In my classroom, I've occasionally come across some of our commercial underglazes (Amaco), that seem very watery.
They seem fine, when the bottles are opened, but over time thin out. I am not watering them down, and I've never seen a student do so either. Most the time, if a glaze or underglaze is too thick, they bring it to my attention. That's why I don't think they have had water added to them.
Is it possible that underglazes can thin out, due to a change in their chemistry? They are not settling/ hard panning.
Is there a solution/ fix to this?
On a somewhat related note, some turn darker in the bottle, regardless of color, and create a lovely smell. The darkness burns out. I'm guess this is due to a bacteria or mold eating some of the materials in the underglaze.