Supercritical carbon dioxide: cleaning and stabilization tests for copper alloys and iron keys from a scientific collection of the Centro Hospitalar Conde de Ferreira (Porto, Portugal)

An increasing number of eco-friendly nanotechnologies for the conservation of cultural heritage materials are becoming available. These include supercritical carbon dioxide (scCO2), which has proven to be a viable green, non-flammable, and ecological method. While previously tested and now widely used in the pharmaceutical and food industries, scCO2 has recently been applied in the conservation of cultural heritage objects, and specifically for their cleaning, consolidation, and stabilization. Experience has shown that scCO2 is a promising alternative to conventional toxic organic solvents for the removal of organic materials, such as oils, grease deposits, and active corrosion products. In this study, we tested and applied scCO2 as a cleaning and stabilization agent on two coils of copper and silk textile wires used in the electrical induction mechanism of a faradic machine and on an iron key from a set of handcuffs. Both objects come from a set of psychiatric instruments and equipment dating from the 19th to the mid-20th century that now form part of the scientific collection of the Centro Hospitalar Conde de Ferreira, which belongs to Museu da Misericórdia do Porto, Portugal. Portable X-ray fluorescence was carried out before and after scCO2 treatment to determine the elemental composition of the metal alloys and the effectiveness of the cleaning process. Before the objects were treated, scCO2 was tested on eight samples of similar material and degradation pathologies. Comparative experiments were performed with and without the addition of a co-solvent to scCO2. The co-solvents included ethanol, hydrogen peroxide, ethylenediaminetetraacetic acid (EDTA), and sodium hexametaphosphate. Different temperatures, reaction times, and pressures were evaluated. Cleaning efficiency was assessed by the visual examination of dirt removal, material stabilization, oxidation stain removal, fiber mechanical strength, and dye loss. The results revealed that scCO2 without the addition of a co-solvent did not have a significant impact on the samples compared to when it was applied with ethanol or EDTA, which as co-solvents improved cleaning and stabilized the corrosion products. The respective formulations proved to be suitable alternatives for the treatment of metal artifacts as well as objects composed of different materials.