This is G2934Y white (with 10% Zircopax). I initially blamed the zircon for the crawling. But, since the slurry had settled somewhat I was able to remove about 15% of the water and replace it with CMC gum solution. The gum addition was not enough to slow down the drying much but it really improved reduced the problem! This likely means that adherence of the dried layer to the smooth bisque was the issue. This being said, there were still a couple of small spots where it crawled. The ultimate solution thus appears to be discovering the percentage of CMC gum and water needed to get the least loss of drying speed and while achieving integrity of glaze coverage. It is best to add the CMC gum as a powder at mixing time and blender mix the slurry thoroughly to be sure that it fully dissolves (watch for a rheology change on aging, that will demonstrate if mixing was thorough enough).

This picture has its own page with more detail, click here to see it.
G2934Y, a variation of the G2934 base, is a good stain matte base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry; these make it susceptible to crawling. This base is normally fine as is, but when opacified or certain stains are added (especially at significant percentages) it can crawl. This has 10% Zircopax. Even though the glaze layer thickens at the recess of the handle join, it is still crawling. We also get this on the insides of mugs where the wall and foot meet at a sharp angle. This was initiated because the glaze cracked here during drying. Normally it would heal, but the zircon stiffens the melt, making it less mobile. The easiest solution is to adjust the specific gravity of the glaze to 1.44 and flocculate it to thixotropic; this assures that the application is not too thick. Another measure is to add a little CMC gum (by replacing some of the water with gum solution). Lastly, use a blend of tin oxide and Zircopax, as in the G3926C version of the recipe, to opacify it.

This picture has its own page with more detail, click here to see it.
The reason for this rather strange-looking wrinkling, experienced by one potter, is not obvious. But I suspected the high clay percentage in the G1214Z matte base glaze recipe (which increases drying shrinkage, thus cracking, leading to crawling). I suspect these wrinkles are actually partially healed crawling cracks. The high-clay glaze shrinks and develops tiny cracks or locally loses its bond with the bisque during drying. During firing, the fluid melt begins to pull away at these fault lines, but then flows enough to heal most of them. What remains are these unusual wrinkles rather than the bare patches normally associated with crawling.
To reduce shrinkage, I supply the kaolin as a mix of raw and calcined. The original recipe called for 37 kaolin; I supplied that with 20 EPK and 15 calcined kaolin (15 is the LOI adjusted amount). For most people, the original 37 raw kaolin version is too high (unless a high solids glaze slurry is thinly applied to porous bisque). 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).
| Materials |
CMC Gum
CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness. |
| Troubles |
Crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk