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Liesegang rings in differential deterioration patterns of lime mortars
G Model
ARTICLE IN PRESS
CULHER-3043; No. of Pages 4
Journal of Cultural Heritage xxx (2016) xxx–xxx
Available online at
ScienceDirect www.sciencedirect.com
Original article
Liesegang rings in differential deterioration patterns of lime mortars José Delgado Rodrigues ∗ Laboratorio Nacional de Engenharia Civil (LNEC), Avenida do Brasil 101, 1700-066 Lisbon, Portugal
a r t i c l e
i n f o
Article history: Received 13 January 2016 Accepted 8 March 2016 Available online xxx Keywords: Differential erosion Liesegang rings Rhythmic precipitation Mortar carbonation Flos tectorii
a b s t r a c t Differential erosion is a deterioration pattern common in stone and mortars exposed to environmental agents. The differential morphology is usually determined by specific intrinsic characteristics. The case analysed here corresponds to a peculiar situation of rhythmic precipitation inside lime mortars following a physical process designated as Liesegang phenomenon. The rhythmic reaction occurs between calcium hydroxide and carbon dioxide in the early steps of the carbonation process, and when appropriate boundary conditions are met, a more or less “perfect” sequence of higher and lower concentration of the carbonated lime (calcite) is formed. This sequence is called a series of Liesegang rings. The rings have distinct hardness and, when exposed to erosion agents, a differential pattern may be formed. In certain regions, this differential erosion pattern was given the name of Flos tectorii. © 2016 Elsevier Masson SAS. All rights reserved.
1. Introduction A recent paper [1], published in the Journal of Cultural Heritage, reports on an extensive study carried out upon several examples of an intriguing deterioration pattern frequently found in renders, especially those made of lime-based mortars. This decay pattern is a typical erosion feature that could well be simply called a differential erosion pattern, following the ICOMOS Glossary [2] recommendations. Even so, this deterioration pattern was given a different designation at a regional level, which, to our best knowledge, got a scarce diffusion outside Italy. There, the pattern is called Flos tectorii, a term attributed by Brancato [3] to designate a kind of flower-like deterioration feature common in decayed lime mortars. Terms with preferential regional acceptance are not uncommon and it could continue that way, but this publication is undoubtedly forcing a wider expansion of this alternative designation, and this has raised my interest to know more about the term and the background that led to its creation. In an immediate reaction, I could perceive that the supporting assumptions to justify the occurrence of this specific decay pattern are very weak and that some essential concepts were not considered, two facts that prompted me to contribute to better interpret the deterioration features and to explain the underlying decay processes. The cited paper [1] reports the study of samples extracted from ridges and troughs of the deteriorated areas, in which soluble salts
∗ Tel.: +351218443351. E-mail address: [email protected]
and microorganisms were identified. The authors concluded that the proximity of the sea and presence of soluble salts are responsible for the observed pattern and considered that microorganisms may have also a relevant role. It is a well-known fact that salts are active decay agents, and their role in this study is undeniable, as the results demonstrate. The question that remains unanswered is: can soluble salts (eventually with synergistic effects with microorganisms), and the proximity to the sea alone explain all the morphological features involved in the Flos tectorii deterioration pattern? My answer is “no”. Salts are widely present and involved in many deterioration patterns, from sand disintegration to powdering and scaling. Flos tectorii can be seen both near the sea and far away from it. To explain Flos tectorii, we need more than these unspecific factors. Using the usual terminology for this deterioration pattern–differential erosion–we can easily conclude that salts, with or without microorganisms, explain “erosion” well, but they are not enough to justify the “differential” component of the term. A contribution to explain the specific morphological aspects described as differential is presented in the following sections.
