The local current density distribution of grain boundaries in differently processed melt-textured YBa₂Cu₃O_7-δ (YBCO) is investigated by a magneto-optical technique and inversion of Biot-Savart's law. By correlating the local current density with the microstructure revealed by a scanning electron microscope, we are able to distinguish the depression of the critical current density j_c at clean grain boundaries (GBs) from that occurring at normal phases and cracks at and in the vicinity of GBs. The GBs are formed by different multi-seeding melt-growth processes using a non-stoichiometric precursor which is optimized with respect to the bulk critical current. In order to obtain GBs with extended regions free of normal phases and with large critical current densities the amount of liquid phase and the density of Y₂BaCuO₅ particles has to be controlled. The properties of GBs can be tailored by both controlling the amount of liquid phase and the seed distance. The related changes in the growth and microstructure are analysed and we show how to grow extended GBs almost free of normal phases with optimized critical current densities in multiple-seeded melt-growth YBCO.