© 2018 IOP Publishing Ltd. Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb3Sn wires, in particular a critical current density (Jc) of more than 1500 A mm-2at 16 T and 4.2 K with an effective filament diameter (Deff) of less than 20 μm. The restacked-rod-process (RRP®) is the technology closest to meeting these demands, with a Jc(16 T) of up to 1400 A mm-2, residual resistivity ratio > 100, for a sub-element size Dsof 58 μm (which in RRP®wires is essentially the same as Deff). An important present limitation of RRP®is that reducing the sub-element size degrades Jcto as low as 900 A mm-2at 16 T for Ds= 35 μm. To gain an understanding of the sources of this Jcdegradation, we have made a detailed study of the phase evolution during the Cu-Sn 'mixing' stages of the wire heat treatment that occur prior to Nb3Sn formation. Using extensive microstructural quantification, we have identified the critical role that the Sn-Nb-Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C-400 °C range - greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu6Sn5) and the more desirable ϵ phase (Cu3Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb3Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn2on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb3Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ϵ phase through better Cu-Sn mixing. At a Dsof 41 μm, the Nausite control heat treatment increases the Jcat 16 T by 36%, reaching 1300 A mm-2(i.e. 2980 A mm-2at 12 T), and moving RRP®closer to the FCC targets.