Pure zinc, sand cast (Specimen Xl) Structure is visible by eye that relatively long, columnar poly-crystals radiates in towards the center of the bar. The grains show varying degrees of brightness depending on their orientation. Poly-crystals results due to a number of clusters as the source of grain nucleation, each with different crystalline orientation. The long columnar shape results from the balance between growth from different directions.

Question: The size of the columnar crystals in specimen X1 is found to increase with increase in the mold size. Could you give a qualitative explanation of this phenomenon?