2. Interpreting the observational data Fig. 1 shows a couple of situations displaying the differential erosion pattern also designated as Flos tectorii. They exhibit the typical series of successive concentric rings of alternating ridges and troughs. One comes from a Roman archaeological site near the sea and the other from a place inland, at least 12 km away from the nearest sea line.
http://dx.doi.org/10.1016/j.culher.2016.03.002 1296-2074/© 2016 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4 2
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
Fig. 1. Differential erosion (Flos tectorii) in renders from the archaeological site of Troia (Portugal), near the sea (top), and in a pargetted merlon-shaped moulding from Torres Vedras, 12 km away from the sea.
Fig. 2 illustrates a slightly different pattern, also included in the Flos tectorii concept, taken in Belém, Pará, Brazil, some 100 km away from the sea. One main feature to highlight in Fig. 2 is a continuous and more resistant network of ridges that form a mosaic of cells where independent sets of roughly concentric rings were formed. Although, in a first analysis, this network could be taken as one more ring of the concentric sequence of rings, these ridges have a different origin and precede the formation of the rings and therefore need to be analysed separately. This network is highlighted in the bottom picture in Fig. 2 to better visualise its spatial distribution, and one can immediately perceive that it reproduces a typical network of shrinkage cracks so frequently seen in renders. Recognizing the direct connection of this network of harder ridges with a primary crack system is essential to understand the whole process, since it is this network that imposes the boundary conditions to the subsequent formation of the ring pattern. Fig. 3 adds some additional situations that put in evidence this direct connection between the shrinkage crack network and the indurated ridges. Fig. 3 (top) shows the much-damaged condition of a similar pattern in a white finishing layer over a darker render. The most resistant parts coincide with the main network of indurated ridges and their connection with the shrinkage cracks is clear
Fig. 2. Differential erosion patterns with the primary network of harder ridges highlighted.
once compared to the corresponding network still preserved in the underlying mortar. Ghosts of the concentric rings can still be perceived in some areas identified with stars. When looking carefully, we can identify the presence of a tiny crack roughly at the middle of all these major indurated ridges. These cracks are identified with arrows in both pictures in Fig. 3. Interestingly, and convincingly, no similar occurrence of hair cracks can be found along the indurated ridges that constitute the concentric rings inside the cells defined by the primary ridge system. This same sequence of primary ridges with perfect middle hair cracks running along them can be seen in Fig. 1B, of the above-cited paper [1]. These observations are sufficient to support an interpretation of the genesis of this primary network of indurated ridges. As is frequently seen in practice, immediately following the application of the mortar, a network of shrinkage cracks is formed. These cracks open the access of CO2 from the air to the interior of the mortar layer, facilitating carbonation. With a faster and more efficient carbonation, the mortar results more resistant along each side of the crack, leading to a subsequent formation of a protruding ridge once the erosive factors preferentially destroy the adjacent weaker zones. The presence of the middle hair crack is evidence of the initial crack and is a strong argument to support this interpretation. In fact, knowing
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
Fig. 3. Differential erosion patterns in mortars. The relicts’ aspect on the top shows a direct connection of the most resistant remains with the underlying crack network. The original crack can still be identified in the middle of the ridges that form the primary network of shrinkage cracks, as seen on the bottom picture, a detail of Fig. 2.
that cracks are always weakness zones, their ubiquitous presence in the middle of the more resistant ridges only would have survived if the neighbouring mortar was really of a very good quality. The major crack system is also clearly responsible for the formation of the series of concentric rings, given the perfect parallelism between the crack distribution and the geometry of the concentric rings. Cracks were a major source of CO2, and therefore, they imposed the boundary conditions for the carbonation process. So far, we believe to have established the genetic process of the overall primary spatial distribution and of the differential pattern created by the main network of indurated ridges, but the genesis of the concentric rings still needs to be ascertained. To do it, some additional considerations are necessary, which will be addressed in the next sections. 3. The Liesegang rings as the most plausible explanatory process Rhythmic or periodic precipitation is a very common process in nature that gives rise to very specific spatial patterns. When the end result of the precipitation is a coloured product, the rhythms become evident and may result in very spectacular visual effects. The phenomenon has been known for more than 150 years and was firstly studied in 1896 by R.E Liesegang [4]. Later on, this was called “Liesegang phenomenon”, and the resulting patterns are also called “Liesegang rings”.
