Ivor and Olive Lewis on mon 30 jan 06
Dear Theresa,
You comment <perfectly shiny! Could it be my calculation software gave me too much =
boric oxide from the Ulexite, or could it be that I over-fired some?>>
Perhaps you are correct, it is possible for an excess of boric oxide to =
increase the degree of solubility in a glaze and prevent Wollastonite =
(Calcium Silicate) from precipitating from the melt as the glaze cools. =
In addition, if cooling is fast there would be insufficient time for =
crystals to nucleate and grow so the result is a transparent glaze. The =
key to understanding this comes from a study of Time Temperature =
Transformation (TTT) graphs. This concept is explored in an article in =
Ceramics Technical, No 13. "Predicting Glaze Phase Transformations", 8 - =
14.
As John, Ron and Mel suggest, most glazes respond well to a slow cooling =
regimen. I would recommend this for glazes that have appreciable =
fraction of Magnesium Ox, Calcium Ox or Strontium Ox.
I am not sure that your glaze calculation software is the problem. =
People who design these electronic tools follow the generalised precepts =
of Glaze Science. Your glaze should have been formulated to fall within =
acceptable limits to achieve your aim. Ulexite is a little more =
problematic. As a natural mineral it may have a composition that is on =
the limit of what is acceptable. It is a hydrated natural Boron salt of =
Sodium and Calcium. If it has become dehydrated then the effective =
proportion of flux will have increased.
Reading "Mastering Cone 6 Glazes" by Ron Roy and John Hesselberth, or =
participating in a workshop with Ian Currie might give you a broader =
knowledge of the craft of glazing.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
| |
|
