Salt Weathering: a Selective Review

Geological Society Special Publication Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies. Vol 205, 51-64.

Salt Weathering: A Selective Review

ERIC DOEHNE

The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA, USA 90049-1684 (e-mail: [email protected])

Abstract: The past decade has seen a growing scientific interest in the still poorly understood subject of salt weathering, a phenomenon with significant cultural and economic consequences. This interest has led to an increase in research results and growing clarification of the roles salts play in weathering and decay. There are now over 1,800 research articles on the topic of salt weathering originating from several disciplines, as well as over 6,000 references on the general problems of building material decay. In order to navigate such a vast collection of data and knowledge, this article describes the multidisciplinary nature of the study of salt damage to porous building materials, provides a framework for considering the complexity of salt damage, and serves as a selective literature survey largely focused on recent work and those articles with relevance for conservation.

"In time, and with water, everything changes" settlements and judgments (Haynes 2002; Kasdan, Simonds et al. 2002). Salt weathering is —Leonardo da Vinci indisputably a process with profound cultural and economic consequences. Over the past ten years, there has been increasing scientific interest in building material decay Salts have long been known to damage porous phenomena, especially the incompletely materials (Herodotus 420 B.C.; Luquer 1895) understood subject of salt weathering. The through the production of physical stress reaction of salts with moisture in rock outcrops resulting from the crystallization of salts in pores produces an intriguing array of weathering forms (Taber 1916; Jutson 1918). Salts can damage such as tafoni, honeycombs and pedestal rocks, stone and other building materials through a as well as abundant rock debris (Goudie & Day range of other mechanisms as well, such as 1980; Mustoe 1982; Smith 1994). Salt differential thermal expansion, osmotic swelling weathering of building materials can be equally of clays, hydration pressure, and enhanced destructive (Haynes, O'Neill et al. 1996). Salt wet/dry cycling caused by deliquescent salts weathering is widely recognized as one of the (Smith 1994; Nash 2000). A 1997 book by primary agents in the deterioration of historical Goudie and Viles summarizing salt weathering architecture, structures in archaeological sites, hazards raised awareness of the problem and and archaeological objects (Schaffer 1932; broadened the field of investigation in recent Lewin 1982). Understanding and finding ways to years. Despite this, many unresolved issues reduce damage caused by salts holds great related to salt weathering remain, ranging from significance for the conservation of material the details of the damage mechanisms (Flatt cultural heritage (Torraca 1982; Amoroso & 2002) to the interactions between substrate, Fassina 1983; Goudie & Viles 1997). Moreover, environment and salt type (Charola 2000; damage to modern concrete building foundations Blaeuer et al. 2001), and the expression of this caused by salts has recently been the subject of process in the landscape (Smith, Warke et al. litigation leading to multi-million dollar SALT WEATHERING ERIC DOEHNE

2000) and in building stone (Kuchitsu, Ishizaki and cultural heritage (Hammer and Tinzl 1996; et al. 2000; Schwarz, Gervais et al. 2000). Arendt and Seele 2000; Kraus 2002).

This article is intended to describe the Nevertheless, much of the conservation literature multidisciplinary nature of the study of salt on salts can be difficult to survey, since many damage to porous building materials; articles are published in conference proceedings communicate the complexity of salt weathering (Price 1996). Two recent summaries of the lit- and discuss open questions; and serve as a erature illustrate the breadth of applicable limited literature survey largely focused on research: an overview of the role of salts in the recent work and those articles with relevance for deterioration of porous materials by Charola conservation. (2000) and a discussion of salts and crusts by Steiger (2002).

Background Following these recent efforts, this paper presents a current survey of the multidisciplinary The majority of the published literature on salt contributions to the field of salts and building weathering originates from a collection of deterioration. A large bibliography of research distinct disciplines, each responsible for specific articles on salts and building deterioration phe- contributions: geomorphology, environmental nomena was assembled in order to facilitate the science, geotechnical and materials science, geo- integration of these related researches. Some chemistry, and, lastly, conservation, perhaps the observations based on this survey are discussed most multi-disciplinary of the fields. There is a below. rich literature in geomorphology on salt weathering phenomena, particularly that A recitation of the sometimes significant, published by scientists from the United Kingdom sometimes subtle, differences in terminology and (Smith 1994; Goudie & Viles 1997). Environ- definitions employed in the literature of salt mental scientists have studied the important role weathering reveals the multidisciplinary nature of salts in atmospheric aerosols and their fallout of the scientific literature. The same basic (Pósfai, Anderson et al. 1995; Torfs & Van phenomena are variously described in different Grieken 1997). The geotechnical field has dealt disciplines and countries as salt attack, salt with expansive sulfate soils, or salt heave, damage, salt crystallization, salt crystallisation (Xiaozu, Jiacheng et al. 1999), concrete decay (UK), haloclastisme (French), and salzsprengung from salt crystallization (Haynes, O'Neill et al. (German). Less common terms include salt burst, 1996; Mehta 2000; Haynes 2002), and testing of salt exudation, haloclasty, salt decay, salt building materials using standardized salt crystallization pressure, crystal wedging, salt crystallization tests such as ASTM aggregate fretting, and salt heave in soils. sound test C88-90 (ASTM 1997), and RILEM PEM/25 (RILEM 1980). Materials scientists In the geomorphology literature, the terms have provided useful insights on the fundamental "honeycomb weathering", "alveolar weathering", mechanisms of salt attack (McMahon, Sandberg and the less common descriptors "stone lattice", et al. 1992; Scherer 2000). In the geochemical "stone lace", and "fretting" (Mustoe, 1982), literature, the concepts of pressure solution, force reference a characteristic form of salt weathering of crystallization, and displacive growth are in stone defined by irregular cavities in otherwise important to understanding that salt attack is a regular rock surfaces, although in recent part of a larger set of interrelated behaviours publications those terms are increasingly (Carstens 1986; Maliva & Siever 1988; Minguez discarded in favor of the term "tafoni" for & Elorza 1994). Finally, the conservation cavities large and small. Substantial research has literature is vast and especially rich in case been performed by geomorphologists on tafoni studies and documentation of methods of (singular: tafone), a term that normally refers to mitigating salt damage (Schwarz & Roesch pits and caverns in rock faces resulting from 1993; Young 1995; Siedel 1996; Charola, extensive salt weathering of stone in salt-rich and Henriques et al. 1998; Price 2000; Unruh 2001). desert environments (Kirchner 1996; Turkington German speaking conservation scientists, 1998). Tafoni also occur in humid temperate conservators and architects have published a environments; however, it is not yet clear that number of important books on the topic of salts salt weathering plays a role in the formation of tafoni in this type of climate (Mikulas 2001).

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move salts over the surface of materials but, as In the conservation literature, the term "rising the name implies, is a surficial process. Salts damp" is widely used to characterize salt such as magnesium sulfate can actually bind weathering, but in Australia this process of together (i.e., cement) previously fractured moisture and salts mobilized by capillarity and material. deliquescence is regionally referred to as "salt damp" (Young 1995). Many buildings and sites are also affected by salt-rich aerosols, which may Understanding the properties that contribute to a accumulate and be mobilized by "falling damp" particular weathering effect, and differentiating or "penetrating damp." between benign and damaging salt behaviors, allows the investigator to focus on the latter. In the concrete literature, the terms "salt attack" Typical damage behaviors or processes for salts and "sulfate attack" are sometimes incorrectly can include surface scaling, deep cracking, used interchangeably (Mehta 2000), and recent expansion, granular disintegration, surface authors have proposed the terms physical salt powdering and microcracking. For particular attack or salt hydration distress as distinct from damage mechanisms, such as crystallization the chemical reaction of salts with concrete pressure, the degree of damage (sometimes (Haynes, O'Neill et al. 1996; Hime, Martinek et measured as the damage factor, damage function al. 2001; Haynes 2002). Stone conservators or dose-response function) is often attributable to typically use the terms desalting, salt extraction, specific properties and can be the single most or desalination, referring to salt reduction important metric (Viles 1997). For example, in methods using immersion, intermittent washing, the case of sodium sulfate crystallization, the poulticing or compress treatments. degree of supersaturation and the location of crystallization appear to be the keys to Given the variety of terms used and the diversity understanding the degree of damage (Rodriguez- of disciplines researching the topic of salt Navarro & Doehne 1999a). In turn, there are a weathering, it seems clear that at least some of few important properties and kinetic factors, the apparent contradiction in the literature is such as the evaporation rate, that largely control merely the result of differences in terminology how much damage results from this mechanism (Hime, Martinek et al. 2001). (Fig. 1).

