The mug is has an unglazed (bare porcelain) outside surface. The cuerda seca lines containing the glaze have a formulation to flux and melt more than what is typical in the classic technique on tiles. And this appears to be reduction-fired porcelain at high temperatures (because of the iron specks and grey body color and the metallic appearance of the lines), in contrast to the normal low fire terra cotta temperatures of typical cuerda seca. The formulation for the lines is likely very high in iron oxide (with some kaolin added for suspension, dry hardness, adhesion and paintability). The fill glazes are a transparent with added stain.
A method of decorating terra cotta clay tiles by flood-filling liquid glaze into areas delineated by colored wax or grease lines.