The mug on the left has been cooled slowly (and crystallizes). On the right it was cooled quickly (and is glossy).
This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.
Medium Temperature Glaze
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.