dévitrifier

A technical term from materials science and ceramics, the opposite of vitrifier: glass or a glaze dévitrifie when it partially loses its smooth, glassy structure and develops a cloudy, crystalline texture over time — generally considered a defect in glassmaking and pottery glazing.

• Ce verre ancien s'est dévitrifié avec le temps. — This old glass has devitrified over time.
• Les céramistes cherchent à éviter que l'émail ne se dévitrifie au four. — Potters try to prevent the glaze from devitrifying in the kiln.
• Le verre dévitrifié perd sa transparence caractéristique. — Devitrified glass loses its characteristic transparency.

This is a narrow technical term for glassmakers and ceramicists, not general vocabulary — a learner won't need it outside a materials-science or pottery context, and shouldn't confuse it with the much more common vitrifier (to varnish a wood floor, for instance).