Below 80 K ordinary water ice (ice Ih) undergoes a phase transition in which the randomly oriented molecules in the crystal adopt an ordered orientation. This ordered form of ice is known as ice XI. In pure ice, the transition is expected to require ~ 100 kyr to complete. In planetary environments, such as the outer solar system, time is no issue, so it is likely that ice XI is very abundant in space. However, the transformation can be sped up with a molecular 'lubricant', such as KOH, which at low concentrations acts to catalyse the ice Ih to ice XI reaction. Even so, on the timescales of typical neutron diffraction studies, the transformation usually around half complete after 5 days. I propose a long-duration experiment (1-2 months) to prepare the most completely transformed ice XI specimen, which is essential for accurate characterisation of the structure and properties.