Craig Martell on tue 21 aug 07
Jeanie in Pa. was asking:
>Can you enlarge on adding cristobalite for glaze fit? That's an entirely
>new concept for me
Hello Jeanie:
I'll do my best but I'm not an Earthenware potter.
The basic idea is to add another silica inversion putting the glazes into
greater compression. You probably know about the quartz inversion which
happens at about 1032F. Upon heating, quartz expands from alpha to beta
quartz and there's an increase of volume, about 2%. When the ware cools
and the glazes have "set" and are bonded to the body the change from beta
to alpha quartz with a reduction in volume will put the glazes into
compressive fit providing they are not too high expansion. Because of the
increased need for high expansion fluxes in Earthenware it is sometimes
hard to get enough compression with quartz. Quartz also needs a lot of
temp to melt so you can't add a lot. Cristobalite will invert too and it
does so at about 435F. The extra silica inversion aids in more glaze
compression. In higher fired bodies, such as stoneware, the cristobalite
inversion can cause cracking and dunting. Especially with ovenware. It's
not a good thing to have in a high fired body.
I've never used Cristobalite to do the above. I have no idea where to get
it or how much one may need to add to a glaze. I just know that it has
been used in Ceramic Industry to aid Earthenware glaze fit.
regards, Craig Martell Hopewell, OR
Jeanie Silver on wed 22 aug 07
Thankyou, Craig
That was a very clear explanation. I have had good results with crazing by
bisquing to o5 oro4 and firing to cone 1, and by increasing silica in 5%
increments until crazing disappears. The cristobalite sounds like an
interesting thing to pursue,although its hard not to think of it as a
villain.
Jeanie in Pa.
Michael Wendt on wed 22 aug 07
The main industrial use I know of for cristobalite is
in investment casting. The investment is fired slowly
to remove water and then allowed to cool to just above
the inversion point for cristobalite. The molten metal
can then be poured into the investment which heats it
up but since it is above its inversion temperature, it
does not crack from thermal shock the way regular
silica might.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
Craig Martell on thu 23 aug 07
Jeanie responded:
>That was a very clear explanation. I have had good results with crazing by
>bisquing to o5 oro4 and firing to cone 1, and by increasing silica in 5%
>increments until crazing disappears. The cristobalite sounds like an
>interesting thing to pursue,although its hard not to think of it as a villain.
Hello Jeanie:
It's hard to not think of cristobalite as a "not so hot" thing to have in
your clay but I think a little is ok with Earthenware. Earthenware is not
tight and dense like stoneware so it can survive the "cristobalite
squeeze." But it sounds like you have no use for this stuff anyway so not
to worry.
regards, Craig Martell Hopewell, Oregon
Jeanie Silver on fri 24 aug 07
I think I would like to do a bit of primitive testing with cristobalite
added to my terracotta body-primitive because the commercial clay body's
formula is proprietary so I couldn't really tinker with the recipe. Does
anyone know of a source for cristabalite? Also, is there a similarity to
colloidal silica? I've crossed the frontiers of my ignorance-someone will
demand my passport soon...
Jeanie in Pa.
Ivor and Olive Lewis on tue 28 aug 07
Dear Jeanie Silver,=20
Though I do not doubt the possibility that adding Cristobalite to a clay =
body might adjust the value of its thermal expansion there is a =
reversible alpha-beta Cristobalite phase change given as 200-280 deg C =
which is rapid that could cause Dunting if a kiln were opened while =
still warm. Colloidal Silica and Cristobalite have the same empirical =
formula, SiO2, but Colloidal silica can adsorb large quantities of free =
water. It is used as a desiccant.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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