Back in the day, one issue with Gerstley was its variable chemical content. Natural materials are always variable over time , depending on what part of mine it came from, etc., but Gerstley was variable from batch to batch, and some even gave it the nickname Ghastley Borate. When Laguna bought the whole mineyard (but not the mine, nothing new coming out of the ground anymore), they processed it all at once and thoroughly mixed it all. So now the many years worth stock in the warehouse is all pretty much the same. But also slightly different than what it was 20 years ago. The published analysis of the current stock compared to ~1999 show somewhat less silica than before, but other fluxes are about the same +/-. So get some Gerstley and make a test batch of your glaze recipe to see if it needs any tweaking.