The grain control in multi-crystalline silicon growth by directional solidification is crucial to the ingot quality. In order to study the nucleation and grain growth behavior, we carried out ingot growth using porous silica coatings, with different Si/SiO2 ratios, and the traditional nitride coating for comparison, at the bottom of a small crucible. It was observed that with more silica in the coating, smaller and uniform grains, as well as columnar growth, could be obtained. As a result, more non-Sigma grain boundaries were found as well. On the contrary, as silicon became a continuous phase in the coating, the grain refinement was less effective and their grain development in the porous coating was examined. The possible grain nucleation and growth mechanisms in the porous coatings were further proposed. (C) 2014 Elsevier B.V. All rights reserved.