'Macrosegregation' represents a class of defects in cast products and is of serious concern to both alloy producers and users. Most types of macrosegregation result from thermosolutal convection that takes place in the solid plus liquid and all-liquid regions of a solidifying alloy. Mathematical modeling of convection in solidifying alloys has been done since about 1970, but only in the past five years has analysis of thermosolutal convection been incorporated into solidification theory. This has resulted in simulations, which treat the solid plus liquid region as a porous medium of variable porosity and permeability. Simulations include scenarios in which the convection is strong enough to make channels in the solid plus liquid region, and these channels are responsible for making localized segregates known as 'freckles.' Using Pb-10 wt pct Sn as a model alloy, we simulated vertical solidification with two different solidification rates; both exhibited freckles. By an increase of the cooling rate at the bottom surface, convection was suppressed for the first minute of solidification, but thereafter the convection was sufficient to cause the formation of freckles.