John Baymore on tue 7 jul 98
------------------
=3Cclip=3EAre most working glazes near the eutectic or the eutectic plus =
more
silica?
Someone said (I think) that the best clear glazes were at the eutectic
point?
=3Csnip=3E
Short of time here....so a quick partial thought:
Glazes that are located right at the eutectic mixture on a phase
equilibrium diagram often are very problematic. They have a VERY precise
firing range and composition, and slight variations from either will result
in wide variation of the fired glaze. An artist/craft potter's nightmare
=3Cg=3E.
If you look at a graph of the melting temperature of a mixture..... the
eutectic is the point that forms the bottom of the valley...... the lowest
melting combination of the particular materials in question. Often the
slope of the line is very steep on either side of the eutectic.......
indicating the wide variation in =22meltedness=22 of the mixture with only a
small change in composition or temperature.
The goal in formulating a glaze is usually to work NEAR the eutectic mix,
but in an area where variation in composition (to cover batching errors)
and variation in firing temperature (to cover things like kiln unevenness)
are less critical. This area will have a more gradually sloping curve on a
graph. Generally speaking... (oh god ....that again) the more gentle the
slope of the graph the more forgiving the glaze. Part of the goal of
testing is to find these less touchy mixtures.
On a phase equilibrium diagram the lines are sort of like the lines on a
topographical map. The closer they are together, the =22steeper=22 the =
slope
of the =22land=22. (You are looking =22down=22 on a graph that looked at =
from the
side would be a steep slope.) So glazes located in areas that have lots of
lines close together are often a bit touchy in practical use.
Best,
...................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
| |
|
