Environ Geol (2008) 56:767–775 DOI 10.1007/s00254-008-1481-z
SPECIAL ISSUE
Chemical and mineralogical characterisation of weathered historical bricks from the Venice lagoonal environment Nick Schiavon Æ Gian Antonio Mazzocchin Æ Fulvio Baudo
Received: 28 April 2008 / Accepted: 7 July 2008 / Published online: 22 July 2008 Ó Springer-Verlag 2008
Abstract Surficial and bulk samples of historical bricks of different age (from XII to XVIII centuries) recovered from a campaign of archaeological excavations recently carried out at the site of a medieval monastery in the S. Giacomo in Paludo Island in theVenice Lagoon have been characterised by FT-IR, TGA-DTG and DTA, XRD, SEM + EDS. The samples belong to a particular brick type commonly used in the Venice region: the ‘‘altinella brick’’. The bulk relative abundance of primary (i.e. calcite and dolomite) and secondary firing minerals (i.e. diopside and wollastonite) in the bricks coupled with their relative geometrical dimensions allows placing the samples in a chronological sequence according to known historical changes in brickmaking firing temperatures and practices. Most of the bricks were used as paving material and have been exposed to the action of seawater salts (chlorides and sulphates) due to periodical submersion following high tide episodes. Salt-laden (gypsum, halite, mirabilite) surficial patinas are indeed present but salt weathering does not appear to have affected the overall structural soundness of the bricks in this now abandoned island as it is the case with brickwork located in other more populated (and polluted) areas in Venice and its lagoon.
N. Schiavon (&) G. A. Mazzocchin Dipartimento di Chimica-Fisica, Universita` Ca` Foscari di Venezia, Calle Larga S. Marta, Dorsoduro 2137, 30123 Venice, Italy e-mail:
[email protected] F. Baudo Dipartimento di Scienze dell’Antichita` e del Vicino Oriente, Universita` Ca` Foscari di Venezia, Dorsoduro 3484/D, 30123 Venice, Italy
Keywords Historical bricks Venice Lagoon Altinella brick Salt-weathering
Introduction Despite sustained international action in recent decades aimed at a worldwide reduction in the emissions of atmospheric pollutants responsible for the (in)famous greenhouse effect, it is now widely accepted that anthropic activities are still actively contributing to global environmental atmospheric changes. While the focus of the international political agenda (coordinated by the UN Framework Convention on Climate Change which fostered a series of international climate conferences, i.e. Rio, Kyoto and recently Bangkok) has been so far predominantly concentrated on the effects that future climate scenarios will have on human life and human activities, research on the effects of global environmental change on cultural heritage is only recently forthcoming (Sabbioni et al. 2006; Brimblecombe and Grossi 2007; Grossi et al. 2007). These studies have indicated the following critical climate change factors as those likely to affect building materials in the near and distant future: increased precipitation in Northern Europe, humidity reduction in Southern Europe, global changes in the water cycle, increased floods and landslides and rising sea-levels in low-standing coastal areas. According to these forecasts, Venice and its lagoon are most likely to represent a critical region in future climate change scenarios for at least two reasons: its unique wealth of cultural heritage buildings and monuments and its location in a lagoonal environment highly subjected to the adverse effects of tidal and sea saltinduced weathering processes (Gatto and Carbognin 1987). Despite the wealth of international research studies currently available on the clay firing parameters, the weathering and
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the conservation of bricks in buildings and monuments across Europe and beyond (Maniatis et al. 1981; Maggetti 1982; Baer et al. 1998; Cultrone et al. 2001; Wolf 2002; Bauluz et al. 2004), remarkably few papers have been focusing on the characterisation of bricks and their weathering products in Venice and its lagoon. It has to be said that a bias towards ‘‘nobler’’ construction materials such as precious ornamental stone seems to have been applied to the Venice case study as a whole. Only recently, attention has been paid to ‘‘poorer’’ (but by no means less important in conservation terms) building materials historically used in the Venice urban area such as bricks (Calliari et al. 2001; Antonelli et al. 2002) and mortars (Sabbioni et al. 2002). The aim of this paper is the characterisation of the composition and decay products of a particular type of historical brick commonly used in the venetian area from Roman to medieval times: the so called ‘‘altinella brick’’ (Fazio et al. 1982). Its name derives from the name of the ancient