Snow Flake Yet Again
Glazes are fired at cone 6 in an electric kiln.
The kiln is cooled slowly, at a rate of 50 deg F an hour in the temperature interval 1850 to 1700.
It is then cooled 25 deg F an hour in the temperature interval 1700 deg F to 1650 deg F.
There is then a 1 hour hold at 1650 deg F.
We last saw snow flake glazes:
We saw an additional test of the first glaze in this series, hiAl_4X_2,
with summary seger formula:
total alkali metals .77
alkaline earths equally divided between CaO and MgO
Alumina .66
Silica 3.3
Titania .23
and tests of two variations.
Additional tests of these two variations were not encouraging. Both the snow
flakes and the rutile blue ground phase are present,
but only with tight
control on glaze application thickness. A thick application is required for
the presence of two distinct phases,
yet just a bit thicker application
and either the glaze will crawl, or show unhealed blisters.
glaze hiAl_4X_2PSi
glaze hiAl_4_2PSi on plate 5 inches in diameter
hiAl_4X_2PSi "is" the glaze hiAl_4X_2 with a small amount of additional
silica.
I added 4 % silica to the glaze, which gave a 6 % increase in
silica in the empirical formula.
The appearance of the glaze is virtually
unchanged,
though if one examines the surface closely with 30x
magnification
the craze pattern is coarser, i.e. the craze lines are
spaced further apart,
giving hope that the additional silica strengthened
the glaze.
glaze hiAl_4Z_0 and hiAl_4Z_1
Both of these glazes have nearly the same summary empirical formula as hiAl_4X_2:
hiAl_4X_2 has Li2O = .2, these glazes have Li2O = .14. the intent is that
less Li2O would result in a glaze that had a broader firing range.
These two glazes differ only in the distribution of the CaO and MgO,
the first
has equal CaO and MgO,
the balance shifts, the second has predominantly MgO.
Although neither glaze has sufficient "flocking" to be interesting as a "snow
flake" glaze,
I found the difference produced by the shift between CaO
and MgO intriguing.
glaze hiAl_4Z_0 plate 5 inches in diameter
equal CaO and MgO
glaze hiAl_4Z_1 plate 5 inches in diameter
MgO dominates alkaline earths
With a shift to a glaze with alkaline earths dominated by MgO, I observe a
shift in color from pale blue to grey.
Additionally, the second glaze has a
smoother surface and is translucent.