Deterioration mechanisms of tuffs in Midas monument

Slightly weathered white and pink tuffs of the Midas monument have deterioration problems. In this study, depths and characteristics of the weathering zones developed within the tuffs are investigated through optical microscopy, X-ray diffractometry (XRD), chemical analyses, scanning electron microscopy (SEM) and some index parameters. Accelerated weathering tests including wetting–drying, freezing–thawing and salt crystallization are performed, and durability assessment methods are used to predict the durabilities of the tuffs. The findings are compared with field observations. By examining quantitative weathering indices and comparing them with thin section studies, it is found that thin section analyses of the crystals, LoI, and WPI are good indicators to quantify the depth of weathering for the tuffs. However, thin section studies have limited value for fine-grained tuffaceous matrix. The chemical weathering of the tuffs produces weathered zones that are 4.5-cm thick within the white tuff and 2.5-cm thick within the pink tuff. Physical weathering causes scaling of outer layers of the tuffs and fracturing of feldspars along their cleavage planes. However, variations of the index properties of the tuffs due to weathering are not so significant to quantify the weathering depths in the tuffs. Among the accelerated weathering tests, salt crystallization is found to be the most destructive environmental condition. Pigeon droppings rich in NO₃¹⁻ are found to be the main source of soluble salt at the Midas monument. The salt transported up by capillary rise due to surface water causes spalling of the tuffs in the capillary zone. Surface water and salt of any kind in the close vicinity of the monument should be totally eliminated for the purpose of conservation. Field observations and the durability equations reveal that the white tuff is less durable than the pink tuff. Wet-to-dry strength ratio yields a better stone durability assessment among various durability methods used in this study.     

Weathering of marbles and granites in marine environment: petrophysical properties and special role of atmospheric salts

 The aim of this paper was to study the weathering mechanism of marbles and granites exposed to the marine environment at the Delos archaeological site. Alterations, as granular disintegration, contour scaling and alveolus formations, can be observed either at the base of blocks and columns, or at the median-high zones of monuments. A white marble, which originated from Naxos Island, had a weak porosity (0.2%). Its porous network was organized into two subnetworks of rectilinear and sinuous cracks, which limited capillary transfer to ∼10 cm. The granite, which originated from Delos Island, had a relatively high porosity (2.15%), and had a well-connected system of cracks and microporous zones of weathered minerals. This homogeneous network allowed good capillary transfer for long distances (1–2 m). For the marble, as for the granite, evaporation occurred mainly at depth in the stone, and encouraged its deterioration. This damage depends on the dissolution and crystallization of salts, which occur in cycles. First, by the capillary transfer of water and salt from the ground, when the stones are located near the sea. This processes can explain the deterioration of the marble and the granite bases, and the decay of granite on all its surfaces. Second, by fixation of water vapor by sea salt deposited on the stone by wind. This phenomenon can explain the degradations observed on the median-high parts of monuments in marble and granite, even though, for the granite, the first mechanism was more active.     

Changing climate, changing process: implications for salt transportation and weathering within building sandstones in the UK

