Devitrification happens from overworking the glass. There are basically two types. One that is not really bad and is from being pulled while the outside is very cool. This causes stress in the glass and looks like the top image.
The other type of devit as we call it happens when much of the sodium is burned out. This also can kinda be fixed by adding more sodium to the glass with table salt, or what Elev8 Premier prefers is borax