Roman town of Altino located in the Venice coastal mainland: the town’s province is regarded as the main source area for the clay originally used as the starting material for the ‘‘altinella brick’’ production. Examples of pavings and brick walls made up of ‘‘altinella bricks’’ can still be found in several sites located in Venice city centre including famous monuments such as the Frari Church in the S. Polo district; remains of an old ‘‘altinella brick’’ paving with its typical herringbone lay out has recently been discovered during restoration works in St. Mark’s Square. Examples of ‘‘altinella bricks’’ can also be found on islands in the Venice Lagoon such as it is the case for the brick samples investigated here (see ‘‘Materials and techniques’’). The aim of the current study was twofold: (a) to carry out for the first time a chemical–mineralogical characterisation of the ‘‘altinella brick’’ that could help in the determination of the possible sources for the original clay material used in brickworks in Venice; (b) to assess the nature and extent of salt decay processes that have affected (and still do) bricks in the unique Venice lagoonal environment. New analytical data on venetian historical bricks and their decay mechanisms are of fundamental importance in assisting the correct choice of brick replacement materials in many buildings and monuments in the lagoonal environment of Venice both from a conservation point of view (i.e. to avoid differential response to weathering processes within the same brick wall and/or paving) as well as from an aesthetical one (i.e to avoid juxtaposition in the same wall facade of bricks incompatible in colour and texture).
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site of a now abandoned medieval monastery and hospice (thirteenth century) in the small island of S. Giacomo in Paludo in the NE section of the Venice lagoon (Figs. 1, 2). The S.Giacomo in Paludo archaeological site has been subdivided into different areas corresponding to building structures built in different ages: samples come from the areas 1000, 3000, and 7000 (Fig. 3). Brick dimensions (in cm) were also accurately measured as this data may provide a direct indication of the age of brick making (Varosio 2001). After mortar removal, the bricks bulk composition has been determined on powder samples collected from small holes drilled at 1 cm depth from the surface. For chemical and mineralogical analyses both untreated samples and samples dried in oven at 60°C for 3 days to remove excess humidity have been analysed by Fourier-Transform Infrared spectroscopy (FT-IR), Thermogravimetric analyses
Fig. 1 Satellite image of the Venice Lagoon showing the location of the S. Giacomo in Paludo Island
Materials and techniques Altinella brick samples have been collected as a result of archaeological excavations carried out in 2002–2003 at the
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Fig. 2 S. Giacomo in Paludo Island, NE section of Venice Lagoon
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Fig. 3 Sampling site: S. Giacomo in Paludo Island. Location of brick collecting areas and archaeo-stratigraphic units (1000, 3000, 7000)
(TGA-DTG and DTA) and X-ray Diffractometry (XRD). Absorption spectra in the IR region were collected using a FT-IR Perkin Elmer Spectrum One spectrometer. Thirtytwo signal-averaged were acquired on the samples. Few milligrams of each sample were added to KBr (IR grade, Merck) and pellets of diameter of about 13 mm were prepared. TGA-DTG and DTA analyses were carried out with a Netzsch STA 429 Instrument. A Philips X’Pert vertical goniometer with Bragg-Brentano geometry, connected to a highly stabilised generator, was used for XRD analysis: Cu Ka Ni-filtered radiation, a graphite monochromator on the diffracted beam and a proportional counter with pulse height discriminator were used. Measurements in the 5° to 60° range were taken with a step size of 0.05° and 2 s by point. The weathered outer surface of untreated brick samples was examined by Scanning Electron Microscopy interfaced with Energy-Dispersive Spectroscopy (SEM + EDS) to check for salt weathering attack. SEM analysis was undertaken using a Jeol (Tokyo, Japan) JSM 5600 LV SEM equipped with Oxford Instruments 6587 EDS microanalysis detector. Images were taken in the back-scattered mode (BSEM) and under low-vacuum conditions and coating of the samples with a high conductance thin film (gold or graphite) was not required. To test the mechanical soundness of the bricks, ultrasonic pulse velocity measurements
were carried out on brick samples from areas 1000, 3000 and 7000 (Fig. 3) using a UPV E49 Controls Instrument running at 55.4 kHz: three measurements were taken for each brick dimension (height, width and length) for a total of nine measurements per brick.
