Effects of Firing
I show the effect of three different firing protocols on a glaze with metallic
surface crystals and leopard spots.
Two of these firing have slowed cooling for temperatures above 1900 deg F and
slightly below turn-down. My kiln will normally cool to 1900 deg F
The firing protocol for the first firing started its retarded cooling too
high - that together with the hold there
The metallic surface crystals are micro-crystals. The black leopard spots are
likely also crystals, as also the sharply defined
The difference in result between the first two firings, both with slower
cooling above 1900 deg F were small compared to
The three firings have the same Up Fire Profile, which is given here:
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
There are three distinct down fire protocols, given here:
300 deg F hr to 2285 deg F then hold 30 minutes
100 deg F hr to 1900 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 Two hour hold at 1700 deg F
300 deg F an hour to 1600 deg F then a two hour hold at 1600 deg F
This was a cone 11 firing.
300 deg F hr to 2210 deg F then hold 30 minutes
80 deg F hr to 1900 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 Two hour hold at 1700 deg F
300 deg F an hour to 1600 deg F then a two hour hold at 1600 deg F
This was a cone 10 firing.
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
300 deg F an hour to 1600 deg F then a two hour hold at 1600 deg F
This was a cone 10 firing.
K2O .1
Al2O3 .66
SiO2 4.04
molecular percent Silica 66%
The inside of the pot is shown on the left; the outside on the right.
Cone 11 and slow cooling from below top to 1900 deg F.
cone 10 and slow cooling from below top to 1900 deg F
cone 10 and no slowdown in cooling above 1900 Deg F
Light reflected off the metallic surface crystals causes the diffuse
fragmented white fog. The light is not reflected sharply
The glaze cooled slowly above 1900 deg F, shows more varied colors, a variety
of oranges and reds, and has halos surrounding the leopard spots.
For these saturated iron glazes high in Phosphorus, it seems that a hotter
firing - more heat work - results in less dramatic metallic crystal growth.
faster cooling above 1900 deg F results in less dramatic metallic crystal
growth.
Thinner glaze application results in less dramatic metallic crystal
growth.
in ~ an
hour and a half. Instead, in the two experimental firings, the time in this
interval is increased to ~ 4 hours.
The third firing is a control: it is
allowed to cool normally, the slow ramps and holds are below 1750 deg F.
resulted in
a cone 11 firing. The second firing was adjusted to avoid that problem.
red/yellow/orange
dendritic like lacy patterns. The slow cooling in a region where crystal
growth is rapid facilitated the growth of
large, coarse crystals.
the difference between the
result in either of these firings and the third firing. The extra time in the
temperature interval
below turn down and 1900 Deg F is more significant for
this glaze than final cone reached.
oxidation firing in an electric kiln
Firing profiles
Up Fire profile
Down Fire Profile 1
Down Fire Profile 2
Down Fire Profile 3
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze composition
Empirical Formula glaze satIron_mashiko_neph_1I_0 :
Na2O .1
Li2O .29
CaO .5
MgO .01
Fe2O3 .24
P2O5 .17
First Firing
Second Firing
Third Firing
bowls are ~ 4 inches in diameter
Remarks
as it would be off a
mirror or glassy surface.
In the
faster-cooled glaze, the background color, is more
uniform.