Michael Banks on sun 13 jan 02
High expansion bodies are no problem to formulate and utilise minerals like
quartz and cristobalite which undergo abrupt volume changes (inversions) as
they cool through certain temperatures (573 and 220 degrees respectively).
Other minerals undergo even more extreme inversions and body recipes can
formulated with these, or the minerals can be induced to form in the body by
sintering (solid-state reactions). Examples of these include anatase
(TiO2), proto-enstatite (MgSiO3), wollastonite (CaSiO3), zirconia (ZrO2),
dicalcium silicate (Ca2SiO3) etc.
The biggest problem with utilising many of these materials in bodies is
getting the ceramic to survive the inversion. Dicalcium silicate can occur
in dolomite refractories and the inversion is so extreme (10% volume change)
that even a strong brick is reduced to fine powder. This fault is known as
To counter your strange frit crazing, add more fine silica to the body. If
you cannot raise the expansion of the body high enough (before it falls
apart from "silicosis"), try adding calcined talc. Calcine the talc to
1260oC. This will convert it to a mixture of protoenstatite and
cristobalite. Add that to your body mix and it should raise your expansion
tremendously. You may have to flux your body with something other than
feldspar or nepheline syenite though. Both of these will eat up all the
ultra-fine cristobalite. Maybe you could use one of your exotic lime-titania
eutectic mixtures or something.
trying to stay out of Mordor..
----- Original Message -----
Khaimraj Seepersad wrote:
> What is the highest expansion clay body that might
> be created [ vitreous if possible ] ?
> Or I am trying to fit some of my strange alkaline
> frits to a body , and need a push start - help .
> Isle of the Valar
> [ The Istari want you ----
> for the war against Melkor and Sauron ]