L TURNER on mon 18 oct 10
"According to common perception, glass has a chemical durability
superior to most other materials. Nevertheless, when a piece of glass
is exposed to an aqueous solution, corrosion takes place." Reinhard
Conrad; "Chemical Durability of Oxide Glasses in Aqueous Solutions: A
Review"; Journal American Ceramic Society 91(3) 728-735 (2008).
It is available from:
http://onlinelibrary.wiley.com/doi/10.1111/j.1551-2916.2007.02101.x/pdf
A recent rant on the subject of glaze durability prompted me to reread
Professor Conrad's review of the subject. The quotation was part of
his opening remarks.
As I expected, corrosion of glass in aqueous solutions is considered
to be a localized interfacial process controlled by the chemistries of
the surface and of the solution. There are reliable models that
predict the probable resistance of the surface to a variety of
potential corrodents. Many of the models are referenced in the paper.
The review is current and includes research ongoing at the time of
publication.
Several years ago John Hesselberth and Ron Roy presented a model for
durable surfaces on pots fired to Cone 6. That model was presented at
NCECA and published in their book "Mastering Cone 6 Glazes" (MCSG).
The MCSG model is a simple, but workable, model of the corrosion (loss
of durability) of a category of glazes in common use by the ceramic
arts community. The approach is to provide guidelines for the
composition (oxide ratios) of glazes intended for functional pottery
with considerations of the aesthetics included as well. In the two
essays on his website (www.frogpondpottery.com) John presents an
explanation of the model, the critical assumptions, and the references
to the technical details that support it. The essays are easy to
read, understand, and to apply. Even if you use commercial or stock
studio glazes, the MCSG model provides insight into why some glazes
are more durable than others.
For those who consider the MCSG model too simplistic for their needs,
( or who just want to explore the science of glass corrosion), I
suggest you first digest the Conrad paper and then dig in and develop
your own model. You might want to polish your fluency of chemical
thermodynamics because that is the language of the subject.
Now, two final thoughts. (1) Models are simplified versions of
reality; therefore, all models are wrong. However, some models are
useful. (2) Paraphrasing Harry Callahan : "you gotta know your
model's limitations."
Regards,
L. Turner
The Woodlands, TX
David Finkelnburg on mon 18 oct 10
Dear Marian,
Interesting diatribe you post. I am surprised that you, who claim
scientific knowledge, would mislead people about glass science. It's not
hocus pocus! Many materials can be used to make glass but the results and
science are knowable, understandable and controllable. As you say, testing
is necessary.
The reason glaze calc is a good predictor of chemical durability has to
do with the fundamental structure of silicate glasses. A silicon atom fits
into the center of a tetrahedron formed by four oxygen atoms. The Si:O
ratio, though, is 1:2 (SiO2). Thus, two of the oxygens are shared with
adjacent tetrahedra in a continuously linked network. The reason silicon
does this has to do with the geometry of the tetrahedron and the radius
ratios of the atoms involved. Ref. see Linus Pauling's rules for atom
packing.
Aluminum atoms are a little smaller than silicon but they are close
enough that one can substitute for a silicon in the center of an oxygen
tetrahedron. However this creates a charge imbalance since Si4+ has one
more positive charge than Al3+. Thus, another cation is necessary, but tha=
t
is nicely balanced by a flux atom, Na+ or K+, or a Ca, Mg, Sr, Ba (all 2+)
balancing two sites.
Chemical durability suffers when there is a lack of aluminum atoms to
balance the fluxes necessarily present to make the glass melt. Then the
fluxes aren't charge bound and can be easily leached out by cleaning
solutions, etc, even water given enough time.
There's a lot more to this all, and you already know all this, but I've
skimmed through it to try to give others who are interested in the subject =
a
glimpse of why glaze calculation is an incredibly effective way to predict
where to begin testing for glaze durability.
Good glazing,
Dave Finkelnburg, watching sunrise over the windfall apple
tree where the bear cub did not come to feed this morning in the north Idah=
o
woods, part of a fall family tour...
On Sun, 17 Oct 2010 Neon-Cat wrote in part:
Ron, I'm so glad you mentioned testing. those who need to reassure
clients should test under conditions normal for their own studio or
pottery. There are some real limitations to trying to predict glaze
durability using mathematical relationships based on silica-alumina
ratios or silica-alumina-flux ratios. The phrase =3D93properly formulated
glazes=3D94 sounds so catchy, but to my sense is kind of useless as used
by some of you guys here. There are lots of great studies on how
glazes weather that have been done, especially since the beginning of
this century in industry for fast and slow-fired floor tiles, sanitary
ware, dinnerware, etc. It is almost a fantasy head-trip to use the
limits you all do without much consideration given to the real
ingredients used in a glaze recipe or without understanding what
products are created in a fired glaze. This kind of an approach is
flawed and leads to wrong assumptions regarding what is and is not a
=3D91good=3D92 glaze or what might be suitable for a saltwater fish tank. W=
hy
you stick with problems related to acidity and never mention the
effects of alkaline cleaning solutions or foods & beverages (and now
saltwater) or even plain water has always puzzled me. The lemon test
misses so much. If you don=3D92t know how to properly formulate a glaze so
that it is resistant in alkaline solutions how can you say one should
look to silica-alumina ratios as the solution to this person=3D92s
problem? Or that reading Mastering Cone 6 Glazes will bring the needed
insight? Some glazes will be durable in acidic but not alkaline
environments, some in alkaline but not acidic environments, some will
fail (or survive) in acidic and alkaline environments, and some will
fail in plain water.
