The almost fabric texture on this mug is an example of glaze thickness sensitivity. This teadust iron glaze (a form of tenmoku) forms a creamy base color in thicker areas, and a dark brown in thinner areas.
The fabric like texture is the result of brushing the glaze on in multiple layers, each layer perpendicular to the previous. The teadust or oil spot character that this glaze is capable of doesn’t like brush strokes. Even if you lay on 4 or 5 coats by brush, you won’t see the same spot effects as when you dip the piece instead of brushing.
Saturated Iron with brushed effects, Gunmetal Green rim