Devitrification- de·vit·ri·fy (d -v t r -f ). tr.v. de·vit·ri·fied, de·vit·ri·fy·ing, de·vit·ri·fies,
The crystallization of a glaze during cooling. A glaze is a fairly quickly cooled "glass" and has an amorphous structure but if allowed to cool slowly, crystals can form making the surface appear matt.
(Matt is a former Clay Coordinator at Penland.)