Melamine-formaldehyde in the conservation of waterlogged archaeological wood: investigating the effect of the treatment on wood residual chemistry with FTIR, 13C NMR, Py(HMDS)-GC/MS and EGA-MS

Abstract

This work examined the chemical interrelations between melamine–formaldehyde (MF) and waterlogged archaeological wood to demonstrate the effect of the MFtreatment on cultural heritage objects. Samples from a Roman waterlogged trunk of Greek fir, were analyzed with Fourier Transform Infrared Spectroscopy (FTIR), solid-state ¹³C Nuclear Magnetic Resonance (NMR), analytical pyrolysis with in-situ silylation Py(HMDS) coupled with Gas Chromatography Mass Spectrometry (GC/MS) and Evolved Gas Analysis-Mass Spectrometry, (EGA-MS) before and after the MFtreatment. FTIR results showed the formation of amide functionalities due to melamine reactions and strong evidence of lignin modification, while the deteriorated cellulose fraction appeared to have undergone further depletion as a result of the MF treatment. The ¹³C NMR spectra of the MF-treated wood clearly demonstrated the presence of the resin within the wood and indicated that MF carbons were strongly interacting with lignin moieties. Spectra also revealed that the retention of the MF resin in the wood was positively correlated to the degree of wood degradation. Py(HMDS)-GC/MS of MF-treated wood provided few peaks attributed to holocellulose or lignin pyrolytic products, and it was not possible to detect any signs of non-MF-modified wood components, as the lignocellulosic wood matrix appeared to have been transformed into a new biopolymer. EGA-MS profiles of the MF-treated archaeological wood showed early evolution of volatiles due to free MF retained in the wood, while its thermal stability appeared increased in comparison to untreated material. Nonetheless, mass peaks indicated that the chemistry of MF-treated wood was completely different from both fresh and untreated deteriorated wood. Overall, results showed that the MF treatment irreversibly modified the residual chemistry of the archaeological material and failed to preserve its original physical and historical integrity. This permanent modification of unknown longevity is considered not in line with conservation ethics and, therefore, inappropriate for the long-term preservation of cultural heritage objects.