Results Samples: area 1000 Samples from this area come from two locations (Fig. 3): the perimetral wall of the monastery in the oldest section of the site (twelfth century to thirteenth century) and the cloister paving (sixteenth century). Average brick dimensions (in cm) differ according to their age and location and are as follows: 17.5 9 8.4 9 4.7 (perimetral wall: twelfth century to thirteenth century), 16.3 9 8.4 9 4.7 (cloister paving: sixteenth century). The bricks appear sound and do not display obvious decay features such as surface granular disintegration, scaling and alveolarization, although a thin, compact surface greyish weathering crust can be observed particularly in the samples used as paving material. Greenish moss patches are also present as a surface coating. Ultrasonic pulse velocity measurements range from 2,200 to 2,600 m s-1 averaging 2,430 m s-1.
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FT-IR spectra of the bulk samples show the presence of water, calcite, quartz, feldspars and iron oxides. In particular, peaks at 3,439 cm-1 are due to O–H stretching frequencies associated with hydroxyls in H2O while the 1,635 cm-1 peak is characteristic of bonded water; peaks at 1,429, 875 and 710 cm-1 are due to carbonate ions and reveal the presence of calcite; peaks of quartz are at 798 and 770 cm-1; the series of peaks ranging from 578 to 1,020 are characteristic of aluminosilicates, i.e. feldspars. Two minor peaks, at 537 and 469 cm-1, can be ascribed to iron oxides. Other minor peaks centring around the 2,900 cm-1 frequencies are due to C–H stretching and can ascribed to the presence of organic compounds, which indeed are macroscopically visible in the hand specimens as greenish organic patinas. In the bricks used as paving material, peaks due to diopside have been recognised while bands that can be ascribed to carbonate ions are either very weak or absent: the absence of carbonate in the latter samples is confirmed by the DTA analysis which does not show endothermic peaks due to decarbonation reactions. In the samples from the perimetral wall, bulk XRD spectra confirm the presence of calcite, quartz, feldspars together with the presence of peaks due to mirabilite (NaSO4 9 10H2O), a salt commonly found at the surface of monuments and buildings in coastal environments. XRD spectra of the samples from the cloister paving (Fig. 4) show well-defined peaks of wollastonite, diopside, feldspar (anorthite), halite and mirabilite; quartz is present in minor amounts, calcite peaks are almost absent. Minor hematite peaks may be present but the evidence is not conclusive. Gehlenite major peak at 2h = 31.4° may be present but due to overlaps with adjacent peaks, the evidence is again not conclusive. BSEM + EDS investigation of the brick outer surface of samples from the perimetral wall shows a widespread
coating by silicate and calcitic desegregate debris evidently derived from the breakdown of the siliceous (brick) and carbonatic (mortar) substrate. Pyritic spherical particles can be seen imbedded within the framework of the weathering patina. Amorphous coatings of organic origin are present. Authigenic halite particles are also present, albeit rare; they have been found not only within the surface patinas but also in the internal side of the brick within intercrystalline pore spaces. BSEM + EDS investigation of the brick surface of cloister samples reveals the widespread presence as weathering products of halite crystals with the distinctive cubic habit (Fig. 5) and of acicular gypsum. Note also the common presence of irregular particles of gold as detected by EDS analysis.
