Dunting

A glaze which dunted - why?

In its first firing, glaze application thin on the inside, and thinner on the outside. Fired slightly differently it dunted.
That extra hold at 1900 deg F could not be the cause, so then what? A third firing with the same profile as the first,
and a slightly thicker glaze application provided the answer. This glaze is underfired at cone 10.

How does underfiring a glaze promote dunting?

Dunting is the result of a shrinkage miss-match between the glaze and clay body. The body shrinks more than the glaze,
and the glaze tears the body apart. This shrinkage miss-match is relevant only after the glaze freezes. An underfired glaze solidifies
only a few degrees below the highest temperature reached, providing a large temperature window in which dunting can occur.

cone 10 oxidation

The two firings:

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 - same

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 three hour hold at 1900 deg F

A half hour hold at 1750 deg F

A three hour hold at 1700 deg F

A one half hour hold at 1650 deg F

The extra 3 hour hold at 1900 deg F of the second firing is the primary difference between these two firings.

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

The first bowl has a second glaze inlay design created by applying a contact shelf paper mask to the bisque before glazing.
Then the glaze and mask are coated with an emulsion wax. The mask is stripped when glaze and wax are dry,
and a second glaze is applied to the now uncovered bisque.

glaze composition

Emperical Formula mashiko_ZS_T_4:

K2O        0.1
Na2O        0.06
Li2O        0.1
CaO        0.27
MgO        0.47

Al2O3        .83
Fe2O3        .22

SiO2        5.82

molecular percent Silica 74 %

The bowls are ~ 6 inches in diameter.

Dunted glaze in Firing Two

Thin glaze in Firing One

Underfired glaze in Firing One