An insight into effectiveness and potential damage in removing limewash from wall paintings. An approach based on model samples

Highlights

• Agar gels and cellulosic pulps were compared for limewash removal on wall painting.

• Limewash removal mainly occurs by layer wetting, weakening and detachment.

• The organic binder of tempera protects hematite against damage by chelating agents.

• Limewash adheres more on the hydrophilic fresco surface than on tempera.

• Operative suggestions were given for the cleaning of a real polychrome object.

Abstract

The mechanism, the effectiveness, and the potential damage during limewash removal from wall painting models were evaluated for agar gels and water-based pads constituted by Arbocel^TM BWW 40 cellulosic fibre. Cleaning materials in different formulations were compared: pure and with additives (ethylenediaminetetraacetic acid, EDTA, and ammonium citrate tribasic, TAC) in different percentages (2% and 3%). The cleaning action was evaluated on laboratory model samples, prepared with hematite a fresco and an egg-based tempera with limewash overlayers. Calcium and iron extracted by cleaning materials were quantified by inductively coupled plasma-mass spectrometry (ICP-MS). The potential damage to the hematite painting layers was also studied by electron paramagnetic resonance (EPR) spectroscopy. A visual observation of the limewash detachment induced by the overall cleaning was also performed. Results suggest that limewash removal mainly occurs by aqueous solution release from the cleaning system, with subsequent layer wetting, probable layer swelling, weakening and complete or partial detachment. A stronger limewash adhesion on the hydrophilic fresco surface than on tempera, was observed. None of the used cleaning materials resulted harmful to the integrity of the hematite layer underneath the limewash. A small damage in terms of extracted iron was detected in the cleaning systems after direct contact with fresco and tempera hematite layers; a “protective” effect by the tempera layer was observed for the pigment, due to the organic binder and triggered by the hydrophobic content of the egg-based medium.

Cleaning materials with additives are more harmful than pure materials, with a greater coordinating ability for EDTA than for TAC, which increases with chelator percentage. Data suggest a more efficient backward transportation of aqueous solutions containing calcium and iron ions towards gels with respect to cellulose, due to their smallest pore size. All these results lead to operative suggestions: for fresco painting layers, pure gel allows both a good limewash removal and a lack of damage on the hematite layers. Instead, for tempera layers a good limewash removal and a negligible damage on the pigment was shown by gel, both pure and additivated with TAC, and pure cellulose. Therefore, the present study allows to identify proper characterization methods for evaluating effectiveness and damage in limewash removal and to give useful suggestions for the planning of repeated cleaning operations on a real polychrome object.

Introduction

Cleaning an artwork surface requires highly effective methodologies, which are respectful of the artefact itself and its constituent materials [1], [2], [3]. The cleaning of mural paintings is a good example of how particularly delicate this process is, as they generally consist of a complex inorganic/organic matrix requiring the cleaning plan to be tailored to the specific conservation issues. The removal of soil, deposits and inorganic salts, having different water solubilities and often mixed with organic residues, is a typical example of such procedures [4]. Another example is the removal of layers applied on purpose during past works, such as lime whitewash or tempera overpaints, that can even reach thickness in the mm range. In these cases, the cleaning procedure very often establishes to firstly reduce the layer total thickness by mechanical methods (blades, sponges, brushes), usually after weakening of the layer cohesion with some chemicals to minimize the mechanical impact over the substrate. Then the cleaning is completed by applying a chemical pad directly onto the cleared painted surface. During these operations, the selectivity is of the utmost importance, to ensure that the chosen chemicals are effective at removing the applied layer without damaging the underlying painted ones. Selectivity requirements are usually very high because the whitewash or the overpaints are often similar in composition to the painted layer (e.g., calcite, casein or other organic binders, gypsum, pigments, calcium oxalate or other decay by-products). A difficult issue is represented by painted substrate covered by a whitewash layer; for instance, this implies calcium and iron in the same stratigraphy, few hundreds μm or less distant, and in a semi-porous system permeable to the used solutions. Both effectiveness and damage issues may be well assessed making use of model samples designed on purpose, since real samples are not enough homogeneous for such studies. Moreover, the Model Samples approach is planned to represent an overall conservation issue, thus it has already been used to study specific cleaning issues of wall paintings [3,5], marble surfaces [6] and stuccoworks [7].

