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11 Historical Uses for Invisible Ink

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The history of invisible ink veers wildly back and forth between high-tech methods and the humblest of approaches. In her book, Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda, Kristie Macrakis traces invisible ink from daring escapes to love affairs to acts of espionage.

1. Ovid's Advice

The Roman poet Ovid, a noted ladies’ man, wrote elaborate instructions to lovers in his Ars Amatoria (Art of Love). In the section directed at women, Ovid taught wives bent upon deceiving their husbands to communicate in secret. Ask a servant “who is in your secrets” to carry missives in her stocking or her bosom, “under a wide shawl.” Or, failing that, try secret writing, which was apparently common knowledge by 18 BCE, when the book was written. “Characters written in fresh milk are a well-known means of secret communication,” the poet wrote. “Touch them with a little powdered charcoal and you will read them.”

2. Mary's Downfall

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The Catholic Mary, Queen of Scots, kept under luxurious house arrest for eighteen years by her Protestant cousin Elizabeth I, used invisible ink and cipher to communicate with Catholic supporters on the outside. Mary advised correspondents to write to her employing two commonly used substances: alum (hydrated potassium aluminum sulfate) or nutgall (the tannic acid secreted in swellings generated by parasitic wasps colonizing oak trees). Letters written in alum required the recipient to soak the paper in water, while nutgall needed a solution of ferrous sulphate as a developer. Eventually, Lord Walsingham, Elizabeth I’s spymaster, who had been breaking Mary’s codes all along, set a trap for Mary, using a double agent to entice her into partially committing to a plot against Elizabeth’s life. Mary was executed on February 8, 1587.

3. Orange You Glad...

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Another Catholic prisoner in England used invisible ink with far happier results in 1597. The Jesuit priest John Gerard came to England in 1588 to carry out a secret mission for the Catholic underground. Caught and detained in the Tower of London, Gerard was tortured for information. The priest befriended his prison guard and began to ask for oranges, whose juice he saved to write to confederates to on the outside. With the help of this guard, Gerard even communicated with a fellow Catholic prisoner whose cell he could see from his own, miming directions for developing the orange-juice letters over flame. The two eventually conspired to escape the Tower, with the help of outside accomplices who brought a rope—a feat made more impressive by the fact that Gerard’s fingers had been wrecked during his torture sessions.

4. Explosive Ink

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During the eighteenth century, the new vogue for popular science in France and England made invisible ink into entertainment, enacted in public in front of wondering eyes. Poor Jean-Jacques Rousseau experimented with a dangerous type of sympathetic ink in 1736. Macrakis writes that Rousseau probably heard of the recipe from a professor friend, or read about it in a book of recreational experiments. The ink was made with quicklime and orpiment (a rare mineral that’s pigmented orange or yellow, and contains arsenic sulfide). When the philosopher mixed the two, the bottle began to fizz uncontrollably, and eventually exploded in his face. “He swallowed so much chalk and orpiment that it nearly killed him,” Macrakis writes. “He couldn’t see for more than six weeks.”

5. Washington's "Medicine"

The Culper Spy Ring were agents for George Washington who circulated in occupied New York City from 1778 to 1783. The group, recruited by Major Benjamin Tallmadge, used pseudonyms and numerical codes to pass information, fearful as they were of being discovered. They also used invisible ink, manufactured for Washington by James Jay, a doctor who was John Jay’s older brother. This combination of precautions meant that they all got through the war without being discovered, and managed to feed Washington some valuable bits of strategic information. Jay didn’t record the chemical makeup of the fluid, which he and Washington called “the medicine” in letters to each other. In the 1930s, curious doctor and photographer Dr. Lodewyk Bendikson performed ultraviolet and infrared tests on letters written using Jay’s invisible ink. Bendikson found that Jay’s formula was an old one: tannic acid from gallnuts, developed with ferrous sulfate.

6. When Life Hands You Lemons

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In World War I, several so-called “lemon juice spies”—German agents operating in England—used citrus as their means of communication. The British government had stepped up its censorship of letters in wartime. One agent, Mabel Beatrice Elliot, flagged letters written by three of these men, heated them up, and unmasked them as spies. The lemon-juice operation was a clumsy one: several spies, once caught, had lemons on their persons, or pens with pulp still stuck on the nibs. In the end, the British executed 11 German spies in the Tower of London in 1915; four of them had used lemon juice. “After the painful and visible loss of … the lemon juice spies,” Macrakis writes, “the Germans began to develop more sophisticated invisible ink methods.”

