March 23, 2001, Newsletter Issue #30: Ceramics-tips Newsletter, Issue : 23, Glaze troubleshooting.

Tip of the Week

Crazing:

The effect of cracks that appear in glaze that are unwanted are called crazing and may be caused from an under-fired clay body. Bisque should be fired one to two cones higher than your glaze firing except for those glazes calling for a special firing.


Crawling:

Crawled glaze is a condition caused by spots on the bisque that are not covered with glaze. This may be form too heavy and application of glaze, oil from the skin left on the piece or hard spots from polishing greenware. Often the re-application of glaze over these spots and refiring will take care of the problem.


Running:

Remember that glazes run when fired so you will want to leave your coats of colour approximately 1/16-1/8” up from the bottom of the piece.


Grinding your pots:

Sharpening stones can be put to great effect by using them to grind off by hand left over bits of grog or alumina on the feet of pots and bowls or other surfaces.


Pinholing:

A glaze fault which causes
pitting of the glaze surface. It is caused
by bubbles rising to the surface during
drying or firing, and being prevented
from healing over because of the
viscosity of the glaze melt, which may
be caused either by its composition, or
by underfiring.

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