Note: There has been much public discussion recently around preservation of public monuments. This blog post is about how science can play an important role in preserving historical monuments. Although people, including scientists, must make judgments about historical preservation, nothing in this post is intended to express any opinion on recent public discussions about those judgments.
It has been said that, at its best, preservation engages the past in a conversation with the present over a mutual concern for the future.William J. Murtagh, Preservationist and First Keeper of the National Register of Historic Places
Have you ever stood in front of an old stone statue or building and had your mind whisked away to another time and place? These tangible examples of our cultural heritage teach us about the beliefs, knowledge, and traditions of the communities who created them. They also tell us about the communities who decided these artifacts were important enough to celebrate and preserve. Some of the most famous examples of built cultural heritage are composed of stone. As the timeline in below illustrates, humans have been creating stone cultural heritage throughout the entirety of recorded human history.
However, stone isn’t forever! Because it is always in contact with the environment, stone can deteriorate and become discolored over time.1,2 This decay is related to a decrease in the strength and toughness of the stone, and an increase in the number and size of pores in the stone, which causes the stone to soak up more water.3 There are several causes of stone decay. Acid rain, which can be caused by heavy industrial activity like the burning of fossil fuels, can dissolve stone materials like limestone or marble.4 This is because the minerals that make up these stones have a high solubility, which makes them prone to dissolving and changing color when they interact with acid rain.
Over time, this can result in sculptures losing their faces, as shown in the image above. Another cause of decay is the biological activity of living organisms like bacteria, fungi, and algae.5 These microorganisms may grow on the surface of the stone, or even inside cracks and pores to cause destruction from the inside. As these microorganisms grow and eat, they secrete enzymes and organic acids that are highly harmful to stone and other artwork.6,7 In addition, some microorganisms like fungi can excrete pigments related to photosynthesis, like green chlorophyll or black melanin.7 This leaves blotches of color all over stone monuments! Finally, water is the enemy of stone! Water trapped inside of the stone will freeze in colder weather and thaw in warmer weather. This freeze-thaw cycle causes cracks in the stone due to the expansion of water when ice is formed. Eventually, the stone can break apart completely. Similarly, water can carry dissolved salts into the stone structure, which later crystallize and expand, causing cracking and breakage.
To address these challenges, stone preservation techniques are used to both restore the structural integrity of the stone and prevent further decay. Preservationists must also consider the fundamental principles of restoration when they design a solution, including compatibility, effectiveness, durability, and the ability to repeat a treatment.8,9 This helps avoid the use of a treatment that doesn’t work, or one that could cause new problems. An exciting new development in preservation is the use of nanoparticles to protect and enhance the material properties of stone cultural heritage. The small size of nanomaterials is useful for filling the tiniest cracks of a damaged stone artifact, while their increased surface area per mass enhances the reactivity of nanomaterials used to combat pollution and microorganisms. There are three major categories of nanomaterial applications in stone cultural heritage preservation: consolidation materials, water repellants, and self-cleaning coatings.10
Nanomaterials used for consolidation can be thought of as “fillers,” like the packing peanuts that fill the empty space in a package sent in the mail. These nanomaterials fill empty pore spaces and mimic the original stone material. One common consolidation material is a product based on calcium hydroxide nanoparticles (Ca(OH)2 NPs).11,12 To restore damaged stone artifacts, we can spray the calcium hydroxide nanoparticles over the stone’s surface or apply them using a brush. The nanoparticles sink deep into the pores and cracks of the stone. The power of these nanoparticles is their ability to transform into calcium carbonate mineral phases such as calcite and aragonite, the major minerals of limestone, through a reaction with atmospheric carbon dioxide in the presence of moisture:
Ca(OH)2, aq + CO2, g → CaCO3, s + H2O
(Calcium hydroxide particles in solution plus carbon dioxide gas goes to calcium carbonate plus water)
In this way, Ca(OH)2 NPs are often used to restore stone artifacts and buildings made of limestone, because the properties and chemistry of the filler will match the original artifact. The Megalithic Temples of Malta, the Bust of Nefertiti, and St. Paul’s Cathedral are all examples of cultural heritage made from limestone. Ca(OH)2 NPs used for limestone consolidation are an excellent example of preservationists matching the filler material to the stone material to achieve better compatibility, effectiveness, and treatment durability.13,14
Nanomaterials used as water repellants prevent water from entering and damaging the stone structure. One common example is silicon dioxide nanoparticles (SiO2 NPs), which are mixed into siloxane polymers. This mixture is applied to the surface of the stone and forms a “superhydrophobic” coating, which allows water to bead up and roll off the stone instead of seeping inside. (See this post for more on superhydrophobicity.) In this way, these coatings repel water drops and leave the stone dry after rainfall. The SiO2 nanoparticles create a roughened surface, while the siloxane polymers further minimize contact with water, resulting in what is called a “Cassie-Baxter scenario.” Here, air is trapped between the droplet of water and the surface of the stone, preventing direct contact. This is illustrated in the figure below, and was also captured invideos by researchers Facio and Mosquera which are available to view free of charge as part of the Supporting Information of their article in ACS Applied Materials & Interfaces.15
Nanomaterials are also used in self-cleaning coatings. Here, we choose nanoparticles that are photocatalytically active, which means that they absorb light to power chemical reactions. For example, my labmate and co-author Aranzazu works with nanoparticles made of a mixture of zinc oxide and magnesium oxide (ZnMgO NPs). The ZnMgO nanoparticles can absorb ultraviolet (UV) light to produce highly reactive oxygen-containing molecules, similar to the molecules that form when you use hydrogen peroxide to clean a cut before putting on a bandaid. These molecules then degrade pollutants on the surface of the stone and can also prevent the growth of fungi, one of the most active microbial colonizers on stone. Aranzazu and her team16 have applied these ZnMgO nanoparticles to two stone materials to test the treatment: Laspra dolostone from Spain and conchuela limestone from Mexico. They have studied their antifungal effectiveness against model species that are especially active in the deterioration of limestone. As they grow, these fungi form dense networks of filaments called hyphae, which help them penetrate deep into the stone. Fungi can be difficult to combat because they can grow even in low nutrient conditions or in the presence of intense sunlight. When fungi attach to surfaces like stone, they can form thin biofilms containing extracellular polymeric substances (EPS). The slimy EPS protects the fungi and helps them survive on the stone surface. However, as shown in the figure below, when treated with the ZnMgO nanoparticles, a biofilm was unable to form and the stone remained fungi-free! Aranzazu’s team is currently examining the use of different nanomaterials to preserve real stone cultural heritage all around the world.
