New Custer Feldspar - update
I just opened a new bag of Custer Feldspar. My last 50 lb bag was about 10
years old - so I recomputed the glazes
mashiko_ZJ_0 and mashiko_ZJ_1,
with essentially the same empirical formula according to
Ron Roy's analysis,
and fired them on ~3 inch diameter bowls.
Next I tested the original recipe with the new bag of Custer Feldspar.
I show pictures of the original glaze recipes mixed with the old bag of
Custer Feldspar, the recomputed glaze recipes
mixed with the new bag of
Custer Feldspar, and the old recipe mixed with the new bag of Custer
Feldspar.
same clay body, same firing protocol, same all other ingredients other than Custer Feldspar.
In all cases the glaze is thinner on the outside of the bowl.
The bowls with the old recipe, new bag of Custer Feldspar have an inlaid glaze
decoration on the inside.
a mask cut of shelf paper is afixed to the
bisque before the glaze is poured. The dried glaze is waxed with an emulsion
wax
before the mask is removed, then the second glaze is applied with a
slip trailer.
cone 10 oxidation
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
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze composition
mashiko_ZJ_0 and mashiko_ZJ_1 are high alumina, moderate silica, saturated iron Glazes:
Emperical Formula mashiko_ZJ_0
K2O 0.12
Na2O 0.1
Li2O 0.1
CaO 0.26
MgO 0.42
Al2O3 .82
Fe2O3 .23
SiO2 5.68
Emperical Formula mashiko_ZJ_1
K2O 0.14
Na2O 0.14
Li2O 0.12
CaO 0.2
MgO 0.4
Al2O3 .82
Fe2O3 .23
SiO2 5.73
The reference glazes with "old Custer" are on ~5 inches diameter bowls.
The glaze is shown on the inside and outside, resp left and right.
Glaze mashiko_ZJ_0
The recomputed recipe with the new bag of Custer Feldspar
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The original recipe with the old bag of Custer Feldspar
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The original recipe with the new bag of Custer Feldspar
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Glaze mashiko_ZJ_1
The recomputed recipe with the new bag of Custer Feldspar
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the original recipe with the old bag of Custer Feldspar
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the original recipe with the new bag of Custer Feldspar
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Commentary
The glaze mashiko_ZJ_0 with the original recipe and the new bag of Custer Feldspar is tolerably the same as with the old bag of Custer Feldspar.Note the prominent oil spots in this glaze. The differences are atributable to glaze application thickness.
One observes the difference between inside and outside of each bowl.
The glaze mashiko_ZJ_1 migrated further than can be accounted for by such
things as glaze application thickness or kiln location.
Even for this
glaze, the glaze with the original recipe and new bag of Custer Feldspar is
closer to the original glaze
than the recomputed glaze. One sees some oil
spots, in addition, the rim is metallic, not transparent.
Some half a dozen other glazes fired with the old recipe and new bag of Custer
Feldspar are unchanged.
These burnished metallic background, oil spot
glazes seem particularly sensitive to small compositional changes.