Firing time vs. temperature
I show the difference between two firing profiles on a saturated iron glaze: a
slow, colder firing, versus a fast hot firing.
The slow, colder firing has more time in the approach to cone 10, a lower
firing temperature and longer holds in the cool-down profile.
Previously I'd shown that contrast
I again show a different glaze in two cone 10 firings.
This glaze exhibits micro-crystalline surface crystals with both firings, but
the glossy, orangey matrix has dissappeared in the low, slow firing.
The first firing is to 2310 deg F,
fast firing to cone 10 at 2310 deg F.
Inside of pot:
Outside of pot:
slow firing to cone 10 at 2220 deg F
Inside of pot:
Outside of pot:
The differences between the first and second upfire profiles are:
The top temperature, 2310 deg F for the first, 2220 deg F for the second.
The temperature ramp is steeper for the first firing, slower for the second.
Faster and hotter:
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 minutes at 2310 deg F
300 deg F an hour to 1750 deg F then a half hour hold at 1750 deg F
300 deg F an hour to 1700 deg F then a three hour hold at 1700 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
Slower and cooler, yet including what I had previously considered the crucial
hold near 1750 deg F for saturated iron glazes.
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 2180 deg F / hold 2 hr
13 deg F an hour to 2220 deg F / hold 3.5 hr
300 deg F an hour to 2000 deg F then hold 3 hours
300 deg F an hour to 1725 deg F then hold 3 hours
25 deg F an hour to 1650 deg F then hold 1 hr
K2O .08
Al2O3 .61
SiO2 4.03
molecular percent Silica 66.7%
This glaze continues my exploration of high phosphorus glazes, as suggested to
me by Hank Murrow.
The design on the second pot is created by adhering a mask and then applying
the main glaze. After drying, the entire pot is coated in wax,
This glaze in my usual firing has a glossy, orangey-brown matrix
with a lacy overlay comprised of irregular black markings framed in
silvery-purple
With a medium thick application as seen on the outside, there are silvery
markings on an orangy background,
In the variant firing, to a lower temperature with a slower ramp, The
orangey-brown brown matrix is gone. The result is a nearly uniform
admixture
Interestingly, this glaze is significantly more thoroughly melted and,
consequently, more homogenized in the lower-temperature, slower firing.
the second slower but only to 2210 deg F.
First Firing
Second Firing
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile 1
Down Fire Profile 1
Up Fire profile 2
Down Fire Profile 2
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze composition
Empirical Formula glaze satIron_mashiko_neph_1I_0I :
Na2O .11
Li2O .28
CaO .51
MgO .02
Fe2O3 .25
P2O5 .15
The first bowls is ~4 inches in diameter, the second is ~7 inches in diameter
then the mask
is removed and the open area filled in with a second glaze.
Remarks
metallic micro-crystals.
Where the glaze is especially thick, the orangy brown matrix is gone, leaving
behind
only the black markings surrounded by the metallic
micro-crystals.
but the black markings are nearly gone.
This is seen in the first pictures.
of lustrous black with purple metallic microcrystals. The
metallic nature of the purple-brown crystals is seen
in the mirror-like like
reflections. This is seen in the second set of pictures.