Glaze Chemistry: The Really Brief Version

Most glazes are slurries of crushed minerals and water. You dip your pot in, it soaks up some slurry, the water dries and you are left with powder on the outside of the pot.

When you fire your pot and the kiln is at temperature the powdered glaze on the outside melts. Some of the powder compounds burn off, but most of it becomes a thick liquid. In that hot liquid the molecules that were bound up in various rocky crystals and other materials are freed to roam around and interact with other molecules. It's a molecule soup.

When the kiln is turned off and the pot cools the molten liquid freezes - that's the glaze. Sometimes the glaze freezes relatively quickly and the molecules are caught off guard, as if they were still in that liquid melt, creating a clear glaze. Other times the molecules have time to find a friend and create a crystal or an optical blip, leading to matte glazes and other effects.

You could think about the molecules in that molten glaze soup in terms of elements, like silicon or iron or aluminum, but there's one element that is almost always attached - oxygen. We live in a world bathed in oxygen and it's very reactive. Oxygen glues itself to other elements and doesn't let go, even at kiln temperatures. So when it comes to glazes it's best to think not about elements but oxides - the element plus it's tag along oxygen.