An induction heated furnace assembly (10) for producing a directionally solidified casting includes a susceptor (22) that tailors strength of the magnetic field within the chamber (12) to provide a desired grain structure in a completed cast part. The susceptor (22) proportionally blocks portions of the magnetic field to provide different levels of magnetic stirring within the molten material at different locations within the furnace assembly (10) so as to create the desired grain structures in the completed cast article. The susceptor (22) may have a varying wall thickness as shown. In another embodiment it may have an open area which varies. In a yet further embodiment, the strength of the magnetic field produced by the induction coil may be varied.