Tom Buck on mon 3 mar 97
Gretchen:
Because he uses and teaches glaze calculation via computer software,
Ron Roy may have assumed that all readers of this post were aware
that he sees a glaze not as a batch recipe but as a Seger (Unity)
Formula, and immediately seeks to see where the glaze fits into a
specific firing range. He does so by comparing the glaze's Seger
formula to well established data in the literature. (See books by
Cooper, Hansen, Hamer, Zakin, others).
In a firing, the raw materials (other than simple oxides) change
into essential oxides that hopefully will melt into liquid at high
temperature and then these oxides, molecule by molecule, will join (be
incorporated into) the growing huge molecule called glass, some
characteristics of which Ron discussed.
The glaze being discussed "Cream Breaking Red" C6 Ox was cited
as the following (Note: materials are converted into "molecular parts" for
listing in the Seger formula by taking the percent composition of each
essential oxide in the material, and dividing this amount by its molecular
weight. Silica excepted, the number is less than 1.0):
Batch recipe Seger formula (boria, B2O3, not a flux)
35 Gerstley borate CaO 0.67 Fe2O3 0.04 SiO2 1.21
15 Kona F-4 feldspar MgO 0.09 B2O3 0.44 SnO2 0.27
10 Whiting BaO 0.08 Al2O3 0.10
5 Barium carbonate KNaO 0.16
10 Flint (Total= 1.00)
13 Tin Oxide
2 Iron oxide red Ratio SiO2:Al2O3 = 12.1
Expansion=8.8x10E-6 without effect of Tin Oxide,
likely to lower it to approx. 8 x10E-6.
Zakin cites R.Schmitt's limit data for Cone 3-5 (Transparent to Glossy)
as follows in molecular (Seger) values:
CaO 0.05-0.4; Na2O 0.05-0.4; K2O 0.05-0.4; Li2O 0.0-0.1; ZnO 0.0-0.1;
BaO 0.0-0.05; MgO 0.0-0.05; PbO 0.0-0.5; SrO 0.0-0.4.
Any combination of these flux oxides that total 1.0 is expected to produce
a "balanced" mature glaze when combined with
Neutral/amphoterics Al2O3 0.2-0.3; B2O3 0.0-0.5; and
Glass-formers: SiO2 2.0 - 3.0.
Using these criteria, the Cream Breaking Red glaze fails in every major
oxide category: In the flux oxide group, CaO is too high; the Al2O3 is
half what it should be; and the SiO2 at 1.2 is well below 2.0 minimum
(although the Tin Oxide compensates in part).
If a glaze contains considerable boria (and this one does), it often
helps to include boria as a flux (as Ron did):
Seger formula, boria as flux
Fluxes: CaO 0.46; MgO 0.07; BaO 0.05; KNaO 0.11; B2O3 0.31 (=1.00);
Neutral/amphoterics: Fe2O3 0.03; Al2O3 0.07;
Glass-formers: SiO2 0.84; SnO2 0.19.
Now, Cooper's limit data applies: Fluxes--CaO 0-0.55; MgO 0-0.325;
KNaO 0-0.375; ZnO 0-0.3; BaO (SrO) 0-0.4; B2O3 0-0.35;
Amphoterics: Al2O3 0.275 - 0.65; Glass-formers: 2.4 - 4.7.
Using Cooper's criteria, the Cream Breaking Red glaze falls within the
limits for flux oxides but is drastically low in Al2O3 and SiO2.
So, as Ron Roy concluded, this glaze is not suitable for coating
functional ware to be used in food service.
It may be useful to see how a likely candidate for food-use might look;
it is one of many that could be designed, using the original Seger and
adjusting it:
Cone 6 glossy glaze based on Cream Breaking Red
Batch recipe Seger formula (boria not a flux)
41.0 G200 (or equal) CaO 0.58 Fe2O3 0.05 SnO2 0.30
22.0 Gerstley borate MgO 0.07 Al2O3 0.28 SiO2 3.11
9.0 Whiting KNaO 0.28 B2O3 0.31
3.0 Strontium carb. SrO 0.07
25.0 Flint
13.0 Tin Oxide
2.0 Iron oxide red Ratio SiO2:Al2O3 = 11.1 Expn= 7.3 x10E-6
I'm sure Ron Roy will add to my input. And further questions on this
topic are welcomed. Good glaze testing.
Cheers Tom.Buck@freenet.hamilton.on.ca
Hamilton, Ontario, Canada
Jim Horvitz on tue 4 mar 97
I found Tom Bucks analysis of the cream breaking red exciting. I am not sure
I completely understood all his comments but I tried too. Which book of those
recommended is best for one new at glaze formulation ? I would love to
attend a workshop--does anyone know of a good one comming up on the West
Coast. Jim Horvitz Rancho Mirage CA.
David Hewitt on thu 6 mar 97
In message , Tom Buck writes
>----------------------------Original message----------------------------
Snip
>Zakin cites R.Schmitt's limit data for Cone 3-5 (Transparent to Glossy)
>as follows in molecular (Seger) values:
>CaO 0.05-0.4; Na2O 0.05-0.4; K2O 0.05-0.4; Li2O 0.0-0.1; ZnO 0.0-0.1;
> BaO 0.0-0.05; MgO 0.0-0.05; PbO 0.0-0.5; SrO 0.0-0.4.
>
>Any combination of these flux oxides that total 1.0 is expected to produce
>a "balanced" mature glaze when combined with
> Neutral/amphoterics Al2O3 0.2-0.3; B2O3 0.0-0.5; and
> Glass-formers: SiO2 2.0 - 3.0.
>
An additional way of looking at these figures is to express them as %mol
parts, basic, amphoteric and acidic and see the range in these terms.
Seger %mol parts Seger %mol parts
Basic 1.0 31.25 1.0 20.8
Ampho. 0.2 6.25 0.8 16.7
Acidic 2.0 62.50 3.0 62.5
____ _____ ____ _____
3.2 100.00 4.8 100.00
If you now use a triaxial graph, with say 100% Acidic at the top point,
100% Amphoteric at the bottom left point and 100% Basic at the bottom
right, you can plot these two points and, if the above limits are
correct, expect to get a mature 3 - 5 cone glaze anywhere between these
points. This in essence is a phase equilibrium diagram and gives you a
visual presentation of the glaze from which you can deduce, for example,
if your glaze is matt if it is due to an excess of alumina say.
Da
vid Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/hewitt.htm
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