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(CNN) --
Amid the thousands of treasures in the Cincinnati Art Museum's East Asian art collection, a small bronze mirror dating from the 15th or 16th century always seemed rather nondescript.
Last on display in 2017, this artifact had spent much of the previous decades in storage, where it sat on a back shelf along with other objects excluded from public display.
But the artifact had a secret hidden in plain sight.
While researching so-called "magic mirrors" (rare ancient mirrors that, in certain light, reveal hidden images or patterns on their reflective surfaces), the museum's curator of East Asian art, Hou-mei Sung, saw something like the examples from the Edo period of Japan.
The mirror, dating from the 15th or 16th century, would probably have hung in a temple or noble house. (Credit: Rob Deslongchamps/Cincinnati Museum of Art)
The item stored in Cincinnati, Ohio was smaller than those found in museums in Tokyo, Shanghai, and New York.
It also featured a more complex style of Chinese writing.
However, Sung recalled that there was something "very similar" about it.
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So, last spring, she visited the museum's cellars accompanied by a conservation expert.
"I asked him to shine a strong, focused light on the mirror," Sung said in a video call from Cincinnati.
"So, she used her cell phone (flashlight) and it worked."
On the wall in front of them was the appearance of a texture in the reflected light, not a clear image, but enough to warrant further investigation.
After experiments with more powerful and focused lights, the mirror finally revealed the image of a Buddha, rays of light emanating from his seated form.
The inscription on the back of the mirror explains who was depicted: Amitabha, an important figure in various schools of East Asian Buddhism.
A close-up of the reflected image, showing rays of light emanating from the figure of a Buddha. (Credit: Rob Deslongchamps/Cincinnati Museum of Art)
The discovery makes the museum one of the few institutions in the world that owns a magic mirror, according to Sung.
The curator is only aware of three others, including the Metropolitan Museum of Art in New York.
"We were very excited," Sung said.
A mystery that had endured
Before the invention of today's glass mirrors, polished bronze was used by people in cultures all over the world, from ancient Egypt to the Indus Valley.
The ancient art of Chinese magic mirrors was first developed during the Han dynasty around 2,000 years ago, although they were also made later in Japan.
To create the mysterious effect, craftsmen began by molding images, words, or patterns onto one side of a bronze plate.
Scientists think they then scratched and scraped the flat surface on the other side, before polishing it until it became reflective like a conventional mirror.
Because the plate was of varying thickness, due to the embossed design, the process created very slight changes in curvature on the seemingly white mirrored side.
A mercury-based substance was then used to create additional surface tensions that were invisible to the naked eye but matched the elaborate patterns on the back, according to an article in Unesco's Courier magazine.
When sunlight hits the reflective surface in a certain way, a hidden image would be revealed, matching the design on the back, giving the illusion that the light was passing directly through the mirror.
For this reason, they are known in Chinese as "transparent" or "light penetrating" mirrors.
(However, in the case of the Cincinnati Art Museum discovery, a second metal plate was probably welded to the back, leaving the original relief Buddha hidden inside.)
A second bronze plaque, which is marked on the back with the name of Amitabha Buddha, believed to have been a soldier, conceals the image of the Buddha. (Credit: Rob Deslongchamps/Cincinnati Museum of Art)
The mirrors baffled Western scientists who found them in the 19th century.
And while its optics are now widely understood, Sung said experts still don't know precisely how the craftsmen worked the metal.
"As much as you can explain theoretically, it all depends on the master polishing the surface, which is tremendously difficult," Aung said.
"That's why they're so rare."
The museum's mirror, which measures 21.6 centimeters in diameter, was probably used as a religious ornament and may have hung in a temple or noble house.
The museum has yet to figure out whether it originated in China or Japan, though Sung thinks it's most likely the former.
The object was first recorded in the museum's Asian art collection in 1961, although the curator believes it may have been acquired much earlier.
He also suspects that other institutions and collectors have magic mirrors, but they haven't realized it.
"I found many in online auctions that are similar in design to ours, but (auction listings) never say they are magic mirrors," she said, adding: "I think there might be some magic mirrors out there and that people don't even know they're magical."
The mirror will be on display at the Cincinnati Art Museum beginning July 23.
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