gresificar, according to the Eurodicautom, is vitrify.
- Glass - enamel(=IC8)

(1)
TERM grésificar

Reference Diccionario de azulejos ceramicos,CEC.1970
(2)
TERM fritar

Reference Diccionario de azulejos ceramicos,CEC.1970
(3)
TERM vitrifiear

Reference Diccionario de azulejos ceramicos,CEC.1970

(1)
TERM to vitrify

Reference Ceramic Tile Dictionary,CEC.1970

On searching for "vitrified ceramics" it leads back to stoneware.

stoneware Materials. 1. a hard, opaque, glassy ceramic ware that is fired at high temperatures and composed primarily of clay, silica, and feldspar.a hard, opaque, glassy ceramic ware that is fired at high temperatures and composed primarily of clay, silica, and feldspar. 2. any of various highly vitrified ceramics with low tensile strengths made of selected clays, feldspar, and silica; resistant to all acids, except hydrofluoric acid; used especially in industrial chemical equipment.any of various highly vitrified ceramics with low tensile strengths made of selected clays, feldspar, and silica; resistant to all acids, except hydrofluoric acid; used especially in industrial chemical equipment.
http://www.harcourt.com/dictionary/def/9/8/6/7/9867700.html

Stoneware: is Vitrified Ceramics; they are no longer permeable to water, and they have been fired to around 1250°C or greater.

Earthenware: is not Vitrified; it is fired lower than Stoneware and can still be porous, which means that it could absorb water.

Stoneware:
Highly vitrified ceramics fired to above 1200oC. Most of the Silica in a fired stoneware body is melted into a glassy matrix and the resulting body is of high density and usually has a water absorption rate of less than 1%.