Figure: 15: The different case studies within the weathering model after Peltier (1950).
Utilizing the classification of Peltier (1950), the different areas investigated in this study are located in regions with different intensities of weathering (Fig. 15). These range from strong chemical wea-thering (Angkor) to moderate chemical weawea-thering in the case of Göttingen, Guadalajara und Gua-nojuato. Moreover, in Göttingen the weathering is also associated with frost action. In the Petra re-gion as well as in Guanojuato weathering is only very slight according to the model of Peltier.
1. 4 Main weathering forms in sandstones and tuffstones
The dominant weathering forms in sandstones are delamination, sanding and flaking. Delamination occurs parallel to the bedding (Fig. 16 a, d and e) and sanding causes a uniform material loss paral-lel to the stone surface (Fig. 16 b). This is similar to flaking whereby some stone types lose even greater fragments, which is the case for the Elbe-Sandstone where the material loss is connected to the flaser-like bedding of the rock (Fig. 16 c). Alveolar weathering, which is a hole-creating phenomenon, often occurs in connection with a high salt contamination (Fig. 16 f and 18 a and b).
Some rock types, especially those that contain calcitic cement show a susceptibility to crust forma-tion, crack development and contour scaling (Fig. 10 a and16 g).
Tuffstones often show scale formation, scale-like deterioration, the weathering out of specific rock components and crack formation (Fig. 17 a, b and c). Even flaking of greater fragments is a com-men weathering form (Fig. 17 d).
Figure 16: Weathering phenomena in sandstones: a) delamination parallel to the surface (Zeitz, Germany). b) Sanding parallel to the surface (Fatehpur Sikri, India). c) Flaking (Wittenberg, Germany). d) Crust formation, cracking and lami-nation parallel to the bedding (Angkor, Cambodia). e) Lamilami-nation parallel to the bedding (Goettingen, Germany). f)
Alveolar weathering (Zeitz, Germany). g) Cracking and contour scaling (Angkor, Cambodia).
Some varieties also exhibit a back-weathering similar to the sanding of sandstones (Fig. 17 f). In rarer cases contour scaling and delamination develops (Fig. 17 c). During the process of alveolar weathering salt contamination occasionally occurs (Fig. 17 e).
1. 4. 1 From description to explanation
Weathering takes place due to frequent stress induced by environmental impacts like heat, moisture, cold, wind, natural, urban and industrial attacks and the interaction of internal compounds and structures.
Weathering forms in stone conservation are described and illustrated in different catalogues and glossaries and include all the different types of rocks (Fitzner, Heinrichs 2004; Vergès-Belmin 2008). The focus of these studies is usually related to the phenomenological aspects (descriptive work) and less to the processes that create the different weathering forms.
Only a few studies deal with the causes or explain the specific weathering forms found in different types of rocks (Snethlage, Wendler 1997; Durnan 2008; Muir 2008; Ruedrich 2003; Kemp 2008:
Graue 2013; Stueck 2013).
Wa n j a We d e k i n d We a t h e r i n g a n d C o n s e r v a t i o n o f M o n u m e n t s
Figure 17: Weathering phenomena in tuffstones. a) Structural cracking and scaling (Rome, Italy). b) Cracking and sca-ling parallel to the surface (Berlin, Germany). c) Cracking and crust formation (Guadalajara, Mexico). d) Structural flaking (Guadalajara, Mexico). e) Alveolar weathering (Guanojuato, Mexico). f ) Disintegration parallel to the surface
(Kassel, Germany).
The knowledge of specific weathering behaviors of different rock types, however, is the main point for generating a suitable conservation treatment. This thesis will focus on three different weathering forms: a) sanding and flaking, with emphasis on structural flaking or disintegration, b) alveolar weathering and c) contour scaling.
1. 4. 1. a) Sanding and flaking
Sanding and flaking only affect the outer 10-20 mm of a stone and is a deterioration form mostly observed in relation to salt weathering (Warke, Smith 2000; Muir 2008; Myrin 2006). Sanding off will occur when the maximum of salt is reached near the surface of the stone (Snethlage, Wendler 1997) as is shown in Figure 16 b. Flaking of sandstone as well as volcanic stone can be traced back to a combined salt contamination, if gypsum and low soluble salts can be found in the urban envi-ronment (Graue 2013). Flaking in tuff appears to be connected to selective dissolution, hygric cy-cling, lichen hyphae, and calcite precipitation in stone fractures under tropical environments (Doene et al., 2005).
In the case of rhyolitic lapilli tuffs structural flaking can reach a material depth of six centimeters.
Cracking and structural flaking is due to a different hydric expansion of the matrix and clast materi-al often combined with smateri-alt crystmateri-allization (this study and Jauregui et materi-al., 2012).
1. 4. 1. b) Alveolar weathering
Alveolar weathering describes a honeycomb-like weathering form that creates local back-weathe-ring with a hole diameter of 10 cm (Fig. 18 b - d). If the weatheback-weathe-ring hole is greater than 10 cm, a cavernous depression is formed which is called tafoni (Turkington 1998). Exemples can be found at the rocks in the Petra region (Fig. 18 a).
