Craig Martell on mon 5 apr 10
cause shivering
Marian was saying:
>Clayart experts write that lithium carbonate is especially prone to
>causing shivering. They cite a high coefficient of diffusion for
>lithium or wrongly proclaim that lithium carbonate is very soluble and
>lay the blame there
Hello Marian:
I was the one who wrote about the Coefficient of Diffusion but said
nothing at all about solubility. The only comment that I can
remember, via Clayart, regarding Li2CO3 solubility is that it is
slightly soluble and I honestly don't remember anyone saying that
this is direct cause of shivering.
Also, I've never thought of myself as an "expert" and no one on the
list has expressed that I am. Mainly, I'm a full time potter that
will study and use science as much as time permits.
>In fact, shivering can occur in glaze systems
>that use the insoluble lithium fluxes spodumene, petalite, and
>lepidolite. Diffusion of lithium into the clay body is not the answer
>to why lithium-based glazes sometimes cause shivering. Diffusion,
>ionic or molecular, stops when the water in the glaze stops vessel
>egress or evaporates
When 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. The best example of the result of
any sort of Diffusion in the ceramic process is the clay/glaze
interface. Lithium will diffuse throughout a glaze with a great
amount of ease (Cullen Parmelee/Cameron Harman PhD, Ceramic
Glazes). The problem with lithium carb is that lithium is very light
and a percent added to a glaze will contain a lot of lithium
molecules. There is evidence that an excess of lithium can result in
the formation of spodumene at the clay/glaze interface. This could
cause shivering and yes, I know the cause of shivering is over
compression of the glaze as in, it don't fit the claybody.
The person who originally proposed the migration of lithium forming
spodumene at the interface was mineralogist Michael Banks. This was
in a conversation we were having on Clayart. It's probably in the archives=
.
>Another flash -- beta-spodumene is anisotropic. You never read about
>anisotropy on-list. In glaze chemistry it's important to understand
>that many minerals and other substances can expand in not just one
>direction but along other planes, too.
I'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 Clayar=
t.
regards, Craig Martell Hopewell, Oregon
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