Slowing the melting - the up fire
Comparing two firings,
a base line firing, and a second modified firing in
which the time between cone 6 and cone 10 is doubled.
I thank Hank Murrow for encouraging me to explore the rate of climb.
cone 10 oxidation
The two firings:
Base line Firing profile
Up Fire profile
150 deg F an hour to 250 deg F
400 deg F an hour to 2050 deg F
120 deg F an hour to 2250 deg F
60 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
Down Fire Profile
A half hour hold at 1750 deg F
A three hour hold at 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
A one hour hold at 1650 deg F
Slow Firing profile
Up Fire profile
150 deg F an hour to 250 deg F
400 deg F an hour to 2100 deg F
120 deg F an hour to 2200 deg F
60 deg F an hour to 2240 deg F
30 deg F an hour to 2310 deg F NO HOLD
Down Fire Same as Above repeated for reference
A half hour hold at 1750 deg F
A three hour hold at 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
A one hour hold at 1650 deg F
The second firing has twice as much time between cone 6 at 2100 deg F and cone
10 at 2310 deg F.
In the Up Fire part of the slow firing note the ramp of 30 deg F an hour
from 2240 deg F to 2310 deg F.
This gives 2 hours and 20 minutes between 2240 deg F ~cone 8 and 2310 which
is cone 10.
The hold at top temperature was eliminated to avoid overfiring.
With a climb rate of 30 deg F an hour at peak,
the hold was thought
unnecessary. The cones were the same in both firings.
cone 10 is horizontal.
I'd intended that more time in the early melted stage
would allow time for the
the various chemical reactions to proceed
to completion before confounding
alternative reactions became posible at higher temperatures.
I show three glazes in each of the two firings. In each of these glazes, the
before and after pictures show
distinctive changes that indicate a more
mature glaze -
a less lumpy surface
- a greater development of the silvery metallic phase
more glossy black and less prominent mud brown
coloration
less bubbling within the glaze
The mud brown is the color of the glaze before the iron has fully disolved into it, The glaze not hot enough to absorb all the iron.
There were several glazes not shown which were nearly identical in the two
firings. I conjecture that those glaze
started melting closer to cone
5 than to cone 8. For these glazes extra time between cone 5 and cone
8
is required to elicit similar effects. I will be testing that
hypothesis, in a future firing.
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
Several bowls have a second glaze inlay design created by applying a contact
shelf paper mask to the bisque before glazing.
Then the glaze and mask are
coated with an emulsion wax. The mask is stripped when glaze and wax are
dry,
and a second glaze is applied to the now uncovered bisque.
glaze composition
Emperical Formula mashiko_ZJ_3:
K2O 0.04
Na2O 0.17
Li2O 0.13
CaO 0.26
MgO 0.4
Al2O3 .83
Fe2O3 .23
SiO2 5.74
molecular percent Silica 74 %
Emperical Formula mashiko_ZV_3:
K2O 0.15
Na2O 0.14
Li2O 0.12
CaO 0.21
MgO 0.38
Al2O3 .83
Fe2O3 .21
SiO2 5.86
molecular percent Silica 74 %
Emperical Formula mashiko_ZS_T_4:
K2O 0.1
Na2O 0.06
Li2O 0.1
CaO 0.27
MgO 0.47
Al2O3 .83
Fe2O3 .22
SiO2 5.82
molecular percent Silica 74 %
Each glaze is shown on the left in the base line firing, and on the right in the slow firing.
Where both an inside and outside picture are shown, the base/slow of inside and outside are shown paired on left and right.
The glaze is often thicker on the inside, which enhances the effect of the differing geometry on the firing results.
Glaze mashiko_ZJ_3
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In the base line image on the left, note the heavy dimpling, the oil spots have the appearance of sitting at the bottom of a lake.
some of the bubbles beneath the oil spots indeed seem to be sitting on the clay.
This appearance in contrast to the slow cooked version on the right which has a satin surface, with oil spots sitting flat on the surface.
Glaze mashiko_ZV_3 inside
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Glaze mashiko_ZV_3 outside
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Note brown tint in the base line firing image on the left, contrasted with the glossy metallic silver surface in the slow fired version on the right.
Glaze mashiko_ZS_T_4 inside
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Glaze mashiko_ZS_T_4 outside
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Note the color of the baseline image on the left, underfired brown, as compared to the silver
and gloss black in the right image.