Firing Differences
cone 10 oxidation
Firing profile One
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
Firing profile Two
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 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
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
I show a glazes from the family of silvery oil spot glazes which have been much discussed and seen here:
Here I show one glaze in two different firings with the same peak
temperature and hold at peak. The cones look the same in both firings.
Both pots have a thick glaze coat on the inside and thinner glaze on
the outside.
In the first firing, the inside of the bowls is satiny, has prominent large oil spots
and a nearly flat surface, which shows only slight puckering,
but isn't
particularly dimpled.
In the second firing, the dimples on the inside of the bowl are prominent, the
oil spots nearly invisible.
The satiny ground to the oil spots seen in the
first firing is replace a a near mirror like full gloss. The outside shows a
shift in texture with the thinner glaze application, yet the highly dimpled
surface is still visible.
In this second firing, a slightly thinner glaze
coat is heavily textured, and nearly raw in appearance.
This glaze has a metallic silvery translucent layer over a brown/black
textured opaque layer. The thickness of the silvey layer
is determined by
both the thickness of the glaze application and the firing. The
differing angle
of the light on the transparent glaze exposes the underlying
brown layer. For this reason, using a ring light to take the pictures, the
differing angle will always make the outsides look somewhat browner than
otherwise.
For this family of glazes, the firing influences the appearance nearly as much as small changes in the composition
glaze composition
Emperical Formula mashiko_ZS_5_EU
K2O 0.12
Na2O 0.09
Li2O 0.1
CaO 0.26
MgO 0.43
Al2O3 .83
Fe2O3 .22
SiO2 5.82
molecular percent Silica 74 %
The bowls are ~ 6 inches in diameter.
The glaze is shown on the inside and outside, resp left and right.
The black markings on the outside of the first bowl are an inlaid
second glaze decoration created by applying a glaze over a mask,
waxing,
removing the mask and filling in the second decorative glaze.
The glaze is thinner on the outside of the each of the bowls
that is not the cause of the color change.
mashiko_ZS_5_EU is translucent, the
glazes in this family are brown with a surface layer of silvery crystals.
Firing One
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Firing Two
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