By focusing on the nature (i.e., moment, location Salt Damage: Dealing with Complexity and type) and, especially, the degree of salt damage, the field of variables, or properties, Two major difficulties encountered by those directly responsible can be narrowed investigating salt weathering are the multiple considerably and investigators can begin to variables involved and the fact that the answer the questions most relevant to specific physiochemical reactions of greatest importance damage types. Why are certain types of stone occur along thin, nanometer-scale films inside a much more vulnerable than other types to salt porous solid. Such interactions are inherently damage (Goudie 1999b)? Why are certain salts difficult to study in situ and in tempo, further much more damaging than other salts (Hoffmann inhibiting understanding of the degree of & Grassegger 1995; Aires-Barros & Mauricio interaction between possible variables. To aid 1997; Rodriguez-Navarro & Doehne 1999a)? Is understanding of the variables, it is useful to damage caused mostly by relatively rare en- classify them as properties of the substrate, vironmental events (rapid cooling) or cumulative solution, salt type, or environment (Fig. 1). The everyday stresses (humidity cycling)? What are properties interact in specific ways determined the long-term effects of various conservation by thermodynamic equilibria and kinetic factors treatments, such as desalination or consolidation, to produce a range of salt behaviours. on salt damage? How can desalination and preventive conservation efforts be enhanced? It is important to note that not all salt behaviours Can general agreement be achieved regarding the result in deterioration. For example, the fundamental mechanisms of salt weathering? production of surface efflorescence is often What are the appropriate equations to use in impressive and highly visible, but generally calculating crystallization pressure (Duttlinger & results in little damage. Likewise, salt creep can Knöfel 1993; La Iglesia, Gonzalez et al. 1997;

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Figure 1. Diagram of properties, factors and behaviours in the salt crystallization process.

Substrate Properties Surface Area, Porosity, Pore Size Distribution, Permeability, Internal Surface Roughness, Adsorption, Surface Energy, Crystal Defects - type and density, Pore Shape, Tensile Strength, Wet Strength, Substrate surface relation to evaporative Salt Behaviours front, Colloidal effects (Clay or Fe) Thermodynamics (Processes) Efflorescence Solution Properties System Equilibrium Subflorescence Surface Tension (salts, solution, environment, substrate) Salt Creep Viscosity Salt Crust development Salt Damage Activity Coefficient of Solute Whiskers - Basal Growth Form Scaling Molar Concentration Whiskers - Tip Growth Form Deep Cracking Thickness of supersaturated zone Kinetic Factors Dissolution of existing salts Uniform Expansion Thickness of diffusion layer Hydration/Dehydration Microcracking Hydraulic pressure Thermal expansion stress Granular Disintegration Vapor Pressure Solution Evaporation Rates Cyclical Processes Delamination Equilibrium Relative Humidity Crystal Nucleation Rates Effect on crack propagation Solubility Crystal Growth Rates Crystal Expansion Rates Crystallization Pressure Sustainable degree of supersaturation Crystal Dissolution Rates Pressure Solution Volume Solution Diffusion Rates Hydration Pressure Capillary Condensation Solution Transport Rates Solubility changes with P or T Solution Vaporization Rates Hygroscopicity Heat Transfer Rates Key to Damage Salt Properties Degree of Supersaturation Crystal Type and Morphology Location of crystal growth Hydration States, Stability Range Density Thermal Expansion Coefficient Important Properties and Factors Environmental Properties For Supersaturation: Wind velocity and direction Evaporation Rate (surface area, temperature, humidity, wind) Turbulence Cooling Rate (temperature, wind) Boundary layer Pre-existing salts (thenardite dissolves, then mirabilite ppts) Evaporative Cooling Nucleation/Growth rates Temperature Humidity (macro and micro) For Location: Support moisture content Evaporation Rate/Solution Transport Rate Solution properties (surface tension, viscosity) Pore size distribution (more small pores = more damage)

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Benavente, Garcia del Cura et al. 1999; Scherer scale thin film between salt and stone can result 1999; Charola 2000)? Can the large number of in stresses that exceed the strength of most laboratory simulation studies of salt weathering porous building materials. Putnis & Mauthe be reconciled with field data and explained using (2001) have noted that halite cementation in existing theories? Can salt weathering forms porous sandstone reinforces the observation that such as tafoni be accurately modeled using larger pores fill before smaller pores, confirming existing knowledge? How do the behaviors of that fluids in small pores can better maintain water, salts, and substrate compare at the higher supersaturation. important mesopore scale (2-50 nanometers) versus the macro scale (Brown 2001; Hochella It has long been known that the solution must be 2002)? These complex questions, together with supersaturated, in order for salt crystallization to such basic questions as how the hydration of damage porous materials (Goudie & Viles 1997). salts progresses and how crystallization What has been unclear until recently is how, in a pressures are sustained in situ, are still in need of material with abundant nucleation sites, such a study (Charola & Weber 1992; Doehne 1994; supersaturated solution was created and Steiger, Beyer et al. 2000). Nevertheless, the maintained. research performed in recent years and surveyed herein has significantly advanced overall under- Dei et al. (1999) investigated the experimental standing of the damage caused by salt crystallization of potassium nitrate salt in Italian weathering processes. marble, sandstone and travertine samples. They found that the pore size distribution was shifted towards larger pores and the porosity was Deterioration due to salts: Theory and reduced after salt crystallization. This appears to Laboratory Observations be the result of blocking of micropores due to crystallization of salts in the zone between large Several important advances in salt damage and small pores. Thermal analysis of the samples arising from theoretical and laboratory research to identify the salts showed that the shape and have application for the conservation field and timing of the exothermic peak indicative of deserve highlighting. The research summarized crystallization varied depending on the pore here addresses the precise nature of the system and stone type. relationship between salt stress and material pore size; how supersaturated solutions develop and The important relationships between the rate at are maintained; the contradictory behavior of the which supersaturation increases, nucleation, and common salts halite and gypsum in the field as pore size distribution was studied by Putnis et al. opposed to the laboratory; and the important role (1995). They found that nucleation was of humidity cycling in salt crystallization. suppressed in fine-grained substrates in experiments with gypsum and other salts. This George Scherer (1999; 2000) has recently work provides provides another explanation for advanced the theoretical framework for un- empirical observations that stones with abundant derstanding stress caused by the crystallization micropores are often have a higher decay rate in of salts in pores. This research helps explain why standard salt crystallization tests. Also, it helps salt crystallization may not damage certain explain how high supersaturations may be ob- materials: in some cases, the local stress field is tained in materials with a high internal surface not large enough to propagate critical flaws in area. the material. While stones with large pores tend to fare better than those with abundant small Kashchiev and Van Rosmalen (1995) present a pores, Scherer's work points out that large pores way to calculate the supersaturation a solution effectively behave like small pores when filled can maintain in a pore of a given radius before with salt. Scherer (1999) finds that the maximum crystallisation occurs. Due to the Laplace effect pressure salt crystallization can achieve is highly of curvature, in small pores (less than 1 micron) dependent on pore size, predicting that most of a higher supersaturation can be reached before the damage occurs when salt growth migrates crystallization begins. Pores also alter the from larger to smaller pores in the size range of 4 crystallization of ice, dramatically reducing the nm to 50 nm. Scherer's work clarifies the visual normal freezing point (Baker et al. 1997). model of how salts growing on the nanometer-

