G2934Y is a fabulous 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. While it is normally fine as is, when you add certain stains to color it (especially at significant percentages) or opacify it using zircon (this has 10%), it can become more susceptible to crawling. On this mug, the glaze layer thickens at the recess of the handle join, that produces crawling during firing. Crawling can also happen on the insides of mugs, where wall and foot meet at a sharp angle. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. Rounding such contours will help. Even better, adjust the glaze recipe so it shrinks a little less on drying (by trading 5% of the raw kaolin for calcined). Adding a little CMC gum (e.g. 0.1-0.2%) will make it adhere better.
G2934Y - Cone 6 Magnesia Matte Low LOI Version
The same chemistry as the widely used G2934 but the MgO is sourced from a frit and talc instead of dolomite. It has a finer surface, less cutlery marking and staining.
G2934Y variations for fired hardness, COE adjustment, less crawling, etc
G2934Y is a popular recipe used worldwide in industry and by potters and hobbyist. This page shows it, and four variations, that adjust for different purposes. All have the same chemistry, but source the needed oxides from different materials.