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SOLID PARTICLES OF ATMOSPHERIC POLLUTION FOUND ON THE HONTORIA LIMESTONE OF BURGOS CATHEDRAL (SPAIN).

ESBERT,R.M. , DIAZ-PACHE, F., ALONSO, F.J., ORDAZ, J. and GROSSI C.M.

Department of Geology. University of Oviedo (Spain)

Key words: Atmospheric pollution, solid particles, deterioration, black crust, monumental stone,

scanning electron microscopy (SEM), microanalysis (EDX), automated particle analysis, Burgos cathedral.

SUMMARY

Several hundreds of solid particles of atmospheric pollution, found on the stone of the Burgos cathedral

(Spain), have been examined by means of SEM+EDX. These particles were collected on the building

stone from the monument (Hontoria limestone) as well as on specimens of the same material placed in different orientations.

The morpho-chemical analyses of the above-mentioned particles have allowed us to establish five

different groups which are thought to be generated by the combustion of different products: mainly fuel oil, coal, petrol and industrial fuel.

Examples of the different morphological and chemical analyses of the solid particles are shown and

discussed. Some analyses of the material which is presently being deposited on quarry rock samples are

also given. That means a first step in determining its role in the formation of black patinas and crusts.

1. INTRODUCTION.

Building stones, when exposed outdoors, develop several forms of deterioration as a result of its interaction

with the environment. Aerosols from atmospheric pollution have a significant influence on the deterioration of

the stone. These aerosols are made up of a set of solid and liquid particles of a mixed nature and with very

variable chemical compositions and sizes.

In this report we characterize the solid particles which have been found on the black patinas and crusts

developed on the Hontoria stone in Burgos cathedral (Fig. 1). This stone is a white limestone, with more than

99% calcite, with fossil remains, coarse and variable grain sizes (2 mm average) and a high open porosity

(21%) (Marcos et al, 1993). The morphological and chemical analyses of the solid particles give data for finding out the sources of the

pollution which has generated them. It is hoped that they will also help to determine the causes and processes

responsible for blackening the stone. The analytical techniques used are scanning electron microscopy and micro-analysis by energy dispersion X­

Ray microscopy (SEM+EDX). Both techniques are very useful for analysing and classifying the particles, as

they permit individual study.

2. PARTICLE ANALYSIS

2.1 Sampling and sample preparation. Particles found on samples taken in black crusts and patinas from different orientations and heights of the

building have been analyzed. Particles were also found in samples of dust deposits on the surface,

efflorescences and alveolized areas. The analyzed particles show different characteristics and concentrations due to their relation to the different

sources of pollution, the prevailing wind and rain-washing. Therefore, it is important to know the precise

position and orientation of these samples. The samples collected have been prepared for analysis by sticking them to a stub and then coating with a

carton layer to avoid the interference that gold coating could provoke in the EDX spectrum.

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2.2 Morpho-chemical analysis: sources of emission.

From the results of the analyses, different types of particles can be established according to their morphology

and chemical composition:

* Type A: Small spherical particles (< 5 microns) showing dense pitting, few circular pores and

sometimes micropores. The main component is sulphur; other elements (such as silicon, calcium and

vanadium) appear in much smaller proportions (Fig. 2). They seem to be generated by fuel oil combustion in

domestic heating systems. * Type B: Small smooth spheres mainly composed of iron oxides (Sabbioni and Zappia, 1993) (Fig 3).

This type of particles can be generated by the combustion of fuel oil in domestic heating systems and also by

the combustion of coal (Sabbioni, 1992).

* Type C: Smooth spherical particles with an essentially aluminosilicate composition (Fig. 4). These

particles are highly characteristic of coal combustion processes (Sabbioni, 1992).

* Type D: Subspherical particles with large holes, characterized by the presence of lead and zinc, along

with iron, sulphur and other elements (Fig. 5). This type of particles is attributed to the combustion of petrols

(Van Bormet al., 1989). There are few of these on Burgos cathedral.

* Type E: Particles of a skeletal appearance and carbonaceous nature. They are spherical, corroded

and have very irregular pores (Sabbioni, 192). The characteristic components are calcium, sulphur and

sometimes vanadium. The probable source of these particles is the combustion of industrial fuel. Only a few of

these particles have been found in the studied crusts. This could be because they are converted into gypsum

and lose their individuality (Fig. 6).

In addition to these groups of particles, which may be considered "specific" to certain combustion processes,

we have also observed other particles which are common to all of these processes. Calcium is the main

element of these particles, sometimes accompanied by silicon and sometimes by sulphur (Connor, 1990).

They can have a variable morphology: compact, corroded, porous, etc., (Fig. 7).

All of the mentioned particle types, after some time, can undergo transformation and coating processes leading

to the formation of gypsum on their surfaces (Fig. 8).

3. PRESENT DEPOSITION OF PARTICLES

The above-analyzed particles have been deposited over a period of several years and their previous

characteristics and deposition rate cannot be determined. Then, some studies are being carried out to find out

the present deposition rate and the origin and composition of the particles.

Samples of Hontoria quarry stone have been placed outside the cathedral sheltered from rainfall, on the

Coroneria fac;ade with the most favourable orientation in terms of pollution (facing WNW). Part of the samples

have been collected every two months for SEM+EDX study. The time of exposure was one year. The particles

are being characterized. Moreover, mappings of several chemical elements -especially sulphur- have been

carried out in order to determine the formation rate of gypsum and other deterioration products.