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Some authors have taken the idea of crystal- and gypsum from sea spray salts and soil lization pressure and applied it to less soluble capillary waters is addressed in a recent case minerals such as calcite (McBride and Picard study documenting the decay history of the site 2000), suggesting that crystallization of calcite in of Civil Palaces in Alicante, Spain (Louis, Del a calcite-rich rock may result in the production Cura et al. 2001). The desalination of porous of tafoni. However, conventional salt building materials typically results in the non- crystallization theory requires the pore wall and proportional removal of salts (Bromblet & the salt to be incompatible in order to generate Verges-Belmin 1996). If gypsum is present, the crystallization pressure (Scherer 2000), so some removal of hygroscopic salts may decrease its other mechanism may be at work. mobility since the solubility of gypsum is significantly increased by the presence of sodium Crystallization can be characterized as needing a chloride and other salts (Robinson & Williams driving force, such as a supersaturated solution, 2000). to create and sustain it. The generally accepted kinetic pathways to produce such a solution or Other open questions pertaining to salt damage mode of supersaturation generation are rapid of porous materials are the role of the range of cooling, for salts with a strong solubility humidity cycling, the potential presence of dependence on temperature, and rapid damage thresholds, and the degree to which evaporation for the remainder of salts damage over the long term can be reduced by (Rodriguez-Navarro, Doehne et al. 1999). A decreasing humidity fluctuations. Experiments third reaction pathway was first noted by on ceramic tiles loaded with NaCl, Na2SO4 and Chatterji & Jensen (1989), more recently by CaSO4.2H2O and simulating the humidity cycles Doehne, Selwitz et al. (2001), and in detail by (43%-55% RH and 0%-11% RH) of museum Flatt (2002). This pathway is the presence of display cases (Nunberg & Charola 2001) over fine-grained pre-existing salts that dissolve the course of six months found that measurable rapidly to produce a supersaturated solution, deterioration of the tiles had taken place. This such as is caused by the wetting of thenardite, ran contrary to expectations that no damage leading to the rapid precipitation of mirabilite would occur in a closed environment with from solution, as observed in the environmental limited humidity cycling well below the SEM (Doehne 1994; Rodriguez-Navarro & deliquescence point of the salt (equilibrium Doehne 1999a). relative humidity or RHeq). The discovery of Another unresolved conundrum is the contrast damage suggests that while environmental between severity of damage in the field caused control significantly reduces the rate of damage, by halite and gypsum (Kirchner 1996), two of additional research is needed to understand the the most common salts, as compared to their limits of environmental control and how the relatively benign behavior in the laboratory damage was produced. In this example, the mode (Smith & McGreevy 1988; Goudie & Viles of solution supersaturation is not clear. However, 1997; Robinson & Williams 2000). the filling of larger pores with salt and the Understanding gypsum's behaviour is especially creation of smaller pores may play a role, as important to stone conservation in view of the might capillary condensation of water in fine common occurrence in the field of "black crust" pores combined with limited humidity cycling containing abundant gypsum, a product of the exerted over time (Rucker, Holm et al. 2000). reaction of regional sulfate air pollution with The results of Nunberg and Charola (2001) building stone (Verges-Belmin 1994; Wittenburg suggests that environmental control may provide & Dannecker 1994). Despite the ongoing less protection than previously believed (Price reduction of sulfate air pollution over the past 20 2000). years, the rate of loss of carbonate stone surface has not slowed proportionally at sites such as St. The standard salt crystallization tests (ASTM Paul's Cathedral in London. This discrepancy has and RILEM) for building materials, where been attributed to salt crystallization damage sample blocks are repeatedly immersed in from pre-existing gypsum (Trudgill, Viles et al. saturated solutions of sodium sulfate and then 1991). dried, are widely used to estimate resistance to frost damage and general durability (Price 1978; Gypsum also plays a significant role in the Marschner 1979). Nevertheless, there has been typical problems of masonry buildings near the some controversy over the applicability of the sea. Salt weathering related to sodium chloride tests, given the severity of the test and the some-

Page 6. SALT WEATHERING ERIC DOEHNE times inconsistent results (Sheftick 1989; Hunt have suffered significant decreases in their 1994). Recent research has addressed this modulus of elasticity values, indicating loss of problem from two directions: examining the strength. Limestone durability was generally way the test actually operates (Rodriguez- found to correlate with high values of modulus Navarro, Doehne et al. 2000a) and proposing of elasticity, lower water absorption capacities, new standard test methods which more closely high densities and low salt uptakes. simulate typical conditions (Benavente, Ordóñez et al. 2001). The assumption of many researchers has been that the damage from this test occurred Deterioration due to Salts: Field Observa- due to the hydration of sodium sulfate. More re- tions cent work suggests that direct crystallization of thenardite (Na2SO4) may occur (Rodriguez- A wide range of important field observations of Navarro, Doehne et al. 2000a) in addition to decay to porous building materials over several rapid mirabilite (Na2SO4.10H2O) crystallization decades has been summarized by Arnold and during immersion (Doehne, Selwitz et al. 2001; colleagues (Arnold & Küng 1985; Arnold & Flatt 2002). Goudie (1999a) found that salt crys- Zehnder 1985; Arnold & Zehnder 1988; Arnold tallization tests of limestone were poor predictors & Zehnder 1991). These observations clearly of frost resistance. Efforts to discover parameters establish the strong relationship between the rate that correlate with building stone durability of salt decay and the environmental conditions. continue, with emphasis on those that can be Arnold discusses the morphology of the salt measured more easily than salt crystallization crystals and its relationship to the support resistance (Moh'd, Howarth et al. 1996; moisture content. With a relatively dry substrate Ordonez, Fort et al. 1997; Nicholson 2001). with a slow evaporation rate, the crystals take on an acicular form; when more moisture is present, a salt crust may form (Arnold & Küng 1985; New Methods for Studying Salt Weath- Arnold & Zehnder 1985). Subsequent ering observations of climate and salt behavior in six churches over the course of five years showed An important turning point in the advancement that… "hygroscopic salts crystallize periodically of salt weathering studies has been the according to the variations of relative humidity development of methods that provide data on and to a much lesser extent of temperature. material behavior in situ and in tempo, such as Within continuously heated rooms crystallization ultrasonic field testing (Goudie 1999b; Simon & and decay are related to variations in relative Lind 1999), acoustic emission (Storch & Tur humidity caused primarily by the heating and 1991; Grossi, Esbert et al. 1997), time-lapse and subordinately by natural variations of the outside Environmental Scanning Electron Microscopy climate. In non heated rooms seasonal variations (ESEM) (Doehne 1997; Rodriguez-Navarro & of temperature also induce periodic crystalliza- Doehne 1999b), and three-dimensional imaging tion." (Arnold & Zehnder 1988). Arnold also methods such as NMR (Rucker, Holm et al. documented the fractionated pattern of salts in 2000) and micro-CT scanning (Degryse, Geet et walls with rising damp, dividing the wall into al. 2001). Such methods add the third and fourth four zones in vertical succession differentiated (time) dimensions to previous studies and aid in by degrees of damage and salt content. The separating the effects of salts from other decay lowest zone was perpetually damp with salts phenomena. For example, Pel (2000) found that remaining in solution, succeeded by the zone of the absorption of a concentrated NaCl solution in greatest damage and, finally, the uppermost zone calcium-silicate brick, as seen when using NMR, marking the limit of rising moisture caused by revealed a sharp wetting front. Unexpectedly, the deliquescent salts. The highest salt content does sodium was clearly seen to lag behind the not always correspond to the greatest damage, moisture profile. Little sodium, if any, was since ion mixtures that are strongly hygroscopic observed near the wetting front, an apparent will not crystallize under normal conditions. The result of the interaction of sodium ions with the sources of the salts are attributed by Arnold to pore surface. In tests of 21 limestones using non- the extensive use of cement-based mortars and destructive methods, Goudie (1999b) noted that concrete in large restoration programs over the even durable stones which showed no visible past 50 years, as well as salts from air pollution decay after salt crystallization tests were found to and rising damp. Soluble salts from cement-

