Evaluation of Cleaning Processes Using Colorimetric and Spectral Data for the Removal of Layers of Limewash from Medieval Plasterwork

Abstract

In the cultural heritage preservation of medieval buildings, it is common to find plaster walls covered in lime, which previously were painted in polychromy. The conservation interventions usually try to remove the whitewash, whilst maintaining the original color of the painted wall as much as possible. However, there is no agreement on which cleaning technique best preserves the original appearance of the colored plaster. Different pigments found below the lime layer may behave differently depending on the cleaning technique used. Usually, colorimetric or photometric area-based measurements are carried out to study the color of the cleaned areas to compare with their original color, obtained from pre-made plaster probes. However, this methodology fails when the mean color difference is not enough to fully characterize the changes in texture and color appearance. This study presents a set of experiments carried out using two different pigments (cinnabar and malachite) covered with lime, and treated with nine different cleaning techniques on plaster probes prepared according to medieval techniques. We have studied the effect of the cleaning process on the color and the homogeneity of the samples using a hyperspectral imaging workflow. Four different analysis methods are presented and discussed. Our results show that the proposed analysis is able to provide a much more comprehensive and diversified characterization of the quality of the cleaning method compared to the commonly used colorimetric or photometric area-based measurements.

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

“…This result shows that limewash is more adherent on hydrophilic fresco model samples, even if weakened by the action of cleaning materials. Thus, pure gel can be proposed as a good cleaning material for the limewash removal both from fresco and tempera layers, in agreement with the limewash removal from other tempera layers based on malachite and cinnabar [29] . Hence, the herereported results can have a general value, even if a different adhesion of limewash on underlying layers can be present in different model samples and in real cases.…”

“…5 ). The edges between the treated and the untreated areas appear quite sharp because small residues of detached limewash were mechanically removed with a scalpel, according to a widespread procedure [29] .…”

“…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: a) the effectiveness of limewash removal from painting layers based on hematite, applied with different techniques ( a fresco and an egg-based tempera ).…”

“…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.…”

Methodology to Evaluate the State of Conservation of Historical Plasterwork and Its Polychrome to Promote Its Conservation

“…Detailed knowledge of traditional execution techniques [29], the history of the building [12,30,31], and the characterization of the plasterwork and its polychromies is essential to prevent the causes of alteration and establish maintenance and preventive conservation plans [32]. It is also relevant when addressing any restoration intervention to choose materials that are compatible with the original ones [33] and/or distinguishable [34], as well as to determine the most suitable cleaning, fixing, and replacement techniques [35][36][37]. Thus, the study of plasterwork has been recently implemented from the approach of the prescription of useful life [28] and the study of the durability of the material.…”