Background to the study
The subject of consolidation forms the basis of this research study where solutions for the mitigation of powdering limestone were considered. Stones which form part of a building or monument on site with this type of deterioration present a particular problem where replacement is unjustified and no intervention objectionable, especially when seen in light of a holistic conservation project. The desirable intervention in this case would call for a reduction in powdering through consolidation of the loose grains to each other and to the parent stone beneath while also slowing down further damage. This would need to be achieved in the presence of naturally-occurring salt mixtures, while still retaining the various properties of the stone including visual, physical and chemical aspects.
The limestone studied in this research is Maltese Globigerina Limestone, the main constituent material of the rich architectural heritage in the Mediterranean Islands of Malta and Gozo located 93 km to Sicily’s south and 288 km to Africa’s north. Globigerina Limestone is a fine-grained stone which is chiefly composed of calcium carbonate in the form of calcite crystals which are cemented together by micrite [1]. This limestone weathers readily in a salt-laden environment with powdering forming one of the main deterioration manifestations. The material has a high porosity that has been reported to vary between 24% [1] and 41% [2], with a pore size distribution where the majority of pores ≤ 4 μm [3]. Further testing on quarry Globigerina Limestone in the initial stages of this research project showed that the porosity accessible to water was 21.36% and the ratio of mesopores (6 nm < Ø < 50 nm) to macropores (Ø > 50 nm) was 1:7 [4].
Ammonium oxalate treatment
The chemical reaction between ammonium oxalate and calcium carbonate to form calcium oxalate is the premise on which this treatment is based. When naturally-formed calcium oxalate patinas on stone first started being investigated at the Opificio delle Pietre Dure in Florence, Italy in 1985, these were found to have improved acid resistance properties [5]. The potential formation of calcium oxalate through artificial means was later investigated [6]. This was developed using ammonium oxalate and marked the beginning of ammonium oxalate treatment in stone conservation. Consolidation properties were also subsequently reported [7].
Further studies were taken up on different marbles and limestones [8,9,10,11,12,13] including Maltese Globigerina Limestone [14]. Continued research on ammonium oxalate treatment to Globigerina Limestone was reported to result in the formation of calcium oxalate as whewellite which had a consolidating action, increased the surface hardness and retained the water transport properties of the stone [15].
The potential benefits of this treatment initiated this research project and commenced with the application of the treatment in the presence of a single salt—sodium chloride—in the laboratory, under controlled conditions [16,17,18]. This study also included different weathering typologies—quarry and weathered stone. The next step in the research led to the inclusion of further single soluble salts—sodium sulfate and sodium nitrate—which were studied immediately after treatment as well as after a fixed duration of 1 year’s site exposure on parallel sample sets [19,20,21]. No deleterious by-products were detected when the treatment was applied on the presence of these salts. Conclusions from this research highlighted the potential benefits of the treatment together with the need for treatment and testing in the field on real buildings in the presence of salt mixtures and fluctuating environments.
These studies led to the large-scale project under discussion which took in three different architectural heritage building sites where three representative environments were chosen: marine [22, 23], urban and garden settings in which the predominant salt present was envisaged to be different in each case, albeit within mixes. Ammonium oxalate treatment was applied without any desalination, in order to evaluate the resulting outcomes for each case scenario. A coastal bastion was selected to represent the marine environment, where the salt mixture included sodium chloride and sodium nitrate and did not contribute towards the formation of any adverse by-products [24] such as sodium oxalate which has been considered to be a potential by-product [14]. Similarly, no unfavourable by-products were formed when the treatment was applied to the urban site which consisted of a church, situated in a heavily polluted urban environment and included gypsum (Dreyfuss in preparation). This paper deals with the garden site where for the first time, a new by-product was formed after ammonium oxalate treatment. By-products—namely newly-formed soluble salts—are crucial to consider in the context of historic porous limestone on site, where water is available in its various forms—rainfall, rising damp, moisture from the air—and plays a key role in the salt crystallization and re-precipitation cycles which promote the deterioration of the stone fabric.
The aesthetic considerations, consolidating effects and water transport properties following ammonium oxalate treatment on this site are discussed elsewhere (Dreyfuss in preparation) and not the focus of this paper here.
St Philip’s Garden, Floriana, Malta
St Philip’s Garden was built within St Philip’s Bastion as a private garden by Grand Master Jean Paul Lascaris [25], who led the Knights of St. John in Malta between 1636 and 1657. It is constructed on a ravelin (triangular bastion), laid out over numerous terraces at varying levels, connected by pathways, steps and ramps which culminate at the Wignacourt Fountain which the garden is best known for. In 1932, the garden was included in the Antiquities Protection List and later scheduled by the Malta Environment and Planning Authority (MEPA) as a Grade 1 national monument in 2009 [25]. Today, it is a public garden.
Conservation works at the garden commenced in 2017. The garden wall chosen for this study was scheduled to be conserved in the later stages of the project. In this way, the study could serve an additional diagnostic as well as research and didactic purpose. The characterisation, treatment and testing discussed in this paper were all carried out before the start of any intervention works.
The garden wall considered in this research is approximately 5 m high and sits on a natural rock outcrop. It is accessible through the internal ramped passageway located within the garden confines (Fig. 1). Both the wall and the rock outcrop are composed of Maltese Globigerina Limestone. The wall is a retaining one which serves to hold the earth and soil located immediately behind it. This wall was selected because of the high incidence of powdering stones which were scheduled for replacement in the future intervention project.
8 stones—indicated with the red arrows in Fig. 2—were characterised as described in “Stone characterization” section below, followed by treatment and testing of the selected stones—enclosed by the black rectangles in Fig. 2, left untreated and right treated—as described in “Treatment” and “Testing methodology” sections.