Page 7. SALT WEATHERING ERIC DOEHNE based mortars and concrete are still today an im- conservation is surprising uncommon, such portant cause of decay to historic building patterns of low rates of decay, followed by a materials, despite efforts to improve cement rapid increase have been noted by Charola quality and reduce the overall use of these (2000) and Snethlage, Wendler et al. (1996). The materials (Moropoulou 2000). fact that field salt weathering rates are typically non-linear and may show a low initial rate of A brief review of other more recent field loss suggests that stone conservators should observations on salt weathering is considered interpret short-term observations of overall next. Some useful insights into rates of salt stability of vulnerable structures, such as wall weathering have recently been gained through paintings, with caution. the measurement of tafoni depth using rock surfaces with a range of known ages (Matsukura One of the advantages marble possesses for & Matsuoka 1991; Sunamura 1996; Norwick & resisting salt damage is its minimal porosity and Dexter 2002). Tafoni are cavities or pits in tendency not to have a strong capillary suction otherwise homogeneous rock surfaces. The (which can lead to rising damp). However, recent current consensus appears to confirm the salt studies of the marble monuments at the weathering origin of most tafoni (Pye & Motter- archaeological site of Delos, Greece have shown shead 1995; Mikulas 2001; Norwick & Dexter that salts deposited from atmospheric aerosols 2002). For tafoni that has developed over the last are responsible for damage of marble sculpture century, linear recession rates are estimated at (Jeannette 2000; Chabas & Jeannette 2001). 0.6 to 5.2 mm/year (Sunamura 1996). Tafoni Additionally, recent work on soil chemistry in depth was also found to develop twice as fast as Hawaii using strontium isotope data has found the width of the pit, providing additional that about 50% of the calcium in soil samples confirmation for the salt weathering origins of from a coastal location originated from salt tafoni. More recent analysis has shown that spray, not soil mineral weathering as expected while there is significant scatter in the depth (Whipkey, Stewart et al. 2000). This suggests measurements, it appears that the weathering rate that salt spray has a strong effect on the local is actually non-linear, with a time lag followed environment, perhaps dependent on overall wind by rapid recession and then a much slower rate and storm patterns. of loss as the tafoni age. This pattern generates a sigmoidal or “S” curve, a typical example of An extensive survey and analysis of bedrock in which is given by the following equation: the Valley of the Kings, Luxor, Egypt recently resulted in warnings concerning the hazards to D = b1 + e(b2+(b3/t)) (1) ancient tombs from swelling clays and salts activated by moisture from the exhalations of where D is the depth of the tafoni, t is the age of tourists as well as flash floods (Wüst & McLane the surface and b1, b2, and b3 are constants 2000; Wüst & Schlüchter 2000). Expansion from solved for mathematically (Norwick & Dexter hydration of anhydrite and crystallization cycling 2002). of abundant sodium chloride where cited as important decay mechanisms in important Royal A speculative explanation for the non-linearity tomb sites, such as Seti I, which have seen a suggests that the rate of loss is first limited by rapid increase in stone loss since their opening low salt concentrations, then becomes dominated by archaeologists. by a positive feedback loop with an increasing rate of damage related to increasing salt Warke and Smith (2000) suggest that "Salt concentration, followed by a negative feedback weathering is a threshold phenomenon where loop as the scale of the tafoni reaches a point decay is manifest only when a stress/strength where it apparently no longer allows efficient threshold is crossed." This conclusion is based salt weathering (Norwick & Dexter 2002). This on the analysis of two similar sandstone blocks, non-linear pattern is typical for many other one sound and one damaged, with similar salt natural weathering processes (Goudie 1995). content and distribution. They found that NaCl Such non-linearity may help explain why salt was deeper and more mobile in the stone (6 cm) weathering rates measured in laboratory settings than expected from surface damage (2 cm). A have been much more rapid than those typically strong correlation of decay rate with envi- found in field studies. While erosion rate data ronmental variation has recently been from the study of stone structures for documented by Cantón, Pini et al. (2001). In a

Page 8. SALT WEATHERING ERIC DOEHNE range of experiments that the weathering rate of presence of bacteria, associated biofilms, and a calcareous mudstone was found to most closely biopolymers greatly increases the damage caused follow the number of wetting/drying cycles. by salts (May, Papida et al. 2000; Papida, Some useful cautionary notes on the real world Murphy et al. 2001). distribution of salts in sandstone can be found in (Turkington & Smith 2000). The researchers The relationship between salts and frost concluded that "The widely held perception that weathering was explored recently by Williams & urban environments are 'dry' with shallow Robinson (2001). The authors extended the surface wetting of building stone does not appear range of salts known to intensify frost to hold true for certain building stone." weathering (potassium and ammonium alums) and confirmed the significance of halite in the Salt weathering is traditionally associated with process. Williams & Robinson (2001) show that coastal and desert environments (Theoulakis & the degree of damage to stone varied greatly Moropoulou 1999), however regions with a depending on the combinations of salts involved. tropical monsoon climate may also be effected While most salts contributed to damage by salt weathering due to the strong contrast proportionally when used in combination, some between the rainy and dry season. Two recent salt mixtures caused intensified damage out of examples are the famous temples at Angkor in proportion to the component salts. The topic of Cambodia (Uchida, Nakagawa et al. 2000) and salt mixtures and their effect on salt damage and the brick monuments in Ayutthaya, Thailand other decay mechanisms is complex since the (Kuchitsu, Ishizaki et al. 2000). At Angkor, one behaviour of the mixture cannot be predicted of the main causes of deterioration was identified from single salt data (Steiger & Dannecker 1995; as crystallization pressure from gypsum and Steiger 1996; Steiger & Zeunert 1996). phosphate minerals growing between exfoliated However, the use of computer models such as layers of sandstone. The sulfur and phosphate are the semi-empirical ion interaction model of derived from rain water leaching of bat guano, Pitzer provides an important predictive tool for which subsequently moves into the stone through studying most field situations where salt rising damp (Uchida, Nakagawa et al. 2000). At mixtures are the rule (Clegg & Brimblecombe Ayutthaya, gypsum efflorescence is found in the 2000; Steiger, Beyer et al. 2000). rainy season, with more soluble salts such as thenardite appearing in the dry season. Most Dragovich (1997) studied the weathering of damage occurs at the beginning of the dry season marble tombstones near Sydney, Australia and to brick surfaces, and Kuchitsu Ishizaki et al. was surprised to measure a low weathering rate (2000) propose methods to reduce water (540 microns/ 100 years). The seaward facing impregnation of the bricks in the rainy season as sides of the marble blocks, which were exposed a conservation measure. to greater salt input, unexpectedly weathered more slowly than the landward side. This suggests that even in coastal environments salt Interrelationship between Salt Weather- weathering is only one aspect of the full ing and Other Decay Mechanisms weathering process.

With an understanding of the properties nec- essary for a particular salt damage mechanism to Prevention, Mitigation, and Treatment be activated, it is possible to contemplate Options modifying those properties to better understand the damage mechanism and to develop Avoiding damp is a fundamental principle for preventive conservation measures. Changing a maintaining any structure where porous materials parameter such as surface tension using surface- are used (Ashurst & Ashurst 1988). The most active agents was recently found to have a strong important mitigation method is prevention, effect on damage from sodium sulfate through the physical separation of building crystallization since it directly affects where salts materials from soil moisture and salts with the crystallize (Rodriguez-Navarro, Doehne et al. traditional "damp-proof course." This is typically 2000b). The authors speculated that biologically an impermeable barrier such as plastic (current produced surfactants might have a similar effect. structures), glazed brick, bitumen, or other Two articles have subsequently found that the material. The service life of the plastic sheeting

Page 9. SALT WEATHERING ERIC DOEHNE used in modern concrete slab construction is not wall. Siedel also noted the difficulty of well known. In existing salt-laden structures, controlling desalination (Siedel 1996), achieving salts can be removed through poulticing and in results that were "contradictory and sometimes some cases the affected masonry may be even undesirable on large monuments." replaced. However, in the past few decades, the cost of such replacement has led to increasing Two desalination methods (total immersion and use of chemical "damp-proof courses" in the intermittent washings) were studied recently for form of injected siloxane. Below, a range of their potential for use in the removal of salts treatment options and research into the reduction from ceramic tiles by Freedland & Charola of salt damage is discussed. (2001). The authors found that for more soluble salts, such as NaCl, repeated washings were Salts tend to cause damage in building materials more effective, while long-term immersion was when activated by a change in humidity, more efficient for less soluble salts such as gyp- temperature or the presence of liquid water. If sum as well as for sodium sulfate in ceramic tiles the composition of the salts is known, then with fine pores. Based on a substantial body of calculating the stability range (RH and experimental work, Charola, Freedland et al. temperature) for the salts allows establishment of (2001) have developed a reproducible method to the appropriate conditions for minimizing calculate the salt remaining in an object after damage due to a change of state. Establishing desalination. such controlled conditions should substantially extend the life of the material. An important new An interesting effort to move sodium chloride report by Price and colleagues (2000) documents from a brick vault into an adsorbing mortar the development of an expert system to provide above the vault used differential humidity to such information and is an extremely useful mobilize the salts (Larsen & Bøllingtoft 1999; repository of detailed thermodynamic and kinetic Larsen 2001). With a low relative humidity information concerning individual salts, salt above the vault and a moist environment beneath mixtures, and their calculated and actual (set below the deliquescence point of the salts), it behavior. Other researchers have also explored was found that the kinetics for NaCl movement this approach (Aires-Barros & Mauricio 1997). solely attributable to humidity are slow, and that Unfortunately, controlled environments for stone NaCl most readily moves into the mortar (from objects and structures tend to be the exception damaged bricks) when liquid water is present. rather than the rule, and field experience suggests that damage tends to occur whenever A newly proposed approach mitigating salt significant concentrations of salts are present in weathering is the reduction of the interfacial porous masonry (Andreas Arnold, personal energy between the salt and the stone using a communication). specially selected non-swelling polymer with the appropriate surface properties (Scherer, Flatt et The practice of poulticing salts using highly al. 2001). By placing such a barrier between the absorbent materials such as clays or wood pulp pore walls and salts, it is anticipated that greater has long been employed by stone conservators to surface energy compatibility may be achieved remove salts from important wall paintings and and the salts should stop growing when they carved stone. Study of the effectiveness of these come in contact with the polymer. methods has shown that, while chloride and nitrate are readily removed, sulfate removal is Long experience with salt weathering of building more difficult. Artificial anionic clays foundations in Adelaide, Australia, a region with (hydrotalcites) are recommended for enhanced a Mediterranean climate and salt-rich soil, has extraction of the less soluble gypsum (Vicente & resulted in a host of practical methods for Vicente-Tavera 2001). Vicente & Vicente- mitigating this damage (Blackburn & Hutton Tavera (2001) also noted the loss of extraction 1980; Young 1995). Young found that decay is efficiency under conditions of high humidity. most commonly found at the base of walls, Simon, Herm et al. (1996) found that a new high where solutions migrating from local salt-rich porosity compress render was generally more soils damage masonry, mostly in cases where a effective at removing salts than a traditional damp-proof course is either lacking, has been sacrificial lime plaster, although the introduction damaged or was bridged by later masonry. The of water to the surface during the treatment most effective treatment is replacement of the increased nitrate concentrations higher up on the salt-laden stone and reinstallation of a damp-