Salt weathering is a crucial process that brings about a change in stone, from the scale of landscapes to stone outcrops and natural building stone façades. It is acknowledged that salt weathering is controlled by fluctuations in temperature and moisture, where repeated oscillations in these parameters can cause re-crystallisation, hydration/de-hydration of salts, bringing about stone surface loss in the form of, for example, granular disaggregation, scaling, and multiple flaking. However, this ‘traditional’ view of how salt weathering proceeds may need to be re-evaluated in the light of current and future climatic trends. Indeed, there is considerable scope for the investigation of consequences of climate change on geomorphological processes in general. Building on contemporary research on the ‘deep wetting’ of natural building stones, it is proposed that (as stone may be wetter for longer), ion diffusion may become a more prominent mechanism for the mixing of molecular constituents, and a shift in focus from physical damage to chemical change is suggested. Data from ion diffusion cell experiments are presented for three different sandstone types, demonstrating that salts may diffuse through porous stone relatively rapidly (in comparison to, for example, dense concrete). Pore water from stones undergoing diffusion experiments was extracted and analysed. Factors controlling ion diffusion relating to ‘time of wetness’ within stones are discussed, (continued saturation, connectivity of pores, mineralogy, behaviour of salts, sedimentary structure), and potential changes in system dynamics as a result of climate change are addressed. System inputs may change in terms of increased moisture input, translating into a greater depth of wetting front. Salts are likely to be ‘stored’ differently in stones, with salt being in solution for longer periods (during prolonged winter wetness). This has myriad implications in terms of the movement of ions by diffusion and the potential for chemical change in the stone (especially in more mobile constituents), leading to a weakening of the stone matrix/grain boundary cementing. The ‘output’ may be mobilisation and precipitation of elements leading to, for example, uneven cementing in the stone. This reduced strength of the stone, or compromised ability of the stone to absorb stress, is likely to make crystallisation a more efficacious mechanism of decay when it does occur. Thus, a delay in the onset of crystallisation while stonework is wet does not preclude exaggerated or accelerated material loss when it finally happens.     

Present and future challenges of urban systems affected by seawater and its intrusion: the case of Venice, Italy

In lagoonal and marine environments, both historic monuments and recent buildings suffer from severe salt damage caused by sea flooding, sea-level rise and frequent storm events. Salt-water contamination of groundwater systems, a widespread phenomenon typical of coastal areas, can lead to a deterioration not only of the quality of fresh groundwater resources, but also of building materials in urban settlements. A general overview is given of the hydrogeological configuration of the subsoil of Venice (Italy), with particular reference to the shallow groundwater circulation. The relationship between the seawater in the subsoil and salt decay processes, due to salt crystallization, is highlighted. These processes affect civil constructions in Venice??s historic center. Perched aquifers, influenced by tide variations and characterized by salt-water intrusion, favor the transport of salts within masonry walls through the action of rising damp. In fact, foundations, in direct contact with the aquifers, may become a preferential vehicle for the transportation of salt within buildings. Decay patterns of different building materials can be detected through non-destructive techniques, which can identify sea-salt damage and therefore assist in the preservation of cultural heritage in coastal areas.     

Dry and wet laboratory tests and thermal fatigue of rocks

Investigation of the weathering and weatherability of three types of igneous rocks (granite, nepheline syenite and lamprophyric-minette dyke) was carried out by means of laboratory tests.For this purpose an experimental device was developed to obtain hot conditions alternating with room temperature and also with wet conditions, by immersion in two types of liquids (distilled water and a solution simulating “seawater”).Beyond qualitative effects emphasized in the text and in a photographic documentary, some quantitative determinations were made (weight loss and chemical decay of main cations of rock-minerals), which enable the authors to present an attempt at an alterability index for classification of the rocks. The research-program outlined on quantitative determinations is suitable for application in problems of engineering geology such as in determining suitability of rocks for use as breakwater stone or riprap for dams or in the stone deterioration measurements of the deterioration of stone used for buildings and ornamental facing stones.     

