Jeff Tsai on fri 17 may 02
HI,
I was thumbing through some books when I came across the expansion
so-efficient for Magnisia and several other glaze oxides. I noticed Magnesia
was relatively low on the scale, at least not extreme.
I know Magnesium is used to effect color and texture on glazes, and really
high magnesium glazes often become crawl glazes. However, I had always
thought that crawling was due to a different rate of expansiona dn
contraction between the glaze and the clay. After the clay had reached some
point where it was no longer shrinking much in the cooling cycle, the glaze
continued to shrink causing the formation of craze line, and inevitably,
platelettes. So I figured that for a glaze to crawl, it needed a high level
of expansion and shrinkage. Maybe I'm wrong there.
Anyway, that's why I was wondering why magnesia, which doesn't seem to have
an extraordinary coefficient of expansion, would be used to make crawl
glazes. Is there something I've totally missed?
I searched the archives and found nothing, though I couldn't really figure
out what to search under after a while, and I can't find it in my books.
-jeff
Clay Coordinator on sat 18 may 02
Jeff,
I believe that magnesium causes crawling because it has a high surface
tension.
There are other reasons for crawling like shrinkage, but remember that
expansion/contraction is not the same as shrinkage.
Differences in expansion/contraction between the clay and glaze cause
problems like crazing and shivering.
Hope that helps,
John Britt
Penland Clay
Snail Scott on sat 18 may 02
At 02:44 PM 5/17/02 EDT, you wrote:
>Anyway, that's why I was wondering why magnesia, which doesn't seem to have
>an extraordinary coefficient of expansion, would be used to make crawl
>glazes. Is there something I've totally missed?
It's not the coefficient of expansion (CoE) that
makes glazes crawl, it's the wet-to-dry shrinkage
in the early stages of drying (and firing) that
makes the glaze split apart. Firing fuses the
platelets of fractured glaze, but if the gaps
are too big, the glaze can't 'heal over', hence
the crawl effects. Some other glazes derive their
crawling properties from high surface tension
instead, and only split apart after melting. (I'm
not sure which sort the high-magnesium glazes are;
I'd bet that some crawling glazes are really both.)
But, I can't think of any which rely on the CoE
to create such effects. I am under the impression
that CoE really only becomes a factor after the
glaze has cooled and become solid. If it's still
liquid enough to pull and move, the CoE is not
yet relevant.
-Snail
iandol on sat 18 may 02
Dear Jeff Tsai,
Good question. Crawling is about surface tension and adhesion rather =
than thermal expansion and contraction. Magnesia, which is highly =
refractory, appears to impart high surface tension as it dissolves into =
a melting glaze. If the glaze has fractured as it dried on the pot, =
leaving a network of fine fissures, these do not heal to retain a =
continuous coating. The glaze starts to ball up and pull in to itself as =
melting proceeds. I would imagine that if there are residues of magnesia =
adhering to the clay they will prevent wetting as well.
Best regards,
Ivor Lewis
Craig Martell on sat 18 may 02
Hi:
John Britt has already pointed out the importance of applying the terms
"shrinkage" and "expansion" to the appropriate scenario so I'll keep quiet
about that.
Magnesia does indeed have a very high surface tension and viscosity. When
you look at the crawled magnesia glazes from a complete glaze view instead
of just one of the oxides, such as magnesia, you will see a real high
surface tension glaze. Carlton Ball's white mud flat looking glaze is
Magnesia and ball clay. Ball clay contributes a lot of alumina which has
the highest surface tension of any oxide used in glazes. The viscosity
being high too does not allow the glaze to flow when melted and keeps the
cracks from healing.
regards, Craig Martell in Oregon
Ron Roy on sun 19 may 02
Hi Jeff,
First of all - it is magnesium carbonate that is used to promote crawling -
because it shrinks so much as the glaze is in the early stages of firing.
The more used the greater the cracking of the glaze. Magnesium in Talc and
Dolomite does not have the same effect.
Most glazes crack up to a certain extent during early stages of firing -
when the alumina or magnesia is high enough the flow rate of the glaze at
the top of the firing can be inhibited enough that those gaps do not heal
over - due to high viscosity and surface tension of both Magnesia and
Alumina.
It is true that Magnesia has a comparatively low expansion rate compared to
the other fluxes we use - this is helpful in controlling crazing - which
happens after the glaze has "frozen" during cooling. High Magnesia glazes
can have low enough CTE's (Coeficient of Thermal Expansion) to cause
shivering and dunting - depending on the CTE of the clay of course.
So crawling happens on the way up - crazing and shivering happen on the way
down due to differences in the CTE of the clay and glaze.
The use of Magnesium is counter indicated with some glaze effects but
necessary in others - knowing which is crucial for anyone wishing to adjust
and invent glazes - see the Hamer book for this kind of information - best
text that I know of.
RR
>Anyway, that's why I was wondering why magnesia, which doesn't seem to have
>an extraordinary coefficient of expansion, would be used to make crawl
>glazes. Is there something I've totally missed?
>
>I searched the archives and found nothing, though I couldn't really figure
>out what to search under after a while, and I can't find it in my books.
>
>-jeff
Ron Roy
RR #4
15084 Little Lake Road
Brighton
Ontario
Canada - K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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