Assessment of the efficacy of agarose and agarose augmented with zinc oxide, carbon dots, and graphitic carbon nitride nanostructures in the restoration of historic tintype

Abstract

Tintype photographs, a popular medium in the 19th century, provide valuable cultural and historical records. However, they are frequently at risk due to surface contaminants, such as soot and dust, which compromise both the visual appearance and the chemical stability of tintypes. The sheet-like nature of tintypes makes them highly susceptible to damage from conventional cleaning methods, which can cause scratching and result in partial loss of the image layer. The present work evaluated efficiency of various cleaning techniques, including swab cleaning, agarose gel, and nanomaterial‑modified agarose gels incorporating zinc oxide (ZnO), carbon dots (CDots), and graphitic carbon nitride (g‑C₃N₄). The nanomaterials and composite gels were characterized in detail using transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential, UV-vis spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Cleaning efficiency was also assessed through portable digital microscopy, colorimetric analysis, and ATR-FTIR spectroscopy, prior to and post cleaning as well as after artificial light ageing. The results demonstrated that nanomaterial-functionalized agarose gels offered superior cleaning efficiency and a protective effect in comparison to conventional methods. Interestingly, agarose with ZnO (ΔE = 3.35), carbon dots (ΔE = 2.7), and graphitic carbon nitride (ΔE = 3.09) efficiently removed dust without inducing noticeable color changes while simultaneously providing an additional barrier against UV radiation. Here, ΔE represents the total color difference calculated according to the CIELAB color space and is a dimensionless parameter. In soot removal, agarose + ZnO (ΔE = 5.84) and agarose + carbon dots (ΔE = 7.73) exhibited maximum performance. Significantly, ATR-FTIR analysis confirmed that no chemical alteration in the tintype layers following treatment or accelerated invasive cleaning of tintypes, confirming the suitability of the methods for use in restoration and conservation.