Tom Buck on sun 18 jan 98
Ron says he is offline for three weeks, so I will provide one answer
to Kris Baum's query.
Over the years, ceramicists (including potters) have kept records
of their successful glaze recipes, ones suitable for use on functional
ware (eg, food-servers) and these recipes, when converted to
Seger (molecular) Formulas, have been assembled into what are called
"Limits" or Limit Tables; and Ron and I and others who do glaze analyses
rely on these Tables to describe a "balanced" glaze.
For example, one such Limit Table (Insight's) lists these limits
for a successful C3-7 glaze (non-lead) ... which only means if you stay
within the limits you have an excellent chance of getting a good glaze on
most claybodies:
C3-7 Limits (Seger Formula)
CaO 0.1-0.7; ZnO 0-0.25; BaO 0-0.3; MgO 0-0.3; KNaO (alkalies) 0.1-0.5;
Li2O 0-0.2; SrO 0-0.4; B2O3 0-0.4; Al2O3 0.2-0.35; and SiO2 2.5-3.5
The first eight oxides above, either "R2O" or "RO", are called the
flux oxides and their total (expressed in moles) in any glaze's Seger is
1.0 (unity); so there is some variability in a recipe as one juggles the
flux oxides.
The remaining oxides are the amphoteric (R2O3) oxides, B2O3 and
Al2O3, and the chief glass-forming oxide, SiO2 (RO2).
As most of us know all too well, a glaze with too little Al2O3
(alumina) will misbehave, either run off the pot or cause other surface
defects. On the other hand, too much alumina can yield a dry matte of some
charm.
Boria (B2O3) is a touchy oxide. In small amounts it can behave as
if it were a flux oxide, and thereby turn a mattish glaze into a glossy
one, or a runny one. But too much boria, and 12% by weight seems where
too much begins, will yield an uncertain result, likely an undesireable
surface. And body/glaze interaction will render the effect of high boria
content quite unpredictable.
And, of course, the basic idea of using Seger Formulas is to make
glazes that will behave in predictable ways, that is, make good glass on
the pot's surface..
Finally, how much silica, SiO2, should be associated with so much
flux oxides to make a "balanced" glaze. For a C3-7 (midfire) firing, the
limits table says the number of moles of silica should be between 2.5 and
3.5. If one goes below 2.5 and above 3.5 the chances are high that a poor
glaze will result. Below 2.5 (assuming the boria and alumina are in the
proper range) one gets into dry mattes with an "open" surface improper
for food use. Above 3.5 moles of silica per mole of fluxes, there is
every likelihood that the glaze materials will not fully melt and form a
uniform glass on the pots' surface ...we call this a "immature" glaze.
Again, such a glaze is not suitable for food use.
Here endeth the lesson for today. Peace.
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).
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