Fig. 4 Typical XRD spectrum of samples from cloister paving structure (area 1000). D Diopside, W Wollastonite, F Feldspar, Q Quartz, Mir Mirabilite, H Halite, Cal Calcite
Fig. 5 Coalescing cubic Halite crystals (H) imbedded within surface weathering patina showing also Biotite flakes (B) and Ilmenite (I) mineral grains. BSEM image
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Samples: area 3000 Samples from this area (Fig. 3) were collected from an E–W running wall whose construction age, albeit uncertain, should date back from the fourteenth century. Average brick dimensions (in cm) were as follows: 17 9 8.5 9 5.5. Under visual inspection, the brick’s original structure and textural soundness seems again to be well-preserved although a thin, irregular, greyish surface crust is present. Ultrasonic pulse velocity measurements range from 2,500 to 3,100 m s-1, averaging 2,830 m s-1. FT-IR spectra show the presence of water, calcite, quartz, feldspars and iron oxides. In these samples, carbonate bands are well developed: this is confirmed by DTA analysis which shows the characteristic peak at 810°C due to the endothermic decarbonation reaction. XRD of bulk samples confirms the presence of wellcrystallised calcite as the predominant constituent with
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subordinate quartz and feldspar (orthoclase) peaks and traces of dolomite. No crystalline salts were detected. BSEM + EDS investigation of the outer surface of the brick samples reveals the presence of a surficial irregular coating made up of quartz, silicate, calcite mineral particles. Samples: area 7000 Samples from this area come from two locations (Fig. 3): from the remains of a paving structure dating back to the sixteenth century and from the remains of a wall whose construction age is uncertain. Average brick dimensions (in cm) were as follows: 16.2 9 8.2 9 5.1. Under visual inspection, bricks appear coated by a grey to black patina which in some cases is associated with surface fracturing and powdering. Ultrasonic pulse velocity measurements range from 2,700 to 3,000 m s-1 averaging 2,860 m s-1. FT-IR spectra show the ubiquitous presence of aluminosilicates (feldspars and diopside), quartz and iron oxides; broad bands centring around 2,900 cm-1 and due to organic compounds (C–H) are again present. In all samples, calcite bands are either absent or very weak. Samples from the wall remains show strong peaks due to the presence of wollastonite (CaSiO3), a calcium silicate whose presence in bricks is due to the transformation after brick firing of calcite and quartz originally present in the source clay (see also ‘‘Discussion’’). In the samples from the the paving, bulk XRD spectra (Fig. 6) confirm the presence in decreasing order of abundance of wollastonite, diopside, feldspars, mirabilite and quartz. XRD spectra of the samples from the wall show well defined peaks of quartz, feldspars and minor peaks due to calcite.
Fig. 6 Typical XRD spectrum of samples from paving structure (area 7000). D Diopside, W Wollastonite, F Feldspar, Q Quartz, Mir Mirabilite, Cal Calcite
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SEM + EDS investigation of the bricks surface of samples from the paving structure shows the presence, upon a predominantly feldspathic and quartzitic substrate, of authigenic crystals of gypsum with a lanceolate habit and of baryte (BaSO4) particles. Of particular interest is the presence of mineral grains of monazite (a phosphate mineral containing rare elements such as Th, Cr, U) imbedded within the weathering surface patina (Fig. 7). In the wall samples, the widespread, striking presence of authigenic sulphide deposits in the form of framboidal pyritic aggregates has been detected (Fig. 8). The widespread presence of aggregates of well-crystallised wollastonite in the internal parts of the bricks from the paving is confirmed (Fig. 9).
Discussion The few previous chemical and mineralogical analyses of historical bricks from buildings in the Venice lagoonal
Fig. 7 a Monazite cristal grain (M) imbedded within surficial weathering patina (7000 area). BSEM image. b EDS spectrum of monazite crystals shown in a
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Fig. 8 a Framboidal pyrite aggregate imbedded within surficial weathering patina (7000 area). BSEM image. b EDS spectrum of pyrite crystals shown in a
Fig. 9 a Well formed wollastonite crystals on the internal side of bricks from the 7000 area. BSEM image. b EDS spectrum of wollastonite crystals shown in a
environment using EDXRF, XRD and OM techniques have shown a fairly uniform chemical composition (Calliari et al. 2001; Antonelli et al. 2002). In particular, analytical data from bricks of different ages from the sixteenth through to the twenty-ninth century have identified quartz, feldspar, calcite and pyroxene as the main mineralogical constituents (Antonelli et al. 2002). The results presented here on the ‘‘altinella bricks’’ are consistent with previous data and show a certain homogeneity in the bulk composition of the bricks suggesting a common local source for the primary clay. The main mineralogical species are again quartz, feldspars and carbonates: these mineral species and their abundances reflect the mineralogical composition of the original source clay. The main differences between the samples are correlated with the presence/absence of secondary minerals produced by firing which in turn provides an indication of the different firing temperature reached in the kiln during the brick-making process (Maggetti 1982; Wolf 2002; Bauluz et al. 2004). The presence/absence of carbonate minerals in the primary clays also plays an