3
Fig. 4. Examples of natural occurrences of Liesegang rings. An agate slice (top) and a natural outcrop in Petra, Jordan (bottom).
Fig. 4 shows two typical examples of Liesegang rings found in natural objects, an agate slice and a natural outcrop of red sandstones. The primary reasons behind this phenomenon are still a matter of debate, but the essential conditions that permit its occurrence are well known and widely accepted: the system includes a support medium, such as a gel phase, and two reactants, one being dispersed within the medium and the other supplied under steady state conditions and the result is a sparingly soluble salt. The physical system is controlled by a diffusion-driven process in the absence of any convective transport. Liesegang rings can be easily prepared in vitro and many recipes to produce them can be found in the literature. For additional information on the theory and rules governing the Liesegang phenomenon, see for instance [5,6]. In the case of lime mortars, the binder is the reaction medium, calcium hydroxide is the dispersed reactant, and carbon dioxide from the air is the second reactant. Liesegang rings were experimentally produced with mortars [7] and the implications in terms of mortar characteristics were discussed [8], highlighting the role of the lime putty aging and the consequent dimensions of the portlandite crystals in the development of Liesegang patterns. The authors observed that Liesegang rings were produced during carbonation of mortars made with lime putty aged for more than 4 years, while lime putty under that age was not able to develop similar patterns. In the Liesegang patterns, the rings correspond to the zones with a higher concentration of the precipitate, while zones between rings are substantially depleted of it. In the case of mortars, the rings correspond to zones with higher concentrations of the carbonated
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
4
binder (higher calcite contents) and therefore showing higher resistance to erosion. The ridges and troughs found in the differential pattern (the so-called Flos tectorii) are the direct consequence of the alternation of more richly and more poorly cemented zones. 4. Discussion and conclusions The visual patterns designated as Liesegang rings are so typical and distinctive that their presence is enough to demonstrate that a periodic or rhythmic precipitation has occurred. This physical phenomenon is in the base of the situations discussed in this paper, namely a specific differential erosion pattern regionally designated as Flos tectorii [1]. As we tried to show above, this pattern has the characteristic concentric rings, which we sustain must be called “Liesegang rings”, but they may include other features that are not necessarily to be included under the same concept. The first order ridges are to be ascribed to the development of a network of shrinkage cracks and the concomitant faster carbonation of their walls, a process that immediately precedes the inset of the Liesegang phenomenon. In the example used, the crack system defines the cells where proper conditions were met to produce the Liesegang rings. From its specific origin, ridges correspond to precipitation rings, meaning higher precipitate (calcite) concentrations, and therefore, a higher cohesion. In this situation, ridges resist weathering better and it is thus normal to have them better preserved than the troughs, where the precipitate is scarcer and the cohesion is lower. The almost perfect parallelism of the indurated rings with the major cracks demonstrates that the cells defined by the crack network act as individual reaction systems, with the cracks defining one of the basic boundary conditions for each system. Another boundary condition is needed to explain why the Liesegang rings are roughly parallel to the primary crack network and not parallel to the renders surface. To be effective, this condition requires that CO2 has a much easier and faster access from the cracks than from the surface, condition that can be foreseeable to occur when a special treatment of the surface is applied, such as a hard finishing detailing, a paint or any other treatment that might significantly reduce the air permeability. Looking carefully to surfaces showing this pattern, a very smooth layer is frequently identified, as for instance in the relicts shown in Fig. 