The Collaborative International Dictionary of English v.0.48:

Pug \Pug\, n.
   1. Tempered clay; clay moistened and worked so as to be
      plastic.
      [1913 Webster]

   2. A pug mill.
      [1913 Webster]

   Pug mill, a kind of mill for grinding and mixing clay,
      either for brickmaking or the fine arts; a clay mill. It
      consists essentially of an upright shaft armed with
      projecting knives, which is caused to revolve in a hollow
      cylinder, tub, or vat, in which the clay is placed.
      [1913 Webster]