A line is crossed

Here I show two pieces with saturated iron background glazes: one is glossy black with silver colored
metallic markings, the other waxy-textured with a variety of browns and a lighter golden brown precipitate.

A line is crossed; two dissimilar appearing glazes have nearly equal total alkali metals and alumina, with small differences
in the other oxides in their empirical formulae. These glazes are in distinct glaze families. By that I mean
that the composition of the phases in these two glazes are different. The first glaze has a silvery metallic
precipitate which forms an irregular connected network, the other a brown pricipitate which occurs only in disjoint,
nearly equally spaced disks. The first is a tenmoku, the second not.

The design, which is not addressed in this analysis, is created by adhering a mask and then applying
the main glaze. After drying, the entire pot is coated in wax, then the mask is removed and the open area
filled in with a second glaze.

Image of the pieces

full view

mug with iron_mashiko_satIron_Z2Y and inlay alexanderBowl_0_Z1Y

mug is ~ 4.5 inches high.

full view

bowl with glaze mashiko_alk_179_1_Z30-0 and inlay hankPaper_Z1R_2C_1

Bowl is ~7 inches in diameter.

oxidation firing to cone 10 in an electric kiln

Firing profiles

Up Fire profile

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

Down Fire Profile

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

Clay body is a grolleg porcelain from Tacoma Clay Art Center.

glaze compositions

Background glazes

Empirical Formula iron_mashiko_satIron_Z2Y :

K2O        0.15
Na2O        0.36
Li2O        0.21
CaO        0.12
MgO        0.16

Al2O3        0.71
Fe2O3        0.21

SiO2        4

molecular percent Silica 67.6%

Empirical Formula mashiko_alk_179_1_Z30-0 :

K2O        0.15
Na2O        0.35
Li2O        0.22
CaO        0.10
MgO        0.18

Al2O3        0.69
Fe2O3        0.2

SiO2        3.84

molecular percent Silica 67%

Inlay Glazes

alexanderBowl_0_Z1Y :

K2O        .09
Na2O        .05
CaO        .70
MgO        .16

Al2O3        .43

SiO2        2.7

molecular percent Silica 65%

Added:

1% Cobalt Carbonate .67% Copper Carbonate

Empirical Formula hankPaper_Z1R_2C_1 :

K2O        0.12
Na2O        0.12
Li2O        0.05
CaO        0.14
MgO        0.02
BaO        0.31
SrO        0.24

Al2O3        0.42

SiO2        2.7
TiO2        0.14

molecular percent Silica 63%

Added:

1% Cobalt Carbonate .67% Copper Carbonate

Remarks

In the computed empirical formulas, the CaO:MgO balance differs; the first glaze, iron_mashiko_satIron_Z2Y,
having slightly higher CaO and lower MgO than the second, mashiko_alk_179_1_Z30-0. The first glaze has slightly higher alumina
and silica along with a slightly higher silica:alumina ratio.

None of these differences seem in themselves to account for the difference in appearance. Because the recipes contain
different materials, inaccurate materials analysis may have contributed to a greater difference in empirical formula
than seen in the computations.

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