7. Bacon's Blunder

National Archives

George Vaux Bacon, an American journalist recruited by Germans to spy in Britain, was the beneficiary of these new ideas. He smuggled a new kind of invisible ink into the country in a novel way. Bacon’s handler asked him to buy black socks, and then impregnated the socks with the paste-like ink, telling him to soak the socks in water once he reached his destination. Bacon was quickly nabbed, after censors suspected his letters based on their address in the Netherlands. The substance on his socks was Argyrol, a drug used as an antiseptic and anti-bacterial that contains a mild silver protein. Bacon didn’t know how to develop the ink, which was totally new to British censors; eventually, British and French chemists figured out a method using electrolysis. A British court condemned Bacon to death, but his sentence was commuted in exchange for his testimony against his German handlers. Later, he claimed that the whole episode had been a stunt, meant to result in a great magazine story about spying.

8. Flu Ink

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During World War II, chemist Linus Pauling worked on an unusual wartime project, formulating new kinds of invisible ink that would resist all known reagents. Pauling and his colleagues experimented with invisible inks made from pneumococcus bacteria—inert, in this preparation, and so unable to spread pneumonia. The ink-ified microbe would react to an antibody, and then become visible once dipped in a dye solution. The ink never passed the experimental stage; neither did the various inks made with radioactive isotopes that were tested by MIT physicist Robley Evans.

9. What is This? Why is it Here?

James Stockdale, a Navy pilot, was shot down over North Vietnam in 1965 and sent to the “Hanoi Hilton,” where he was to stay for seven and a half years. With the help of U.S. Naval Intelligence, his wife Sybil initiated secret communications with Stockdale by enclosing a photograph of her mother in a letter to him. He was confused, but (as he said later) he thought: “It’s dumb to throw away something from the States without doing more with it. James Bond would soak it in piss and see if a message came out of it.” So he did. After it dried, print appeared on the back, establishing the code that he later used to communicate with the Navy, informing them of conditions at the prison.

10. Ultraviolet Exposure

The East German secret police—the Stasi—prescreened 90,000 pieces of mail every day during the 1980s, using an elaborate conveyor belt system. Agents steamed open letters in bulk, identified suspicious pieces with indentations or scratch marks, then glued them shut, in assembly-line fashion. Renate Murk, a Stasi captain who examined the letters that were under suspicion, used new technology also favored by the CIA to reveal secret writing without using a reagent. (If you chemically develop a letter, that’s an irreversible process, and you can’t send the letter on to its intended recipient; any element of surprise is lost.) Murk used ultraviolet scanners and the Nyom invisible impressions detector to find invisible writing without developing the letter.

11. Prison Speak

In the 1990s, the Aryan Brotherhood used citrus juice and urine to send messages between prisons, orchestrating violent action. In 1997, Brotherhood leader T.D. Bingham, imprisoned at the Supermax prison in Fremont County, Colorado, sent a letter to an outside courier, who passed it to Brotherhood members imprisoned in Lewisburg, Penn. The letter was written in urine, and revealed its secrets upon being “toasted” over a flame. The message: “War with DC Blacks, T.D.” When Bingham and other Brotherhood leaders were tried for ordering this and other attacks in 2006, embarrassed government censors had to admit that they’d missed the message completely.

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Brigido Lara, the Artist Whose Pre-Columbian Fakes Fooled Museums Around the World
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In July 1974, Mexican authorities sent a man named Brigido Lara to jail. His crime wasn't a violent one, but it was serious nonetheless: Archaeologists from the Instituto Nacional de Antropología e Historia (INAH), a Mexican federal government bureau devoted to preserving the nation's heritage, claimed that Lara had been found with ancient ceramic artifacts looted from archeological sites in the state of Veracruz.

Lara was convicted of stealing and smuggling antiquities, but he insisted he wasn't a thief—and he could prove it. All he needed were tools and some clay brought to his jail cell. 