important role in ceramic firing phase reactions (Cultrone et al. 2001). The presence of diopside (and of gehlenite) for example, detected in the samples from the cloister in the 1000 area (together with the absence of calcite in the same samples), indicates firing temperatures [800°C (Cultrone et al. 2001): the presence of wollastonite and anorthite in the same samples and in the samples from the 7000 area (Figs. 5, 9) suggests that, in these cases, kiln temperatures may have reached and exceeded 900–1,000°C. The absence of hematite in these samples is also consistent with high temperatures ([1,000°C) produced during the firing of calcareous clays (Maniatis et al. 1981; Cultrone et al. 2001). On the other hand, the persistence of calcite as a brick main mineralogical constituent after clay-firing in samples from the perimetral wall in the area 1000 and from the 3000 area indicate kiln maximum temperatures not exceeding 750–800°C. Such estimates of ancient firing temperatures based on mineralogical assemblages may in turn provide an indication of the relative age of brick production: it is well known infact that the maximum firing
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temperature reachable during historical brick production has been steadily increasing from Roman to modern times in line with technological improvements in kiln construction and firing parameters. These results are consistent with age assignments obtained from archaeological evidence based on brick average dimensions and on stratigraphical data (Gelichi et al. 2004). Despite the extremely complex water chemisty of the Venice lagoon (with its natural as well as anthropic influxes from the nearby chemical industrial area of Porto Marghera), a remarkably low mineralogical variety of surface salt weathering products have been found on the brick samples examined here. Sulphates (gypsum and mirabilite) and chlorides (halite) are the only salts detected: their abundances vary between the samples with halite being present in higher amounts, particularly in paving samples (Fig. 6). The presence of authigenic halite has been reported in bricks in which seawater was used to mould bricks (Rye 1976): its detection on the surface weathering patinas in the samples examined here however suggests a salt weathering origin in our case. Other salts, commonly reported as weathering products on bricks, (and other man-made materials) such as niter, thaumasite, ettringite, epsomite or thenardite have not been found. The occurrence of salt weathering on bricks and mortars associated with the contemporaneous presence of gypsum and halite has indeed been reported in other areas in the Lagoon (Antonelli et al. 2002; Sabbioni et al. 2002). Experimental and modelling studies on the crystallisation parameters of mixed salt solutions (and in particular of the Na–Ca–Cl–SO4–H2O system) related to conservation science (Price and Brimblecombe 1994; Klenz Larsen 2007) have pointed out that the solubility of gypsum, when mixed with sodium chloride, is increased up to four times while its crystallisation humidity for a saturated solution is lowered from close to 100% to 75% RH: at the latter RH value also halite will precipitate. These studies also indicate that, in line with the fact that the solubility of gypsum rises with falling temperature, the precipitation of gypsum is inhibited (and, instead, mirabilite start precipitating) below 0°C. The above-mentioned theoretical considerations based on models which take into account the RH and temperature as controlling factors in salt crystallisation, may explain why in the cases examined here gypsum abundance at the surface of most samples, as detected by XRD and BSEM + EDS, is low with respects to halite and mirabilite despite no shortage in the supply of dissolved sulphate from the lagoonal pore waters as evidenced by widespread pyrite precipitation. Moreover, the presence of pyrite framboids, both as single particles and as aggregates at the surface of many samples (Fig. 8), suggests that the bricks have been subjected to periodic burial episodes under a thin layer of recent lagoonal sediments following tidal excursions (Gatto
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and Carbognin 1987). Framboidal pyrite and other monosulphide species are indeed a common feature within the upper layers of recent clay-silt sediments of the Venice Lagoon and of other lagoonal and intertidal coastal environments where they are usually associated with clayey organic matrix and/or shell debris and plant cell material (Curtis and Spears 1968; Pye et al. 1990; Frizzo et al. 1991; Bertolin et al. 1995; Zaggia and Zonta 1997). Pyrite growth is due to seasonally adjusted anoxic conditions that affects surface sediments: under reducing conditions, iron is mobilised from ferric hydroxide coatings on detrital grains [Fe(OH)3] absorbed on clays or from Fe, Mg silicates and it then reacts with H2S, produced by sulphate reducing bacteria in the zone of sulphate reduction, to precipitate pyrite and other iron sulphide species. An alternative explanation for the occurrence of authigenic pyrite could be provided by reports that indicate its occurrence within weathering patinas on the surface of monuments in heavily polluted urban atmospheres (Schiavon and Zhou 1996): these authors explained pyrite