One of my students worked with a well-known glaze: https://glazy.org/recipes/16712
She put it in her fridge with some rhubarb-leftovers and after 2 days it looks white instead of blue. The colorants were leached out.
So I redid this test... and yes it is leaching... But why? And how?
In this glaze are three colorants: rutile, copper and cobalt.
To figure out or the colorants weaken this glaze I made a triangle to test it: