Nick Schiavon

Nature and decay effects of urban soiling on granitic building stones

A detailed microscopical and chemical examination by SEM + EDAX, XRD and PyGC-MS of black patinas coating the surfaces of urban granitic monuments in Aberdeen and Dublin has revealed a variety of decay features together with a complex chemical composition. Beside sulphate-rich thick patinas composed of a framework of gypsum crystals in which both particulate and gaseous inorganic/organic pollutants are deposited and which are similar to gypsum crusts developing under polluted atmospheric conditions on calcite-bearing stones, thinner, gypsum-free, black surface layers made up of particulate matter embedded in a fine-grained, iron-rich matrix are commonly found. These soiling layers show, relative to the substrate, higher % of P, Ca, S, and Cl. The occurrence of a patinas type against the other is probably controlled by the availability of local SO2 sources. Physical and chemical decay effects on the granitic substrate are more severe when gypsum is present but dissolution and micro fracturing episodes are also observed in iron-rich patinas. The chemical inorganic/organic composition of these surficial layers suggests that pollution induced soiling processes are the main factors resoonsible for their build-up on building facades. On the other hand, microscopic and preliminary chemical evidence does not exclude biological activity as an important agent in the development of iron-rich thin black layers.

Biodeterioration of calcareous and granitic building stones in urban environments

Abstract Scanning electron microscopy in secondary (SEM) and back-scattered (BSEM) mode associated with energy-dispersive X-ray spectroscopy has been used to investigate decay features associated with biological colonization of calcareous and granitic building stones in monuments and rock outcrops in the UK, Portugal and Spain. In combination with physical decay caused by lichen hyphae penetrating the stone substrate, this study reveals how chemical attack by organic acid exudates derived from metabolic activity of lichens and leading to the crystallization and growth of inorganic salt compounds within the stone microfabric is contributing to the destruction and weakening of the building stone surface. BSEM analysis is particularly useful in showing extensive dissolution and corrosion of mineral surfaces underneath biological patinas. Both Ca-sulphate (gypsum) and Ca-oxalate (weddellite) precipitates are clearly associated with the presence of fungal hyphae and bacterial activity particularly in the case of calcareous building stones; the Ca-oxalates present in the example examined are then the result of biomineralization processes and do not derive from past restoration treatments or from air pollution as suggested by many authors. In urban locations, lichenous cover may facilitate the deposition of particulate airborne pollutants on the stone surface. Biological patinas are by no means forming a protective layer on the biocolonized substrate; as is the case with sulphate or non-sulphate soiling patinas from urban air pollution, they act as localized sites of intense desegregation of the stone underneath. Their promot (but careful) removal and a biocidal treatment of the infested sites in the building is then suggested to avoid permanent loss of detail, particularly harmful in the case of carved surfaces.

Self-organization origin of wood-grained chert, Portland Limestone Formation (Upper Jurassic), southern England

Wood-grained chert is an unusual type of banded nodular chert that is characterized by alternating thin dark-coloured bands and thicker light-coloured bands, which give the chert a texture reminiscent of the growth rings of wood. The wood-grained texture found in some chert nodules in the Portland Limestone Formation of southern England is the result of variations in the concentration of calcite inclusions within the chert. Light-coloured bands contain more abundant calcite inclusions than adjoining darker coloured chert bands. The Portland Limestone wood-grained cherts formed by the late diagenetic replacement of a highly compacted carbonate sand, in which abundant siliceous sponge spicules were the silica source. Oxygen and hydrogen isotope data (δ 18 O=27.0 to 29.3‰; δ 18 O=−71 to −91‰ V-SMOW), indicate that either chert formation or, more likely, the opal-CT to quartz transformation, occurred in meteoric or mixed marine and meteoric pore waters at temperatures in the range of 20–60°C. The wood-grained texture likely formed by a self-organization process. A feedback mechanism is proposed for the origin of the texture in which calcium and carbonate ions released during the replacement of the host limestone resulted in calcite supersaturation along the boundaries of growing chert nodule. Calcite inclusion-rich chert bands formed within the calcite supersaturated zone and inclusion-poor bands formed outside of the calcite-supersaturated zone. Wood-grained chert provides another example of how the coupling of reaction and transport can produce repetitive patterns in otherwise unordered sediments or rock.

