Effective cleaning of rust stained marble
Calcareous materials, like marble used in connection with cultural heritage objects such as statues and pedestals, or as wall facings on buildings, often show a brownish staining owing to contact with iron metal or iron-containing minerals in the stone. The discolouration alters the appearance of the stone, which is undesirable from an aesthetic point of view. Despite rust staining being a conspicuous phenomenon and numerous works that have dealt with the problem of removing rust stains, a simple and non-toxic method has so far been missing. This paper describes a highly efficient method for cleaning rust stains from marble by introducing the chelating amino acid cysteine in a Laponite poultice in combination with the strong reducing agent sodium dithionite.
Results
Cleaning experiments were performed on artificially discoloured samples of various types of Carrara Bianco marble and on naturally rust stained marble. To begin with, solutions of cysteine in combination with sodium dithionite and ammonium carbonate were tested by immersion of samples into the different solutions. Secondly, solutions of cysteine and sodium dithionite with and without buffering were used in a poultice consisting of Laponite® RD, Arbocel® BC1000 and CMC. The poultice was applied on three different marble types: Carrara Fabricotti, Carrara Vagli and Carrara La Piana. Thirdly, the optimized method was tested on original rust stained material of luxury marble, which has been used as wall facing, and finally in situ in Copenhagen on a larger area of The Marble Church showing rust stains due to pyrite oxidation. The cleaning results were evaluated by visual observations, cross sections, and etching of the surface by testing on high gloss marble.
Conclusion
Cleaning of iron-discoloured marble surfaces has been investigated and a new method for removal of rust stained marble has been developed. A solution of 0.1 M cysteine and 0.1 M sodium dithionite in a poultice consisting of Laponite® RD/Arbocel® BC1000/CMC = 10:10:1 has shown to be a fast, simple, cheap, and non-toxic, do-it-yourself method.
Since ancient times, white marble has been used as a popular material for sculptural artefacts such as statues, busts, and friezes as well as an architectural building material with numerous applications from flooring, wall facings, and pedestals, to columns and fountains. Although marble is a relatively stable material, the desired white surface is unfortunately prone to tarnishing when used in outdoor environments [1]. One of the major sources of tarnishing is iron. In addition to the oxidation of internal iron compounds present in stone like pyrite (FeS2) and siderite (FeCO3) [1, 2], contact with iron-rich ground water when Full Body Marble is used in, for example, garden fountains, results in severe and unsightly discolouration [3]. Another cause is the proximity to iron metal, which is oxidized by air in the presence of rain. The solubilized ions are then transported by rain onto the marble surface, resulting in rust formation [4].
The detailed mechanism for rust formation is highly complex; depending on the pH value, different species, all characterized by a brownish colour, are formed. The atmospheric corrosion of iron, regardless of the pH value of the reaction may, however, be summarized by the overall stoichiometric reaction (1) where the product FeOOH represents the generic formula for rust [5].
The general name rust consists of a variety of iron(III) oxyhydroxides or hydrated oxides of high stability and low solubility. The actual species formed depend as mentioned on the pH value and the presence of different anions [6–8]. The thermodynamic parameters and solubility products have been estimated for many of the rust species, such as ferrihydrite and α-, β- and γ-FeOOH (goethite, akaganeite and lepidocrocite). These investigations have shown that goethite defines a thermodynamic minimum of the rust system [7, 9] and the solubility product of goethite (Ksp = 10−41) is the lowest among the different rust species [7]. This means, from a thermodynamic point of view, that rust can be examined as goethite, and thus the cleaning of rust can be considered as removal of goethite.
Rust discolouration of marble is characterized by areas or stains having an orange to brownish colour, which alters the appearance of the stone. From an aesthetic point of view, the discolouration is undesirable and stone conservators and conservation scientists have therefore worked for several decades with various cleaning methods in attempts to remove rust stains from marble and calcareous stone materials [3, 10–12].