The Orange-Brown Patina of Salisbury Cathedral (West Porch) Surfaces: Evidence of its Man-Made Origin (5 pp) |
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Authors: | Email author" target="_blank">Jesus?Martín-GilEmail author F J?Martín-Gil M C?Ramos-Sánchez P?Martín-Ramos |
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Institution: | (1) Jesus Martín-Gil Laboratory of Environment Assessment ETSIIAA Avenida de Madrid, 57 Palencia-34004 SPAIN,;(2) F.J. Martín-Gil Laboratory of Environment Assessment ETSIIAA Avenida de Madrid, 57 Palencia-34004 SPAIN,;(3) M.C. Ramos-Sánchez Laboratory of Microbiology Hospital Universitario Del Río Hortega Cardenal Torquemada, s/n 47010-Valladolid SPAIN,;(4) P. Martín-Ramos Laboratory of Environment Assessment ETSIIAA Avenida de Madrid, 57 Palencia-34004 SPAIN, |
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Abstract: | 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 accord 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 CO32– ions, but not necessarily from the limestone. They could
be attributed to carbonated hydroxyapatite through the substitution of groups PO43– for CO32– 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. |
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Keywords: | bone Salisbury cathedral XRD and XRF ATD-FTIR stone phosphates sulphates oxalates patina |
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