The ligand is applied in the newly developed poultice consisting of Laponite ® RD, Arbocel ® BC1000, and CMC (carboxymethyl cellulose, sodium salt) which has recently been introduced for the cleaning of rust stained marble. In this paper the bidentate ligand ethylenediamine (en, ethane-1,2-diamine) will be introduced as an effective cleaning agent for removal of copper stains on calcareous stone. Recently the use of the amino acids alanine (ala), cysteine (cys) and tyrosine (tyr) has been investigated, and the mixture of alanine + ammonia in a poultice of sepiolite and Arbocel ® BC1000 has shown effective cleaning results. The hexadentate ligand edta (ethylenediaminetetraacetic acid) has also been used, although its affinity towards Ca 2+ is troublesome. NH 3 can be applied either from an aqueous solution of ammonia or from a solution of ammonium carbonate ((NH 4) 2CO 3), which forms an ammonium/ammonia buffer in solution. Ī standard ligand used for the removal of copper stains from stone surfaces is ammonia, NH 3, due to the formation of the well-known blue tetraamminecopper(II) complex. Furthermore the ligands have to be non-toxic for the conservators, simple to handle and dispose, kinetically fast-reacting, easily obtainable and economical affordable. The ligands have to show high affinity and stability towards the metal ion in question and low affinity towards the stone material itself, mainly Ca 2+. Several criteria must be met when selecting ligands for chemical cleaning. The cleaning of stained stone material is mainly performed by chemical cleaning methods using ligands that coordinates to the metal ion causing the stains, and thereby dissolving the stains. Dissolution of the patina by rainwater transports the soluble ions to the nearby porous stone material, where reprecipitation occurs and greenish stains are subsequently formed in the pores and on the surface of the stone material. The patina consists mainly of salts of basic copper(II) sulphates, such as brochantite (Cu 4(SO 4)(OH) 6), and to a lesser extent of basic copper(II) chlorides like atacamite (Cu 2Cl(OH) 3) and in rarer cases also malachite (Cu 2(CO 3)(OH) 2). The first step is oxidation of copper metal by atmospheric dioxygen (O 2), which creates a layer of cuprite (Cu 2O), which in turn reacts with rain and atmospheric impurities to subsequently build up a layer of greenish patina. The corrosion mechanism observed in connection with uncoated bronze statues and monuments is generally well understood. ĭiscolouration due to metal corrosion is observed when composite artefacts of uncoated metal and porous stone are located in an outdoor environment. The green stains alter the appearance of the monument, and therefore stone conservators and conservation scientists are constantly working with the cleaning of stone monuments as well as the development of new efficient cleaning methods. Light calcareous stone materials used in connection with cultural heritage objects, such as pedestals, or used as wall facings in connection with bronze or copper statues or joints often show green staining due to the corrosion products from the copper-containing materials. A solution of 0.1 M ethylenediamine in a poultice consisting of Laponite ® RD/Arbocel ® BC1000/CMC has shown to be an effective, economical, and fast do-it-yourself method. The chemical cleaning of copper stained calcareous stone surfaces has been investigated and a new method for removal of copper stains has been developed. The cleaning results were evaluated by visual observations. ResultsĬleaning experiments were performed on marble and on a calcareous sandstone plinth contaminated with natural copper corrosion products due to outdoor exposure of an untreated bronze statue. This paper describes a new, highly efficient method for cleaning copper stains from calcareous stone by introducing the chelating agent ethylenediamine in a poultice consisting of Laponite ® RD, Arbocel ® BC1000 and CMC. Due to this, several cleaning methods, often involving ammonia, have been developed for removal of copper stains from stone surfaces. The green stains alter the appearance of the monument and thus disrupt the original intention. Light calcareous stone materials used in connection with cultural heritage objects, such as pedestals, or used as wall facings in connection with bronze statues or joints often show green staining due to the corrosion products from the bronze elements.
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