G2931F Ulexite-based transparent bubbles, G2931K frit-based version does not
I melted these two 9 gram GBMF test balls on tiles to compare their melting (the chemistry of these is identical, the recipes are different). The Ulexite in the G2931F (left) drives the LOI to more than 14%. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass. Notice the size of the F is greater (because it is full of bubbles). While this seems like a serious problem, in practice the F fires crystal-clear on most ware.
LOI Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Glaze Bubbles Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Frit Frits are used in ceramic glazes for a wide range of reasons. They are man-made materials of controlled chemistry with many advantages or raw materials.