内蒙古赤峰召庙花岗岩表面的冰川壶穴、泄水槽、盐风化穴和剥离面理

在内蒙古赤峰市巴林左旗林东镇以南约20 km的召庙, 出露一片面积约20 km2的花岗岩, 称为召庙花岗岩体。该岩体被至少2组垂直节理及与地面起伏一致的剥离面理切割成大小不等的弧形岩块, 构成高于周围地表300~400 m的花岗岩山区。在多条花岗岩山脊上保存有大小不等的壶穴, 山脊两侧则发育从上到下紧密排列的泄水槽, 而泄水槽则被剥离面理横向切割, 沿剥离面理的露头线发育大小不等的风化穴。通过详细的野外调查, 认为花岗岩山脊上的壶穴不是风蚀和风化所致, 而是晚更新世冰盖上的冰川融水下泄冲刷形成的; 山脊两侧的泄水槽也是冰川融水下泄冲刷的结果; 剥离面理是第四纪冰川发育导致地表之下岩石温度变化的热变应力所致, 可用于恢复第四纪冰川作用时期的基岩古地貌; 盐风化穴是第四纪冰川退缩后本区重要的物理风化类型, 优先沿着剥离面理发育。本区3种微型地貌景观和大型的剥离面理共生组合记录了更新世以来召庙花岗岩山区的地貌演化史。     

Wetland Loss Patterns and Inundation-Productivity Relationships Prognosticate Widespread Salt Marsh Loss for Southern New England

Tidal salt marsh is a key defense against, yet is especially vulnerable to, the effects of accelerated sea level rise. To determine whether salt marshes in southern New England will be stable given increasing inundation over the coming decades, we examined current loss patterns, inundation-productivity feedbacks, and sustaining processes. A multi-decadal analysis of salt marsh aerial extent using historic imagery and maps revealed that salt marsh vegetation loss is both widespread and accelerating, with vegetation loss rates over the past four decades summing to 17.3 %. Landward retreat of the marsh edge, widening and headward expansion of tidal channel networks, loss of marsh islands, and the development and enlargement of interior depressions found on the marsh platform contributed to vegetation loss. Inundation due to sea level rise is strongly suggested as a primary driver: vegetation loss rates were significantly negatively correlated with marsh elevation (r ²?=?0.96; p?=?0.0038), with marshes situated below mean high water (MHW) experiencing greater declines than marshes sitting well above MHW. Growth experiments with Spartina alterniflora, the Atlantic salt marsh ecosystem dominant, across a range of elevations and inundation regimes further established that greater inundation decreases belowground biomass production of S. alterniflora and, thus, negatively impacts organic matter accumulation. These results suggest that southern New England salt marshes are already experiencing deterioration and fragmentation in response to sea level rise and may not be stable as tidal flooding increases in the future.     

The effect of air pollution on stone decay: the decay of the Drachenfels trachyte in industrial, urban, and rural environments—a case study of the Cologne, Altenberg and Xanten cathedrals

Severe stone deterioration is evident at the Cologne cathedral. In particular, the “Drachenfels” trachyte, which was the building material of the medieval construction period, shows significant structural deterioration as well as massive formation of gypsum crusts. The present article investigates crust formation on limestone, sandstone, and volcanic rock from the Cologne cathedral as well as from the Xanten and Altenberg cathedrals. These three buildings, showing varying degrees of deterioration, are located in different areas and exposed to varying industrial, urban, and rural pollution. Thin laminar and black framboidal crusts form on calcareous as well as silicate stone. The lack of a significant intrinsic calcium and sulfur source for the formation of the gypsum crusts on the Drachenfels trachyte indicates major extrinsic environmental impact: a sufficient offer of SO _x from pollutant fluxes as well as external calcium sources (e.g., pollution, mortars, neighboring calcite stones). Chemical analyses reveal strong gypsum enrichment within the crusts as well as higher concentrations of lead and other pollutants (arsenic, antimony, bismuth, tin, etc.), which generally can be linked to traffic and industry. The formation of weathering crusts in an industrial environment is clearly distinguishable from that in rural areas. Scanning electron microscopy observations confirm that the total amount of pollution is less at the Altenberg cathedral than at the Cologne and Xanten cathedrals. XRF analyses show that the formation of gypsum occurs in lower amounts at Altenberg. This correlates well with the measured SO₂ content and the intensity of the decay at the different locations. Furthermore, the different types of crusts, e.g., framboidal and laminar, can be differentiated and assigned to the different locations. The black weathering crusts on the silicate Drachenfels trachyte contribute to the degradation of the historic building material. They enhance mechanical moisture-related deterioration processes and the decay by chemical corrosion of rock-forming minerals. Although SO₂ concentrations in air have shown a strong decrease over the past 30 years, degradation in connection with weathering crusts is still observed. This indicates that not only contemporary or recent emissions, but also past pollutant concentrations have to be considered.     

