Crack Pattern

Here is a glaze, longquan_iron_1_1 with a pattern comprised of a series of branching edges which appears to be
craze lines. Virtual craze lines as it happens.

Closer perusal with a jewler's loop, 30x magnification, indicated pale yellow lines on a darker background.
These lines don't break the surface of the glaze. Yet closer inspection with a digital microscope indicates that the
lines are not sharp, rather the yellow diffuses into the background glaze, like watercolors on wet paper.

I show images of the bowl and a micro-graph from the digital microscope.

The image from the digital microscope has been much edited to emphasize the virtual crack pattern seen in the
inside of the bowl. Note the diffuse bleeding of the yellow from the pattern into the background glaze.

While at the upper edge of the inside of the bowl these lines mimic cracks, in the enlarged micro-graph,
it is clear they are not even straight lines.

Bowl with glaze longquan_iron_1_1

full view

inside

full view

outside

micro graph showing enlargement of crack pattern

bowl is ~ 3.5 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

Empirical Formula longquan_iron_1_1 :

K2O        0.13
Na2O        0.27
Li2O        0.12
CaO        0.38
MgO        0.06
ZnO        0.04

Al2O3        0.56
Fe2O3        0.17

SiO2        3.3

molecular percent Silica 65.7%

Remarks

This crack pattern is the result of non-uniform solidification of the glaze. As the glaze cools,
the various phases of which the glaze is comprised have their own distinct glass transition temperatures. As a result
each phase will undergo its transition to a solid at a different temperature and time.

A possible scenario for the effect seen above:

The earliest solidifying phase shrinks as the remaining liquid cools. If that phase is in contact with the
clay body beneath the glaze, cracks form in the glaze as a result of differential shrinkage. Because a liquid phase
remains, it fills in those cracks before final solidification of the remaining glaze.

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