Page 10. SALT WEATHERING ERIC DOEHNE proof course. The high cost of this treatment is measurements of field weathering rates and limits its use to severe cases or situations where laboratory studies using methods such as three- sufficient funding is readily available. The most dimensional imaging to further illuminate the common treatment is injection of waterproofing relationships between salt weathering agents (siloxane) to provide a chemical damp- mechanisms and the properties of solution, proof course and desalination of the salt-laden substrate and environment, and to contribute to a masonry. The longevity of these treatments better understanding of the action and kinetics of remains uncertain, with some failures attributed salt mixtures in weathering stone. Finally, to problems with desalination or incomplete improving methods to prevent and mitigate salt injection. Saline soils are an increasing regional weathering of building materials clearly deserves problem in south and southeast Australia, and more effort and attention. salt weathering of heritage structures in this region has begun to mount (Spennemann 2001). The author is indebted to Dr. Michael Steiger and Dr. Andrew Putnis for their comments and Planetary Salt Weathering critical reviews, which greatly improved the manuscript. This paper was supported by the Salt weathering is a global phenomenon, present Getty Conservation Institute under the Salt in Antarctica as well as Death Valley. This Research project. Also, the support of Stefan diversity has been appreciated by geologists and Simon, William Ginell, and Jeanne Marie planetary scientists, who have suggested that Teutonico and Alberto de Tagle is salt-weathering mechanisms may help explain acknowledged. Konrad Zehnder generously unusual features on Mars (Malin 1974). contributed many useful references. The full Photographs from Mars taken at the Pathfinder building materials decay bibliography is landing site reveal features remarkable similar to available from the author. honeycomb weathering structures on Earth (Rodriguez-Navarro 1998). McCord (2001) found that under conditions typically found on the Jovian moon Europa, magnesium sulfate References could remain hydrated on a geologic time scale. Aires-Barros, L. & Mauricio, A., 1997. Transition frequencies of evaporitic Summary and Future Work minerals on monuments stone decay. In: A. Moropoulou, F. Zezza, E. Kollias As of this writing, there are over 1,800 ref- and I. Papachristodoulou (Editors), 4th erences in the scientific literature on the topic of International Symposium on the salt weathering of porous materials and over Conservation of Monuments in the 6,000 references addressing general problems of Mediterranean. Texniko Enimeahthpio, building material decay. Many of these articles Rhodes, Greece, pp. 33-51. describe unique case studies that may be difficult Amoroso, G.G. & Fassina, V., 1983. Stone to interpret or apply to other situations. In order Decay and Conservation: Atmospheric to navigate this vast collection of data and Pollution, Cleaning, Consolidation and knowledge borne of disparate disciplines, this Protection. Materials Science article serves as an organizing framework to con- Monographs, 11. Elsevier, Amsterdam sider the complexity of salt damage, presents an and New York, 453 pp. overview of multidisciplinary research and open questions on this topic, and provides a limited Arendt, C. & Seele, J., 2000. Feuchte und Salze literature survey largely focused on recent work in Gebäuden: Ursachen, Sanierung, and those articles with relevance for Vorbeugung (Damp and salts in conservation. The research surveyed here has buildings - causes, rehabilitation, clearly advanced our understanding of the prevention). Verlagsanstalt Alexander damage caused by salt weathering processes, and Koch, 167 pp. raises important questions for the conservation of Arnold, A. & Küng, A., 1985. Crystallization material heritage. Future investigation into salt and habits of salt efflorescences on weathering should include more quantitative walls. Part I: Methods of investigation

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and habits. In: G. Félix (Editor), 5th Blackburn, G. & Hutton, J.T., 1980. Soil International Congress on Deterioration conditions and the occurrence of salt and Conservation of Stone, Lausanne, damp in buildings of metropolitan 1985. Presses Polytechniques Adelaide. Australian geographer, 14, Romandes, Lausanne, pp. 255-267. 360-365. Arnold, A. & Zehnder, K., 1985. Crystallization Blaeuer Boehm, C., Kueng, A. et al., 2001. Salt and habits of salt efflorescences on crystal intergrowth in efflorescence on walls. Part II: Conditions of historic buildings. Chimia, 55(11): 996- crystallization. In: G. Félix (Editor), 5th 1001. International Congress on the Bromblet, P. & Verges-Belmin, V., 1996. Deterioration and Conservation of L'élimination des sulfates sur la Stone, Lausanne, 1985. Presses statuaire calcaire de plein air: une Polytechniques Romandes, Lausanne, habitude discutable" = The removal of pp. 269-277. sulfates from calcareous stone outdoor Arnold, A. & Zehnder, K., 1988. Decay of stony statuary: a questionable practice, Le materials by salts in humid atmosphere. dessalement des matériaux Poreux. 7es In: J. Ciabach (Editor), Proceedings of journées d'études de la SFIIC, Poitiers, the 6th International Congress on 9-10 mai 1996. SFIIC, Champs-sur- Deterioration and Conservation of Marne, pp. 55-63. Stone. Nicholas Copernicus University, Brown, G.E.J., 2001. How minerals react with Torun, Poland, pp. 138-148. water. Science, 294, 67-70. Arnold, A. & Zehnder, K., 1991. Monitoring Cantón, Y., Pini, R. et al., 2001. Weathering of a wall paintings affected by soluble salts, gypsum-calcareous mudstone under The Conservation of Wall Paintings. semi-arid environment at Tabernas, SE Proceedings of a symposium organized Spain: Laboratory and field-based by the Courtauld Institute of Art and experimental approaches. Catena, 44, the Getty Conservation Institute, 111-132. London, July 13-16, 1987. Getty Carstens, H., 1986. Displacive growth of Conservation Institute, Marina del Rey, authigenic pyrite. Journal of Sedi- pp. 103-135. mentary Petrology, 56, 252-257. Ashurst, J. & Ashurst, N., 1988. Practical Chabas, A. & Jeannette, D., 2001. Weathering of Building Conservation. Gower marbles and granites in marine Technical Press, Aldershot, England. environment: Petrophysical properties ASTM, 1997. ASTM C88-90, Standard test and special role of atmospheric salts. method for soundness of aggregate by Environmental Geology, 40, 359-368. use of sodium sulfate or magnesium Charola, A.E., 2000. Salts in the deterioration of sulfate, Annual Book of ASTM porous materials: An overview. Standards 4.2. ASTM, pp. 37-42. Journal of the American Institute for Baker, J.M., Dore, J.C. and Behrens, P. (1997) Conservation, 39, 327-343. Nucleation of ice in confined geometry. Charola, A.E., Freedland, J. et al., 2001. Salts in J. Phys. Chem. B, 101, 6226-6229. Ceramic Bodies IV: Considerations on Benavente, D., Garcia del Cura, M.A. et al., Desalination. Internationale Zeitschrift 1999. Thermodynamic modelling of f ü r B a u i n s t a n d s e t z e n u n d changes induced by salt pressure Baudenkmalpflege = International crystallization in porous media of stone. Journal for Restoration of Buildings Journal of Crystal Growth, 204, 168- and Monuments, 7, 161. 178. Charola, A.E., Henriques, F.M.A. et al., 1998. Benavente, D., Ordóñez, S. et al., 2001. The Tower of Belem Exterior Quantification of salt weathering in Conservation Project. Internationale porous stones using an experimental Zeitschrift für Bauinstandsetzen 4. continous partial immersion method. Jahrgang = International Journal for Engineering Geology, 59, 313-325. Restoration of Buildings and Monuments (IJR), 4, 587-610.