Patterns of halite (NaCl) crystallisation in building stone conditioned by laboratory heating regimes

The crystallisation of soluble salts within the pores of the stone is widely recognised as a major mechanism causing the deterioration of the stone-built architectural heritage. Temperature, in turn, is one of the main controls on this process, including salt precipitation, the pressure of crystallisation and the thermal expansion of salts. Most laboratory experiments on decay generated by salts are just carried out with convective heating regimes, while in natural environments building stones can undergo radiative and convective heating regimes. The thermal response of stone to these different heating regimes is noticeably different and might influence the crystallisation patterns of a salt within a stone. The aim of this work is to raise awareness on the different patterns of crystallisation of NaCl within a porous stone tested with different heating regimes (convection and radiation) and the implications that this could have on the design of experimental modelling of natural weathering conditions in laboratory simulations. Results show that heating regime affects the sodium chloride distribution within a stone with high percentage of microporosity. In this case, radiation heating facilitates the generation of subefflorescences, while convection heating promotes efflorescences. This has a clear implication both on the stone decay in natural environments and on the methodologies for testing salt decay, as subefflorescences are more destructive than efflorescences. In this sense, the use of convective heating in laboratory experimentation might underestimate the potential damage that sodium chloride may generate. This counsels the use of radiation heating test methods in addition to convection for the laboratory study of salt crystallisation.     

Damage evaluation and conservation treatment of the tenth century Korean rock-carved Buddha statues

This study focuses on the lithological characterization, quantitative deterioration assessment and conservation treatment of the tenth century rock-carved Buddha statues in Korea. The Buddha statues were carved on light gray macrocrystalline biotite granite, and features microcline phenocrysts and pegmatite patches. The rock-forming minerals are quartz, plagioclase, microcline and biotite showing micrographic and porphyritic textures. Feldspars and biotite in the host rock have been partly altered into sericite and chlorite by weathering. The surface of the Buddha statues is remarkably irregular due to granular disintegration and the differential weathering of the quartz and microcline. In addition, horizontal and vertical cracks in the host rock have promoted mechanical weathering. Biological colonization of the statues was serious, and featured dark gray and yellowish green lichen, dark green bryophyte and some plants. The Buddha statues were evaluated as the highly weathered rock by ultrasonic survey. The lower parts under the influence of water and areas with severely broken surfaces showed a much more advanced weathering grade. Therefore, scientific conservation treatment was carried out for the long-term maintenance and conservation of the Buddha statues. After pretests, dry, wet and chemical cleanings were applied to the statues. Next, joining of cracked parts and surface consolidating were executed. Finally, the site environment was improved by installation of drain and trimming trees around the statues to ensure more stable long-term conservation.     

Evaluation of the physico-mechanical parameters affecting the deterioration rate of Ahlat ignimbrites (Bitlis,Turkey)

The paper principally focuses on the durability assessment of various stratigraphic levels of Ahlat ignimbrites collected from the eastern region of Turkey. A total of four different ignimbrite types with dissimilar color, texture and particularly welding degree were tested in laboratory. The laboratory tests performed on the ignimbrite specimens indicate that the welding degree as well as the lithic material content mainly controls the strength and capillarity properties of the ignimbrites. In addition, the durability of highly porous ignimbrites strongly depends upon the degree of welding. The effect of several weathering agents on the ignimbrites was evaluated on the basis of decay constant parameter. Accordingly, salt and ice crystallization pressures are a couple of major destructive agents acting within the micropores of the ignimbrites. Conversely, the investigated specimens are relatively durable against cyclic wetting–drying. Statistical evaluations reveal that the pore diameter is the major controlling factor on the deterioration rate of the ignimbrites after specifically recurrent freeze–thaw cycles. Moreover, the dry unit weight of the ignimbrites is more significant than the uniaxial compressive strength considering the deterioration rates during wetting–drying and salt crystallization. A less significant relationship was obtained between pore diameter and salt crystallization decay constant.     

