The Hidden Past of Invisible Ink

Kristie Macrakis. American Scientist. Volume 102, Issue 3. May/Jun 2014.

For many people, the words invisible ink readily conjure up images of childhood fun with lemon juice and a candle, or spy messages during wartime. And there is a good reason for this association, because secret writing has long been part of the worlds of both magical entertainment and international intrigue. But it also has an overlooked long, colorful, and little-known history.

The tale begins with tire discovery of a magical appearing and disappearing ink, and crests with a convergence of trends during the end of the Enlightenment in the 18th century. During that era of intellectual ferment, a rise of stage magic coincided with a revival of so-called natural magic-an occult tradition that sought power from natural sources, such as herbology and astrology-alongside an increased demand for public scientific demonstrations. It was against the backdrop of this witches’ brew of circumstances that invisible ink flourished.

It was a heady time, an era when popular science became intoxicatinglow-tech entertainment in a low-key world. But other historical and cultural circumstances carried the inks to 20thcentury America, and ultimately into modem popular culture.

Invisible Landscapes

I’ve been fascinated by the history of invisible ink for a number of years, and have even worked with my colleague Jason Lye, a color chemistry expert, to re-create some traditional ink formulas. That is why I was excited when, a few years ago, I had the opportunity to visit Schneeberg, Germany (below), one of the birthplaces of these magical substances, and a rich source of inspiration for modem chemistry.

I have fond memories of hiking up and down the trails surrounding that mountaintop village, where shop windows are filled with hand-carved nutcrackers, smoking men, candelabras, angels, and pyramids. Villagers there still use the miners’ greeting Glück auf!-Godspeed-and the hammer and pick symbol still decorates knickknacks, buildings, walls, and fences. It is easy to imagine the music of Bach wafting through the cobblestone streets of this Baroque village.

Located in the Saxon Ore mountain range, the Schneeberg Mountains once contained rich sources of silver, bismuth, and cobalt hidden beneath the bucolic exterior of verdant hills and valleys. Until the 16th century, Schneeberg was known as a a silver-mining town, but after workers depleted the known veins of that metal they turned their attention to cobalt, which was in high demand for coloring glass and other goods. Miners removed hundreds of thousands of kilograms of cobalt from the mountain. Streets such as Silver Road and Cobalt Road still snake through the Schneeberg landscape and define its past. Although silver brought riches to the town, cobalt produced many more exotic marvelsincluding invisible ink.

The 16th century was the heyday of “books of secrets,” compilations of technical and medicinal recipes and magic formulas. These books occasionally included directions for creating invisible ink, written as a basic kitchen recipe using simple and readily available ingredients. For example, a recipe would suggest dipping an alum-written letter in water, where the writing would turn from white to black; or it might advise rubbing powder on fig-tree milk to make it visible. Other directions advised the experimenter to paint, sponge, or spray a liquid on the secret letter, or wave it in the air, sprinkle powder on it, hold it over fire, or dip it into water, depending on what kind of ink was used to write the invisible message.

Cobalt allowed invisible ink to become much more advanced. In 1705 a mysterious female German alchemist seems to have been the first person to identify bismuth-cobalt as a valuable substance from which to make invisible ink. This alchemist was also the anonymous author of three books, including one with the alluring title On the Key to the Cabinet of the Secret Treasure Room of Nature, which included a discussion of the changing bismuth-cobalt colors. The author’s name was listed only by the initials DJW. Historical research has shown that she might have been Dorothea Juliana Walchin (or some other form of the name, such as Wallich or Wallichin), but little else is known about her.

This obscure woman alchemist seems to have discovered a cobalt mineral that appeared red in a solution and was supposed to be the “first matter” from which all other elements formed-the Holy Grail for the alchemists. The cobalt mineral also displayed remarkable visual qualities: Its color changed from rosy red to grassy green to sky blue when heat was applied. When the cobalt was prepared and turned into a solution with which to write, it was clear, but it produced a fabulous blue-green color when heated. The writing disappeared when cooled.

