Abstract
This paper represents the introduction to the volume focused on the various aspects of natural stone research. From the topic issues studied recently, four major aspects are covered: (1) availability of certain stone types for monuments, (2) strategies aiming to strengthen our knowledge of past resources through the establishment of natural stone databases and inventories, (3) evaluation of natural stone properties and assessment of the compatibility of repair stones, and (4) decay studies of natural stone on monuments and on natural exposures. This paper also aims to highlight the importance of the use of local stone resources, a practice that has seriously declined during the 20th century in most of the industrialized countries. The availability of natural stone from monuments is also discussed in terms of sandstones and travertine in the Czech Republic and Hungary as a typical example.
Natural stone has become the expression to describe the versatile, durable and aesthetically plausible building materials (Currier 1960). From the very beginning of civilization, important structures and monuments were built from or were principally based on natural stone. The use of local stone resources was mostly in balance with the local environment. Although highly durable when properly applied, no stone type can be considered immortal (Schaffer 1932) and most of the stone varieties are affected by a polluted atmosphere (Winkler 1997). Deteriorated stone on monuments should be preferably replaced by stone varieties of the same composition from the same quarries. However, this is often not possible, and so stone having similar properties and appearance must be sourced (Přikryl 2007). The main aim of this research is to find compatible stones in terms of appearance and geochemical-physical properties. This task requires a thorough understanding of both the rock properties and its response to external conditions and weathering agents.
Availability of natural stone
General
Availability of traditional building stone for the restoration of monuments is rarely discussed in the scientific literature although it represents one of the key points of the conservation of built heritage (Ashurst & Dimes 2004; Snethlage 2005). Exploitation and utilization of local resources of natural stone are a typical feature of world and European history until the turn of 19th/20th century (e.g. Frangipane 2004 and this volume). The tradition of natural stone utilization dramatically declined during the first half of the 20th century when new, artificial materials and previously un-used and alien stone varieties were introduced. Häfner (2007) provides an example of this decline from the sandstones of the Rhine area (Germany) where only 16 operations now exist when previously there were about a thousand quarries until relatively recent times. A similar situation can be found in the most European countries; the Czech Republic has access to about 5–10% of the originally available natural stone varieties (Přikryl et al. 2003; Přikryl 2004a). New materials such as concrete or imported stones are probably acceptable for new buildings in rapidly expanding suburbs but restored monuments and historic city centres require the use of traditional materials (Dreesen & Dusar 2004; Carta et al. 2005).
A common question that architects and restorers often ask geologists concerns the availability of the traditional stone types. The availability is limited not only by geological factors but is regulated by government requirements, local planning, natural and water resources protection (Häfner 2007). Even if the original stone types are still being quarried (which is not commonly the case in most of the industrialized countries), the quality of recently available stone varieties can be different from those exploited in the past. The quantity of exploitable reserves is another key issue when considering the suitability of a certain site to supply material for monuments (Selonen et al. 2000). Small scale, non-continuous operation of local quarries, which has been practised in the past, does not fit modern, industrial-scale exploitation of internationally traded stone varieties. A similar contradiction applies to stone manufacturing for which diamond-saw cutting now prevails (Shadmon 1989). Hand-dressed stone elements are often required for most of the restored monuments, a fact hardly realized in practice.
Recultivation and/or renaturalization of ancient quarry sites and conversion of the land for other purposes is common. As well as agricultural or developer activities, numerous abandoned quarries have also become sites of ‘natural protected’ status. The protection of these sites is very often not due to the geological record but to other non-geological values (e.g. botanical, zoological) (Přikryl 2009). The question regularly arises as to whether or not small-scale operations aiming to exploit limited amounts of stone for a particular monument will be acceptable on those sites. The public responses, particularly in developed countries, are very aware of the potential environmental impact of any mining/quarrying activity.
In cases where the original stone is no longer available, interest commonly focuses on whether any alternative material can be supplied and used without changing the character of the monument. Even if the alternative stone types show similar petrographic macroscopic character, it can produce a significantly different appearance when exposed to weathering conditions. A typical example of such behaviour is provided by Blows et al. (2003) describing the currently unavailable Caen stone (cream-coloured fine-grained French limestone) that has often been replaced by Lepine limestone. The latter exhibits significantly different weathering features. Compatibility assessment is therefore an important research topic in recent natural stone studies as shown by several papers in this volume (Beck & Al-Mukhtar 2010; Nijland et al. 2010).