3 (top) and in Fig. 1 (bottom). The other pictures show surfaces where such a finishing layer can be perceived, but the heavy erosion turns it less conspicuous and more difficult to see. In the above-cited paper [1], a similar situation is described for the studied areas, where an underlying layer of a coarser mortar and lower binder/aggregate ratio is covered with a finer and richer rendering mortar, thus constituting a clearly less permeable surface boundary. In spite of being relatively easy to find examples of this deterioration pattern in lime renders worldwide, the situations tend to be small in size (at least to our best knowledge) suggesting that perfect situations to develop this mechanism tend to occur in restricted areas only. On the other hand, these situations correspond to the
best-preserved areas of the exposed renders, suggesting that they may result from a specific and fortunate coincidence of a good quality mortar and a well-finished surface. In the absence of a surface less pervious layer, the carbonation front would progress inwards parallel to the surface. When proper conditions are met, Liesegang rings may be formed as well, creating a sequence of more and less cohesive fine layers. The aspect expected to result as a deterioration pattern would be a fine delamination of the mortar layer. This deterioration pattern exists, but the aspect is much less striking and the usually small thickness of the mortar layers make it difficult to document. Salts are widespread deterioration agents of porous building materials and they are frequently responsible for severely deteriorated conditions. Salts are commonly identified in efflorescences when crystallisation occurs at the surface of the objects, and are usually associated with powdering, scaling, honeycomb weathering and other deterioration patterns. Their role as a major deterioration factor is perfectly established and therefore it is highly likely to have them as a major agent in the formation of this differential erosion pattern regionally called Flos tectorii. The type of salts present, the possible role of microorganisms, the effect of winds or other external agents can help to explain the decay rates, including eventual differences from place to place, but alone they are not enough to explain the occurrence of the concentric distribution of ridges and troughs in some specific situations like those illustrated in this paper. The effect of salts and other agents is superimposed on a differential spatial distribution of some specific intrinsic factors whose geometry is defined by a precise physical process called “Liesegang phenomenon”. Salts may eventually be more abundant in troughs than in ridges, but this fact has to be interpreted as a consequence, rather than a cause, of the specific geometrical distribution in concentric rings. Salts attack preferentially the weaker areas of materials, thus enhancing the differential distribution of internal characteristics. In conclusion, they are responsible for “enhancing” the differential appearance, but they are not responsible for the specific geometry of concentric rings. References [1] L. Randazzo, G. Montana, R. Alduina, P. Quatrini, E. Tsantini, B. Salemi, Flos tectorii degradation of mortars: an example of synergistic action between soluble salts and biodeteriogens, J. Cult. Herit. 16 (2015) 838–847. [2] ICOMOS-ISCS: Illustrated glossary on stone deterioration patterns. Monuments and Sites XV, ICOMOS International Scientific Committee for Stone (ISCS). Sept, 2008. [3] F.S. Brancato, Patologia di un degrado, Flos tectorii, Recuperare 22 (1986) (Marzo–Aprile, Milano, Italy, cited in [1]). [4] Liesegang rings (geology), 2016 (Wikipedia. Accessed January 10, 2016) https://en.wikipedia.org/wiki/Liesegang rings (geology). [5] F. Izsáka, I. Lagzib, A new universal law for the Liesegang pattern formation, J. Chem. Phys. 122 (2005) 184707. [6] P. Hantz, Pattern formation in a new class of precipitation reactions, University of Geneva, 2006 (PhD thesis, No. 3747). [7] O. Cazalla, C. Rodriguez-Navarro, E. Sebastian, G. Cultrone, M.J. de la Torre, Aging of lime putty: effects on traditional lime mortar carbonation, J. Am. Ceram. Soc. 83 (2000) 1070–1076. [8] C. Rodriguez-Navarro, O. Cazalla, K. Elert, E. Sebastian, Liesegang pattern development in carbonating traditional lime mortars, Proc. R. Soc. Lond. A 458 (2002) 2261–2273.
