Tom Buck on fri 6 oct 00
Will Edwards:
You ask about this recipe:
43.5 Gerstley Borate
43.5 Plastic Vitrox
13.0 Zircopax
This mix likely was invented a generation ago by a California
potter since both the GB and the PV were mined there for many years.
Glenn Nelson in the 1970 edition of Ceramics a Potter's
Handbook gives the composition of Plastic Vitrox and describes
it as "a complex mineral...used in both glaze and body formulas
as a source of silica, alumina, and potash. It is similar to both
potash feldspar and Cornwall stone."
Gerstley Borate, the mine now abandoned, is being replaced
by other materials. But from the 1970s til 1999, the composition
of GB underwent a fairly steady increase in alumina and silica.
(see below).
To estimate whether this glaze is a sound one when fired to Cone 6
in oxidation, one has to examine the nature of the components.
Firstly, let us be on the same page when we calculate
the Seger (aka unity) Forumla. To do so, we have to agree on
the analyses of the components.
Gerstley Borate 1997 (Hammill & Gillespie):
CaO, 19.1 weight percent; MgO, 4.1%; K2O, 1.4; Na2O 4.4;
Fe2O3, 0.5; B2O3, 27.7; Al2O3, 1.6; SiO2, 11.7; Loss on Ignition
(LOI), 22.5.
The same material had this composition in the 1970s:
CaO, 15.9; MgO, 3.5; K2O, unstated but likely 0.2; Na2O 4.9;
Fe2O3, 0.3; B2O3 30.4; Al2O3 1.1; SiO2 9.6; LOI 34.1.
Plastic vitrox was cited by Nelson (1970) as:
SiO2, 75.6; Al2O3, 14.9; CaO, 0.2; MgO, 0.2; K2O, 6.8;
Na2O, 0.3; LOI, 2.0.
Recent analysis gives these values:
SiO2, 78.8%; Al2O3, 12.1; Fe2O3, 0.1; CaO, 0.5; MgO, 1.1;
K2O, 5.1; Na2O, 0.3; LOI, at least 2.0.
Now, let us calculate the Seger as of 1970+ - it works out
as: Fluxes Modifiers Formers
CaO 0.54 B2O3 0.92 SiO2 2.95
MgO 0.17 Al2O3 0.29 ZrO2 0.29 (special)
K2O 0.13
Na2O 0.16 Ratio SiO2/Al2O3 = 10 Coef.Expansion=6.4x10E-6
Flux Total 1.0
The Seger today would be close to this:
CaO 0.72 B2O3 0.77 SiO2 3.77
MgO 0.06 Al2O3 0.39 ZrO2 0.32
K2O 0.11
Na2O 0.11 Ratio= 9.8 Expansion= 5.9x10E-6
We now have the needed information to discuss the nature of this mix.
In the 1970s mix, the flux oxides are ballpark for Cone 6, although
the MgO is a bit high (fortunately so). The alumina and silica are
mid-range, according most limit tables for good glass. The Boria
(B2O3) is well above the normal range for good glass at Cone 6 ---
at this level the molten glass would flow off the pot were it not for
the high MgO and the presence of refractory Zircopax. This material,
ZrO2.SiO2, is treated by many glaze calculation programs as if it
were two oxides going into the glaze, hence the separate listing in
the Seger for ZrO2. In reality, at Cone 6, the Zircopax is largely
untouched by the fluxes and glasses being formed (boro-silicate,
alumino-silcate, boro-alumino-silicate). Overall, though, one would
expect to obtain a glossy white glaze somewhat mobile if the coat
on the pot were too thick. The glaze would be durable but would tend
to be marked by knife or fork were it to be used on plates, etc.
Today's mix (using 1997 data for GB) is closer to common base
glazes in use (eg, Hansen's 5x20 base). Note that the B2O3 has declined
as has the MgO, while the Al2O3 and the SiO2 have increased, and the
two have moved beyond the usual 0.35/3.50 often stated as the upper
limit for good glass. But the Seger as given by the glaze calculation
needs some "interpretation" -- the Zircopax distorts the SiO2 value,
as calculated, making it appear high. Its actual value is approx. 3.5,
ie, in the ballpark for Cone 6. The two oxides, B2O3 and Al2O3, affect
the molten glaze in opposite ways, the Boria makes the glaze more fluid
while the second, Al2O3, makes it stiffer. Tests will likely show
the mix makes good glass, harder than before but still marked by
a knife or fork.
Finally, how much additive(s) can be incorporated in this
mix and remain fixed in place and not react with acidic foods?
Because of the Zircopax, suspended in molten glass on the pot,
the glaze would tend to have a reduced ability to accept colourant
oxides. I'd try up to 4% Iron Oxide Red, up to 3% Copper Oxide,
1-2% Cobalt Oxide, 1% Chrome oxide, perhaps 5-6% rutile. But it is
difficult to be certain, so test glazes with colourants should undergo
leach tests.
Good pots with PV & ??? BFN. Peace. Tom B.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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