Availability of natural stone for the restoration of monuments: a case example of the Charles Bridge in Prague
The stone Charles Bridge, the oldest preserved bridge in Prague (Czech Republic), represents one of the iconic monuments of the city that shows long-term deterioration of its natural stone facing masonry. The bridge presents a mosaic of numerous types of local sandstones (quartz arenites, litharenites and arkosic arenites from Carboniferous and Upper Cretaceous sediments in the Prague neighbourhoods) that were utilized either during the original construction during 1357–1402 period or for later repairs (Fig. 1a). Based on the detailed petrographic research of stone samples from the bridge and geotechnical survey (Drozd & Přikryl 2003; Drozd et al. 2005), seven quarry areas that provided two major rock types, Carboniferous arkoses and Cretaceous sandstones (Table 1), can be traced. Categorization of these stones was facilitated by a lithotheque of historical dimension stones of the Czech Republic (Přikryl et al. 2001, 2004a), a fact-documenting exercise through study of local stone resources and their presentation on the form of stone databases, lithotheques and catalogues (see papers written by Cooke 2010; Frangipane 2010; Kampfová & Přikryl 2010 and/or Allocca et al. 2010). Until about the mid-19th century, the sources of stones for the Charles Bridge remained unchanged but, in the late 19th and 20th centuries, repairs did not respect the original types of stones and new stone varieties were introduced (Table 1). This was partly due to economic reasons but also to the fact that the 20th century repairs suffered from the closure of all sandstones quarries in the Prague area.
The sandstone facing masonry of the Charles Bridge in Prague (Czech Republic) (a) has often been repaired using different types of local stone resources. The stone used during the latest repair in 1960–1970 (b) shows odd colours (beige to rusty yellow) and rock fabric which differentiate it from the original sandstones (which have a dark grey surface due to the polluted atmosphere in Prague).
Summary of natural stone types used for the construction and repairs of the Charles Bridge in Prague (modified after Přikryl 2006a)
The Charles Bridge also suffered from incorrect maintenance during the 20th century. Major repairs were conducted after the damaging floods in 1890 (repairs continued till 1910) and again in 1960–1970. Between and after these repairs, there was no ordinary maintenance of the facing masonry partly caused by missing stoneworks of Charles Bridge and non-availability of the original stone. The most serious impact of improper modern repairs is linked to the fact that the original types of natural stone were not employed. As stated before, this was mainly caused by the closure of original quarries. As a consequence, other types of sandstones showing different quality (in terms of physical properties and of appearance – see Fig. 1b) were introduced which probably accelerated the deterioration of the facing masonry. These stones show pronounced granular disintegration by salt weathering followed by the surface retreat of 1 cm (rate 0.3 mm/a) for Božanov arkosic sandstone after 30 years of service and up to 3 cm retreat for Hořice sandstone after 100 years of service. Use of Portland cement-based concrete for fixing of newly inserted ashlars worsened the state of the bridge further.
Discussions for the new repair and maintenance plan opened the question of whether or not the dominant traditional natural stone (Carboniferous arkoses) could be used for the replacements (Přikryl 2004b, 2006a) in spite of the fact that its exploitation finished at the beginning of 20th century. In 2004–2008, an extensive desk study and field reconnaissance focused on several areas of the Czech Republic where this stone type was quarried in the past. From eleven abandoned quarries, two sites provided promising results; the possible re-opening of these sites is being discussed. Although the macroscopic similarity to the original stone was the main criterion, the physical properties, durability and amount of reserves were the main decision-making criteria for selection of these sites.
Property evaluation
Assessment of natural stone properties became routine practice throughout the 19th/20th centuries. The testing of physical properties, both index and mechanical, relies on procedures developed for other artificial inorganic silicate materials, namely concrete, bricks and glass. The specific features of natural stone concerning their genesis and wide range of composition makes ‘blind’ adoption of testing procedures often questionable and often requires test procedures that at least be modified. This also affects the evaluation of the test results. This broad issue is discussed below in the context of rock durability.
Durability of natural stone reflects its ability to withstand the external pressures leading to the deterioration of the physical properties, partial decomposition and physical breakdown. The durability is therefore proportional to the period during which the stone can preserve its properties, both physical and aesthetical. The resistance of natural stone to weathering action is a function of:
internal parameters of the rock which encompass genetic conditions, mineralogical and chemical composition, rock fabric (understood as spatial arrangement of rock-forming minerals and pore space – see discussion provided by e.g. Přikryl 2006b) and isotropy or anisotropy of rock fabric;
external factors that can generally be described as the environmental conditions to which the stone is exposed (climatic conditions, composition of atmosphere including presence of pollutants, presence of water, biospheric influence and/or interaction between stone piece and other materials present in the construction, Smith et al. 2001).