Page 12. SALT WEATHERING ERIC DOEHNE

Charola, A.E. & Weber, J., 1992. The hydration- Microscopies. Kluwer Academic dehydration mechanism of sodium Publishers, pp. 45-62. sulphate. In: J.D. Rodrigues, F. Doehne, E., Selwitz, C. et al., 2001. Damage to Henriques and F.T. Jeremisas (Editors), monuments from the crystallization of 7th International Congress on mirabilite, thenardite and halite: Deterioration and Conservation of mechanisms, environment, and Stone, Lisbon Portugal, 15-18 June preventive possibilities, Eleventh 1992, Proceedings. Laboratorio Annual V. M. Goldschmidt Conference. Nacional de Engenharia Civil, Lisbon, Geochemical Society, pp. 3212.pdf. Portugal, pp. 581-590. Dragovich, D., 1997. Weathering of marble Chatterji, S. & Jensen, A.D., 1989. Efflores- tombstones in a near-coastal cences and Breakdown of Building environment, Australia. In: P.G. Materials. Nordic Concrete Research, 8, Marinos (Editor), Engineering geology 56-61. and the environment. Proc. symposium, Clegg, S.L. & Brimblecombe, P., 2000. Pitzer Athens, 1997. Balkema, pp. 3129-3134. model of electrolyte solutions. In: C.A. Duttlinger, W. & Knöfel, D., 1993. Price (Editor), An expert chemical Salzkristallisation und Sal- model for determining the zschadensmechanismen, Jahresberichte environmental conditions needed to aus dem Forschungsprogramm prevent salt damage in porous materials. "Steinzerfall - Steinkonservierung," European Commission Research Report Band 3 - 1991. Ein Förderprojekt des 11, Protection and Conservation of Bundesministers für Forschung und European Cultural Heritage. Archetype Technologie. Verlag Ernst & Sohn, Publications, London, pp. 13-18. Berlin, pp. 197-213. Degryse, P., Geet, M.V. et al., 2001. Microfocus Flatt, R.J., 2002. Salt damage in porous ma- Computer Tomography as a Qualitative terials: how high supersaturations are Approach to Frost Damage in Modern generated. Journal of crystal growth, Restoration Mortars. Internationale 242(3-4): 435-454. Zeitschrift für Bauinstandsetzen und Freedland, J. & Charola, A.E., 2001. Salts in Baudenkmalpflege = International Ceramic Bodies III: An Experimental Journal for Restoration of Buildings Study on Desalination. Internationale and Monuments, 7, 47. Zeitschrift für Bauinstandsetzen und Dei, L., Mauro, M. et al., 1999. Growth of Baudenkmalpflege = International crystal phases in porous media. Journal for Restoration of Buildings Langmuir, 15(26): 8915-8922. and Monuments, 7, 147. Doehne, E., 1994. In situ dynamics of sodium Goudie, A., 1995. The Changing Earth: Rates of sulfate hydration and dehydration in Geomorphological Processes (The stone pores: Observations at high Natural Environment). Blackwell. magnification using the environmental Goudie, A.S., 1999a. A comparison of the scanning electron microscope. In: V. relative resistance of limestones to frost Fassina, H. Ott and F. Zezza (Editors), and salt weathering. Permafrost and The conservation of monuments in the Periglacial Processes, 10, 309-316. Mediterranean Basin. Proceedings of the 3rd International Symposium, Goudie, A.S., 1999b. Experimental salt Venice, 1994. Soprintendenza ai Beni weathering of limestones in relation to Artistici e Storici di Venezia, pp. 143- rock properties. Earth Surface 150. Processes and Landforms, 24, 715-724. Doehne, E., 1997. ESEM development and Goudie, A.S. & Day, M.J., 1980. Disintegration application in cultural heritage of fan sediments in Death Valley, conservation. In: P.L. Gai (Editor), In- California, by salt weathering. Physical Situ Microscopy in Materials Research: Geography, 1, 126-137. Leading International Research in Goudie, A.S. & Viles, H., 1997. Salt Weathering Electron and Scanning Probe Hazards. John Wiley, Chichester, 241 pp.

Page 13. SALT WEATHERING ERIC DOEHNE

Grossi, C.M., Esbert, R.M. et al., 1997. Acoustic http://www.kasdansimonds.com/news2. emission monitoring to study sodium html. sulphate crystallization in monumental Kashchiev, D. & Van Rosmalen, G.M., 1995. porous carbonate stones. S t u d . Effect of Pressure on Nucleation in Conserv., 42, 115-25. Bulk Solutions and Solutions in Pores Hammer, I., Tinzl, C., 1996. Salzschäden an and Droplets. Journal of Colloid and Wandmalereienn (Salt damage to wall Interface Science, 169: 214-219. paintings). Bayerisches Landesamt für Kirchner, G., 1996. Cavernous weathering in the Denkmalpflege (Ed.) Arbeitsheft 78 des Basin and Range area, southwestern Bayerischen Landesamtes für USA and northwestern Mexico. Denkmalpflege, 222 pp. Zeitschrift für Geomorphologie, Haynes, H., 2002. Sulfate attack on concrete: Supplement 106, 73-97. Discrepancy between laboratory and Kraus, K. (Editor), 2002. Salze im historischen f i e l d e x p e r i e n c e . Concrete Natursteinmauerwerk (Salts in historical International, 24, July, 64-70. natural stone and brick-work). IFS- Haynes, H., O'Neill, R. et al., 1996. Concrete Tagung 2002, Beriecht Nr. 14. IFS, deterioration from physical attack by Institut Fur Steinkonservierung, 88 pp. salts. Concrete International, 18, 63-8. Kuchitsu, N., Ishizaki, T. et al., 2000. Salt Herodotus, 0420 B.C. History: "I observed ... weathering of the brick monuments in that salt exuded from the soil to such an Ayutthaya, Thailand. Engineering extent as even to injure the pyramids." Geology, 55, 91-99. 2.12 (Book 2). La Iglesia, A., Gonzalez, V. et al., 1997. Salt Hime, W.G., Martinek, R.A. et al., 2001. Salt crystallization in porous construction Hydration Distress. Concrete materials I. Estimation of crystallization International, 23, 43-50. pressure. Journal of Crystal Growth, Hochella, M.F., Jr., 2002. There's plenty of room 177, 111-118. at the bottom: Nanoscience in Larsen, P.K., 2001. The Development and geochemistry. Geochemica et Cos- Testing of a Salt Extracting Mortar. mochimica Acta, 66(5): 735-743. Internationale Zeitschrift für Hoffmann, L. & Grassegger, G., 1995. Sal- B a u i n s t a n d s e t z e n u n d zsprengeffekte an Naturbausteinen mit Baudenkmalpflege = International verschiedenen umweltrelevanten Journal for Restoration of Buildings Salzen, Umwelteinflüsse erfassen, and Monuments, 7, 79. simulieren, bewerten. 24. Jahrestagung Larsen, P.K. & Bøllingtoft, P., 1999. der GUS der FhG. 22.- 24. März 1995, Desalination of a painted brick vault in Pfinztal/Karlsruhe, pp. 23-13. Kirkerup Church, Preprints, 12th Hunt, B., 1994. There's no alternative. Stone Triennial Meeting, ICOM Committee Industries, February, 5, 37-39. for Conservation, 29 August - 3 September. ICOM, Lyon, France, pp. Jeannette, D., 2000. Originalite des mecanismes 473-477. d'alteration sur les vestiges archeologiques de Delos (Cyclades, Lewin, S.Z., 1982. The mechanism of masonry Greece) Delos (Cyclades, Greece). decay through crystallization. Comptes Rendus de l'Academie de Conservation of historic stone buildings Sciences - Serie IIa: Sciences de la and monuments. National Academy of Terre et des Planetes, 330, 683-688. Sciences, Washington, D.C., 120-144 pp. Jutson, J.T., 1918. The influence of salts in rock weathering in sub-arid Western Louis, M., Del Cura, M.A.G. et al., 2001. The Australia. Royal Society of Victoria, Civil Palaces in Gravina Street, Ali- Proceedings, 30, 165-172. cante: building stones and salt weathering. Materiales de Construcción, 51, Kasdan, Simonds et al., 2002. Sulfate Attack to 23-37. Concrete. World Wide Web Address: http://www.kasdansimonds.com/sulfate Luquer, L.M., 1895. The relative effects of frost _attack.html and sulphates of soda efflorescence tests