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
ARTICLE IN PRESS
CULHER-3043; No. of Pages 4
Journal of Cultural Heritage xxx (2016) xxx–xxx
Available online at
ScienceDirect www.sciencedirect.com
Original article
Liesegang rings in differential deterioration patterns of lime mortars José Delgado Rodrigues ∗ Laboratorio Nacional de Engenharia Civil (LNEC), Avenida do Brasil 101, 1700-066 Lisbon, Portugal
a r t i c l e
i n f o
Article history: Received 13 January 2016 Accepted 8 March 2016 Available online xxx Keywords: Differential erosion Liesegang rings Rhythmic precipitation Mortar carbonation Flos tectorii
a b s t r a c t Differential erosion is a deterioration pattern common in stone and mortars exposed to environmental agents. The differential morphology is usually determined by specific intrinsic characteristics. The case analysed here corresponds to a peculiar situation of rhythmic precipitation inside lime mortars following a physical process designated as Liesegang phenomenon. The rhythmic reaction occurs between calcium hydroxide and carbon dioxide in the early steps of the carbonation process, and when appropriate boundary conditions are met, a more or less “perfect” sequence of higher and lower concentration of the carbonated lime (calcite) is formed. This sequence is called a series of Liesegang rings. The rings have distinct hardness and, when exposed to erosion agents, a differential pattern may be formed. In certain regions, this differential erosion pattern was given the name of Flos tectorii. © 2016 Elsevier Masson SAS. All rights reserved.
1. Introduction A recent paper [1], published in the Journal of Cultural Heritage, reports on an extensive study carried out upon several examples of an intriguing deterioration pattern frequently found in renders, especially those made of lime-based mortars. This decay pattern is a typical erosion feature that could well be simply called a differential erosion pattern, following the ICOMOS Glossary [2] recommendations. Even so, this deterioration pattern was given a different designation at a regional level, which, to our best knowledge, got a scarce diffusion outside Italy. There, the pattern is called Flos tectorii, a term attributed by Brancato [3] to designate a kind of flower-like deterioration feature common in decayed lime mortars. Terms with preferential regional acceptance are not uncommon and it could continue that way, but this publication is undoubtedly forcing a wider expansion of this alternative designation, and this has raised my interest to know more about the term and the background that led to its creation. In an immediate reaction, I could perceive that the supporting assumptions to justify the occurrence of this specific decay pattern are very weak and that some essential concepts were not considered, two facts that prompted me to contribute to better interpret the deterioration features and to explain the underlying decay processes. The cited paper [1] reports the study of samples extracted from ridges and troughs of the deteriorated areas, in which soluble salts
∗ Tel.: +351218443351. E-mail address: [email protected]
and microorganisms were identified. The authors concluded that the proximity of the sea and presence of soluble salts are responsible for the observed pattern and considered that microorganisms may have also a relevant role. It is a well-known fact that salts are active decay agents, and their role in this study is undeniable, as the results demonstrate. The question that remains unanswered is: can soluble salts (eventually with synergistic effects with microorganisms), and the proximity to the sea alone explain all the morphological features involved in the Flos tectorii deterioration pattern? My answer is “no”. Salts are widely present and involved in many deterioration patterns, from sand disintegration to powdering and scaling. Flos tectorii can be seen both near the sea and far away from it. To explain Flos tectorii, we need more than these unspecific factors. Using the usual terminology for this deterioration pattern–differential erosion–we can easily conclude that salts, with or without microorganisms, explain “erosion” well, but they are not enough to justify the “differential” component of the term. A contribution to explain the specific morphological aspects described as differential is presented in the following sections.
2. Interpreting the observational data Fig. 1 shows a couple of situations displaying the differential erosion pattern also designated as Flos tectorii. They exhibit the typical series of successive concentric rings of alternating ridges and troughs. One comes from a Roman archaeological site near the sea and the other from a place inland, at least 12 km away from the nearest sea line.
http://dx.doi.org/10.1016/j.culher.2016.03.002 1296-2074/© 2016 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4 2
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
Fig. 1. Differential erosion (Flos tectorii) in renders from the archaeological site of Troia (Portugal), near the sea (top), and in a pargetted merlon-shaped moulding from Torres Vedras, 12 km away from the sea.