The programme FEA TU RESCAN -incorporated into the SEM+EDX- is being used to count and characterize

the particles. This programme, firstly processes the SEM images. From these images the programme detects,

measures and analyzes the particles. Furthermore, about 250 parameters of their morphology and composition are automatically given.

Statistical analyses of combined morphological and chemical data can be performed, the results being

displayed on histograms or XY diagrams. It is also possible to group the particles according to their morpho­

chemical characteristics, and to assign a colour to each group, both in the processed images and in the correlation graphs (Figs. 9 and 10).

The main elements of the particles analyzed over a period of several months are sulphur and calcium; and in

lesser proportions silicon, potassium, iron, chlorine, sodium and aluminium. Magnesium, phosphorus and

metal elements, such as titanium, vanadium, copper, chromium, lead, zinc and nickel, have been found more sporadically.

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Work is also being carried out to determine the characteristics of the particles that will permit the definition of groups according to their origin. Using this knowledge, it is possible to determine the most significant sources of pollution, which would make preventive action possible. After four months of exposure to the open air, gypsum has been detected on the samples, and its concentration has increased over subsequent months. Sometimes we can see how the gypsum grows around the particles (Fig. 11), especially when the latter are metallic (Rodnguez-Navarro and Sebastian, 1994). On the whole, according to the studies carried out so far, a significant particle deposition on the stone of Burgos Cathedral has been verified (Fig. 12). In one year, the white quarry samples have suffered a very considerable blackening process.

ACKNOWLEDGEMENTS

Our thanks to the Comisi6n lnterministerial de Ciencia y Tecnologfa (CICYT), Project CC95-SEC 0501 : "Protecci6n de rocas monumentales emplazadas en ambientes urbanos, frente al dep6sito de partf culas s61idas de contaminaci6n".

REFERENCES

Connor, M. (1990): "Les aerosols antropogenes et l'alteraction de la pierre". Project de Diplome. Laboratoire de la Conservation de la pierre. Ecole Politechnyque de Laussanne (82 pp.).

Marcos, RM.; Esbert, RM.; Alonso, F.J. y Dfaz-Pache, F. (1993): "Caracterfsticas que condicionan el comportamiento de la caliza de Hontoria (Burgos) como material de construcci6n". Boletfn Geol6gico y Minero. Vol. 104 n°5, 587-597.

Rodriguez-Navarro, C. y Sebastian, E. (1994): "Pollution-derived heavy-metal enrichment on building stones". Goldsctimictt Conference Edinburgh 1994. Mineralogical Magazine, Vol 58A (781-782).

Sabbioni, C. (1.992): "Characterization of atmospheric particles on monuments by scanning electron micr~py I energy dispersive X-ray analyses". Electron Microscopy, Vol 2, EUREM 92. Granada (773-777).

Sabbipni, C. y Jappia, G. (1993): "Characterization of particles emitted by domestic heating units fueled by distilled .oil';. Atmos. Environ. Vol. 27A, n° 8 (1331-1338).

V.an.Borm, WA; Ad,ams, F.C. y Maenhault, W. (1989): "Characterization of individual particles in the Antweip aerosol". Atmos. Environ. Vol. 23, n° 5.

Figure 1: General aspect of the main fayade (Santa Marfa) of Burgos Cathedral.

Figure 2: A sphefical particle with few circular pores.

Its main component is sulphur and it is attributed to

the combustion of fuel oil in heating systems.

Figure 3: A solid particle from the combustion of

fuel oil and coal, containing a large quantity of iron.

Counts ( x 10 • I

ii: . _,-' \ S f\ T i F • Fe

,.,,,..,,~~"""r ;.-,,...~,"r.:.=.-,.....,_...-r",,.,_,-rr~..,...-,.,...,.,_.,.,...,.J,.. 0

0 I 0

Ranee < keV I

Figure 4: a) A smooth spherical particle with an incipient covering. b) Its main components are silicon and aluminium. They are highly typical of the

combustion "coal.

Figue 5: a) A subspherical particle with large holes. b) It is characterized by the presence of lead and zinc. A particle originated from the

combustion of petrols.

NZ•

Figlre 6: A carbouac:euus skeletal particle. b) It shcM's a large quantity of calcium and, to a lesser extent, sulphur. Its origin is attributed to the

Counts (XlO' l c ..

z

s

St

0 2 4

398

6 8 10

Range (keVl

Figure 7: Particles rich in calcium with several

morphologies. Some are spherical with few pores (a),

others are more porous and corroded and also

have a solid appearance (b). They an have a

composition close to that of the analysis shoYm (c).

Their origin canr-a be detemlined.

Figure 8: Gypsum covering solid

particles on Hontoria limestone.

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Figure 9: a) SEM Image or a filter v.tlere particles of pollution can be seen. b) The same image treated by FEA TURESCAN; the smaller particles

have been eliminated, and those remaining have been highlighted and numbered.

Figure 11 : A SEM image of a sample of Hontoria stone

e>cposed outdoors for six months around BurQOS Cathedral.

Figure 1 O: Histogram showing the particles in

Figure 9, grouped according to their

chemical characteristics and their

hydraulic radius.

Figure 12: A small gypsum aggregate found on a sample

from Hontoria exposed to the open air for six months.

N<Xe the gypsum surrounding a particle or pollution.