Direct observation of conservation treatments by single-sided NMR relaxometry

Non-destructive techniques are key to assessing the effectiveness of treatments and the state of conservation of stones. Nuclear magnetic resonance (NMR) relaxometric profiling is an innovative, non-invasive, and non-destructive candidate to address these issues directly in situ using single-sided portable devices. In this study, low-field 1H NMR relaxometry was used to investigate conservation treatments on stone samples by depth profiling up to 25 mm. Two types of stone strengthening consolidants (KSE 100 and KSE 300) and a hydrophobic agent (Funcosil SNL) were tested on four types of sandstone: Schilf sandstone, Bunter sandstone, and two types of Stuben sandstone. The evolution of conservation treatments was monitored over time, also considering the influence of sediment bed direction and stone granularity. The depth distribution and the curing of the products could be followed until 74 hours after application. NMR can localize the agent because the NMR signal of the hydrogen nuclei of the solvent is acquired as a function of depth in the stone, monitoring the product during the application and curing of the treatment directly in situ with a non-invasive technique. The hydrophobic effect of stone strengtheners shortly after treatments in limiting and slowing down water ingress has also been assessed 50 days after treatment with capillary suction tests. Additionally, NMR was used to assess water distribution within the porous structure of sandstones samples from the Hoppenlau Cemetery, Stuttgart, currently undergoing restoration. Water content was measured as a function of distance from the external surfaces, alongside NMR parameters related to the properties of the stone's porous structure. By characterizing both water content and the penetration dynamics of conservation products, this study highlights the potential of NMR as a diagnostic tool for evaluating the effectiveness of treatments and the state of conservation of historical stone materials. This information is useful for restorers in designing and implementing appropriate restoration strategies.