The selectivity issue is strongly related to the nature of the chemical pad directly applied onto the original surface. Cellulose pulp mixed with water and various chemicals is extensively used by conservators as liquid reservoir and absorbent pad for both stone substrates [8], [9], [10] and painted plasters [11]. Pure water or its mixture with polar solvents, such as ethanol and acetone, is normally used. Non-polar solvents are also used in the practice, with the addition of surfactants [11]. However, cellulose pulp pads do not effectively retain the solvents, since their release to the absorbing substrate is a quite fast kinetic mechanism, with limited control on the liquid diffusion [12].

To overcome this drawback, conservators are more and more making use of gels, both synthetic and natural, in the attempt to improve cleaning performances. Synthetic gels include networks formed by both a single type of polymer and semi-interpenetrating polymers showing optimal mechanical properties and retentiveness [13], [14], [15], [16]. On the other hand, natural polysaccharide gels are highly effective, versatile, low-cost, and gentle on the artworks [6,[17], [18], [19], [20]]. Among natural polymers, agar gels are currently the most used material by Italian restorers for removing soiling [17] and metal stains from artwork surfaces [18, [21], [22], [23], [24], [25]].

Both cellulose and gel pads may be implemented by adding chelating agents [26,27], which showed outstanding practice improvements, worth a full investigation. Chelators are already used in the conservation of stone materials, thanks to their effectiveness in removing newly formed calcium compounds, such as black crusts mainly composed of gypsum and calcite, and white calcite thin layers or encrustations [3]. Agar gels with ethylenediaminetetraacetic acid (EDTA) and ammonium citrate tribasic (TAC) were used for the removal of brochantite stains from the marble base of the bronze statue portraying Napoleon by Antonio Canova, located in the Brera Gallery courtyard, Milan (Italy) [28]. On the other hand, the use of EDTA and citrate has been only recently reported for the removal of layers containing calcium salts on wall paintings [3].

For what concerns the selective removal of lime whitewash (hereinafter “limewash”), there is no agreement on which cleaning technique best preserves the coloured underpaintings [29], nor a systematic comparison has been investigated yet between cellulose pads and gels, evaluating their action mechanism, benefits, and limits. Thus, the present paper will study the cleaning mechanism of agar gels and cellulose pulp pads and will compare them in terms of:

• the effectiveness of limewash removal from painting layers based on hematite, applied with different techniques (a fresco and an egg-based tempera). Hematite was chosen because it is a very common iron-based pigment in mural paintings; moreover iron forms stables complexes with the used chelating agents. A fresco and a tempera layers were chosen in order to evaluate the protecting action of the different binders with respect to the chelating species;

• the damage on the hematite painting layers underlying the limewash (first step of limewash removal);

• the potential damage on hematite painting layers when they are directly contacted with the cleaning pads (second step of improving limewash removal).

These objectives were achieved by comparing the cleaning action of different formulations of agar gels and cellulose pulp, both pure and with additives (EDTA and TAC) in different percentages (2% and 3%). Cleaning was performed on laboratory model samples under well-controlled and reproducible conditions.

For the evaluation of limewash removal, different techniques were used: colorimetric measurements fail, since the mean colour difference in painting layers is not high enough; on the other hand, hyperspectral imaging techniques gave different results according to the used analysis methods [29]. In the present paper, both the effectiveness of limewash removal and the potential damage to hematite painting layers were evaluated in cleaning materials as extracted calcium and iron, respectively. Metal concentration was quantified by inductively coupled plasma-mass spectrometry (ICP-MS). This analytical method allows to obtain quantitative data, useful to compare some aspects of the effectiveness and the damage issues; it was successfully used for the systematic comparison of different agar gel formulations in the removal of brochantite stains from marble surfaces [6, 28]. A visual observation of the limewash detachment induced by the overall cleaning was also performed, which is the usual type of empirical evaluation made by conservators, in the attempt to link the scientific analytical approach to the practical one. The potential damage to the hematite painting layers was also evaluated by electron paramagnetic resonance (EPR) spectroscopy.