Neon-Cat on sat 17 apr 10
s cause shivering
Hi Craig,
You wrote: =3D93When I was writing about Diffusion, I didn't mean
diffusion related to glaze application. I was talking about diffusion
during the firing when the glaze is molten.=3D94
Good of you to clarify where and when you were thinking lithium
diffused readily. Lithium diffusion into a body being fired, if it
occurs, might be minimal. For a sintering and expanding clay body that
has given up much of its porosity at elevated temperatures and whose
ions are already involved in reactions and phase transformations (a
porcelain stoneware or kaolin-based clay body may already have formed
its stable crystalline and amorphous phases by 1050 C =3D96 quartz,
feldspar, hematite and mullite) only positive ions that are larger
than those initially in the clay will diffuse into the clay body. The
larger ions trade places with smaller ions near the surface of the
clay body at the clay-glaze interface. As the clay body and glaze
cools these ions no longer fit; they=3D92re trapped in the interface
structure and contribute to surface compression.
Lithium in a glaze may trade places with ions in the clay body that
are larger than its ion (sodium, potassium, barium, calcium,
manganese, iron 2+, and maybe titanium). The ionic sizes, in nm:
lithium (Li+) 0.068; sodium (Na+) 0.097; potassium (K+) 0.133; barium
(Ba2+) 0.134; calcium (Ca2+) 0.099; manganese (Mn2+) 0.080); iron
(Fe2+) 0.074; iron (Fe3+) 0.064; titanium (Ti4+) 0.068; magnesium (Mg
2+) 0.066; aluminum (Al3+) 0.051; silicon (Si 4+) 0.042, and boron
(B3+) 0.023. Diffusion may occur outward from the clay body back to
the glaze, too. Diffusion models and theories abound but the fact
remains lithium (introduced as carbonate or some other way) does not
wander off by itself. The old tried and true model of =3D91hopping=3D92 sti=
ll
seems to hold. Lithium moves in the network it is in by rules and ion
exchange is one of the mechanisms.
So, viewed as just a flux, without a lot of concern for diffusion or
potential diffusion, lithium carbonate contributes to early lowered
viscosity of glazes and reactions and transformations primarily in the
glaze as opposed to clay-body interface or clay body. Lithium is so
reactive that at higher temperatures if it does not become involved in
sintering reactions, much is lost (volatilization) with the escaping
gases from the earlier decomposition of such things as magnesium and
calcium carbonates and fluxes.
You (Craig) wrote: =3D93I've never heard the term anisotropic with regard
to thermal expansion stuff. Did you mean to say "anisometric?"
Anisometric is the only term I've ever encountered regarding expansion
on different planes and It's not found in many ceramic texts. Ceramic
Engineering texts yes. That's where I encountered the term and a lot
if info about it. You are correct though, I've never seen this
discussed on Clayart.=3D94
No, Craig, I meant to write =3D93anisotropic=3D94.
Anisometric is a term that refers to a structure having unsymmetrical
parts or unequal measurements, as in a crystal with unequal axes.
Anisotropic is a term that refers to a substance that has different
physical properties in different directions. We fire a crystalline
substance with or without symmetrical axes and it expands at different
rates along the different axes -- the response to stress is unequal.
My problem with a strictly linear approach to thermal expansion as
often discussed on clayart is with the hysteresis anisotropy
introduces in dilametric testing, the limitations of TDA (especially
single contact-pushrod older model units), and the shortsightedness of
the entire linear approach to thermal expansion imposes on thoughts
about clay body and glaze chemistry. Anisotropy is far too important
not to be considered and the failure to do so leaves those content
with linear expansion vulnerable to mistakes of all sorts. Optical
dilametric testing is now the norm in ceramic industry because it can
measure the anisotropy of a material right through an entire firing
sequence from its inception to final form. New theories of just what
forces hold our clay bodies together are quite interesting and
fascinating and utilize some of the concepts of stresses set up by our
materials that do have anisotropic properties.
