The aim of the invention is a separation rate of foreign particles which is as high as possible in a tun dish with a minimised production of inclusions. Said aim is achieved, whereby the lined interior of the tun dish (1), depending on an operating bath level (h), fills the condition that a dimensionless relationship ( kappa ) between the lined surface (Aref) and the filling volume (V), defined by said lined surface and the free surface (ATop), dependent on the bath level derived from the relationship in formula (I), lies between 3.83 and 4.39.