The orange-brown patina of Salisbury Cathedral (West Porch) surfaces: evidence of its man-made origin.

Goal, Scope And Background: In this paper, we attempt to elucidate the composition and origin of the orange patina on the surfaces of the West-Porch of Salisbury Cathedral by comparison to other known patinas: (i) the orange-brown patina on the marble surfaces of the Acropolis in Athens and the Arch of Titus in Rome whose analyses have shown very high amounts of phosphates, and generally amino acids from animal-skin glue or other protein binders; (ii) the phosphated patinas which also contain oxalates, found in 1996 on Catalonian calcareous sandstones and in the calcareous dolomites of the Monastery of Silos, Spain, whose origin is either the application of calcium caseinate, or egg yolk and animal glue; and (iii) the patinas with only oxalates found in some of Verona's monuments (St. Zeno) and Spanish sites as in the Monastery of Guadalupe and Cuenca cathedral, formed either by the mineralization of algal filaments or by biological reactions yielding oxalate from yolk egg (added to stone as part of preservative empirical treatments).

Methods: In the winter of 2003, the West-Porch of Salisbury Cathedral received conservation works, but the old patina was not entirely removed. This fact has allowed us to collect the samples for its study. The IR spectra were registered with a Golden Gate ATR Mk II system using attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectrometry. Mineral composition was determined by XRD (Philips PW 1710 spectrometer with Cu tube), whereas major and trace elements analyses were performed by XRF (Philips PW1480 PW). Microscopy examination was performed on a Leica M655 microscope. Phosphate, oxalate, calcium and sulphate contents were analysed by usual chemical methods.

Results: ATD-FTIR spectra of the Salisbury's patina exhibit peaks at 2361, 2341 and 671 cm(-1) (assigned to phosphates); 3410, 1680, 1620, 1122 and 602 cm(-1) (assigned to sulphates); and 1447/1437 and 876 cm(-1) (attributed to carbonates). The little peaks at 1620 and 798 cm(-1) could be assigned to oxalates. XRD and XRF have led to identify the carbonates, phosphates and sulphates as pertaining to the species dolomite, hydroxyapatite and gypsum, respectively. Oxalates are detected only in small amounts by chemical analyses but wewellite and weddellite have not been well identified. The interface between the patina and the calcareous dolomite is very uneven and full of cavities in certain cases, but well-defined and rather smooth in other cases. In accordance with the very small amounts of the oxalates found, remnants of micro-organisms are not detected in the patinas.

Discussion: The Salisbury's patina is a composite material formed by particulates and matrix constituents. Regarding the patina particulate, e.g. animal bones, it is necessary to refer to the apatite phase composition. The bone mineral contains 4-8 wt % of carbonate in animal body and its presence in the apatite phase is advantageous as it increases the mechanical strength. We think that FTIR bands at around 1440 and 876 cm(-1) arise from vibration of CO3(2-) ions, but not necessarily from the limestone. They could be attributed to carbonated hydroxyapatite through the substitution of groups PO4(3-) for CO3(2-) in the lattice of hydroxyapatite. Concerning the matrix and also from the FTIR spectra, the absence of specific bands of the following species: proteins (3350-3225, 1660, 1550-1535, 1270-1230 and 620 cm(-1)), oils (1778, 1738 and 1051 cm(-1)), bee waxes (3000, 1470, 720-730 and 1700 cm(-1)) and aged egg-yolk (2954, 2920, 2850, 1650, 1549, 1465 and 1240 cm(-1)) had led us to exclude these usual binders. On the other hand, the amount of sulphates in the paste that covers the walls of the Salisbury's Cathedral is excessively high (above 20% in weight) to consider it as a biotransformation product of calcium oxalate from fungal biofilms. Consequently, we must think that the gypsum found in the samples has a man-made origin (it was deliberately added as part of a protective paste) and that it is the matrix searched for. Thus, we deduce that the patina of Salisbury's Cathedral is a special stucco made mixing plaster with powdered bone (the colour of the bones is the same that it exhibits in the patina), low quantities of an uncharacterized binder (collagen, possibly) and water.

Conclusion: We believe that the patina of the Salisbury's Cathedral is a variant of the Greco-Latin empirical protective treatment that included bone as a hardening material. Nevertheless, we also think that the presence of the bones in the paste could be related to an aesthetical intention: gaining a warm tone for the original stone through the ochre colour of the bones.

Recommendation And Perspective: Our results have been an excuse to contribute to the controversy started at the 80's on the origin of orange-brown patinas observed on stone surfaces of Greco-Latin and medieval monuments. There are two major theories on provenance: biological vs. man-made. In Salisbury Cathedral, neither of them has been proven through scientific evidence as yet. Our opinion is that Salisbury patina can be classified into the man-made group.