The rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride and silicon carbide. As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation setup which has been developed in order to measure the undercooling versus impurity concentration is presented. The impurity concentration in the levitated Si drop is controlled by the partial pressure of nitrogen or hydrocarbon gas. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of carbon impurities on the undercooling of silicon droplet is discussed.