FORGING A CAREER

Lara grew up in Veracruz, in the village of Tlalixcoyán. While his parents were peasant farmers, Lara showed artistic talent—specifically, a knack for creating figurines from clay. Veracruz is home to many archaeological sites that date back to hundreds and thousands of years before the arrival of before Christopher Columbus, and the young Lara would often find ancient terra-cotta figurines in the fields and near rivers. He claims that by the time he was 9 years old, he was making versions of these artifacts using clay harvested from a local stream.

As Lara grew older, his skill set expanded. He reportedly taught himself how to prep and oven-fire local clay, and began making objects that mimicked those of several ancient Mesoamerican cultures—imitation Olmec pots, Maya polychrome vessels, and figurines in the Aztec, Mayan, and Totonac styles. He began specializing in replicating works by the Totonacs, a culture that flourished in central Veracruz until the Spanish Conquest introduced diseases that ravaged the communities. These figurines ranged in size from large to tiny, and often depicted mythological gods wearing masks and headdresses.

It's not entirely clear whether Lara began making these figurines for fun or profit. But according to the man himself, traveling dry-goods merchants had noticed his talents before he had even reached his teens. They accepted his "interpretations," as he called his early work, in lieu of cash—then sold them on the black market. Looters also came to Lara, asking him to fix and restore stolen works. Eventually, the artist wound up working in a Mexico City atelier that produced forgeries.

No detail was too tiny for Lara. He visited archaeological sites to study just-dug-up artifacts, and harvested clay from the surrounding region to sculpt exact likenesses. He later told Connoisseur magazine that for true authenticity, he even crafted his own primitive tools and stockpiled 32 grades of cinnabar—a reddish form of mercury used by the Olmec, an ancient Mesoamerican civilization that existed between 1200 BCE and 400 BCE—for precise pigmentation. He finished his works with a ancient-looking patina made from cement, lime, hot sugar water, urine, and other ingredients, and coated the final products with a seal made from dirt and glue.

But even though Lara was a stickler for the details, he also took artistic liberties with some of his "interpretations," adding elements that wouldn't have appeared on the original artifacts. Sometimes he would include a fanciful new detail from his imagination: a winged headdress, or one that writhed with serpents; a duck-billed mask, or a dramatic, lifelike pose.

Lara didn't consider himself a forger. "My style was born with me," he told The New York Times in 1987. "I didn't learn from anyone. I studied the pre-Columbian pieces in my town that came from the burial mounds, and I used the ancient techniques. I made these pieces and I am very proud."

But by young adulthood, he'd also become a businessman, selling his unsigned pre-Columbian replicas to middlemen who re-sold them to illegal art collectors both domestically and abroad. "I was aware that many buyers then sold them as authentic pre-Hispanic works," Lara admitted to Art & Antiques magazine years later.

COMING CLEAN

Lara's forgery career may have continued undetected had he and four of his buyers not been apprehended in 1974 and charged with trafficking in pre-Columbian works. The police didn't consider Lara an artist or a forger—his works looked so real, the authorities thought they'd been dug right out of the ground.

Lara was sentenced to 10 years in jail. To regain his freedom, he devised a plan: He asked law enforcement officials to grant his lawyer permission to bring him clay and art tools. Right there in his cell, Lara created replicas of the antiques he'd reportedly stolen. Experts from the INAH examined the earthen artworks, and declared them "genuine" ancient artifacts.

The stunt worked. Lara had proven he had made the works himself, not smuggled them out of ancient sites. Finally convinced of his innocence, prison officials released him in January 1975 after he'd served only seven months of his sentence.

After his release, Alfonso Medellín Zenil, head of the Museo de Antropología de Xalapa, offered Lara a job. "Our policy is, when you can't beat them, hire them," Fernando Winfield Capitaine, then the museum's director, joked to Connoisseur.

The Museo de Antropología is home to an extensive collection of artifacts from Mexico's Gulf Coast produced by ancient indigenous peoples such as the Olmec, the Huastec, and the Totonac. Lara was hired to restore these works as well as to make replicas for the museum's gift shop.

But his career as a forger wasn't behind him quite yet.