presence with the iron sulphide crystals being a residue from past incomplete combustion of fossil fuels. In the cases examined here, though, airborne atmospheric SO2 from fossil fuel combustion processes is likely to have played only a minor role in the precipitation of sulphides and sulphate salts, taking into account the remote location of the island in the Lagoon, far away from the main SO2 pollution sources, both domestic and industrial, active predominantly elsewhere in the Venice city area. Accordingly, in the brick samples from S. Giacomo in Paludo Island, only rarely the development of a thin superficial sulphate patina can be seen which, in any case, does not seem to affect the overall soundness of the brick texture. Infact, decay phenomena commonly associated with salt weathering mechanisms (i.e. crystallisation pressure and/or hydration-dehydration cycles) such as scaling and granular disintegration are not present confirming also historical reports on the durability and weathering resistance of the ‘‘altinella brick’’. Ultrasonic velocity values (from 2,200 to 3,100 m s-1) are indeed within the range typical of new and sound brick material. This is in contrast with the fact that well-known severe sulphation-induced decay effects are so commonly found on monuments and building stones across Venice and also with the experimental evidence showing Na sulphates (such as mirabilite and its anhydrous equivalent thenardite) as the most damaging salts as far as building materials are concerned (Rodriguez-Navarro and Doehne 1999). Evidently, the particular environmental conditions experienced by the brick samples examined here (high RH values between 75 and 100%, low temperatures, negative Eh values within the burying upper sediment layer) have inhibited the onset of the hydration/dehydration cycles and oxidation reactions, which are well-known decay inducing factors.
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The imbedding within surface weathering patinas from samples of the 1000 area of particles of Au appears rather intriguing. The source of such particles in the Lagoon is not clear. It is interesting to note, though, that the S. Giacomo in Paludo Island site, albeit distant, as already mentioned, from the main sources of industrial air pollution in the Venice region, is, nevertheless, quite close to the Murano Island: the latter island harbours some of the most famous ornamental glass making factories in the world, which indeed utilise gold mineral flakes as a common minor constituent during the glass manufacturing process. Another interesting finding is the presence of monazite particles imbedded within weathering patinas from the paving samples from the 7000 area: sand grains made up of this heavy mineral have been reported as mineral markers of fluvial deposits of the river Brenta, which has been discharging fluvial sediments into the Venice Lagoon until the sixteenth century (Jobstraibizer and Malesani 1973). It is then likely that the monazite found in the superficial weathering patinas in the S. Giacomo Paludo Island bricks may also derive from reworked fluvial sediment material from the same local fluvial source.
Conclusions This study reveals the complexity and historical changes in environmental conditions (air and water pollutants, sediment chemical composition) to which the building materials (such as the historical bricks here investigated) have been subjected in the Venice Lagoon. The bulk mineralogical characterisation of the bricks reveals the presence of new mineral phases such as diopside, wollastonite, anorthite that can give clues as to the firing temperature to which the original clays were subjected and the carbonate content of such clays; these results are consistent with age assignments obtained from archaeological evidence based on brick average dimensions and on stratigraphical data. Despite the detection of salt mineral phases, such as mirabilite and halite in surface patinas, the bricks do not show signs of severe weathering as it is the case with brickwork located in other more populated (and polluted) areas in Venice and its lagoon: this is probably due to the particular environmental conditions experienced by the brick samples examined here (high RH values between 75 and 100%, low temperatures, negative Eh values within the burying upper sediment layer), which have inhibited the onset of decay-inducing hydration/ dehydration cycles and oxidation reactions. Further work is needed to extend the chemical/mineralogical characterisation of ‘‘altinella bricks’’ in other sites within the Venice Lagoon. From a conservation point of view, the goal of this future research will be twofold: (a) a
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more precise identification of the source clay quarries originally used for historical brickmaking; the results will be of paramount importance in order to be able to reproduce the ‘‘altinella bricks’’ for restoration purposes; (b) the assessment of the nature and extent of salt weathering attack on this type of bricks in other more polluted areas of Venice. It is worth mentioning that the ‘‘altinella brick’’ has been extensively used in the past in monuments and buildings in Venice and in its Lagoon, not only for paving and walling purposes but also as part of building foundations: its decay and conservation is therefore very important for the future and survival of Venice and its unique cultural heritage.
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