A lot of devitrification resembles dirty smears over the glass that will not clean away. This kind of devitrification results from inadequate cleaning.
The glass needs to be made “squeaky clean”. The glass needs to be free of dust, oils and minerals before firing. An initial wash of the glass with a minimum amount of liquid soap will dispose of the dust and oils. However it may leave behind minerals and additives from the soap and water, so a rinse in clean water followed by a polishing with unprinted paper towels or lint free cloths washed without softeners. As the glass dries you may very well hear the squeak of glass that is well polished to dry.
If there are still residues of labels or markers, use of a spirit may be required to remove these marks. Then the glass will have to be cleaned again in the normal way to remove the residues from the spirits.
If you are fortunate to be in an area with very few minerals in the water, you will not have to take as many precautions as those in areas with hard water. If you have hard water, you may need to think about using distilled water for the final rinse if you have streaks of devitrification after the standard cleaning process. The use of spirits is not necessary. The glass still needs to be polished dry with unprinted paper or dedicated towels.
An alternative (that I use most often) is to use a window cleaner without additives, such as supplied by glaziers. This avoids the local water supply, and most often is sufficient to remove dust and oils.