Non-destructive characterization of archeological Cu-based artifacts from the early metallurgy of southern Portugal

In this study, Monte Carlo (MC) simulations combined with energy dispersive X-ray fluorescence (EDXRF) spectroscopy have been used to characterize non-destructively a collection of Cu-based artifacts recovered from two archeological sites in southern Portugal: (a) the Chalcolithic E.T.A.R. site of Vila Nova de Mil Fontes and (b) the Middle Bronze Age site of Quinta do Estácio 6. The metal artifacts show a multilayered structure made up of three distinct layers: (a) brownish carbonate soil-derived crust, (b) green oxidized corrosion patina, and (c) bulk metal. In order to assess the reliability of the EDXRF-based Monte Carlo simulations to reproduce the composition of the alloy substrate in archeological bronze artifacts without the need to previously remove the superficial corrosion and soil derived patinas, EDXRF analysis together with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) was also performed on cleaned and patina-/crust-coated areas of the artifacts. Characterization of the mineralogical composition of the corrosion products in the surface patinas was further determined by means of X-ray diffraction (XRD). Results suggest that the adopted EDXRF/Monte Carlo protocol may represent a safe and fast analytical approach in the quantitative characterization of the bulk chemical composition of Cu-based metal artifacts even in the presence of fairly thick corrosion patinas and/or soil-derived encrustations at the surface of the archeological objects.

Efficacy of laser cleaning in the removal of biological patina on the volcanic scoria of the rock-hewn churches of Lalibela, Ethiopia

The Lalibela rock-hewn churches are one of the most important religious pilgrimage sites in Ethiopia. These churches are carved from the scoriaceous basalt rock substrate, which has been exposed to attack by biological agents with significant loss of surface material. Particularly, the widespread growth of lichens and other microorganisms on the carved surfaces of the churches has proven to represent a substantial threat for the preservation of the site. In this study, laboratory tests have been conducted to assess the feasibility of using laser technology as an efficient cleaning method of biological patina from polymineralic stone substrates. Multi-analytical techniques were applied for the characterization of the stone samples collected from two of the Lalibela churches: Bete Giyorgis and Bete Amanuel. Stone samples artificially inoculated with bacteria, yeast, fungi isolates, and lichen-encrusted samples were laser cleaned using UV and IR laser wavelengths. The high content of Fe and Ti oxides and the high porosity have made the stone surfaces easily susceptible to low-energy laser treatment. Results indicate that laser cleaning can be applied to polymineralic lithotypes and UV irradiation can successfully remove lichen colonies. Further studies need to be conducted to optimize the laser procedure in polymineralic, high porosity stones.

Synchrotron-based XRD from rat bone of different age groups

Synchrotron-based XRD spectra from rat bone of different age groups (w, 56 w and 78w), lumber vertebra at early stages of bone formation, Calcium hydroxyapatite (HAp) [Ca10(PO4)6(OH)2] bone fill with varying composition (60% and 70%) and bone cream (35-48%), has been acquired with 15keV synchrotron X-rays. Experiments were performed at Desy, Hamburg, Germany, utilizing the Resonant and Diffraction beamline (P9), with 15keV X-rays (λ=0.82666 A0). Diffraction data were quantitatively analyzed using the Rietveld refinement approach, which allowed us to characterize the structure of these samples in their early stages. Hydroxyapatite, received considerable attention in medical and materials sciences, since these materials are the hard tissues, such as bone and teeth. Higher bioactivity of these samples gained reasonable interest for biological application and for bone tissue repair in oral surgery and orthopedics. The results obtained from these samples, such as phase data, crystalline size of the phases, as well as the degree of crystallinity, confirm the apatite family crystallizing in a hexagonal system, space group P63/m with the lattice parameters of a=9.4328Å and c=6.8842Å (JCPDS card #09-0432). Synchrotron-based XRD patterns are relatively sharp and well resolved and can be attributed to the hexagonal crystal form of hydroxyapatite. All the samples were examined with scanning electron microscope at an accelerating voltage of 15kV. The presence of large globules of different sizes is observed, in small age groups of the rat bone (8w) and lumber vertebra (LV), as distinguished from, large age groups (56 and 78w) in all samples with different magnification, reflects an amorphous phase without significant traces of crystalline phases. Scanning electron microscopy (SEM) was used to characterize the morphology and crystalline properties of Hap, for all the samples, from 2 to 100μm resolution.

Identification Techniques II

An overview of applications of Small Angle X-ray Scattering (SAXS) techniques to topics of interest in the field of Cultural Heritage is presented. The basic concepts of the technique, a description of sources and laboratory instrumentation and some models and methods for data analysis are discussed. Applications of SAXS to various kinds of materials are reported to obtain information on the structure useful to shed light in some subjects such as preparation, physical treatment, traceability and degradation of materials.