中国不同气候带盐风化作用的地貌特征

盐风化作用是地球表面普遍存在的一种物理风化作用,由于盐类的周期性结晶作用而造成地表岩石和建筑材料的破坏,形成诸如风化穴或蜂窝石构造等地貌景观。盐风化作用也是差异风化的主要表现形式之一。然而,到目前为止盐风化作用在中国地学界仍然被严重忽视,以至于盐风化作用造成的地貌景观常常被地学研究者和科普人士误读为海浪冲蚀、流水侵蚀、风蚀作用等。经过近十年的野外观察与探讨,笔者等对盐风化的形成机理和表现形式有了深入的理解。本文以中国境内东部海岸带、华北半干旱区、西北干旱区和东南湿热气候带基岩露头为例,系统地分析了盐风化作用的机理及其在不同气候带的表现形式。盐风化的必要条件是:适当的可溶性盐类(如Na_2SO_4、NaCl等)供应、周期性的干湿交替和温度变化。盐风化作用主要在发育可渗性孔隙的砂砾岩类和富含微裂隙的花岗岩类之露头表面表现明显,可以形成特征显著的盐风化穴。盐风化作用形成的地貌景观在东部海岸带和西北干旱区表现尤为明显,常常形成蜂窝石构造和大型风化穴,与风蚀作用的痕迹明显有别;而在华北半干旱区和南方湿热气候带虽然受到降雨等其他因素的影响而常常遭受改造、叠加甚或清除,但在某些露头区仍然保留有重要的识别标志,形成大型风化穴以及小型蜂窝石构造。笔者等强调:地表各种地貌景观形成过程中都有盐风化作用的贡献,而建筑物和景观保护也必须考虑到盐风化作用的影响。建议地学同仁重视盐风化作用的普遍性和重要性,在相关教材中补充更新盐风化的概念,并以科普的方式通过多种媒体纠正过去的错误认识。     

Distribution of rare earth elements and oxyhydroxide phases within a weathered felsic igneous profile in Hong Kong

Variation in type and abundance of oxyhydroxide phases and the fractionation of rare earth elements (REE) within a weathered felsic pyroclastic profile in Hong Kong were investigated in order to assess the key factors and micro-environmental conditions controlling their distribution during weathering at rock and mineral scales. Three types of oxyhydroxide phases (Fe-, Mn- and Ce-rich) were recognized based on their most abundant elements. The Fe-rich types appeared in all samples at different stages of weathering. The Mn-rich types, however, were limited to samples at advanced stages of weathering, while the Ce-rich types were found solely in samples from oxyhydroxide-rich zones. Fractionation of REE likely started at the early stages of weathering, but became evident only at the advanced stages. Characteristics of REE patterns, especially the direction (negative or positive) and extent of Ce-anomalies, correlated very well with the appearance and abundance of Mn-oxyhydroxides, and were attributed to the co-existence of distinct micro-environmental conditions within the profile. These conditions resulted from two major competing processes, “leaching and fixation” and “oxidation (precipitation) and reduction (dissolution)”, which operated simultaneously within the profile at any given stage of weathering.     

