Evaluation of Chloride Extraction Rates from Copper Compounds Under Subcritical Conditions

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ABSTRACT This paper presents a critical review of the reasons behind the initiation of this project to find a more sustainable, cost efficient, and safer form of chloride extraction that would result in stabilisation of archaeological copper and its alloys with tin and zinc. The imperative has become increasingly strident with cutbacks in funding for museum conservation facilities and with the reduction in staff, leaving less time for routine analyses of traditional immersive washing treatments that could take several years to effect. Owing to the faster oxidation and reduction kinetics of copper, compared with tin and zinc ions, the problems of uncontrolled corrosion of copper alloys in a chloride-rich environment have led to unacceptable levels of decay in heritage collection items. This study looked at the chloride extraction rates under subcritical conditions for both primary and secondary chloride-rich mineral species nantokite, clinoatacamite, and atacamite and determined the impact of changing the solution pH from 8 to 10, while also addressing the impact of temperature. The range of temperatures between 130 and 230°C was assessed at 50°C intervals at both levels of acidity of the leaching liquors. For the nantokite treatment, the formation of a cuprite patina which replaced the waxy-grey initial corrosion products was regarded as a bonus. The nature of the patina and changes in surface textures resulting from the treatment are critically reviewed and a series of recommendations for future work are included in the conclusion.