REVELATIONS AND REFLECTIONS

In the early 1980s, Veracruz governor Agustín Acosta Lagunes began repatriating pre-Columbian works from abroad, expanding the collections at the Museo de Antropología de Xalapa. But when Lara saw some of these imported works, which had been purchased at Sotheby's auction house in New York City, he pronounced them fakes. He knew, he said, because he'd made many of them—including a figure of a male dancer that had been exhibited at the Los Angeles County Museum of Natural History’s “Ancient Art of Veracruz” exhibit in 1971.

Little by little, it emerged that Lara’s works might have made their way into pre-Columbian art collections around the world, including in prestigious museums such as the Dallas Museum of Art and the Saint Louis Art Museum, as well as in renowned private collections. Lara claimed credit for a 3-foot statue of the Mexican wind god Ehecatl in New York City's Metropolitan Museum of Art, and out of approximately 150 works on display in the "Ancient Art of Veracruz" exhibit, asserted that he had made about a dozen.

Among the most notorious fakes Lara claimed to have created were three life-size ceramic sculptures in the Dallas Museum of Art that had once belonged to film director John Huston. "If you look at them closely, they are copies," Lara told the Associated Press in 1987. The works were attributed to the Totonac, and thought to have been made between 600 to 900 CE. Lara, however, claimed to have produced them during the 1950s: "The details are different than the originals … the details in the breast decorations, in the shoulder patches and so on," he said. "They are very different. They are originals of course—my own."

As news spread about Lara’s forgeries, the Saint Louis Art Museum, the Met in New York City, and the Dallas Museum of Art responded to the controversy by taking works off display. "All three museums acknowledged that many of the Veracruz-style objects in their collections were problematic," Matthew H. Robb, a former curator at the Saint Louis Art Museum who is now chief curator at the Fowler Museum at UCLA, tells Mental Floss.

Nobody knows exactly how Lara’s creations made their way into American museums (Lara blamed various high-profile art traffickers and dealers), but experts say they noticed when suspicious artifacts resembling his work first began popping up in the 1950s, as pre-Columbian art was becoming more and more popular among American art collectors. "They appeared out of nowhere, resembling nothing previously excavated," Edmund Carpenter, a New York archeologist, told The New York Times. "I saw some in New York, Los Angeles, Paris. Museums bought them, big collectors bought them. But nobody asked, 'How come a big find like this?'"

Bryan Just, a curator and lecturer on pre-Columbian art at the Princeton University Art Museum, chalks the phenomena up to scholarly ignorance. At the time, "there wasn't a lot of material available for comparison," he tells Mental Floss. "There are many regions, including Veracruz … where not a whole lot of archeology had been done. So for a lot of these [new] artworks, there weren't great sources to reference that answered questions like, 'How should this stuff really look?' And at that time, what had been excavated may not have been published."

There was also a shortage of experts to consult because the very idea of pre-Columbian relics as art was still relatively new. Connoisseurs only began collecting and selling these works in the early 20th century, and university scholars didn’t begin offering pre-Columbian art history courses until the 1950s, according to Just.

Not that collectors were necessarily consulting scholars in the first place: "If you were considering work that was offered to you by a dealer, you may have not wanted to consult a colleague who's an expert in that particular area if they work at a collecting institution," Just says. "You know, out of concern that they might snag it up before you do."

Fortunately, modern scholars have access to a greater body of knowledge about pre-Columbian art than their predecessors. "In retrospect, when I see Lara's stuff now, it seems pretty obvious to me that it's wrong," Just says. "It doesn't make sense when you think about it in terms of the broader context of what we know about these particular traditions."

But even today, it isn't always easy to ascertain what's real and what's not when it comes to pre-Columbian art. Experts sometimes use thermoluminescence tests, which involve removing a tiny piece of the object, grinding it up, heating it in a furnace, and observing how much light it emits. Ideally, this process can measure how long ago the clay was fired, but the results can be skewed if a work was recently exposed to extreme heat or had been cleaned.

Another issue is that "lots of these complicated ceramic sculptures are pastiches," Victoria Lyall, a curator of pre-Columbian art at the Denver Art Museum, tells Mental Floss. Artists "will use bits of older sculptures and put them back together. So you have to test a lot of different spots to really get a better sense of whether the entire piece is fake."