Page 14. SALT WEATHERING ERIC DOEHNE

on building stones. American Society of Mehta, P.K., 2000. Sulfate attack on concrete: Civil Engineers - Transactions, 33, 235- Separating myths from reality. Concrete 247 and 247-256. International, 22, 57-61. Malin, M.C., 1974. Salt Weathering on Mars. Mikulas, R., 2001. Gravity and oriented pressure Journal of Geophysical Research, 79, as factors controlling "honeycomb 3888-3894. weathering" of the Creataceous Maliva, R.G. & Siever, R., 1988. Diagenetic castellated sandstones (northern replacement controlled by force of Bohemia, Czech Republic). Bulletin of crystallization. Geology, 16, 688-691. the Czech Geological Survey, 76, 217- 226. Marschner, H., 1979. Application of salt crystallisation test to impregnated Minguez, J.M. & Elorza, J., 1994. Diagenetic stones. In Researchs in Archaeology volume-for-volume replacement - force and Conservation, Mishra, Phanikanta, of crystallization and depression of ed. pp. 70-77. dissolution. Mineralogical Magazine, 58, 135-142. Matsukura, Y. & Matsuoka, N., 1991. Rates of tafoni weathering on uplifted shore Moh'd, B.K., Howarth, R.J. et al., 1996. Rapid platforms in Nojima-Zaki, Boso prediction of Building Research Peninsula, Japan. Earth Surface Establishment limestone durability class Processes and Landforms, 16, 51-56. from porosity and saturation. Quarterly Journal of Engineering Geology, 29, May, E., Papida, S. et al., 2000. Comparative 285-297. studies of microbial communities on stone monuments in temperate and Moropoulou, A., 2000. Reverse engineering to semi-arid climates. In: O. Ciferri, P. discover traditional technologies: a Tiano and G. Mastromei. (Editors), Of proper approach for compatible Microbes and Art : the role of microbial restoration mortars. PACT (Athens, communities in the degradation and Greece), 58, 81-107. protection of cultural heritage Part 1. Mustoe, G.E., 1982. The origin of honeycomb Ecology of microbial communities weathering. Geological Society of developing on art works. Kluwer America Bulletin, 93, 108-115. Academic/Plenum Publishers, New Nash, D., 2000. Arid Geomorphology. Progress York, pp. 49. in physical geography, 24, 425-443. McBride, E.F. & Picard, M.D., 2000. Origin and development of Tafoni in Tunnel Spring Nicholson, D.T., 2001. Pore properties as Tuff, Crystal Peak, Utah, USA. Earth indicators of breakdown mechanisms in Surface Processes and Landforms, experimentally weathered limestones. 25(8): 869-879. Earth Surface Processes and McCord, T.B., Sieger, M.T. et al., 2001. Landforms, 26, 819-838. Thermal and radiation stability of the Norwick, S.A. & Dexter, L.R., 2002. Rates of hydrated salt minerals epsomite, development of tafoni in the Moenkopi mirabilite, and natron under Europa and Kaibab formations in Meteor Crater environmental conditions. Journal of and on the Colorado Plateau, Geophysical Research E: Planets, 106, northeastern Arizona. Earth Surface 3311-3319. Processes and Landforms, 27, 11-26. McMahon, D.J., Sandberg, P. et al., 1992. Nunberg, S. & Charola, A.E., 2001. Salts in Deterioration mechanisms of sodium Ceramic Bodies II: Deterioration due to sulfate. In: J.D. Rodrigues, F. Henriques Minimal Changes in Relative Humidity. and F.T. Jeremisas (Editors), 7th Internationale Zeitschrift für International Congress on Deterioration B a u i n s t a n d s e t z e n u n d and Conservation of Stone, Lisbon Baudenkmalpflege = International Portugal, 15-18 June 1992, Proceedings. Journal for Restoration of Buildings Laboratorio Nacional de Engenharia and Monuments, 7, 131. Civil, Lisbon, Portugal, pp. 705-714. Ordonez, S., Fort, R. et al., 1997. Pore size distribution and the durability of a

Page 15. SALT WEATHERING ERIC DOEHNE

porous limestone. Quarterly Journal of RILEM, 1980. Recommandations provisoires / Engineering Geology, 30, 221-230. Tentative Recommendations. (RILEM) Papida, S., Murphy, W. et al., 2001. Commission 25-PEM. Protection et Enhancement of physical weathering of erosion des monuments: Essais building stones by microbial recommandés pour mesurer l'altération p o p u l a t i o n s . International des pierres et évaluer l'efficacité des Biodeterioration and Biodegradation, méthodes de traitement. Matériaux et 46, 305-317. Constructions, 13, 175-253. Pel, L., Kopinga, K. et al., 2000. Saline Robinson, D.A. & Williams, R.B.G., 2000. absorption in calciumsilicate brick Experimental weathering of sandstone observed by NMR-scanning. Journal of by combinations of salts. Earth Surface Physics D: Applied Physics, 33, Processes and Landforms, 25, 1309- 1380–1385. 1315. Pósfai, M., Anderson, J.R. et al., 1995. Rodriguez-Navarro, C., 1998. Evidence of Compositional variations of sea-salt- honeycomb weathering on Mars. mode aerosol particles form the North Geophysical Research Letters, 25, Atlantic. Journal of Geophysical 3249-3252. Research, 100, 23,063-23,074. Rodriguez-Navarro, C. & Doehne, E., 1999a. Price, C.A., 1978. The use of sodium sulphate Salt weathering: influence of crystallisation test for determining the evaporation rate, supersaturation and weathering resistance of untreated crystallization pattern. Earth Surface stone, Alteration et protection des Processes and Landforms, 24, 191-209. monuments en pierre: Colloque Rodriguez-Navarro, C. & Doehne, E., 1999b. International : Paris, du 5 au 9 juin 1978 Time-lapse video and ESEM: = Deterioration and protection of stone Integrated tools for understanding monuments : International Symposium : processes in situ. American Laboratory, Paris, June 5th-9th 1978. Unesco- May, 28-35. Rilem, Paris, pp. 10. Rodriguez-Navarro, C., Doehne, E. et al., 1999. Price, C.A., 1996. Stone Conservation. An Origins of honeycomb weathering: The Overview of Current Research. Getty role of salts and wind. Bulletin of The Conservation Institute, J. Paul Getty Geological Society of America, 111, Trust, 7-9 pp. 1250-1255. Price, C.A., 2000. An expert chemical model for Rodriguez-Navarro, C., Doehne, E. et al., 2000a. determining the environmental How does sodium sulfate crystallize? conditions needed to prevent salt Implications for the decay and testing of damage in porous materials. European building materials. Cement and Commission Research Report 11, Concrete Research, 30, 1527-1534. Protection and Conservation of Rodriguez-Navarro, C., Doehne, E. et al., 2000b. European Cultural Heritage. London: Influencing crystallization damage in Archetype Publications, 136 pp. porous materials through the use of Putnis, A., Prieto, M. et al., 1995. Fluid surfactants: Experimental results using supersaturation and crystallization in Sodium Dodecyl Sulfate and porous media. Geological Magazine, Cetyldimethylbenzylammonium 132(1): 1-13. Chloride. Langmuir, 16, 947-954. Putnis, A. & Mauthe, G., 2001. The effect of Rucker, P., Holm, A. et al., 2000. Determination pore size on cementation in porous of moisture and salt content rocks, Geofluids, 1, 37-41. distributions by combining NMR and Pye, K. & Mottershead, D.N., 1995. Honeycomb g a m m a r a y m e a s u r e m e n t s . weathering of Carboniferous sandstone Materialsweek - Munich, September. in a sea wall at Weston-super-Mare, Schaffer, R.J., 1932. The Weathering of Natural UK. Quarterly Journal of Engineering Building Stones, Special Report No.18, Geology, 28, 333-347. Building Research Establishment, Garston, Watford.

