When Does a Stored SnO₂ Sol Stop Ageing?: Settling Times for Optical Gap, Crystallite Size, Hydrodynamic Diameter and Zeta Potential over 540 Days of Shelf Storage Resolving to When Does a Stored SnO₂ Sol Stop Ageing?: Settling Times for Optical Gap, Crystallite Size, Hydrodynamic Diameter and Zeta Potential over 540 Days of Shelf Storage…