X-rays are a good way to spot a composite, but they interfere with thermoluminescence test results, putting conservationists between a rock and a hard place. Furthermore, clays from certain regions—like the clay Lara worked with in Veracruz—reportedly aren't as conducive to thermoluminescence testing.

A LEGACY OF LIES

Lara is now in his mid-70s. He no longer restores antiques at the Museo de Antropología de Xalapa full-time, but he still works as a consultant there, and he continues to make art under his own name. However, his legacy will forever be tied with the difficult history of pre-Columbian artwork. According to experts, it's possible that his artworks are still masquerading as artifacts around the world, and that he may have even helped shape modern scholars' perception of pre-Columbian art from Veracruz.

However, it's also feasible that Lara's stories are a composite of fact and fiction—just like his work. The artist claims to have made thousands of forgeries (one estimate places the number at more than 40,000 pieces), but some experts say it would have been nearly impossible for Lara—who was only in his 30s when he was arrested—to have produced so many works in just a few decades.

Plus, the timelines don't always add up: Lara "was about 8 years old at the time that the [Ehecatl statue] was supposedly manufactured and purchased by the Met," Lyall says.

Lara also claims to have been self-taught, but some have speculated that he's stretched the truth about his natural talent. He may have instead learned his trade by apprenticing at a young age in a Veracruz workshop that specialized in forgeries, theorizes Jesse Lerner, a professor of media studies at Pitzer College. Lara "denies all that, but it's hard to know … Just by the nature of his business, it's kind of shady," Lerner tells Mental Floss. (Lerner's 1999 documentary Ruins—a look at the history of Mexican archeology and the traffic in fakes—features an interview with Lara.)

This workshop might have sold both Lara's wares and similar works to international collectors through an established underground market. Such a scenario would explain the artist's familiarity with pieces in faraway collections, like the Met's statue, which he could describe in great detail despite likely having never produced it with his own hands. Because forgeries aren't exactly signed, it's difficult to know for sure which pieces are Lara's and which may have been made by other forgers.

Either way, Lara's frauds are a reminder to avoid believing everything you read—even if it's a label in a museum. And they offer another lesson, too.

"The types of ancient works that Lara and other forgers were imitating, they weren't intended as aesthetic objects," Lerner says. "They weren't for museums. They were representations of this whole world view of cosmic forces."

That makes forgeries like Lara's particularly problematic. "If the only way we can access that worldview is through these objects that survive, [Lara] is just adding bad data to the pool of data that we have available. He's messing up everyone's understanding of who these figures are representing, and how their universe was understood and functioned."

In other words, sometimes fakes don't just fool art lovers—they can also change our understanding of history.

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10 Cold War Artifacts From the Real-Life Bridge of Spies Auction
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U-2 designer Kelly Johnson with Francis Gary Powers (left to right).
U.S. Air Force, Wikimedia Commons // Public Domain

The 2015 Steven Spielberg film Bridge of Spies tells the story of a captured U.S. Air Force pilot who was released from Soviet prison in exchange for the freedom of a KGB spy during the Cold War. To mark the 55-year anniversary of that historic prisoner swap, Guernsey’s is auctioning off items that belonged to the real-life military hero Francis Gary Powers.

The artifacts, donated to the auction house by Powers’s son, depict what life was like for an American spy during the Cold War. The Espionage Collection includes gear built for high-altitude covert missions, as well as documents written by Powers during his imprisonment. Here are some of the most fascinating objects going up for bid on October 6 and 7.

1. HIGH-ALTITUDE FLIGHT SUIT

Flight suit used on air force jet.

On May 1, 1960, Francis Gary Powers attempted to fly deeper into Soviet airspace than any U.S. reconnaissance mission had gone before. The U-2 plane was famously shot down and its pilot captured, turning what should have been a covert operation into a historic Cold War pressure point. When the auction opens in October, one lucky bidder will snag a perfect copy of the flight suit Powers wore that day. Never-been-worn, the suit still includes its two intact air hoses, similar to those that kept Powers alive when flying through the upper atmosphere. Similar technology was used to build the gear worn by astronauts and cosmonauts during the space race.

Starting bid: $5000

2. INFLATABLE RAFT

Raft used on military jet.

