Vitreous paint is glass ground up to a very fine powder- it is then mixed with gum and lavender oil, clove oil, or H2o and applied to the glass. It is then fired in a glass kiln at approx. 1250 degrees. At that high heat the paint is "fused" into the glass so it never chips or flakes off. In earlier times some of the kilns that were used were not as elaborate as we have access to now and the firings may have been a little off in degrees hence there are some older pieces that the paint is sometimes not stable. In restorations it is wise to have an experienced and reputable studio examine any historical painted glass for paint stability before having anything done.