Invisible ink, such a simple and yet crafty way to keep secrets. You may know that it was used in wars such as the American Civil War, the American Revolutionary War and both World Wars, but did you know it was being used thousands of years ago by ancient civilizations? In the first century AD, Pliny the Elder wrote in his Natural History, an early encyclopedia, about how the milk of the tithymallus plant could be adapted as an invisible ink. Ovid spoke about secret ink in his Art of Love. Ahmed Qalqashandi, a medieval Egyptian writer and mathematician, described several types of invisible ink. And recently an article published in LiveScience explored a startling new discovery at Cambodia’s famous Angkor Wat temple regarding invisible ink. Ancient invisible ink didn’t always start out as invisible and in this case the ancient artists probably had no idea that their stunning works would one day be hidden to the naked eye.
For centuries, the walls of the ancient temple of Angkor Wat have held a secret. This secret was written in paint that had faded so much over time that what was depicted was virtually invisible. Each year millions of visitors tour the 12th-century Khmer temple, but the walls appear blank to them. That is, until Noel Hidalgo Tan, a rock-art researcher at the Australian National University in Canberra, noticed traces of red pigment on the walls. There were quite a few of them—enough to make him curious.
Tan was working nearby at an excavation site in Angkor Wat in 2010. To his trained eye, these markings on the temple walls did not look like graffiti. Pilgrims would sometimes write personal messages on the walls of the holy places where they worshipped, possibly prayer requests or praises. Researchers term these markings graffiti. As Tan examined the walls more closely, though, he realized that most of these pigment traces were not random. They were intricate and artistic. He immediately took pictures of the walls containing the red pigment, planning to digitally enhance them to see if more detail could be revealed.
What showed up in these digitally enhanced photos amazed and excited Noel Tan. Hidden in plain sight were carefully drawn wall paintings. These were actual murals, picturing people riding horses, elephants, lions, boats, buildings, deities—including a Hindu monkey-god named Hanuman—and orchestral ensembles.
After this mind-boggling discovery, Tan returned to the site in 2012 to conduct a scientific survey coordinated with Cambodian colleagues from APSARU*, a cultural heritage preservation organization. Tan applied a technique called “decorrelation stretch analysis,” which is used in rock-art research, to all the painted temple walls. By exaggerating subtle differences in color, this technique makes the faint images stand out from the underlying rock. This powerful tool has even been used to clarify images that were taken by NASA’s Opportunity Rover of the surface of Mars.
We know now from Noel Tan’s work that there are 200 invisible paintings on the walls of the Angkor Wat temple, most probably created sometime after the 15th century when the temple was abandoned. There seem to be two types of invisible paintings. Some of them were probably graffiti left by pilgrims who journeyed to the empty temple. These seem amateurish. The rest, however, are exquisitely painted and seem to have a different purpose. For example, in a chamber located high in the temple’s center tower, named the Bakan, there is an ornate mural showing a “pinpeat,” which is a traditional Khmer musical ensemble made up of wind instruments, xylophones, gongs and various other percussion instruments. Also in that chamber is a finely detailed picture of people riding horses between two buildings that appear to be temples. These paintings have been done so well that Tan and his fellow researchers have decided that the painters were carefully decorating the walls of the temple in an effort to restore its function as a house of worship.
In trying to determine exactly when these invisible paintings were created, Noel Tan suggests that they might have been commissioned between 1528 and 1566. There is evidence that Cambodia’s King Ang Chan tried to restore the temple—perhaps to transform it into a Buddhist pilgrimage site. This is supported by the fact that some of the invisible paintings contain Buddhist icons, such as a stupa, which is a mound-like monument.
Invisible ink is meant to start off dark so that the writer or painter can view his or her work, but as it dries it fades until it is invisible to the naked eye. Some types of invisible ink can be read if illuminated by ultraviolet light, others when treated with chemicals or heat. However, ancient civilizations would have had no way to predict how certain types of paint would react to time and the elements. At the Angkor Wat temple, the restoration efforts unwittingly resulted in a beautiful hidden secret for us to uncover so many years later through modern digital enhancement.
What other incredible discoveries await us as we apply modern technology to our cultural heritage? It is an ongoing quest by archaeologists and science to find the mysteries of the ages, whether hidden under centuries of earth or, as with the temple at Angkor Wat, hidden in plain sight. Stay tuned…there are exciting adventures ahead!
*The Authority for the Protection and Management of Angkor and the Region of Siem Reap
Make Your Own Invisible Ink and Secret Message
What you’ll need:
- Half a lemon
- White paper
- Lamp or other light bulb
- Squeeze some lemon juice into the bowl and add a few drops of water.
- Mix the water and lemon juice with the spoon.
- Dip the Q-tip into the mixture and write a message or draw a picture onto the white paper.
- Wait for the juice to dry so it becomes completely invisible.
- When you are ready to reveal your hidden message or work of art, heat the paper by holding it close to a lit light bulb. Be careful not to get the paper too close to the bulb so as not to burn the paper (or yourself!).
What Is Happening When the Ink Reappears?
Lemon juice is an organic substance that oxidizes and turns brown when heated. Diluting the lemon juice in water makes it very hard to notice when you apply it the paper. No one will be aware of its presence until it is heated and the secret message is revealed. Other substances that work in the same way include orange juice, honey, milk, onion juice, vinegar and wine. 
 Kahn, D. (1996). The codebreakers: the story of secret writing ; [the comprehensive history of secret communication from ancient times to the Internet] ([Rev. and updated] ed.). New York, NY: Scribner. Pg 522.
 Gannon, B. (2014, May 27). Hidden Paintings Revealed at Ancient Temple of Angkor Wat. LiveScience. Retrieved July 29, 2014, from http://www.livescience.com/45909-hidden-paintings-revealed-at-angkor-wat.html
 Create invisible ink with lemon juice – Fun Science Experiments for Kids. (n.d.). Create invisible ink with lemon juice – Fun Science Experiments for Kids. Retrieved July 29, 2014, from http://www.sciencekids.co.nz/experiments/invisibleink.html