Many ceramic materials will show different values for changes in the
direction of expansion based on their structure. Quartz, for example,
shows a thermal expansion coeffecient of 14 X 10(6)/C along the c axis
but 9 14 X 10(6)/C parallel to the c axis. Calcium carbonate, CaCO3,
shows -6 X 10(6)/C along the c axis but 25 X 10(6)/C parallel to the c
axis. Titania, TiO2, 6.8 X 10(6)/C along the c axis but 8.3 X 10(6)/C
parallel to the c axis (it is sometimes hard to format science
expressions into clayart posts -- just make the best of this notation
method). Even the bricks used in kilns, depending on composition, can
have anisotropic behavior perpendicular and parallel to the pressing
direction used during their forming and this will dictate their
properties during use and good installation practices will keep brick
anisotropy in mind. So, when applying only a linear idea of thermal
expansion to glaze and clay body fit we=3D92re missing the 3-D aspect
(among other things) involved with fit. Besides, the values reported
for a material are only valid for the temperature and time at which a
sample was fired =3D96 glazes and clay bodies will have different values
depending upon firing conditions with changes in clay body coefficient
of linear thermal expansion more pronounced than changes in glaze
specimens. The average linear thermal expansion coefficients of
ceramic materials may have an extended range within a specified
testing range of temperature and samples will show non-linearity
around phase transition temperatures. The coefficient of linear
thermal expansion is but one tool that was commonly used in the past
and a pretty limited tool at that.
As an example in the context of this thread, the structure of
beta-spodumene consists of corner sharing silicon and aluminum
tetrahedra that form interconnected five-membered rings that lead to
little channels in the beta-spodumene structure. We say the ultra-low
thermal expansion of beta-spodumene is due to anisotropy: we have the
release of strain in the five-membered rings; the =3D91a=3D92 and =3D91b=3D=
92 axes
contract, and corner-connecting oxygen atom angles increase, and the
=3D91c=3D92 axis expands. This is spodumene getting comfortable in its
surroundings and the available space it has. This =3D91getting
comfortable=3D92 and the transformation of alpha to beta spodumene results
in a conformation with net ultra-low thermal expansion for
beta-spodumene. Anisotropy is not a novel concept; I smash my finger
hammering old street bricks into grog for speckling a clay body and my
finger swells all over, it does not just lengthen.
Anyway, sorry for the slow response and the post that I hacked to make
fit our limit back when (the original post did address ion exchange) =3D96
I=3D92ve been off-line making some life changes, putting the finishing
touches on recluse spider bite healing (those spiders can kick a--),
working on my new =3D93Iron Maiden=3D94 claybody using a novel clay, and
having a delightful spell of handbuilding. I thought lithium glazes
interesting at the time they came up so wanted to share some different
thoughts with the list -- no big deal. Shivering just makes more sense
to me knowing the end products we form during firing than by trying to
predict shivering potential by considering either diffusion at any
point during glaze application and firing or the linear expansion of
oxide proportions. We all think about things differently, one of the
beauties and strengths of clayart.
Marian
Neon-Cat
Neon-Cat on sun 16 may 10
s cause shiv ering
Wow, Craig, this is like a blast from the past -- with worlds upon
worlds of thoughts in the meantime.
One should never need feel the need to apologize for not knowing
something. Mel's idea of 'learning together' is not a bad concept.
As to ceramic engineering, beats me. Thinking about expansion and
contraction and related interactions in "3-D" seems an old idea to me.
Back when I worked with boilers and electricity the idea was there
although few "fancy" terms were employed. The idea and term crop up
among amateur rock hounds and geology buffs, too. It was there again
later when I started with first year undergrad college chemistry,
organic and inorganic. In human interactions of all sorts,
particularly with reference to expansion, expansion in all directions
can be good. Now when hand building in clay I instinctively consider
not only how things might expand and contract linearly, but also
vertically and in all directions -- it's how I balance work and think
of it making it through firing. I just find it odd and limiting that
here on clayart during clay science chat the trend is to think along
lines of linear expansion only.
Anyway, tonight I'm wondering why time itself seems to expand and drag
in all dimension while waiting for that last 400 degrees of kiln heat
to dissipate before kiln opening. I might have something interesting
to share -- or not -- guess morning will be soon enough...I'm all worn
out waiting...
Thanks for the note.
Happy clay-making!
Marian
Neon-Cat
On Sat, May 15, 2010 at 9:46 PM, Craig Martell wrote:
> Hello Marian:
>
> This is response is so late I'm wondering if anyone remembers what
> was up in the first place.
>
> Anyway, I wanted to thank you for your email and apologize for not
> knowing what anisptropy is. =3DA0Actually, prior to your email Dave
> Finkelnburg wrote and clued me to that fact that this is a
> reality. =3DA0I looked everywhere in my arsenal of references and found
> nada. =3DA0Dave said that anisptropic stuff is a course of study for
> graduate students of Ceramic Engineering. =3DA0So it goes.
>
> regards, Craig Martell =3DA0Hopewell, Oregon
>
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