iandol on fri 5 apr 02
Dear Richard Aerni,
Books on the topic. I use my rather out of date Britannica for a lot of =
the information I learn, but I did teach Senior High School Geology and =
Earth Science for a few years and did the Open U foundation Science =
Course which deals with these things. There is a wonderful self testing =
programmed text by Cox and Cox but after not teaching the topics for =
twenty years I cannot recall the title. "Understanding the Earth" Ed I. =
G. Glass is a good selection of essays but will be dated now. I have an =
elementary understanding, sort of pre undergraduate if you want to pin =
me down. Cross check everything I say.
Metamorphism is, as I understand it, graded into High Grade and Low =
grade. So you get combinations of temperature and pressure. High =
temp/high press to low temp/low pressure and stages in between, with and =
without the intervention of water. I think stoneware glaze would fit =
into high temp/low press pigeon hole.
The ACS Phase Equilibrium Diagrams are listed on their web site, about =
$10 a piece. Because they are large scale with one per cent scale =
divisions they can be interpolated to one tenth of one per cent. Anyone =
who has to teach glaze compounding and calculation would find them =
useful (Until bench analysis comes into the picture I would not call it =
"Glaze Chemistry"). I also have the 1964 Handbook of Phase Equilibrium =
Diagrams for Ceramists which ACS publish. It cost rather a lot but is =
worth its weight in Platinum. Has a diagram of the CaO-P2O5-SiO2-Al2O3 =
system showing the immiscible liquid phases.
The most important thing to remember is that Phase Equilibrium Diagrams =
are for guidance. But you have to learn how to read them. How many =
Ceramic arts course teach that skill? They tell us what will happen to a =
homogeneous solution, formed by totally melting and mixing the =
ingredients, as it cools slowly. We can only rely on it telling us when =
a glaze will melt and mature if we start with the Mineral species or =
Chemical compounds named in the phase fields. We cannot do this for any =
other combination of compounds even though they may contain the correct =
oxides in their proper numerical relationship in a Seger or Unity =
formula. My favourite example of this is the 1170 Eutectic in the =
CaO-Al2O3-SiO2 diagram. The raw oxides or their precursors do not melt =
at the eutectic temperature of 1170. But given a little time, the right =
proportions of Quartz, Anorthite and Wollastonite will fuse well at a =
slightly higher temperature. I know this because I did the prac. To say =
that a mixture of this, that and another oxide melts at a certain =
eutectic temperature is logically erroneous and ignores the rules and =
definitions.
Hope that helps to put things into perspective for you.
Best regards,
Ivor Lewis. Redhill, South Australia
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