shivering in a slow-up firing
zincSr_Z3S is a high zinc glaze, and shivering is a known risk factor for such
glazes. Yet I have
Only in the one firing, with the profile below, have I seen shivering. In the
image below, the network
I have stained these lines with ink, and show images of this, as no glaze has
as yet popped
A final observation - these are not the cracks of shivering, rather of
dunting. I observed that
Firing 1 has a normal upfire, a 20 minute hold at peak temperature of 2310
degF, and then is turned off.
Firing 2, which resulted in the dunting, achieves cone 10 at 2220 degF, a
temperature which is
The bowls are ~6.5 inches in diameter.
150 deg F an hour to 250 deg F
400 deg F an hour to 1800 deg F
300 deg F an hour to 2050 deg F
120 deg F an hour to 2310 deg F with a hold of 20 min at 2310 deg F
Turn Kiln Off
150 deg F an hour to 250 deg F
400 deg F an hour to 1850 deg F
120 deg F an hour to 2130 deg F
13 deg F an hour to 2190 deg F / hold 2 hours
13 deg F an hour to 2220 deg F / hold 3 hours
Turn Kiln Off
K2O 0.09
Al2O3 0.54
SiO2 2.78
molecular percent Silica 64.2%
Added:
2.5% cobalt carbonate
K2O 0.18
Al2O3 0.58
SiO2 3.61
molecular percent Silica 65.9%
Both bowls have a splash of a high rutile decoration glaze beneath the primary
cobalt glaze on the
Notice that this glaze in the first firing has a coarse texture, and is smooth
and nearly as glassy as
I might speculate that in the second firing, a more melted, glassier result is
pre-shrunk, thus has lower
Speculation aside, visibly the structure of this glaze differs in these two
firings, resulting in
fired this glaze successfully many times with a variety of
firing profiles, all of which get to cone 10 at 2310 degF
at a moderate ramp.
of fine white lines indicate the cracks in the
glaze.
off the pot. Running ones fingers over the cracks, one
feels the uplift of the glaze.
the inked crack lines later appeared on the inside of
the bowl, so filled it with water, and placed it on
a paper towel. This pot
leaks, those cracks go all the way through the pot. The remaining puzzle,
I've not previously seen dunting where the pot didn't crack apart.
commonly used for cone 6 firings. It achieves this
with a glacially slow upfire for the last 90 degF of
the firing. The
rate of climb is 13 degF an hour, in addition, there is a two hour hold at
2190 degF
and another three hour hold at 2220 degF.
Image of the glaze zincSr_Z3S in Firing 1
Image of the glaze zincSr_Z3S in Firing 2
Photo Ehanced Image of the glaze zincSr_Z3S in Firing 2
close up of a small section of an uplifted glaze crack
stained network of spidery cracks in glaze
oxidation firing to cone 10 in an electric kiln
Firing profile 1
Up Fire profile cone 10
Down Fire Profile cone 10
Firing profile 2
Up Fire profile cone 10
Down Fire Profile cone 10
Clay body is a grolleg porcelain from Clay Art Center in Tacoma, WA.
glaze compositions
Empirical Formula zincSr_Z3S:
Na2O 0.37
CaO 0.02
MgO 0.17
SrO 0.15
ZnO 0.20
decoration Glaze:
Empirical Formula hiAlk_Z3P_Z3V_0Mng:
Na2O 0.35
CaO 0.45
MgO 0.02
P2O5 0.07
TiO2 0.21
Remarks
inside of the bowl.
obsidian in the second firing. As the highest
temperature reached in the second firing is so very
much lower, likely the
structure of the interface between the glaze and body differs from that in the
first firing.
coefficient of thermal expansion below the glass
transition temperature, thus making shivering
more likely.
distinct physical properties, i.e. distinct thermal
expansion.