Some authors (e.g. Warke 1996) also distinguish between factors that influenced the rock before its emplacement in the construction (i.e. mode of the extraction, dressing of the stone) and factors that modify the stone during its service. The latter case has been found extremely significant for stone sculptures which have experienced numerous restoration/conservation treatments in the past (Přikryl et al. 2004b).
Weathering as a process leading to both functional and aesthetical loss of the original value of the stone (Smith & Přikryl 2007) is often manifested by numerous so-called weathering phenomena that have been studied in detail during the last few decades both from the point of view of the mechanisms of formation and the methodology of assessment (e.g. Warke et al. 2003). Monuments represent highly prized cultural objects that do not allow detailed study and sufficient sampling in most cases; the study of weathering processes on natural objects therefore presents a welcome alternative (see e.g. Siedel 2010).
The susceptibility of natural stone to weathering agents was tested by several approaches during past decades. Along with normalized procedures that focused on the salt crystallization action and freezing/thawing of water in pore systems of natural stone, widely adopted in practical assessment of recently produced stone types (EN 12370 2000; EN 12371 2002), numerous scientific experimental approaches have also been applied. The latter tests mostly rely on accelerated decay in laboratory conditions and employ just one type of weathering agent (e.g. variable types of salts) (see Angeli et al. 2010; Yu & Oguchi 2010; Oguchi & Yuasa 2010). Numerous studies published during the past decade document a significant increase in knowledge both regarding the processes and respective stone type response (Turkington & Paradise 2005).
Some of the tests are interpreted according to the old practices which, unfortunately, are linked to certain types of environment or climate and cannot be easily extrapolated and generalized. Assessment of the rock susceptibility to the frost action is a typical example (Thomachot & Jeannette 2002). When assessing this parameter, the rock specimens are tested for their uniaxial compressive strength in the dry state and after being subjected to 25 freeze-thaw cycles. The number of cycles – 25 – was originally suggested by Hirschwald (1911) based on an evaluation of the number of frost days that might have had an impact on natural stone in Germany (Berlin) during the period 1884–1892. The lowest number of frost days (Hirschwald considered the days during which the temperature was below 0 °C and were preceded by a rainy period which caused wetting of the rock) was 14 and the highest 25. Based on this observation, Hirschwald (1911) proposed to subject the rocks to 25 freeze-thaw cycles which should correspond to 1–3 years of real conditions. It is obvious that this can only be valid for regions having similar climatic conditions as Berlin in the late 19th century. The freeze-thaw impact of one winter in for example, New York area can be better deduced from 12–16 cycles as shown by Bortz & Wonneberger (1997).
The dynamics of the changes during the above-mentioned tests presents another important issue. It has been shown by numerous theoretical considerations and practical tests that the decrease of stone properties due to accelerating weathering is neither linear nor continuous (e.g. Smith et al. 1992). This is another weak point in the practical assessment of the quality of natural stone by standard procedures when only the values before and after the cycles are compared. In contrast, experimental studies often evaluate changes after each cycle (e.g. Goudie 1999).
The real weathering conditions involve several processes. The test design should therefore combine those processes that are known or likely expected from the site where the natural stone will be emplaced (Duffy & O'Brien 1996). The real conditions can be better modelled using large climatic chambers allowing control of several factors (temperature, humidity, concentration of salts etc.) (Taylor-Firth & Laycock 1999).
Performance in use and compatibility of different varieties of natural stone
From a great variety of stones, only one example is given in the wide use and compatibility of stones, namely travertine. Travertine is a common building and dimension stone that has been explored and used in many countries from prehistoric times (Pentecost 2005). Travertine was one of the favourite stones of the Roman Empire and historic monuments were made from this stone in the heart of the Empire in Rome (e.g. the Colosseum). Understanding the properties and workability of this stone allowed the Romans to search for similar deposits throughout the entire Roman Empire. This resulted in the widespread use of travertine during Roman times not only in Rome but far away from the ‘locus typicus’ of travertine in Europe (e.g. present day London) and Asia (e.g. Hierapolis in Turkey). The fate of the Roman Empire brought a drastic decline in the use of travertine. Far fewer monuments and structures were built from this unique stone in the following periods. Nevertheless, numerous Renaissance and Baroque buildings made of travertine are known. The previous Roman travertine quarries (near Tivoli) were partly abandoned but later re-opened and new quarries were developed, especially near Rome. The existing deposit, which provides the source material, is therefore always available but its use depends on the traditions and understanding of workability of stones. Travertine is also gaining increasing popularity in modern architecture, especially when newly built emblematic structures are considered. From many possible examples just two are given: travertine slabs and blocks were used at the Metropolitan Opera House in New York and at J. Paul Getty Museum and Conservation Center in Los Angeles, to where large quantities of travertine were transported from Rome.
