Oxamate salts as novel agents for the restoration of marble and limestone substrates: case study of ammonium N-phenyloxamate

The ammonium salt of N-phenyloxamic acid (AmPhOxam) was synthesised, characterised by FT-IR, FT-Raman, UV-Vis, 1H-NMR spectroscopic methods and single crystal X-ray diffraction, and evaluated as protective and consolidating agent for calcareous stone substrates under mild conditions. Hydroalcoholic solutions of AmPhOxam were tested for the treatment of naturally weathered white Marble and biomicritic limestone. Mercury intrusion porosimetry, FT-NIR spectroscopy measurements and SEM microscopy showed the formation of a superficial protective layer of crystals of the corresponding monohydrated calcium salt, CaPhOxam, on both treated stones.