Jean Hellot (1685-1766), the French scientist who did more than anyone else to investigate, promote, and advance the subject of invisible ink during the 18th century, referred to another person-an artist, name also unknown-when allocating credit for the discovery of the magical qualities of cobalt. Hellot lived most of his professional life as an industrial chemist during the early period of the French Enlightenment. He was a pioneer in the technical chemistry of dyeing, mining, and assaying. Contemporary accounts refer to him as a short and rather chubby man, made memorable by eyes that sparkled with liveliness. It is easy to imagine a twinkle in his eyes when he started playing with this unusual new substance.

In the summer of 1736, the anonymous German artist mentioned by Hellot showed some members of the Royal Academy of Sciences in Paris the magical cobalt phenomenon. The artist had climbed the hills of Saxony in search of the source rock. He dubbed it Minera Marchassitae, based on a general term for ore including bismuth, cobalt, iron, and other minerals. The curious properties of this material enchanted Hellot, who experimented assiduously during his leisure time. By the next summer he had written a paper on the subject for the Academy’s journal.

Soon after Hellot’s pathbreaking paper was published, German scientists objected that a Frenchman got credit for “discovering” this substance as a new phenomenon. They claimed that a Professor Teichmeyer in Jena had, in fact, first demonstrated the same cobalt ink to his students six years earlier. Of course, there was also that mysterious German lady who had described similar activity in 1705. Nationalistic pride became part of this priority dispute. The Germans surely were galled when cobalt ink became known as “Hellot’s sympathetic ink” in the centuries that followed, an honor implying French discovery. (Experimenters at the time described the inks, especially those that involved multiple substances, as working with each other by “sympathy.” The term soon led to sympathetic ink becoming a synonymous name for invisible ink.)

Historians now know that, even earlier, Italian scholar Giambattista della Porta (1535-1615), had written about invisible writing, and had observed that some such inks developed by heat, others through application of a glutinous material, and still others through application of a “liquor” that worked only when paired with a certain chemical. The latter ink that needs a specific developer, known as a reagent, is the most important and secure kind of pairing. Robert Boyle, known as the father of modem chemistry, used these substances to make an ink “confess its secrets” in the late 17th century.

Finally in the mid-1700s Hellot developed a full classification system for the three common developer types as described by della Porta. Hellot also added two new categories of his own. In the first new type, air developed some dyes, and in the second one, some inks simply appeared and disappeared spontaneously. The common denominator of all the old secret-writing pairings was that once developed, the writing stayed visible even when cooled, unlike the new sympathetic ink.

Hellot experimented with different mineral and solution pairings to come up with his sympathetic ink. At the time, a scientist could not obtain a solution of cobalt chloride at the druggist, but had to make it. Hellot bought the mineral from various local apothecaries or received samples of cobalt pieces from his scientific friends in different countries.

Hellot became so enchanted with cobalt’s varied color possibilities that he started thinking about painting landscapes with the inks or dyes. He dreamed of creating a winter landscape that would transform instantly into a springtime scene when it was heated. The idea caught on, and continued to captivate the public for the next two centuries.

By around 1746, sympathetic ink was fashionable in Paris, and changeable landscape fire screens became all the rage. A barren winter landscape with tree trunks and branches was painted on a fire screen with ordinary India ink. Artists then painted a solution of cobalt chloride on the screen to create lush shrubs and greenery. They used acetate of cobalt to paint blue features such as the sky. The cobalt was invisible initially, but as soon as the heat from the fire reached the screen, the barren landscape turned a verdant green. When the heat source was removed, the scene became winter again. By the 19th century, women used similarly transforming handheld paper fire screens to shield their faces from intense fireplace heat.

Later in the 19th century chemists began to realize that the color change associated with cobalt compounds depended not only on heat but also on the humidity in the air. When the paper turned from blue or green to a rose color, it indicated high humidity. This discovery led to the creation of dolls and floral arrangements that doubled as weather indicators. The doll’s dress or flowers’ petals were impregnated with cobalt chloride so that they changed color from blue to pink when there was an increase in the amount of moisture in the air and thus a chance of rain. These devices were often erroneously labeled barometers, even though they measured humidity rather than atmospheric pressure.