Fig. 2 illustrates a slightly different pattern, also included in the Flos tectorii concept, taken in Belém, Pará, Brazil, some 100 km away from the sea. One main feature to highlight in Fig. 2 is a continuous and more resistant network of ridges that form a mosaic of cells where independent sets of roughly concentric rings were formed. Although, in a first analysis, this network could be taken as one more ring of the concentric sequence of rings, these ridges have a different origin and precede the formation of the rings and therefore need to be analysed separately. This network is highlighted in the bottom picture in Fig. 2 to better visualise its spatial distribution, and one can immediately perceive that it reproduces a typical network of shrinkage cracks so frequently seen in renders. Recognizing the direct connection of this network of harder ridges with a primary crack system is essential to understand the whole process, since it is this network that imposes the boundary conditions to the subsequent formation of the ring pattern. Fig. 3 adds some additional situations that put in evidence this direct connection between the shrinkage crack network and the indurated ridges. Fig. 3 (top) shows the much-damaged condition of a similar pattern in a white finishing layer over a darker render. The most resistant parts coincide with the main network of indurated ridges and their connection with the shrinkage cracks is clear
Fig. 2. Differential erosion patterns with the primary network of harder ridges highlighted.
once compared to the corresponding network still preserved in the underlying mortar. Ghosts of the concentric rings can still be perceived in some areas identified with stars. When looking carefully, we can identify the presence of a tiny crack roughly at the middle of all these major indurated ridges. These cracks are identified with arrows in both pictures in Fig. 3. Interestingly, and convincingly, no similar occurrence of hair cracks can be found along the indurated ridges that constitute the concentric rings inside the cells defined by the primary ridge system. This same sequence of primary ridges with perfect middle hair cracks running along them can be seen in Fig. 1B, of the above-cited paper [1]. These observations are sufficient to support an interpretation of the genesis of this primary network of indurated ridges. As is frequently seen in practice, immediately following the application of the mortar, a network of shrinkage cracks is formed. These cracks open the access of CO2 from the air to the interior of the mortar layer, facilitating carbonation. With a faster and more efficient carbonation, the mortar results more resistant along each side of the crack, leading to a subsequent formation of a protruding ridge once the erosive factors preferentially destroy the adjacent weaker zones. The presence of the middle hair crack is evidence of the initial crack and is a strong argument to support this interpretation. In fact, knowing
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
Fig. 3. Differential erosion patterns in mortars. The relicts’ aspect on the top shows a direct connection of the most resistant remains with the underlying crack network. The original crack can still be identified in the middle of the ridges that form the primary network of shrinkage cracks, as seen on the bottom picture, a detail of Fig. 2.
that cracks are always weakness zones, their ubiquitous presence in the middle of the more resistant ridges only would have survived if the neighbouring mortar was really of a very good quality. The major crack system is also clearly responsible for the formation of the series of concentric rings, given the perfect parallelism between the crack distribution and the geometry of the concentric rings. Cracks were a major source of CO2, and therefore, they imposed the boundary conditions for the carbonation process. So far, we believe to have established the genetic process of the overall primary spatial distribution and of the differential pattern created by the main network of indurated ridges, but the genesis of the concentric rings still needs to be ascertained. To do it, some additional considerations are necessary, which will be addressed in the next sections. 3. The Liesegang rings as the most plausible explanatory process Rhythmic or periodic precipitation is a very common process in nature that gives rise to very specific spatial patterns. When the end result of the precipitation is a coloured product, the rhythms become evident and may result in very spectacular visual effects. The phenomenon has been known for more than 150 years and was firstly studied in 1896 by R.E Liesegang [4]. Later on, this was called “Liesegang phenomenon”, and the resulting patterns are also called “Liesegang rings”.