There is another aspect of the use of travertine, which is its long-term performance. Travertine is generally considered a durable stone, although air pollution-related weathering is commonly observed and black crust-covered façades are common in urban areas (Sidraba et al. 2004; Török 2006, 2008). In comparison with other porous limestones, travertine normally performs better than many of its porous carbonate counterparts in extreme stress conditions (Török 2004). A good example is from Budapest, where the façade of Parliament House (late 19th century building) was originally made of porous Miocene oolitic limestone. This showed rapid decay after only 20 years from completion of the building. Due to the deterioration (Török 2002) and structural damage (Török 2003a) of the porous limestone and unavailability of durable porous oolitic limestones, a decision was made (after the Second World War) that a type of a Hungarian travertine would be used as a replacement stone. Consequently, during the restoration works the entire façade will be replaced by travertine (Török 2008) (Fig. 2). The improved long-term performance of the high-quality travertine has been demonstrated by checking other built examples. At Mathias Church (Budapest) the parts that are the most exposed were built from travertine (window frames, gargoyles, footing), while the rest of the church façade is covered by the less durable porous limestone. This understanding of the long-term durability of this stone resulted in use of travertine in monuments throughout the country for centuries. Long transport distances did not hamper the application of this workable but durable stone, as it is clearly seen on the map of the country where the two regions with travertine quarries and the localities where travertine in monuments are found are indicated (Fig. 3).
Stone replacement at Parliament House, Budapest: the porous limestone façade is replaced by more durable travertine.
Map of Hungary showing travertine quarries and localities where travertine is used in the monuments. The wide distribution of monuments indicates that travertine was a popular dimension and building stone from the Roman period and that the raw material was also transported longer distances.
Although the Hungarian travertine is very similar to that of Italy and Turkey in terms of the origin (Török 2007), since it was also precipitated from lukewarm waters (Korpás 2003; Török 2003b) the selection of proper quality travertine must be based on quality checks and laboratory tests. These types of travertines are less porous and more durable than calcareous tufas of recent or relatively recent stream deposits. The use of travertine as building and/or replacement stone depends on its mechanical properties and on the long-term behaviour, which is primarily controlled by water content and its influence on rock strength, that is, freeze-thaw durability or thermal behaviour (Gomez-Heras et al. 2006).
Conclusions
The late 20th and 21st century brought a new era in the use of natural stones. Due to the availability and bulk mass of dimension stones, the use of imported stones has significantly increased in most industrialized countries. These almost unlimited sources of cheap stones and the strict environmental legislations resulted in the closing of many long-existing quarries in industrialized countries, which led to a shortage of available local stone resources. This lack of locally available stones is a significant drawback in monument restoration practice, since replacement stones are no longer available from the original source. Consequently, researchers and restorers are trying to solve this problem by various different approaches. One solution is the identification of old quarries and deposits that have similar stones to the monument. The re-opening of ancient quarries or beginning new quarrying activity at known deposits can provide material for use as replacement stones. Another approach is the in situ preservation of monumental stones by understanding the decay mechanism and using preventive conservation methods or techniques to slow down the decay processes. This approach is often not possible and the long-term performance of such interventions (e.g. consolidants) can only be judged after years. The role of scientists and practitioners in preserving our built stone heritage is therefore clear; it has to focus on: (i) providing sound scientific background and data based on the availability of possible replacements stones (ii) evaluating the properties of new stones and stones that are found in the structure; (iii) judging the risks of application of replacement stones or the long-term durability of historic stone structures; and (iv) modelling the long-term effect on any conservation intervention.
Acknowledgments
This paper benefited from financial support from the project of the Ministry of Education, Youth and Sports of the Czech Republic: MSM 0021620855 ‘Material flow mechanisms in the upper spheres of the Earth’ and from Hungarian Scientific Research Fund (OTKA, grant no. K63399).
- © The Geological Society of London 2010
References
Natural stones for monuments: their availability for restoration and evaluation