Kaolinisation of granite in an urban environment

Under natural acid and wet conditions, one of the main weathering processes affecting granitic rocks is the kaolinisation of Na, Ca and K-feldspar minerals by H₂O + CO₂ attack. We here report the occurrence of authigenic kaolinite on the surface of an eighteenth century granitic monument covered with sulphate-rich weathering patinas. We suggest that, in humid mesothermic climates, anthropogenically derived gaseous SO₂ from air pollution is responsible for accelerated kaolinisation of feldspars in an urban environment; SO₂ from air pollution thus plays a dual role in the weathering of silicate rocks being responsible for the well known sulphation of Ca-bearing materials leading to the formation of sulphate salts such as gypsum as well as the weathering of feldspar minerals to kaolinite. The kaolinisation reaction weakens the granite microfabric leading to enhanced decay of the building stone.     

海岸壶穴与风化坑的成因差异及证据——以广东沙扒镇和庙湾岛海岸为例

风化坑和壶穴都是常见的岩石坑穴地形,其形成原因和形态完全不同,但容易被误解和混淆。广东西部沙扒镇海岸和珠江口外庙湾岛海岸同时发育了风化坑和海岸壶穴。近年来我们对两处地区进行了考察,从地貌学、沉积学、岩石化学和矿物学等各个方面对风化坑和海岸壶穴的形成过程和影响因素进行了论证,并将两者进行了对比。研究表明在不同的岩石(火成岩和变质岩)上均能形成风化坑或海岸壶穴,各种证据显示风化坑形成于岩石表面积水的风化作用,海岸壶穴则是由海浪的侵蚀作用而形成:1)风化坑可以形成于高潮线以上的各个高度平坦的岩石面上,平均高潮线以下不能形成风化坑,而海岸壶穴可形成于平均高潮面之下,其形成高度不超出高潮面; 2)海岸壶穴与风化坑在形态上有明显区别,如不同的侧壁形态、底部、口径边缘及宽深比等; 3)海岸壶穴中堆积物颗粒磨圆度和分选度好于风化坑中颗粒; 4)风化坑内堆积物的化学蚀变指数CIA值均高于附近地面岩石的CIA值,而风化坑内堆积物中矿物的英长比高于地表岩石的英长比,但CIA值和英长比均不能区分海岸壶穴和风化坑中的堆积物; 5)风化坑内颗粒含大量的粘土矿物,而海岸壶穴中无粘土矿物; 6)风化坑和海岸壶穴的堆积物具有显著不同的化学元素迁移特征。     

Preliminary observations on the impact of complex stress histories on sandstone response to salt weathering: laboratory simulations of process combinations

Historic sandstone structures carry an inheritance, or a ‘memory’, of past stresses that the stone has undergone since its placement in a façade. This inheritance, which conditions present day performance, may be made up of long-term exposure to a combination of low magnitude background environmental factors (for example, salt weathering, temperature and moisture cycling) and, superimposed upon these, less frequent but potentially high magnitude events or ‘exceptional’ factors (for example, lime rendering, severe frost events, fire). The impact of complex histories on the decay pathways of historic sandstone is not clearly understood, but this paper seeks to improve that understanding through the use of a laboratory ‘process combination’ study. Blocks of quartz sandstone (Peakmoor, from NW England) were divided into subsets that experienced different histories (lime rendering and removal, fire and freeze–thaw cycles in isolation and combination) that reflected the event timeline of a real medieval sandstone monument in NE Ireland, Bonamargy Friary (McCabe et al. 2006b). These subsets were then subject to salt weathering cycles using a 10% salt solution of NaCl and MgSO₄ that represents the ‘every-day’ stress environment of, for example, sandstone structures in coastal, or polluted urban, location. Block response to salt weathering was monitored by collecting, drying and weighing the debris that was released as blocks were immersed in the salt solution at the beginning of each cycle. The results illustrate the complexity of the stone decay system, showing that seemingly small variations in stress history can produce divergent response to salt weathering cycles. Applied to real-world historic sandstone structures, this concept may help to explain the spatial and temporal variability of sandstone response to background environmental factors on a single façade, and encourage conservators to include the role of stress inheritance when selecting and implementing conservation strategies.     

