Once the crystals reached the centre of the cell, the absorption at 0.5 THz and 1 THz decreased (see Figure ) as the probed sample became more ordered and thereby the VDOS was depleted, while the absorption at 1.6 THz increased as the peak emerged. On the other hand, the liquid phase concentration seemed to increase at 0.5 THz and 1.0 THz when crystals started to grow in the field of view of THz-TDS. This was in line with a decrease of the hydration shell size, and a potentially denser liquid in the area of forming crystals that was probed with THz-TDS. The absorption coefficient at the peak at 1.6 THz clearly increased, and this effect was accompanied by a decrease in MgSO 4 concentration. Once the crystals covered the centre of the cell and the system reached an equilibrium state, the changes at all frequencies became subtle again.