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. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. To solve this issue one could use a mix of tin oxide and zircon (tin does not stiffen the melt, try 4% tin, 6% zircon, for example). Or, calcine more of the kaolin in the recipe (to reduce drying shrinkage).
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.