Figure 18: Tafoni and alveolar weathering. a) Tafoni weathering in the region of Petra (Jordan), b) alveolar weathering in Petra, c) alveolar weathering of tuff at the San Francisco Church in Zamora, Michoacan/Mexico and d) alveolar
wea-thering at the historical city wall of Avignon (France).
Various theories account for alveolar weathering: Some authors assume that chemical weathering takes place due to so-called core weathering (Abu-Safat 1988) or partly results from the reaction of water and organic acids with iron and silica (Johnson 1974). Various mechanisms are assumed to be responsible for the physical forms of weathering as well. Some authors assume that alveolar wea-thering is caused by abrasion due to wind (Quayle 1992). Others consider the pressure induced by swelling clay minerals in combination with the effects of salts to be the major causes (Pye,
Motters-Wa n j a We d e k i n d We a t h e r i n g a n d C o n s e r v a t i o n o f M o n u m e n t s
head 1995). However, the majority of authors are convinced that salt weathering due to salt crystal-lization is responsible for the development of tafoni especially in coastal and desert environments (Bradley et al., 1978; Matsukuta, Matsuoka 1991; McGreevy 1985; Smith 1978, Kirchner 1995, Wedekind, Ruedrich 2006). Recent research states that different drying processes in the affected zones cause tafoni weathering (Huinink et al., 2004). The largest amount of salt settles in zones in which the drying process is slow and therefore causes damage.
During the process of alveolar formation due to salt weathering, it is assumed that the damage-crea-ting salts concentrate in the back-weathering zones due to moisture transport and drying, thus gi-ving rise to the described damages. The pore space in the back-weathering zones has specific pore radii (micropores), which need to be critically assessed in regards to the explosive affect associated with salt crystallization. In rock zones that have a corresponding microporosity or a low average pore radii an explosive affect through salt crystallization and back-weathering preferentially occurs (Seidel 2004, Siedel 2010 a).
1. 4. 1. c) Contour Scaling
According to Fitzner et al., (1995), contour scaling describes the detachment of large, platy stone elements parallel to the stone surface, but not following the stone structure (Fig. 19). Contour sca-ling is often observed on buildings, especially built from clay-rich sandstones (Fig. 19 b). Scales show that a soluble salt enriched zone and a decrease of the flexural strength below the stone sur-face can be detected even if damage is not yet visible (Wendler et al., 1991). Some authors assume that contour scales will form when the crystallization of salt is situated in the interior of the stone (Snethlage, Wendler 1997).
Clay might be an explanation for the swelling, but not nessesarily the reason for the formation of crusts that are creating contour scaling. In many studies contour scaling can be observed in cases of a clay-rich arkose or greywacke-like sandstone (Sebastián et al. 2007, Leisen 2002). However, the interplay of a soluble mineral like calcite for crust formation was only taken into account by a few authors (Hosono et al., 2006). Many of the stone types that show contour scaling as the dominant weathering form contain a significant calcite content. This is the case for the sandstone of Puerta Perdón (Sebastián et al., 2007) 5-10 % CaO, for the Darney Stone, a Carbonifrous sandstone (Smith, Mc Greevy 1988), oolitic limestones (Smith et al., 2003), Brownstone (Wangler, Scherer 2008 a and b) and nearly all sandstones found in Angkor, where contour scaling is the main damage form.
Some authors take into account the changes in porosity induced by salt, mainly gypsum in the for-mation of crusts and contour scaling (Adamovic et al., 2011).
Figure 19: a) Scale formation due to gypsum accumulation at the monument of independence in Guadalajara, Mexico (tuff), and b) perpendicular to the bedding due to gypsum accumulation at a sandstone tomb in Göttingen, Germany. c) Contour scaling perpendicular to the bedding due to calcite accumulation at a tomb in Guadalajara, Mexico (tuff). d) Massive crust formation and contour scaling at a tomb in Lugano, Switzerland (clay-rich sandstone). e) Contour scaling
due to hydrophobization at the Templo Mayor in Mexico City, Mexico (tuff), and f) at the Charlottenburg Gate in Ber-lin, Germany (tuff) as well as g) at the Servatius Church in Duderstadt, Germany (sandstone).
According to their study, armored rock crusts on fine-grained clayey sandstone show a reduced vo-lume and size of macropores, since these are filled with clay mineral aggregates and gypsum crys-tals. In another study, near-surface temperature cycling on limestone was shown to lead to stress by expansion and the implication of scales (Smith et al., 2011), probably due to the high thermal ex-pansion coefficient of calcite.
Crust formation and contour scaling can also take place due to the input of material by conservation treatments. Crust formation could be observed due to hydrophobization and consolidation (see Fig.
19). Contour scaling often takes place combined with the accumulation of salt crystallization.
1. 4. 2 Weathering forms and case studies
Sanding and structural flaking are discussed in chapters 3.2, 3.4 and 3.5 for sandstones and tuffsto-nes corresponding to the Mexico City, Guadalajara and Goettingen case studies. Alveolar
weathe-Wa n j a We d e k i n d We a t h e r i n g a n d C o n s e r v a t i o n o f M o n u m e n t s
ring is discussed in the chapter outlining the sandstones of Petra and contour scaling in the chapters for sandstones and tuffs in Guadalajara and Angkor. The causes and processes that are responsible for their development and their consequences for conservation will be discussed for the Angkor case studies.