3
Fig. 4. Examples of natural occurrences of Liesegang rings. An agate slice (top) and a natural outcrop in Petra, Jordan (bottom).
Fig. 4 shows two typical examples of Liesegang rings found in natural objects, an agate slice and a natural outcrop of red sandstones. The primary reasons behind this phenomenon are still a matter of debate, but the essential conditions that permit its occurrence are well known and widely accepted: the system includes a support medium, such as a gel phase, and two reactants, one being dispersed within the medium and the other supplied under steady state conditions and the result is a sparingly soluble salt. The physical system is controlled by a diffusion-driven process in the absence of any convective transport. Liesegang rings can be easily prepared in vitro and many recipes to produce them can be found in the literature. For additional information on the theory and rules governing the Liesegang phenomenon, see for instance [5,6]. In the case of lime mortars, the binder is the reaction medium, calcium hydroxide is the dispersed reactant, and carbon dioxide from the air is the second reactant. Liesegang rings were experimentally produced with mortars [7] and the implications in terms of mortar characteristics were discussed [8], highlighting the role of the lime putty aging and the consequent dimensions of the portlandite crystals in the development of Liesegang patterns. The authors observed that Liesegang rings were produced during carbonation of mortars made with lime putty aged for more than 4 years, while lime putty under that age was not able to develop similar patterns. In the Liesegang patterns, the rings correspond to the zones with a higher concentration of the precipitate, while zones between rings are substantially depleted of it. In the case of mortars, the rings correspond to zones with higher concentrations of the carbonated
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002
G Model CULHER-3043; No. of Pages 4
ARTICLE IN PRESS J. Delgado Rodrigues / Journal of Cultural Heritage xxx (2016) xxx–xxx
4
binder (higher calcite contents) and therefore showing higher resistance to erosion. The ridges and troughs found in the differential pattern (the so-called Flos tectorii) are the direct consequence of the alternation of more richly and more poorly cemented zones. 4. Discussion and conclusions The visual patterns designated as Liesegang rings are so typical and distinctive that their presence is enough to demonstrate that a periodic or rhythmic precipitation has occurred. This physical phenomenon is in the base of the situations discussed in this paper, namely a specific differential erosion pattern regionally designated as Flos tectorii [1]. As we tried to show above, this pattern has the characteristic concentric rings, which we sustain must be called “Liesegang rings”, but they may include other features that are not necessarily to be included under the same concept. The first order ridges are to be ascribed to the development of a network of shrinkage cracks and the concomitant faster carbonation of their walls, a process that immediately precedes the inset of the Liesegang phenomenon. In the example used, the crack system defines the cells where proper conditions were met to produce the Liesegang rings. From its specific origin, ridges correspond to precipitation rings, meaning higher precipitate (calcite) concentrations, and therefore, a higher cohesion. In this situation, ridges resist weathering better and it is thus normal to have them better preserved than the troughs, where the precipitate is scarcer and the cohesion is lower. The almost perfect parallelism of the indurated rings with the major cracks demonstrates that the cells defined by the crack network act as individual reaction systems, with the cracks defining one of the basic boundary conditions for each system. Another boundary condition is needed to explain why the Liesegang rings are roughly parallel to the primary crack network and not parallel to the renders surface. To be effective, this condition requires that CO2 has a much easier and faster access from the cracks than from the surface, condition that can be foreseeable to occur when a special treatment of the surface is applied, such as a hard finishing detailing, a paint or any other treatment that might significantly reduce the air permeability. Looking carefully to surfaces showing this pattern, a very smooth layer is frequently identified, as for instance in the relicts shown in Fig. 3 (top) and in Fig. 1 (bottom). The other