Jim Murphy on mon 2 jun 03
Alrighty then,
Maybe CaO - alone - is a refractory but soluble Group 2 Alkali Metal Earth
Group Oxide (RO). Thanks for the refresher Ivor.
With deference to pure oxides, If one's goal is to be able to better harness
certain combinations of raw materials' capability to promote better fusion
and/or melting behavior at Cone 6, then essentially CaO compounds like
Limestone/Whiting, Dolomite, and Wollastonite are to be considered as
"auxiliary" fluxes while other Group 1 Alkali Metal Group Oxide (Li2O, Na2O,
K2O) raw materials need to serve as primary flux providers.
We're gonna have a hard time developing Cone 6 glazes based on CaO as the
sole "unity" formula flux material. I'm alright with that.
I presume Anorthite (Calcium Feldspar?) for CaO-Al2O3-SiO2 eutectic testing
had been sugggested - not solely for its high CaO content - but also for its
inherent Na2O/K2O (Group 1 Alkali) content to promote melting.
Sounds like for us to take advantage of the good things - like durability,
hardness, scratch and acid resistance, etc., - attributed by some to CaO
(not REALLY the "pure oxide" though), we'll naturally need to sway away,
conceptually, from the CaO-Al2O3-SiO2 eutectic.
I've read a bit about Cone 10 starting-point glazes based on 70% Potash
Feldspar & 30% Whiting (0.3 K2O 0.7 CaO) with a proper balance of Alumina
and Silica.
What about CaO content at Cone 6 though. Ron Roy noted, "One of the
interesting part of Johns experiments was the good effect
CaO had on stability - some of the glazes in our book are way over the
limits for CaO and yet are still stable." I'll go out a limb and presume the
higher CaO levels at Cone 6 work in the presence of little or no MgO.
Sure, Neph Sy (high Na2O content) works to promote fusion/melting at Cone 6.
What about combos of CaO and K2O at Cone 6 though to harness better
melting/fusing ? Some literature suggests lime causes a sharp increase in
the fusibility of potash feldspar but not of soda spar.
Perhaps use of a "fine" grade of Potash Feldspar with low pyrometric cone
equivalent (PCE) - like a fine grade low-PCE Volcanic Ash or Pumice - along
with Wollastonite to source CaO is worthy of consideration/testing for Cone
6 glazes.
Jim Murphy
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