What is devitrification or devit?
Devitrification (aka devit) is the growth of crystalline structures within or on the surface of glass. ("Vitri" = glass; "devitrification" = the process of becoming un-glasslike.) This problem can result from firing too slowly through the devitrification range of temperatures. It can also result from firing contaminants like oils and fingerprints onto glass and thus seeding the growth of crystals.
To learn about firing safely through the devitrification range of temperatures, see TechNote 4: Heat & Glass. To learn about removing surface contaminants from glass before firing, see Glass Cleaning Basics.