Dunting in Calico Glazes

firing to cone 10 in oxidation

slow downfire at 50 deg F an hour in the interval 1850 deg F to 1700 deg F

slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F

One hour hold at 1650 deg F

The calico glazes exhibit separation into phases with distinctive colors and textures forming large regions separated from each other with sharp boundaries.

To go with these unusual visual effects, these glazes seem unusually prone to shivering and dunting.

It is my current conjecture that with strongly demarkated phase separation comes additional strains within the glaze, leading to the defects observed.

Here as support for this conjecture, are several calico glazes which dunt, and several which don't.

Compositional Parameters

Alumina between .4 and .45

silica alumina ratio between 6 and 7

alkali metals either equal to alumina or a bit lower.

The bases of these glazes all contain some Zinc Oxide, and Barium Oxide.

The images below show inside and outside of each pot.



Dunting

glaze c_404_Z12_3

A close look at the matt part of the surface of this glaze shows both crystals, and the granular surface of a rocky conglomerate. The popped bubbles on the outside of the pot have the irregular shape which suggests voids remaining as a result of incomplete melting.

The glossy rim is a fully matured glaze. This glaze has two components nearly divorced from each other, and both present in significant volume fraction.



glaze c_404_Z12_1

In the image of the back of the pot, where the glaze terminates at the foot a full gloss transparent amber phase is visible. One observes the faint varnish where most of that glossy phase has soaked into the pot.



glaze c_404_Z12_4

As with the glaze C_404_Z12_3 we see a full gloss phase so liquid the matt phase is sliding into the bowl. Not visible in this picture, the evidence for the internal strains withiin this glaze are the horizontal cracks in the bits of gloss phase in the center of the bowl. Those bits of amber glass will crack.



No Dunting

glaze c_404_Z12_2

The glaze is not translucent on the rim indicating that a transparent full gloss is not present in as large a volume fraction as in e.g. c_404_Z12_4.

In comparing the inside and outside of this pot, we note that the calico markings are prominent only where the glaze is thin.

This in contrast with the next glaze, where the calico markings are most prominent with a thicker glaze application.



glaze c_404_Z12_0

Another glaze where the non-transparency of the glaze on the pot rim indicates less extreme phase separation.

As mentioned above - the calico markings are more prominent with a thicker glaze application.



glaze cooper_404_10_0

As above, the transparent full gloss phase while present, is nowhere dominant. Note that the rim is not transparent.

The strangely shapped brown ovoid is a leaf print. The main vein bisecting the two sides of the leaf is almost visible with suspension of disbelief.

These calico glazes act like blotting paper, and do not take sharp leaf prints.



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