A mould for casting a component in a directional solidification casting process having a preferred direction of grain growth (non-axial ) comprises a shell defining a cavity for receiving molten material. The cavity defines a three dimensional shape made up of a finished component geometry portion (42, 43, 44) and a sacrificial geometry portion (45) wherein the sacrificial geometry portion (45) includes a notch (48) which is shaped and positioned so as to, in use, contain high angle grain boundaries between dendritic growth in the preferred direction (non-axial ) and dendritic growth in a competing direction to the preferred direction (axial ) within the sacrificial geometry portion of a casting solidifying in the mould.