They fired the glaze powder onto the sample tiles, as one would do with one’s ceramic vessels - then pulverized the resulting glazed tile back into a colored powder before mixing it with the tea, to improve its reactivity?

I appreciate that the point of this work must be to identify which degradation products to be on the look out for in the first place, not to suggest the rate at which they might form under actual use. Still, in the real-life applications the authors are worried about, surely the fused glaze is doing some degree of work to physically bind its constituents in place.