Feasibility of subcritical fluid technology to stabilize archaeological copper alloy artifacts.

A PhD involving collaboration between Clemson University (USA) and Cardiff University (UK) is investigating the potential of subcritical fluid technology in the treatment of heritage copper alloys at the Warren Lasch Conservation Center. The work reported here examines the impact of temperature, pH and time on the chloride bearing compounds nantokite (CuCl), clinoatacamite1 (Cu2 (OH)3 Cl) and atacamite (Cu2 (OH)3 Cl), of which the latter two are also referred to as trihydroxychlorides. These compounds were individually compressed into pellets, using calcium carbonate (CaCO3 ) as a binder where necessary and subjected to continuous fixed flow of sodium carbonate (Na2 CO3 ) 0.01 percent (w/w) (pH 10) and sodium bicarbonate (NaHCO3 ) 0.00193 percent (w/w) (pH 8) solutions in subcritical conditions. Differing sample sets were run at 130°C, 180°C and 230°C for 1 and 5 hours. Eluate was sampled for chloride content at selected values of time. X-Ray diffraction and micro-Raman spectroscopy were used to investigate compound transformations that occurred during treatments. Nantokite and atacamite released significant amounts of chloride in all treatment environments. Varying degrees of transformation occurred for both compounds depending on parameters. Nantokite transformed to cuprite (Cu2 O) and copper trihydroxychloride (Cu2 (OH)3 Cl) while atacamite transformed to tenorite (CuO). Significant amounts of chloride detected in the eluate from clinoatacamite treatments could be assigned to residual sodium chloride (NaCl) from its synthesis as no compositional changes were detected, suggesting differing behavior from its polymorph atacamite. Hydrolysis of nantokite in high relative humidity produces atacamite and clinoatacamite, which physically disrupts patinas and alters the object aesthetics. Conversion to unreactive compounds in subcritical conditions offers the basis for developing a predictive evidence-based treatment option for the heritage sector