hydroxyls of another. It will be hard to disrupt
these bonds although kaolin can be delaminated by milling and
grinding, for example, or other means. Electrons might best be thought
of as residing in regions like a swarm of gnats and may be
concentrated or dispersed. Within our imaginary swarm the oxygen with
its greater electronegativity (3.5 vs 2.1) will have a greater number
of electrons than hydrogen. This leads to separation of charge and
undirected attraction between the hydroxyl portion of one 1:1
octahedral layer and the structural oxygen of the tetrahedral layer of
another 1:1. In kaolin this distance of separation between the basal
plane of oxygen atoms of one 1:1 layer and that of an adjacent layer
is measurable and is referred to as the d-spacing. This d-spacing for
kaolinite (0.71 =3D96 0.73 nm) does not vary no matter where the kaolinite
ends up. Because the kaolinite crystals are electrically neutral we
don=3D92t think in terms of residual partial positive or negative charges,
instead we should think in terms of delocalized bonding through
overlapping of the orbitals of the ions involved with a resulting net
lowering of energy once the 1:1 sheet layers start bonding together.
For comparison it might be said that the charge distribution arising
from the electrons involved with hydrogen bonding can be interpreted
as about 80% ionic and 20% covalent in character. Another comparison
has hydrogen bonding in plain water about one twentieth the strength
of the polar covalent O-H bond in a water molecule. For those working
with clay, hydrogen bonding, although considered weak, isn=3D92t a bad
thing since it is one factor that makes forming and working with clay
possible.
Dr. Spiller, a dentist, has an easy-to-understand illustration I like
that shows the octahedral and tetrahedral layers of a kaolinite model
and how kaolinite 1:1 entities connect through hydrogen bonding:
http://www.doctorspiller.com/images/Porcelain/Kaolin_structure.jpg
(from: http://www.doctorspiller.com/ceramics_1.htm)
As for the pug question one should look at the chemical reactions that
our ceramic materials undergo while keeping an eye on the dominant
species that we will likely find in the pH ranges we encounter. For
example, Li+, Na+, K+, Mg2+, and Ca2+ ions exist primarily as hydrated
cations throughout the pH range. So there is no reaction between say,
a sodium ion, and the aluminum of a pug barrel. Sodium is just
innocently bouncing around (sodium is often referred to in chemical
reactions as a spectator ion, one taking no active part in a
reaction). The damage (and increase in pH) is initiated by other
species derived primarily through hydrolysis and oxidation reactions
between water and the ceramic materials involved (feldspars, plastic
vitrox clay, kaolinite, etc.). Acid increases feldspar weathering and
Mg2+, for some, encourages chemical reaction. And no, Ivor, you cannot
look to the non-existent residual charges on the structural OHs as a
source of corrosion should the 1:1 tetrahedral-octahedral layers
somehow become separated. There is a promising, non-toxic,
easy-to-apply coating that might work on the barrels, but who knows if
companies or potters would undertake coating trials. Here I=3D92ll leave
you to keep thinking about the pug affair; I=3D92ve got some of my own
work to happily pursue...
(firing test pieces in the pellet wood stove =3D96 it=3D92s a blast! and
educational, too. I=3D92ve got a new, lovely, well-behaved olive-green
terra sig that holds its color and glossy shine after burnishing and
hours of heating while showing flashes of orange and pink. It=3D92s from a
novel material I=3D92m testing in clay bodies and slips, etc.)
Marian, Neon-Cat
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