While the flight suit above was never actually worn by Powers, this raft was likely on board his U-2 with him the day it was shot down in 1960. Experts believe the item was salvaged and displayed in a Soviet museum following the failed reconnaissance mission. After the museum was looted, the raft was allegedly used by local fishermen for years before it was returned to Powers’s family. The flotation device measures 7-by-15-feet folded and features its original plastic inflation tube still bearing old teeth marks.

Starting bid: $750

3. SOVIET PROPAGANDA POSTER

Soviet propaganda poster.

The Bridge of Spies auction features several Soviet Union propaganda posters, including prints of Lady Justice and Vladimir Lenin. This Soviet workers poster, with a gold hammer and sickle against a field of grain, showcases some of the most iconic imagery from the era.

Starting bid: $250

4. FLUID CONTAINMENT VESSEL

Fluid containment vessel used by air force pilots.

When you've got to go, you've got to go—even if you’re a spy in the middle of a top-secret mission thousands of miles above Soviet soil. For long U-2 flights, Powers zipped this container into one of the pockets of his suit and pulled it out when he needed to relieve himself. As the item description reads, “This historic and straightforward piece of equipment proves that smart design can take you great distances.”

Starting bid: $500

5. HOUSE OF UN-AMERICAN ACTIVITIES BOOKS

Six Cold War-era booklets spread out on a table.

The U.S. government formed the House Un-American Activities Committee in 1938 to investigate citizens and organizations with suspected Soviet leanings. These six booklets dictate hearings from the committee in 1960 and outline the “subversive activities of anyone believed to have ties to Communist organizations.” They provide important context for how Cold War tensions manifested in America the year Powers was captured.

Starting bid: $200

6. POWERS’S PRISON JOURNAL

Cover of an old notebook.

Faced with up to 10 years in USSR custody, Gary Powers started teaching himself Russian shortly after he was sentenced. His notebook from that time contains two pages of English words paired with their Russian definitions handwritten in pencil. Despite his status as a prisoner of war, this behavior actually got him into trouble back in the States. Anti-Soviet sentiments were so high at the time that Powers was branded a traitor for daring to learn the language of the enemy. Fortunately this label didn’t last forever, and today he’s considered a military hero.

Starting bid: $750

7. GARY POWERS’S TRENCH COACH

Tan trench coat on a coat hanger.

Nothing says “spy” like a good, vintage trench coat. This tan London Towne jacket worn by Powers in the 1960s was originally made for Army officers. The durable cotton garment features one visible button at the neck and four concealed lower buttons for a sleek, mysterious look.

Starting bid: $600

8. FALLOUT FORECAST MAP OF THE U.S.

Map of the U.S. framed behind glass.

Nuclear conflict wasn’t just a vague threat for many Americans during the Cold War—it was an imminent reality they needed to prepare for. This atmosphere of terror produced such items as this nuclear fallout map, published by the District of Columbia Office of Civil Defense in 1960. The 28.5-by-40.5-inch map depicts the contiguous United States with the longitude and latitude points of potential radioactive decay.

Starting bid: $200

9. PIECE OF THE IRON CURTAIN

A section of lattice iron fencing.

In addition to being one of the most powerful metaphors of the Cold War, the Iron Curtain was also a literal barrier that divided Eastern and Western Europe in some rural parts of the continent. This rare 11-by-16-inch patch of iron fencing shows the welded design that prevented many Eastern Europeans from escaping Communist rule from the late 1940s to 1991.

Starting bid: $350

10. A LETTER FROM POWERS TO HIS WIFE

Pages of a letter and a postcard.

Powers wrote a letter to his wife Barbara on December 13, 1960, seven months after his capture. He wrote of his condition in the facility as well as his outlook on his situation. "As long as I do not lose hope everything will be alright,” he said. “I haven't lost hope yet." He was also interested in the new president-elect John F. Kennedy, who would come to have a major impact on the Cold War following his inauguration in 1961. Powers wrote: "Would you send me a copy of Kennedy's inauguration speech and any other statements he makes. I am very interested in what his views are and especially what his foreign policy will be..." As well as giving insight into Powers’s state of mind during this time, the document is also a Russian artifact, sporting full-color Soviet-era stamps.

Starting bid: $1500

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