If glaze is applied too heavily, the consequence can be seporation of the glaze from the clay. It looks as if the glaze crawled away from an area. It may look like the remnent of a greasey finger print. Bisque clay readily absorbs oil from finger prints, nose or elbow bumps. These are commons reasons for the glaze to crawl away from an area ; leaving bare spots on a glaze fired piece. For more information: TUTORIALS CRAWLING TUTORIAL
When the fired glaze retracts to expose the bare body.
A condition where fired glaze separates into clumps or islands leaving bare clay patches showing in-between. More prevalent in once fired ware. There are many causes for crawling (typically glazes shrink too much during drying and don't have a good bond with the bisque) in the Magic of Fire book. Some times glazes are made to crawl intentionally. One technique to make this happen is to add 15-20% magnesium carbonate (testing required to determine amount) to a low fire transparent glaze.