Lee Love on tue 19 oct 10
On Mon, Oct 18, 2010 at 11:11 PM, L TURNER wr=
=3D
ote:
> Now, two final thoughts. =3DC2=3DA0 (1) Models are simplified versions of
> reality; therefore, all models are wrong. However, some models are
> useful. =3DC2=3DA0(2) Paraphrasing Harry Callahan : "you gotta know your
> model's limitations."
All cognitive/intellectual processes are based upon models of reality.
It is the basis of the Buddha's insight, "All is Mind." Science is
just beginning to discover this aspect of our experience.
Here is a quote from Soulcraft that is my current status message at Faceboo=
=3D
k:
=3DE2=3D80=3D8E"Our ability to make good judgements is holistic in characte=
r, and
arises from repeated confrontations with real things: comprehensive
entities that are grasped all at once, in a manner that may be
incapable of explicit articulation. This tacit dimension of knowledge
puts limits on the reduction of jobs to rule following." --Matthew B
Crawford
Working in "real life" (the way traditional knowledge is acquired),
puts experience before theory.
--
=3DC2=3DA0Lee, a Mashiko potter in Minneapolis
http://mingeisota.blogspot.com/
=3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim th=
em. Fe=3D
el
the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi
James Freeman on wed 20 oct 10
David...
I just wanted to thank you for this post. This was the best
description of what is going on chemically in a glaze to cause
stability. Best yet, you did it in three short paragraphs, without a
lot of unnecessary stuff borrowed from PhD dissertations, and above
all without hiding behind a wall of jargon that would be meaningless
to most of this audience (One of the full-blooded, bona fide chemists
on the list once told me, paraphrasing Samuel Johnson, that "jargon is
the last refuge of the scoundrel").
With few and succinct words, you have painted a vivid and useful
picture of what (in a greatly simplified but completely apropos way)
is happening in the glaze matrix. I can now see in my mind's eye a
vast webwork of oxygen atoms, each satisfying the linking points of
the silicon atoms which they surround, but also linking to and
satisfying neighboring silicon atoms, thus holding the whole group
together. I see an occasional aluminum atoms slipping into the
writhing mass in place of a silicon, and dragging along a couple of
flux or colorant atoms who bond and join in the fun, adding spice and
variety. Like some vast chemical orgy, as long as everybody's, um,
uh... "bonding points" are satisfied, they all stay at the party,
happy and content, so don't wander off bored and end up floating about
in our orange juice. If there aren't enough suitable partners to
satisfy all of the fluxes and colorants, they might leave the party
and seek some action elsewhere.
You also explained quite well what glaze calc and limit formulae do:
If you follow these very general rules and patterns, you have a pretty
good chance of satisfying all of your participants, and if they are
all satisfied, the chances of them staying at the party are pretty
good. Sure, you can alter the ratios of your invited guests most any
way you wish. You might end up with some pretty wild, interesting,
and even unexpected couplings and interactions by so doing, but you
may also end up with some participants without suitable partners, and
those unhappy folk might just wander away and start causing trouble!
Excellent work, David. You have simplified a very complex topic to a
point where we can comprehend it in a way that is quite sufficient for
what we do. Sure, we could all study chemistry in order to understand
it more completely and more accurately (as you have done), but this
would be massive overkill for those of us who just want to make a
beautiful and reasonably stable glaze. This is not
anti-intellectualism nor anti-science. It is rather a case of "learn
the deeper stuff if you are curious, but it is not necessary to get
the job at hand done". Thank you once again.
All the best.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice.=3DA0 I
should not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
On Mon, Oct 18, 2010 at 10:49 AM, David Finkelnburg
wrote:
.
> =3DA0 =3DA0The reason glaze calc is a good predictor of chemical durabili=
ty h=3D
as to
> do with the fundamental structure of silicate glasses. =3DA0A silicon ato=
m =3D
fits
> into the center of a tetrahedron formed by four oxygen atoms. =3DA0The Si=
:O
> ratio, though, is 1:2 (SiO2). =3DA0Thus, two of the oxygens are shared wi=
th
> adjacent tetrahedra in a continuously linked network. The reason silicon
> does this has to do with the geometry of the tetrahedron and the radius
> ratios of the atoms involved. =3DA0Ref. see Linus Pauling's rules for ato=
m
> packing.
> =3DA0 =3DA0Aluminum atoms are a little smaller than silicon but they are =
clos=3D
e
> enough that one can substitute for a silicon in the center of an oxygen
> tetrahedron. =3DA0However this creates a charge imbalance since Si4+ has =
on=3D
e
> more positive charge than Al3+. =3DA0Thus, another cation is necessary, b=
ut=3D
that
> is nicely balanced by a flux atom, Na+ or K+, or a Ca, Mg, Sr, Ba (all 2+=
=3D
)
> balancing two sites.
> =3DA0 =3DA0Chemical durability suffers when there is a lack of aluminum a=
toms=3D
to
> balance the fluxes necessarily present to make the glass melt. =3DA0Then =
th=3D
e
> fluxes aren't charge bound and can be easily leached out by cleaning
> solutions, etc, even water given enough time.
> =3DA0 =3DA0There's a lot more to this all, and you already know all this,=
but=3D
I've
> skimmed through it to try to give others who are interested in the subjec=
=3D
t a
> glimpse of why glaze calculation is an incredibly effective way to predic=
=3D
t
> where to begin testing for glaze durability.
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