the effect of fire

Word:

vitrify
vit·ri·fy \ˈvi-trə-ˌfī \
to convert into glass or a glassy substance by heat and fusion; to become vitrified

Source:

Vitrified glazes appeared during the second millennium b.c., adding color and texture to the surface and exalting the inherent qualities in the clay. The whole range of possibilities for ceramic working was investigated — form was paramount, superfluous decoration dispensed with, and the effect of fire on raw clay cherished... Working with clay puts us in touch with the elements. Water and air provide the plasticity for working with soft clay, while fire brings permanence to the earth we transform (pp. 6–7)