This problem is almost always caused by glazes shrinking too much during drying and then cracking. Those cracks become the crawl points during firing. Excessive shrinkage is normally a product of too much raw clay in a glaze. Even glazes a marginally high clay can crack if applied too thickly. It is likewise with multi-layer application done without consideration for the specific needs of that process (e.g. failure to use a base coat glaze for the first layer). Multi-layering of glazes rewets the first layer, stressing its bond with the body and pulling it away from the body as it shrinks. Base coat glazes have better adherence. Crawling is quite prevalent in once-fired ware since glaze bonding is more tenuous. And in zircon opacified glazes.
To state again: Glazes contain clay to suspend their slurries. Clay shrinks when it dries. Some shrinkage can be tolerated but when it is excessive, something has to give. As glaze layer thickness increases, it is afforded more and more power to impose its shrinkage on the bond with the body. At some point, that bond will be compromised in places where cracks occur to release the tension. Even if these do not appear on dry ware, bond compromise can still exist.

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The reason the G1214Z recipe became G1214Z1 was the high clay percentage (which increased drying shrinkage, thus cracking, leading to crawling). The partial healing of the drying cracks during melting produced these unusual wrinkles (rather than the bare patches normally associated with crawling).
The original recipe called for 37 kaolin. That is too high. The fix is to supply kaolin as a mix of raw and calcined (to reduce drying shrinkage). I adjusted to 20 EPK and 15 calcined kaolin (15 being an LOI-adjusted amount).
If you need to prove this, glaze identical test pieces with all-raw G1214Z and raw/calcined G1214Z1, at the same measured thickness and SG. Before firing, inspect them with a magnifier, not just when they first appear dry, but again 5-15 minutes later (high-clay coatings can continue shrinking after they look dry).

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This problem only happened once in a while at a production pottery. On the rims of thick or thin pieces. The ware is glazed inside and out, the rim sponge-cleaned after each. Finally, the rim was dipped (after a period of drying). Glaze thickness and layering did not affect the appearance of the issue. They control glaze rheology carefully (according to specs). It appeared the problem had been solved when it was discovered that a worker was using hand cream and getting it on the rim while handling the ware (the touch points being the crawl points). Further, ware was not being kept clean and dust-free. But these were not the issue. In the end this was solved by bisque firing. Yes, they were glazing ware in the dry, or green state. The fact that they could accomplish this on thin-walled ware is amazing. Their experience is a testament to the value of bisque firing for smaller production facilities having limited equipment and resources.
| Troubles |
Crawling
Crawling glazes pull into islands, leaving bare spots on the body. Asking yourself the right questions will surface the real cause and help you deal with the problem, not the symptoms. |
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