Crawling is a glaze defect in which the glaze separates, leaving portions of bare clay exposed. This can be a few tiny areas, or it can extend across the surface of a pot. Extreme cases may result in the glaze leaving the surface of the pot completely. Droplets or even puddles of glaze may end up on the kiln shelf.
Reasons for Crawling
Photo © 2008 Beth E Peterson
Crawling occurs when the glaze hasn't properly adhered to the bisqued clay. The most frequent cause is that the the glaze was applied to dirty, dusty, or greasy pots. Other causes include:
- too thick a glaze coat
- glaze coats which have dried too quickly, such as forced drying in a warm kiln
- second glaze coats applied too wet to an already dry layer of glaze
- raw glaze coats which dry excessively and begin cracking on the bisqueware before the glaze firing
- heavy applications of underglaze
- too much opacifier in the glaze
- a burnished or too smooth surface on the bisqueware
- a lack of adhesiveness in the raw glaze.
Matte glazes that have a high clay or calcium borate content are more likely to crawl, as are overly viscous glazes.
Solving Crawling
The very first thing to do is make certain that bisqueware is completely clean before applying glazes. Other measures include:
- if the dry glaze coat cracks, rub it gently to fill in the cracks
- apply thinner glaze coats
- add gum to the raw glaze to increase its adhesion to pottery surfaces
- allow glazes time to dry enough to loose its sheen, but not become completely dry before you apply a second glaze coat
- if there are large differences in the thickness in glaze coat layers, rub to bevel the edge where the one coat overlaps the other
- use thinner coats of underglaze
- do not grind glaze material too finely.