The wetting of glass surfaces by ionic magnetic fluids (ferrofluids) is studied, using an improved birefringence technique, as a function of the concentration and size of the colloidal grains, and the pH of the solution. The problem is considered as a phase transition induced by the surface field on a thin ferrofluid layer which is in contact with the glass walls. The induced optical phase shift increases with the grain concentration. The penetration length in the bulk, induced by the ordered surface ferrofluid layer, calculated assuming that the order parameter at the surface is independent of grain concentration, is of the order of 2-4 grain diameters. The grain size, in the range from 2.5 to 9.5 nm, and the solution pH do not seem to be relevant parameters for the induced phase shift.