Lichenic growth as a factor in the physical deterioration or protection of Cappadocian monuments

Cappadocian monuments that are within tuff have serious deterioration problems. Understanding the effects of lichens on the tuffs is of utmost importance. In this paper, interactions between lichen-coated surfaces of Cappadocian monuments and the rock substratum have been studied. Thin sections and freshly cut sections were investigated by light and scanning electron microscopy. The presence of clay minerals and zeolites as weathering by-products produced by alteration and hydrolysis of glass and feldspars was assessed by XRD patterns. Furthermore, some physical properties of the tuff coated by and free of lichens were measured and compared. Analysis of mineralogical data has shown that the main weathering mechanism can be identified as hydrolysis of some of the rock-forming minerals. The lichenic coat reduces the presence of water inside the rock, thus protecting the rock material from physical decay and disintegration.     

微生物对硅酸盐矿物风化作用研究进展

微生物对硅酸盐矿物风化的影响研究取得了一系列重要进展。在贫营养环境中,微生物风化硅酸盐矿物获取营养物质,加速了硅酸盐矿物的风化;由于微生物的作用,矿物的风化会不遵循正常的矿物化学风化序列,表现出稳定矿物比不稳定矿物更易风化的特征。微生物风化硅酸盐矿物时会在硅酸盐矿物表面留下痕迹,即富集或转移相应的元素和矿物,而且还能改变硅酸盐矿物的化学组成和结构。微生物的上述行为受营养基质含量、有机酸、生物膜、胞外聚合物以及氧化还原作用的影响。     

Application of a fluorescent technique to the study of the weathering process

Processes and rates of weathering in representative tuff obtained from a Green Tuff region were directly examined using a new fluorescent approach. This approach was developed to visualize microscopically the microcracks and micropores that contribute to deterioration. The following observations were made. Progression of tuff weathering is caused by a delicate balance between chemical alteration and physical disintegration. Weathering occurs in many hidden microcracks and micropores not detected under natural light, but which can be clearly visualized under ultraviolet light. Water pathways, such as microcracks and cavities, accelerated the chemical alteration by increasing the effective surface area of rocks in contact with water. As the reaction proceeds, the constituent materials loosen and alteration products become widespread in the matrix. Secondary amorphous to poorly crystallized materials, such as iron hydroxide and aluminosilicate, precipitate on the fracture surfaces, slowing the progress of weathering. At the ultimate stage of weathering in tuff, all cracks and most of the micropores are filled with secondary materials. These observations on a microscopic scale during tuff weathering agree with the assessment of weathering obtained by measuring porosity, P-wave velocity and tensile strength.     

Influence of surface heterogeneities of building granite on its thermal response and its potential for the generation of thermoclasty

Surface properties, especially albedo, and aspect are widely accepted as strong influences on the surface thermal response of building stone to insolation. However, the influence that adjacent areas of stone with very different surface properties may have on the thermal response of a patch of stonework, and the ways in which spatial variation in thermal characteristics might enhance stone decay has received relatively little attention. This paper examines the differential thermal response of granite used in construction that results from the presence of dark coloured micro-granular enclaves within a leucocratic host. Surface temperatures and temperature differences between enclaves exhibiting mico-spalling, enclaves with no spalling and the surrounding stone were measured for different aspects and seasons on a 20th century building in Madrid. These data were used to calculate a number of “indices” related to short-term temperature cycling and temperature gradients that have the theoretical capability of generating irreversible deformation of the stone. These indices suggest that micro-spalling of enclaves, compared to a lack of similar decay on the host-rock, is related to their differential thermal response to insolation, most importantly the lower albedo and thermal conductivity values of the enclaves. However, these factors are not sufficient on their own to trigger spalling, and breakdown was only observed where enclaves also experienced repeated, short-term surface temperature cycling caused by, for example, temporary shading by adjacent vegetation. These rapid temperature reversals are identified as a key contributory factor to the thermally driven decay observed on some of the enclaves.