How can nanotechnology preserve stone cultural heritage sites?

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.

Timeline of Famous Examples of Stone Cultural Heritage (Figure by Alyssa Deline * )

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.

A medieval-era sculpture eroded by acid rain. (image by Slick )

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...