The sample was dissolved in Milli-Q water (IQ 7000, Merck, Darmstadt, Germany, resistivity 18.2 MΩ • cm) in a beaker, which was then left on a magnetic stirrer until the crystals were fully dissolved. After several rounds of preliminary experiments, three concentrations (mass ratio of MgSO 4 heptahydrate to water) were chosen for further repeats: 1.41:1, 1.29:1, and 1.20:1. This was based on the time and temperature observed for crystallisation.