Patricia Warke

Effects of direct and indirect heating on the validity of rock weathering simulation studies and durability tests

Rock surface and subsurface temperature responses in samples exposed to direct heating (insolation) under natural hot desert conditions reveal considerable variability between lithologies related to differences in thermal properties, especially albedo and thermal conductivity. However, when the same samples are heated indirectly by air in an oven-based environmental cabinet, lithological differences in temperature response disappear and all samples attain temperatures similar to the air temperature within the cabinet. Rates and patterns of rock decay produced in such environmental cabinets may not, therefore, reflect those encountered under natural conditions, where breakdown is related to micro-environmental conditions at the rock/air interface and where rock temperature is one of the most important controlling factors. In addition to implications for assessment of weathering effectiveness, use of only indirect forms of heating affects the determination of comparative rock durability because all rock types are cycled through the same temperature regimes. Because temperature exerts such a major control on rock breakdown through its control on physical and chemical weathering processes, all significant factors influencing it must be included in the design of weathering simulations and durability tests.

THERMAL RESPONSE CHARACTERISTICS OF STONE: IMPLICATIONS FOR WEATHERING OF SOILED SURFACES IN URBAN ENVIRONMENTS

Soiling of stone surfaces by particulate deposition increases absorption of radiant energy, raises surface/subsurface temperature gradients and accentuates rates of surface temperature change. Short-term fluctuation of raised surface temperatures, in response to variations in windspeed and cloud cover, may ultimately contribute to stone breakdown through ‘fatigue’ effects which reduce cohesive strength of intergranular bonds and initiate microfracture development. The effects of soiling are particularly marked for stone with low thermal conductivity and high albedo when clean. Albedo change has implications for the effectiveness of weathering processes and the durability of building stone by creating microenvironmental conditions which are more severe than those indicated by macroenvironmental regimes.

Condition Assessment for building stone conservation: a staging system approach

Abstract Trofimov and Phillips (Geomorphology 5 (1992) 203) suggest that the ultimate goal of any science is to predict the behaviour of entire systems. With regard to the decay of building stone, making accurate predictions of stone behaviour remains an elusive goal but given our improved understanding of decay dynamics it should be possible to provide a forecast of likely system behaviour. However, forecasting system behaviour requires classification of the system state with the classification, whether formal or informal, founded on knowledge of the factors that control response. In the context of building stone decay these controlling factors include, structural properties, mineralogical properties, inheritance effects, contaminant loading and natural change. In trying to formalise building stone condition assessment and incorporate a forecast component, an analogy can be made between the requirements for classification and treatment determination of cancer patients and the approach to condition assessment and conservation of stone structures. In medicine, one of the most widely used and refined patient assessment schemes is the TNM Staging System. The rationale underpinning the TNM Staging System has many similarities with approaches to building stone assessment in that it seeks to impose a more formal structure on condition assessment that provides a commonality of approach, language and meaning and a procedure for forecasting the extent of remedial intervention required and outcome in terms of ‘life expectancy’.

Modelling the rapid retreat of building sandstones: a case study from a polluted maritime environment

Abstract Sandstones are widely used as building stones throughout NW Europe. Unlike limestone, sandstones tend to experience episodic and sometimes rapid surface retreat associated with the action of salts and often leading to the development of hollows/caverns in the stone. The unpredictability of these decay dynamics can present significant problems when planning conservation strategies. Consequently, successful conservation requires a better understanding of the factors that trigger decay and determine the subsequent decay pathway. An overview of results from previous studies provided the basis for simulation experiments aimed at identifying the factors that (a) initiate decay and (b) permit the continuance of salt weathering despite rapid loss of surface material. These simulation studies involve investigation of changes in micro-environmental conditions as surface hollows develop and examination of salt weathering dynamics within such hollows. These data combined with knowledge gained from previous work have allowed the refinement of a conceptual model of rapid sandstone retreat. In this model decay is linked to the establishment of positive feedback conditions through interactions between factors such as porosity, permeability, mineralogy and their effect on salt penetration.

Limestone weathering in contemporary arid environments: a case study from southern Tunisia

Weathering features are described from a meteorologically arid area in southern Tunisia. Active weathering is concentrated in topographic lows (pits and pans) that concentrate available moisture and are associated with endolithic and epilithic algae responsible for algal boring, plucking and etching of the limestone substrate. Deposits of gypsum have concentrated through evaporation within hollows and occur as isolated patches of tabular crystals indicating an additional weathering agent characteristic of arid environments. The active development of a complex microkarst and associated salt weathering are combining to destroy linear karstic features such as rillenkarren, widespread case-hardening and in places patches of iron- and manganese-rich rock varnishes. The complexity of active and inherited weathering features belies the perceived simplicity and lack of weathering opportunities associated with aridity. Instead it focuses on the need to consider the nature of microclimatic conditions at the rock/air interface especially when seeking to define desert weathering environments. It also highlights the difficulties of interpreting polygenetic landforms and the importance of microscale studies for identifying the impact of contemporary climatic conditions.

