Origin of films on monumental stone
Citation:S. Pavia and S. Caro `Origin of films on monumental stone? in Studies in Conservation, 51, 2006, pp 177-188
PaviaSinConsJanuary2006TARA.pdf (published (author copy) peer-reviewed) 533.2Kb
This paper investigates the origin of superficial films on different rock types from rural and urban locations in Spain and Ireland, focusing mainly on oxalate films. There is little agreement on the origin of oxalate films coating building stone. Some authors state that they are man originated whereas others claim that they are naturally produced by micro-organisms. This paper evidences the existence of three different types of film, each type with a different origin, composition and microstructure, and concludes that oxalate films can be both man originated and biologically produced. Analytical techniques including X-ray diffraction, petrographic microscopy, scanning electron microscopy with an energy dispersive X-ray diffraction attachment, inductively coupled plasma mass spectroscopy, Atomic absorption spectrophotometry, percentage weight loss on ignition, colorimetry by titration and conductivity measurements coupled to expandable ion analysis were used to study films on sandstone, limestone, dolomite and granite from a number of monuments and quarries. The results evidence that the films studied fell into three distinctive types. A crystalline calcite film resulting from weathering was found in quarries, an oxalate film of biogenic origin was found on rural Irish monuments and man-made, oxalate films containing gypsum were present on Spanish monuments. The organic oxalate film displays a penetrative, irregular contact with the underlying stone, a high water and organic content and a mat of oxalate crystals, entwined within which are minerals from the underlying substrate. In contrast, the man-made oxalate films are uniform, linear layers displaying fractures and striae, consisting of gypsum and oxalates as well as elements typically found in milk and egg white. These were probably fabricated by mixing inorganic binders such as lime and gypsum with organic agglomerates such as milk and egg white and historically applied as masonry finishes.
Type of material:Journal Article
Series/Report no:Studies in Conservation
Availability:Full text available