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.
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.
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 . 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 . 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 .
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 .
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 . 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].
Due to the nature of the discoloration and the possibility of damaging the stone, the stain can only be removed by chemical cleaning. The current method for rust cleaning involves application of different ligands and reducing agents mixed in a poultice and placed onto the stone surface. One of the ligands most widely used is the citrate ion [10, 11, 13], though salts of other carboxylic acids, such as oxalic and tartaric acid, have also been used . Other methods involve the use of fluoride  or EDTA . A relatively new method is the use of the hexadentate ligand tpen, which, in contrast to EDTA, has a high affinity towards iron and a low affinity towards calcium . This ligand has shown excellent results when tested on a discoloured marble fountain, however this method is rather expensive. The ligands are used either alone or in combination with reducing agents like thiosulfate, dithionite or polythiophene [3, 10]. Thioglycolic acid and ammonium thioglycolate have been applied in several conservation treatments of calcareous stone . Thioglycolate is presumably the most efficient ligand for cleaning rust stained marble [12, 13]. However, thioglycolic acid is a toxic chemical, and is thus difficult to acquire for private stone conservators without access to a laboratory. In addition to this, a slightly violet colour may appear on the marble when cleaning with thioglycolic acid, which demands a second cleaning .
In this study, we have aimed to investigate and develop a new method for rust cleaning of simm marble. The focus has been on the use of cheap and commercially available chemicals. Another target was reduction of Fe(III) to Fe(II) while cleaning. Efficient removal of a slightly soluble material requires a ligand having an overall stability constant comparable to the reciprocal value of the solubility product in order to achieve a favourable equilibrium constant. Based on the solubility product of goethite, efficient removal of rust in Fe(III) stage requires a ligand having a stability constant approaching 1041, whereas removal of Fe(OH)2 only requires a stability constant of 1014. Additionally, the ligand should possess low affinity towards Ca(II) to prevent dissolution of calcite.
Introducing new chemistry for rust cleaning
In the search for an efficient method for rust cleaning, the focus has been both on a ligand showing strong complex formation with iron and weak binding to the major constituent ions in marble i.e. Ca(II) and Mg(II), as well as on the identification of a fast reducing agent able to reduce Fe(III) to Fe(II). Among the reducing chemicals, sodium dithionite (SD), Na2S2O4, has been successfully used in combination with different ligands as a dissolving agent for goethite in soil analyses [14, 15] and for removal of rust from paper . Furthermore, the use of dithionite in conservation science in general is well described .
The standard reduction potential, e°, of dithionite in the basic solution given in Eq. (2) has been determined to −1.12 V (vs. NHE) [15, 17] and is thereby one of the strongest reducing agents among the simple, cheap, commercial reagents. The reducing power decreases with lower pH values and using pKa2 = 7 for hydrogen sulphite the potential can be calculated to e°′ = −0.29 V at pH = 7.
In aqueous solution dithionite partly dissociates, forming the highly reactive monomeric sulphur dioxide radical anion with the dissociation equilibrium constant K = 10−9 .