Lichen-induced biomodification of calcareous surfaces: Bioprotection versus biodeterioration

Studies demonstrate the active and passive capability of lichens to inhibit or retard the weathering of calcareous surfaces. Lichen coverage may actively protect a surface through shielding by the thallus and the binding and waterproofing of the rock surface and subsurface by fungal hyphae. Passive protection of rock surfaces may be induced by the formation of an insoluble encrustation, such as calcium oxalate, at the lichen-rock interface. Recent research suggests that the decay of hyphae, induced by changes in microenvironmental conditions, necrosis, parasitism or the natural physiological traits of particular lichen species, may expose a chemically and physically weakened substrate to dissolution, triggering relatively rapid weathering-related surface lowering. Consequently, certain epilithic crustose and endolithic lichens may induce a period of surface stability throughout the course of their lifespan, followed by a phase of instability and rapid episodic microtopographical evolution after death and decay. A series of conceptual models is proposed to illustrate this idea over short (single lichen lifespan) and long (multiple lichen lifespans) timescales. The models suggest that the microscale biogeomorphological system of lichen-rock interaction is underpinned by non-linear dynamical system theory as it exhibits dynamical instability and is consequently difficult to predict over a long timescale. Dominance by biodeterioration or bioprotection may be altered by changes in lichen species or in environmental conditions over time.

Controls on Stone Temperatures and the Benefits of Interdisciplinary Exchange

Abstract Data derived from a series of field and laboratory studies of the influence of albedo and thermal conductivity on stone temperatures are reported. The data indicate the complexity of surface and subsurface temperature response characteristics of different stone types exposed to the same conditions. They also highlight the influence of albedo and thermal conductivity on microenvironmental conditions at the rock-air interface. These conditions have significant implications for the nature and rate of weathering activity and may, over time, affect any treatments applied to stone surfaces. Although the studies reviewed were carried out within the subject area of geomorphology, the data reported and the implications for stone weathering arising from them may be of some relevance to the conservation science perspective on deterioration of contemporary, historical, and archaeological stonework.

Complex weathering effects on durability characteristics of building stone

Abstract Durability characteristics of five stone types are assessed and compared using the standardized sodium sulphate salt crystallization test and a modified laboratory weathering simulation in which a combination of salt weathering (Na2SO4) and freeze-thaw cycles are used. Data indicate significant differences in durability rankings between the two test methods especially in lower-order durability stone types. Both the standard salt crystallization test and the modified durability test identify Leinster Granite and Stanton Moor B Sandstone as the most durable of the five stone types, with the granite performing well under both sets of conditions. Discrepancy between rankings arises in the lower orders, with Portland Limestone, Stanton Moor A Sandstone and especially Dumfries Sandstone responding differently to the two sets of experimental conditions. In the modified durability test the range of permeability values for each stone type produced the same ranking as that indicated by mean percentage weight change values but mean permeability values for each stone type do not appear to be reliable predictors of weathering response. Differences in durability rankings between the two test regimes are attributed in the first instance to the temperature conditions used, with more extreme and unrealistic heating to 103 °C in the standardized test ‘over-weathering’ stone while conditions in the modified test allowed the development of stone-specific decay characteristics. Inclusion of salt weathering and freeze-thaw cycles in the modified test introduced complexity into the decay process that more accurately reflects ‘real-world’ conditions. Data also indicate that relatively minor structural and mineralogical differences between samples of the same stone type can significantly influence weathering behaviour, resulting in distinct rates and patterns of breakdown.

Controls on permeability: implications for stone weathering

Abstract In the light of a well-researched relationship between rock properties and susceptibility of stone to weathering, the role of permeability in weathering is examined. A review of weathering studies indicates the varied use and nature of porosity data, but the paucity of permeability studies in weathering trials. Key factors that control porosity and permeability, depositional characteristics and diagenetic processes are discussed and investigated, with a view to discussing the implications for stone weathering. Results from experimental studies on a range of rock types comprising sandstone, limestone and granite are presented. The relevance of permeability measurement is explored in terms of spatial mapping and quantitative assessment of the deterioration of natural building stone. Increased knowledge and appreciation of the inherited characteristics of a rock is demonstrated to provide valuable insight and a greater understanding of how natural stone heterogeneity is accentuated and exploited by weathering and continued exposure to moisture and salts. Mapping the spatial distribution of permeability provides greater insight into the extent of variability in stone deterioration and presents the possibility of monitoring and predicting the hydraulic properties of stone and how these are modified by weathering processes.

Weathering of igneous rocks during shallow burial in an upland peat environment: observations from the Bronze Age Copney Stone Circle Complex, Northern Ireland

Abstract The stone circle complex at Copney, County Tyrone is a key Bronze Age site that forms part of the Mid-Ulster stone circle complex. This site was excavated in 1995 and since then, deterioration of the archaeological stonework has been a serious problem. Deterioration is visible as splitting/fracturing of stones, the development of a bleached outer margin, surface scaling and granular disintegration, and, in some cases, complete disintegration of individual stones to sandy regolithic material. Environmental conditions at this site exacerbate the deleterious action of weathering processes, with periodic waterlogging and sub-zero temperatures during winter months. The Copney stones comprise two igneous lithologies: quartz porphyry and porphyritic andesite, both Lower Ordovician (470 Ma) in age. Both rock types show extensive alteration by hydrothermal processes resulting in weakened stone fabrics exploited by weathering processes during burial and subsequent exposure. Mineralogy of buried and exposed material indicates that approximately 2500 years of burial in a peat bog has produced secondary porosity comprising extensive microfracture networks and dissolution voids, which permitted further ingress of moisture and acidic waters, thus, promoting chemical alteration of mineral constituents. The occurrence of completely grussified stones and arenisation of boulder surfaces at Copney indicates that the processes of grussification are not solely restricted to deep weathering environment but can be achieved by burial in a shallow, aggressive environment over periods measured in thousands of years.