A shielding method for achieving highly uniform temperatures in liquids during microwave heating by substantially enhancing vertical mixing currents in said liquids, making determinations of final temperature reached in said liquids either by touch or the use of a temperature indicator efficacious, comprising: a electrically conductive shield having very low impedance at microwave frequencies; having, a generally cylindrical shape, and dimensions chosen to accommodate a variety of microwaveable containers. said shield: to be concentric with a microwaveable container, containing a liquid to be heated by microwave radiation; and, to be located so as the top edge of said shield is at or above a vertical level corresponding to the level of the liquid in said container; and, to be of sufficient height as to cover at least 10% of the height of the liquid in said container. 2. a shield according to claim 1, having; a height "h", covering between 10% and 90% of the height of the liquid contained in the container to be heated; and, a circumference/length "L" covering at least 90% of the circumference of the container to be heated.