Overall, the use of nanomaterials in cultural heritage conservation is an exciting application of cutting-edge technology to preserve our connection to the past. This will make it possible for our children and grandchildren to enjoy the stone statues, monuments, and buildings that we admire today!
* Image sources for stone cultural heritage timeline: Temples of Malta, Statue of David, Taj Mahal, Sphinx, Piedra del Sol, Statue of Lincoln, Manzanar Cemetery Memorial
- Ruedrich, J., Kirchner, D., & Siegesmund, S. Physical weathering of building stones induced by freeze-thaw action: a laboratory long-term study. 2011, Environmental Earth Sciences, 63: 1573-1586. doi: 10.1007/s12665-010-0826-6
- Fort, R., de Buergo, M. A., & Perez-Monserrat, E. M. Non-destructive testing for the assessment of granite decay in heritage structures compared to quarry stone. 2013, International Journal of Rock Mechanisms and Mining Sciences. 61: 296-305. doi: 10.1016/j.ijrmms.2012.12.048
- Siegesmund, S., & Snethlage, R. (Eds.) Stone in architecture: Properties, Durability. 2011, Springer, pp. 227-316. doi: 10.1007/978-3-642-14475-2
- Farkas O. et al. Geochemical and mineralogical composition of black weathering crusts on limestones from seven different European countries. 2018, Environmental Earth Sciences, 77, 211. doi: 10.1007/s12665-018-7384-8
- Baggaley, K. The world’s art is under attack – by microbes. Popular Science. https://www.popsci.com/art-attack-microbe/
- Ortega-Morales, B.O., et al. Bioweathering of Cultivable Fungi Associated with Semi-Arid Surface Microhabits of Mayan Buildings. 2016, Frontiers in Microbiology, 7. 201. doi: 10.3389/fmicb.2016.00201
- Gadd, G. M. Geomicrobiology of the built environment. 2017, Nature Microbiology, 2, 16275. doi: 10.1038/nmicrobiol.2016.275
- Lazzarini, L. & Tabasso, M. Il restauro della pietra. 2010. UTET, isbn: 978-88-598-0543-4
- da Fonseca B. S. et al. Challenges of Alkoxysilane-Based Consolidants for Carbonate Stones: from Neat TEOS to Multipurpose Hybrid Nanomaterials. 2018, Advanced Materials for the Conservation of Stone. Springer, Cham, 185-207. doi: 10.1007/978-3-319-72260-3_9
- Sierra-Fernandez, A., et al. New nanomaterials for applications in conservation and restoration of stony materials: A review. 2017, Materiales de Construcción, 67, 325, e107. doi: 10.3989/mc.2017.07616
- Chelazzi, D., et al. Hydroxide nanoparticles for cultural heritage: Consolidation and protection of wall paintings and carbonate materials. 2013, Journal of Colloid and Interface Science, 392, 42-49. doi: 10.1016/j.jcis.2012.09.069
- Ziegenbalg, G. Colloidal calcium hydroxide: A new material for consolidation and conservation of carbonatic stones. In Proceedings of the 11th International Congress on Deterioration and Conservation of Stone, September 2008, Torun, Poland. pp. 1109-15. isbn: 9788323122364
- López-Arce, P., et al. Influence of porosity and relative humidity on consolidation of dolostone with calcium hydroxide nanoparticles: Effectiveness assessment with non-destructive techniques. 2010, Materials Characterization,61 (2), 168-184. doi: 10.1016/j.matchar.2009.11.007
- Gomez-Villalba, L. S., López-Arce, P., Fort, R. Nucleation of CaCO3 polymorphs from a colloidal alcoholic solution of Ca(OH)2 nanocrystals exposed to low humidity conditions. 2012, Appl. Phys. A 106, 213–217. doi: 10.1007/s00339-011-6550-6
- Facio, D. S. and Mosquera, M. J. Simple strategy for producing superhydrophobic nanocomposite coatings in situ on a building substrate. 2013, ACS Appl. Mater. Interfaces, 5, 15, 7517-7526. doi: 10.1021/am401826g
- Sierra-Fernandez, A., et al. Synthesis, photocatalytic, and antifungal properties of MgO, ZnO and Zn/Mg Oxide nanoparticles for the protection of calcareous stone heritage. 2017, ACS Appl. Mater. Interfaces, 9, 29, 24873-24886. doi: 10.1021/acsami.7b06130