In this paper the thermal dependences of the viscosity, density and surface tension of several solutions of the (0.4Na(2)O-0.6B(2)O(3)) - BaO - ((6-x)Fe2O3 + xCoSnO(3)) system containig four different degrees of substitution of Fe3+ by Co2+ + Sn4+ have been determined experimentally. On the basis of all these sets of results, the solution compositions with the highest solvent contents have been proposed for the growth of BaFe12-(x+y)CoxSnyO19 using a TSSG/slow cooling methodology.