FFC relaxometry was used to generate nuclear magnetic resonance dispersion (NMRD) curves , i.e., a plot of the relaxation time T 1 as a function of the Larmor frequency ν. Salt crusts of NaCl and Na 2 SO 4 were prepared by evaporation from a column filled with fine quartz sand F32 (Quarzwerke Frechen, Germany) and a porous glass plate of pore size class P2 (40 -100 µm, Robu GmbH, Germany), respectively. The columns were connected to a storage vessel via a tube ensuring that evaporation took place under wicking conditions (i.e., constant hydraulic and near-saturated conditions). The concentration of the solutions was 4 M for NaCl and 1 M for Na 2 SO 4 , just below the respective saturation concentrations of the two salts. The evaporation was stopped after several days. In the case of NaCl, small granules made of pure efflorescent crust were collected and placed in a 10 mm NMR tube. In the case of Na 2 SO 4 , the crust was carefully removed from the porous support plate and glass walls and transferred to a 10 mm NMR tube at constant relative humidity near saturation to prevent dehydrated thenadite species from forming . FFC experiments were performed with a Stelar Spinmaster II (Stelar s.r.l., Mede, Italy) operating with a magnetic field for relaxation B rlx corresponding to a Proton Larmor frequency ranging from 10 kHz to 20 MHz. In a first step, the raw data, i.e., the free induction decay curves as a function of time period τ and magnetic field B rlx were analyzed with the Stelar software, thus yielding the magnetization in the z-direction, M z , at time zero. In a second step, the relaxation curves (i.e., M z versus time τ ) were analyzed by fitting a mono-exponential function to the data yielding the relaxation time T 1 . We found that in all cases such a fit was a sufficient representation of the data. Finally, the NMRD profiles were obtained by plotting the T 1 relaxation times as a function of the Larmor frequency ν. All measurements were done at room temperature (20 • C).