I could be mistaken but the 11 cent tiles (and almost all others in my possession) are hard porcelain. But as I have done a lot of ceramic and glass chemistry there is a point where the pot is “vitrified” and indeed it shrinks as that air leaves, but there is usually a point that whatever holes (air) still exist will be cut off from each other and the outside.
The glaze will usually melt well below the body and be thin enough that all the bubbles will pop. Plus it will contain (usually ) less aluminum oxide that is what stiffens the clay body (usually), but a fully vitrified pot will not absorb any water even without a glaze and it is possible that both clay and glaze to have no silica at all though it would be a high temp porcelain for the clay body to be so.
Frits of ground glass are frequently used , especially with enamel and low temp glazes but I would hardly expect to see them in any mass production tile manufacturing process.
I would love to see if anyone else here has a laser tile they messed up and access to an x- ray lab to just leave it about where it would get a good dose over time and see if it went back to white. Leaving it out in the sun also is an easier experiment but could take a lot longer if it worked at all.
Many reactions reverse in radiation like the amethyst but not knowing the exact chemistry of the tiles it would just be an interesting experiment.