Cultural Heritage

 
Relaxation time distributions before and after 
hydrophobic treatment
  
  

Magnetic Resonance Imaging applications
  
  

POSTERS

 
 
Relaxation time distributions before and after 
hydrophobic treatment
   Research in collaboration with L. Appolonia, Safeguarding Cultural Heritage Department, Aosta, and G. Rezzaro, CNR Laboratory, Safeguarding Cultural Heritage Department, Aosta, Italy.
  [Magn. Reson. Imaging 19, 2001, 509-512]

     
T₁ and T₂ relaxation time distributions for three Travertine and Three Pudding stone samples, fully saturated with water, are shown before treatment (solid line) and after treatment (dashed line).  There is much variation among the three samples of either type of rock,  but also there are some general differences between the two types. The features of relaxation time distributions correlate with known physical features of the samples and give information on classes of pores from which water is excluded by the hydrophobic treatment.     
        

    
  
  
Magnetic Resonance Imaging applications
  Research in collaboration with Mara Camaiti CNR-ICVBC, Sesto Fiorentino (FI)
[La Chimica e l'Industria 81, 1999, 729-731; J. Cultural Heritage 1, 2001, 127-132;
Giornale delle Prove non distruttive Monitoraggio Diagnostica XXIII (3), 2002, 34-40;
Giornale delle Prove non distruttive Monitoraggio Diagnostica XXIV (4), 2003, 69-72;
Atti II Congr. Naz. AIAR pagg 133-148, Patron Editore, 2002
Addison Wesley pagg 241-261, 2003; Studies in Conservation 48, 2003, 1-6]
     

The structure of a porous stone saturated with water can be directly visualized by MRI. As a first approach, simple images of adjacent internal sections can be acquired, where the signal is roughly proportional to the density of ¹H nuclei. As for medical diagnosis, where images of internal sections of the human body are obtained, also in this case the acquisition is clearly non destructive and gives an unique way to see inside the rock. 
MR images of one section each of two samples of Lecce Stone after fixed times of capillary water absorption show the non-homogeneity of the stone that is due to the presence of shells.
   

With this method the local porosity of internal sections can be evaluated. An example is given of two decayed marble samples from the Cathedral of Santa Maria del Fiore in Florence. The figure shows four adjacent sections of the two samples after 80 minutes of capillary water absorption. The heavily decayed sample (D₂) shows higher signal intensities than the slightly decayed sample (S₁). The signal intensity is not uniform and shows the non homogeneous porosity of the stone. The analysis allows us to compute the amount of pore space filled by liquid water. The data obtained after 8 days of water absorption are in good agreement with the known total porosity of  the samples. The inner parts of the samples show lower porosities than the outer parts.
  
  

MRI also provides direct evidence of protective performance. It gives images of the spatial distribution of liquid water, providing new insight into the study of hydrophobic treatments, getting indirect information on the distribution of the hydrophobic product. As an example, the figure  shows a proton density image of an internal section of a Lecce Stone (parallelepiped 5x5x2cm³) before (top) and after (bottom) treatment. A distribution profile of the product is clearly evident. It is visualized by the inhomogeneous distribution of water after 24 hours of capillary water absorption from the untreated face of the sample. After 8 days of absorption the distribution profile is still visible, although water is now present in all the regions of the sample. It is interesting to note that a result of that study was evidence of a notable decrease in the hydrophobic efficacy after wetting-drying cycles, attributed to a loss of adhesion of the polymer to the substrate, promoted by the action of water. [Magn. Reson. Imaging 19, 2001, 513-516]