Dislocation clusters in multicrystalline silicon are known to be detrimental for photovoltaic cell efficiency. They reduce minority carrier lifetime and are able to build shunts. To get control of them is one of the main purposes of recent developments in crystal growth. For reducing dislocation generation and multiplication there are several issues to focus on during silicon block casting like crystallization condition, grain structure evolution, history of temperature field and phase boundary shape. Different industrial standard ingots were investigated by optical photography. It is shown, that dislocation cluster reduction can be achieved by using suitable conditions during ingot growth.