Calcium oxalate films on works of art: A review

Abstract

This work presents a review of the findings of calcium oxalate films, a widespread decay phenomenon recovered on stone and other substrates (mortars, wall and easel paintings, written materials, glass). The specific attention given to the issue in the 1990s has decreased, although articles have documented films until the present day. The review provides critical insights into the literature, focusing on the general properties of films, numerous case studies, the instrumental techniques used to characterise and date films, insights into the possible origin of the decay phenomenon, and an evaluation of the protective role of calcium oxalate layers. The practice of creating artificial films on stone surfaces for protective purposes is also considered. An evaluation of the literature over the last few decades shows various open issues. The origin is still up for debate, and the issue is still of major concern to conservation scientists, conservators, and restorers. The scientific community tends to attribute a biological origin to these films, however further studies are needed to study exactly how they form, focusing for example on simulation tests of the chemical and atmospheric pathway. How these films protect the artwork in terms of the mechanical properties of the surface underneath deserves more study. This would also help restorers to reproduce the calcium oxalate. The bibliography highlights the prevalence of calcium oxalate findings in the Mediterranean Basin and the formation of the least stable form, i.e. weddellite, which has yet to be explained.

Introduction

The presence of calcium oxalate films on monuments was first documented by Justus Von Liebig in 1853 [1]. He studied marble fragments from the Parthenon (Athens, Greece) that presented a dark ochre colour, and determined that the main component was a calcium oxalate mineral. He called the compound Thierschite in honour of Friedrich Von Thiersch, the conservator of the scientific collections of Bavaria who gave him the marble samples. Thierschite proved to be almost identical to whewellite [2]. Liebig described the phenomenon: “Under the microscope, when illuminated by sun-light, it appeared as a lustrous, opaline aggregate of warty nuclei, exhibiting a concentric structure. When seen by the naked eye, it appeared an uneven layer of a dingy-grey colour, covering the whole outer surface of the stone” [3]. He attributed the formation of calcium oxalate to a series of generations of lichens vegetating on the surface, and excluded the possibility that it could be a pigment intentionally applied to the surface. The residue completely covered the stone surface, until the lichens could no longer find the carbonate surface necessary for their survival. Much later, other authors confirmed the presence of calcium oxalate films on the Parthenon, in some cases ascribing the phenomenon to the degradation of ancient treatments [4], [5], [6]. The subsequent finding of similar films was documented by Cipriani and Franchi on the Colosseum in Rome in the 1950s [7] and attributed to the lichens, which covered much of the surface of the monument.

Since the typical appearance of the films has been identified, several investigations have been conducted on the unique coloured “layers” (from ochre to dark brown) observed on numerous monument surfaces from different locations all over the world, but particularly in the Mediterranean Basin (Greece, Italy, Spain) (Fig. 1). Different terminology was initially used to refer to the same phenomenon on stone, such as oxalate film, patina, scialbatura, crust, skin, epidermis, coating, Mediterranean patina, At some point the term “film” appears to prevail [UNI 11182:2006 Cultural Heritage – Natural and artificial stone – Description of the alteration – Terminology and definition"].

The oxalate films were found to be so widespread that they became one of the most assessed weathering processes, and researchers began to examine the compositional and morphological features, the effects on the stone surface and the mechanisms of formation [8], [9], [10], [11], [12]. The debate was further opened up after the Centro CNR “Gino Bozza” (Milano, Italy) organized two international conferences to compare ideas and on-going research [13], [14], [15]. Scientific interest in the issue has increased over time, as oxalate films are continuously documented and the discussion about the origin of the films is still open (Fig. 2).

This paper focuses on naturally-occurring calcium oxalate films, and describes the findings in the literature. Section 2 discusses the compositional and morphological features of layers, which can be investigated using various analytical and mineralogical techniques. Section 3 outlines the various substrates (marble, limestone, wall and easel paintings, mortars, written materials, and glass) on which films have been documented. The two main theories regarding the origin of the films, suggest that natural processes are behind the formation of calcium oxalate, and these theories consider the reaction either as a transformation of the original substrate by bacterial activity or a consequence of the degradation pathway of past organic-based surface treatments. These two theories are reviewed in Section 4, which also discusses alternative hypotheses on the possible atmospheric origin of oxalic acid or a combination of the anthropogenic and natural standpoints previously described. Section 5 reports some studies that emphasize the significance of films in protecting the stone underneath considering several case studies that assess their protective role over time. These observations have led to the intentional creation of calcium oxalate on stone surfaces for conservation purposes. Although this review was intended as a report on naturally-occurring films, Section 6 briefly discusses artificially-induced calcium oxalate films in the light of their importance in the field of conservation. Finally, Section 7 concludes the assessment of past and present research by considering the dating techniques used to approximate the time period during which rock paintings covered by oxalate films were produced.

Articles referring both to calcium oxalate films and deposits were considered, as researchers do not always conduct analyses necessary for defining the presence of calcium oxalate actually as a “film”. Moreover, whenever calcium oxalate is found on a substrate, it is clearly a compound external to the original materials and thus is worth considering, because it can contribute to the origin of the films. Deposits of calcium oxalate may in fact depend on “formation conditions and not formation causes”, as suggested by Monte [16].