Camouflaged concrete : evaluating the state of conservation of the Maltese sound mirror ‘Il-Widna’ and its painted surface

Abstract

The Maltese sound mirror, Il-Widna, has a surviving and unique painted surface that has never been studied. It is well-established that the British military built this monumental concrete structure from 1934- 1935 in Malta to detect and relay the sounds of incoming enemy aircraft. However, the significance, materiality, techniques, and condition of the painted camouflage surface and its relationship with its concrete support is unexplored. This study focuses primarily on the painted surface of the Maltese sound mirror while also acknowledging the concrete support and the intrinsic and external factors influencing the holistic condition of the structure. Specifically, it illuminates and elucidates the stratigraphy and original materials and techniques of Il-Widna and provides an overall condition assessment of the structure and its painted surface. Archival investigations and interviews with cultural heritage professionals and military historians were conducted to establish theoretical information of the site and its painted camouflage. Visual examinations and infrared thermography (IRT) on site were then conducted to establish the physical evidence of Il-Widna. Afterwards, stratigraphic samples were taken and observed under a Nikon SMZ800 stereomicroscope, prepared into cross-sections and analysed under a Nikon Eclipse CiPOL polarized light microscope. Selected samples were then subjected to a Zeiss Evo 15 scanning electron microscope (SEM), coupled with EDX analysis. The results show that a hydraulic binder, possibly Portland cement, and local limestone aggregates were used in the concrete. A white layer of calcium carbonate on top of the concrete is visible on both sides of the mirror. A grey sandy layer, found only on the north face, is composed of a hydraulic binder and probably sand containing aluminium and silicon-based aggregates. The five coloured paints are possibly cement paints, i.e., Portland cement-based with sand containing aluminium-silicon-based aggregates. Various pigments have been identified. In particular, the pigment of the green paint is a chromium oxide green. This is particularly relevant because chromium pigments were heavily restricted at the start of World War II and therefore helps to convey that the paint is part of the original technique. The red, yellow, light brown, and dark brown paints each include a different iron-oxide pigment. The sound mirror’s functionality and its need for camouflage affected the different material stratifications and techniques used on the north and south faces of the structure. Additionally, the deterioration phenomena of the paint are occurring in relation to their original materials and underlying concrete support, as well as their locations on the large and long concrete structure, which is influenced by different environmental and external factors. The major sources of these deterioration phenomena appear to be high temperatures, fluctuations in relative humidity, solar radiation, and biodeterioration. Hence, measures should be taken to safeguard Il-Widna’s current state of conservation.