This is one of my dark blue crystalline glazes. The secondary development here is particularly strong, there being several different species of crystal present. I used the Canon 65mm f/2.8 5x macro lens to capture the crystals in detail. The images below are all details taken from the main image.
The centres of the primary crystals often contain structures that give clues to how and at what time in the firing they started to grow (nucleated).
At least three different types of secondary crystal can be seen here.