Page 16. SALT WEATHERING ERIC DOEHNE

Scherer, G.W., 1999. Crystallization in pores. Smith, B.J., 1994. Weathering processes and Cement and Concrete Research, 29, forms. In: A.D. Abrahams and A.J. 1347-1358. Parsons (Editors), Geomorphology of Scherer, G.W., 2000. Stress from crystallization Desert Environments. Routledge of salt in pores, 9th International Chapman & Hall, London, pp. 39-63. Congress on Deterioration and Smith, B.J. & McGreevy, J.P., 1988. Contour Conservation of Stone, Venice, 19-24 scaling of a sandstone by salt June, pp. 187-194. weathering under simulated hot desert Scherer, G.W., Flatt, R. et al., 2001. Materials conditions. Earth Surface Processes science research for the conservation of and Landforms, 13, 697-705. sculpture and monuments. Materials Smith, B.J., Warke, P.A. et al., 2000. Limestone Research Society Bulletin, 26, 44-50. weathering in contemporary arid Schwarz, H.J., Gervais, A. et al., 2000. Natural environments: A case study from stone conservation in St. Michaelis southern Tunisia. Earth Surface Cloister, Hildesheim, Proceedings of Processes and Landforms, 25, 1343- the 6th International Congress on 1354. Applied Mineralogy ICAM 2000. Snethlage, R., Wendler, E. et al. (Editors), 1996. Rotterdam, Göttingen, Germany, pp. S. Tenside im Gesteinsschutz-Bisherige 1031 - 1038. resultate mit einem neuen Konzept zur Schwarz, H.J. & Roesch, H., 1993. Damage to Erhaltung von Denkmilern aus frescoes caused by sulphate-bearing Naturstein. Natursteinkonservierun in salts: where does the sulphate come der Denkmalpflege. Ernst & Sohn, from? Studies in Conservation, 38, 224- Berlin, pp. 127-146. 230. Spennemann, D.H.R., 2001. The creeping Sheftick, 1989. Na2SO4 Soundness Test disaster: dryland and urban salinity and Evaluation. Cement, Concrete and its impact on heritage. Cultural Aggregates, 11, 73-79. Resource Management, 24, 22-25. Siedel, H., 1996. Experiences from desalting of Steiger, M., 1996. Distribution of salt mixtures tuffstone and sandstone monuments by in a sandstone monument: Sources, compresses, Le dessalement des transport and crystallization properties. matériaux poreux. 7es journées d'études In: F. Zezza (Editor), EC Research de la SFIIC, Poitiers, 9 - 10 mai 1996. Workshop on Origin, Mechanisms and SFIIC, Champs-sur-Marne, pp. 191- Effects of Salts on Degradation of 198. Monuments in Marine and Continental Environments. European Commission, Simon, S., Herm, C. et al., 1996. Desalination Protection and Conservation of the and control of salt transport phenomena: European Cultural Heritage. Research Experiences with compress renderings Report 4. Bari, pp. 241-46. in the ring crypt of St. Emmeram, Regensburg, Le dessalement des Steiger, M., 2002. Salts and Crusts. In: P. matériaux poreux. 7es journées d'études Brimblecombe (Editor), The Effects of de la SFIIC, Poitiers, 9 - 10 mai 1996. Air Pollution on the Built Environment. SFIIC, Champs-sur-Marne, pp. 147- Air Pollution Reviews. Imperial College 159. Press, London. Simon, S. & Lind, A.-M., 1999. Decay of Steiger, M., Beyer, R. et al., 2000. Data limestone blocks in the block fields of Compilation and Experimental Karnak Temple (Egypt): non- Determinations. In: C.A. Price (Editor), destructive damage analysis and control An expert chemical model for of consolidation treatments, ICOM determining the environmental Committee for Conservation, Triennial conditions needed to prevent salt meeting (12th), Lyon, 29 August - 3 damage in porous materials. European September 1999: preprints. Vol. 2, pp. Commission Research Report 11, 743-749. Protection and Conservation of European Cultural Heritage. Archetype Publications, London, pp. 19-37.

Page 17. SALT WEATHERING ERIC DOEHNE

Steiger, M. & Dannecker, W., 1995. Turkington, A.V., 1998. Cavernous weathering Hygroskopische Eegenschaften und in sandstone: Lessons to be learned Kristallisationsvergalten von from natural exposure. Quarterly Salzgemischen, Jahresberichte aus dem Journal of Engineering Geology, 31, Forschungsprogramm Steinzerfall 375-383. Steinkonservierung. Ernst und Sohn, Turkington, A.V. & Smith, B.J., 2000. Berlin, pp. 115-128. Observations of three-dimensional salt Steiger, M. & Zeunert, A., 1996. Crystallization distribution in building sandstone. Earth properties of salt mixtures: comparison Surface Processes and Landforms, 25, of experimental results and model 1317-1332. calculations. In: J. Riederer (Editor), Uchida, E., Nakagawa, T. et al., 2000. Proceedings of the 8th International Deterioration of stone materials in the Congress on Deterioration and Angkor monuments, Cambodia. Conservation of Stone: held in Berlin, Engineering Geology, 55, 101-112. 30 Sept. - 4 Oct. 1996. Möller Druck, Unruh, J., 2001. A revised endpoint for ceramics Berlin, pp. 535-544. desalination at the archaeological site of Storch, W. & Tur, P., 1991. Untersuchung Gordion, Turkey. Studies in einiger Schädigungsvorgänge an Conservation, 46, 81-93. N a t u r s t e i n e n m i t t e l s Verges-Belmin, V., 1994. Pseudomorphism of Schallemissionsanalyse, Jahresberichte gypsum after calcite, a new textural Steinzerfall - Steinkonservierung 1989. feature accounting for the marble Ein Förderprojekt des Bundesministers sulphation mechanism. Atmospheric für Forschung und Technologie, pp. Environment, 28, 295-304. 189-195. Vicente, M.A. & Vicente-Tavera, S., 2001. Clay Sunamura, T., 1996. A physical model for the poultices in salt extraction from rate of coastal tafoni development. ornamental stones: a statistical Journal of Geology, 104, 741-748. approach. Clays & Clay Minerals, 49, Taber, S., 1916. The growth of crystals under 227-235. external pressure. American journal of Viles, H.A., 1997. Group Report: What Is the science, 41, 532-556. State of Our Knowledge of the Theoulakis, P. & Moropoulou, A., 1999. Salt Mechanisms of Deterioration and How crystal growth as weathering Good Are Our estimates of Rates of mechanism of porous stone on historic Deterioration? In: N.S. Baer and R. masonry. Journal of Porous Materials, Snethlage (Editors), Saving our 6, 345-358. Architectural Heritage. The Torfs, K. & Van Grieken, R., 1997. Chemical Conservation of Historic Stone relations between atmospheric aerosols, Structures: Report of the Dahlem deposition and stone decay layers on Workshop, Berlin, March 3-8, 1996. historic buildings at the Mediterranean John Wiley & Sons Ltd., New York, pp. coast. Atmospheric Environment, 31, 95-112. 2179-2192. Warke, P.A. & Smith, B.J., 2000. Salt Torraca, G., 1982. Porous Building Materials. distribution in clay-rich weathered Materials Science for Architectural sandstone. Earth Surface Processes and Conservation. 2nd ed. ICCROM, Landforms, 25, 1333-1342. International Centre for the Study of the Whipkey, C.E., Stewart, B.W. et al., 2000. The Preservation and Restoration of Cultural importance of sea spray to the cation Property, Rome, Rome. budget of a coastal Hawaiian soil: A Trudgill, S.T., Viles, H.A. et al., 1991. Trends in strontium isotope approach. Chemical stone weathering and atmospheric Geology, 168, 37-48. pollution at St Paul's Cathedral, Williams, R.B.G. & Robinson, D.A., 2001. London, 1980-1990. Atmospheric Experimental frost weathering of Environment, Part A, 25A, 2851-3. sandstone by various combinations of

Page 18. SALT WEATHERING ERIC DOEHNE

salts. Earth Surface Processes and Landforms, 26, 811-818. Wittenburg, C. & Dannecker, W., 1994. Salt enrichment in building stones by deposition of sulfur and nitrogen containing species from urban atmospheres. In: V. Fasina, H. Ott and F. Zezza (Editors), Proceedings of 3rd International Symposium on the Conservation of Monuments in Mediteranean Basin, pp. 179-183. Wüst, R.A.J. & McLane, J., 2000. Rock deterioration in the Royal Tomb of Seti I, Valley of the Kings, Luxor, Egypt. Engineering Geology, 58, 163-190. Wüst, R.A.J. & Schlüchter, C., 2000. The origin of soluble salts in rocks of the Thebes mountains, Egypt: The damage potential to ancient Egyptian wall art. Journal of Archaeological Science, 27, 1161-1172. Xiaozu, X., Jiacheng, W. et al., 1999. Mechanisms of Frost Heave and Salt Expansion of Soils. Science Press, Beijing/New York, 167 pp. Young, D., 1995. Rising Damp and Salt Attack, State Heritage Branch, Department of Environmental and Natural Resources; South Australia, Adelaide. http://www.environment.sa.gov.au/herit age/pub_desc.html#rising_damp

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