pictures show surfaces where such a finishing layer can be perceived, but the heavy erosion turns it less conspicuous and more difficult to see. In the above-cited paper [1], a similar situation is described for the studied areas, where an underlying layer of a coarser mortar and lower binder/aggregate ratio is covered with a finer and richer rendering mortar, thus constituting a clearly less permeable surface boundary. In spite of being relatively easy to find examples of this deterioration pattern in lime renders worldwide, the situations tend to be small in size (at least to our best knowledge) suggesting that perfect situations to develop this mechanism tend to occur in restricted areas only. On the other hand, these situations correspond to the
best-preserved areas of the exposed renders, suggesting that they may result from a specific and fortunate coincidence of a good quality mortar and a well-finished surface. In the absence of a surface less pervious layer, the carbonation front would progress inwards parallel to the surface. When proper conditions are met, Liesegang rings may be formed as well, creating a sequence of more and less cohesive fine layers. The aspect expected to result as a deterioration pattern would be a fine delamination of the mortar layer. This deterioration pattern exists, but the aspect is much less striking and the usually small thickness of the mortar layers make it difficult to document. Salts are widespread deterioration agents of porous building materials and they are frequently responsible for severely deteriorated conditions. Salts are commonly identified in efflorescences when crystallisation occurs at the surface of the objects, and are usually associated with powdering, scaling, honeycomb weathering and other deterioration patterns. Their role as a major deterioration factor is perfectly established and therefore it is highly likely to have them as a major agent in the formation of this differential erosion pattern regionally called Flos tectorii. The type of salts present, the possible role of microorganisms, the effect of winds or other external agents can help to explain the decay rates, including eventual differences from place to place, but alone they are not enough to explain the occurrence of the concentric distribution of ridges and troughs in some specific situations like those illustrated in this paper. The effect of salts and other agents is superimposed on a differential spatial distribution of some specific intrinsic factors whose geometry is defined by a precise physical process called “Liesegang phenomenon”. Salts may eventually be more abundant in troughs than in ridges, but this fact has to be interpreted as a consequence, rather than a cause, of the specific geometrical distribution in concentric rings. Salts attack preferentially the weaker areas of materials, thus enhancing the differential distribution of internal characteristics. In conclusion, they are responsible for “enhancing” the differential appearance, but they are not responsible for the specific geometry of concentric rings. References [1] L. Randazzo, G. Montana, R. Alduina, P. Quatrini, E. Tsantini, B. Salemi, Flos tectorii degradation of mortars: an example of synergistic action between soluble salts and biodeteriogens, J. Cult. Herit. 16 (2015) 838–847. [2] ICOMOS-ISCS: Illustrated glossary on stone deterioration patterns. Monuments and Sites XV, ICOMOS International Scientific Committee for Stone (ISCS). Sept, 2008. [3] F.S. Brancato, Patologia di un degrado, Flos tectorii, Recuperare 22 (1986) (Marzo–Aprile, Milano, Italy, cited in [1]). [4] Liesegang rings (geology), 2016 (Wikipedia. Accessed January 10, 2016) https://en.wikipedia.org/wiki/Liesegang rings (geology). [5] F. Izsáka, I. Lagzib, A new universal law for the Liesegang pattern formation, J. Chem. Phys. 122 (2005) 184707. [6] P. Hantz, Pattern formation in a new class of precipitation reactions, University of Geneva, 2006 (PhD thesis, No. 3747). [7] O. Cazalla, C. Rodriguez-Navarro, E. Sebastian, G. Cultrone, M.J. de la Torre, Aging of lime putty: effects on traditional lime mortar carbonation, J. Am. Ceram. Soc. 83 (2000) 1070–1076. [8] C. Rodriguez-Navarro, O. Cazalla, K. Elert, E. Sebastian, Liesegang pattern development in carbonating traditional lime mortars, Proc. R. Soc. Lond. A 458 (2002) 2261–2273.
Please cite this article in press as: J. Delgado Rodrigues, Liesegang rings in differential deterioration patterns of lime mortars, Journal of Cultural Heritage (2016), http://dx.doi.org/10.1016/j.culher.2016.03.002