But there was still the question of how the inks worked. Back in Hellot’s time, a number of distinguished chemists, such as the Frenchman Pierre Joseph Macquer (1718-1784), described and promoted Hellot’s sympathetic ink in newly created dictionaries and textbooks for chemistry. In this way, such inks migrated beyond the world of magic and became the focus of serious scientific attention.

In the end, Hellot found the color changes inexplicable scientifically. Early experimenters simply attributed the color changes to the effect of heat and cold. By the late 18th century a color chemist, Edward Hussey Delavel, postulated that the salt attracted moisture in the air when cold and expelled it when heated. This theory was the closest early scientists came to matching modem explanations: When the salt is heated it loses water-an anhydrous state-and when it is cooled the water returns to the salt-a hydrous state. (See pages 202-203for more on the chemistry of invisible inks.)

Science and Magic

Hellot’s discovery had a wide-reaching impact, far beyond this circle of chemists. By the last decade of the Enlightenment, science had spread from private laboratories and elite royal societies to the popular press and urban culture. Philosophical salons, cafés, and musées dotted the streets of Paris, and these new institutions for popular science came alive with scientific lectures, experiments, and demonstrations of science. Invisible ink fit right in with these pastimes.

The rise of popular science was not all harmless fun, however. Jean-Jacques Rousseau (1712-1778), the famous French philosopher, experimented with the dangerous solution of quicklime and orpiment. While a young man, in 1736, he tried to reproduce a sympathetic ink experiment with directions from a professor of physics and with the help of Jacques Ozanam’s Mathematical Recreations. After he had filled a bottle with the mixture, it began to “effervesce violently.” He ran to uncork the bottle, but it was too late. It burst in his face “like a bomb.”

Rousseau swallowed so much chalk and orpiment that it nearly killed him. He couldn’t see for more than six weeks. His health also declined after this event. He felt “short of breath, had a feeling of oppression, sighed involuntarily, had palpitations of the heart, and spat blood; a slow fever supervened,” from which he never fully recovered. Although early popular science books use the recipe freely and frequently, that experience is one reason my colleagues and I have not reproduced this experiment at home or in the laboratory.

But the type of science that most called out to pedestrians on the streets of Paris in the 1700s was not of the typical academic variety. It was alternative science, sometimes even what we would today call pseudoscience. Franz Mesmer mesmerized Parisians with his experiments on animal magnetism, Jean Nollet electrified spectators with shows in which a charge pulsated through a row of people, and the public was blown away by the ballooning craze started by the Montgolfier brothers. Science’s power became intoxicating.

As science became part of public culture, it provided entertainment as well as general education. Leading the charge were scientist-magidans such as Henri Decremps (1746-1826), a self-styled “professor and demonstrator of amusing physics,” and Joseph Pinetti (17501800), known as the Professor of Natural Magic. They both toured Paris and London performing what was billed as “amusing physics and various entertaining experiments.” But Pinetti was really only a stage magician. Decremps took it upon himself to unmask charlatans; he exposed and explained Pinetti’s experiments as mere tricks. This upheaval ruined the career of the leading magidan in Paris because his displays were no longer a mystery.

Decremps also included sympathetic inks among the secrets he exposed. By now ordinary people at home could choose five kinds of sympathetic inks developed by liquid, air, powder, or fire, and create amusements such as the changeable landscapes or various fortune-telling games that involved writing a question in regular ink and an answer in sympathetic ink. Nevertheless, magidans employed the compounds in unexpected ways to appear as if they had extraordinary powers.

For instance, in one trick called a book of fate or fortune, the trickster was instructed to make a book of about 70 or 80 pages, with a secret compartment built into the back cover. The “magician” then wrote a question in regular ink, with an answer in sympathetic ink made from litharge of lead or bismuth. The visible questions were listed in the table of contents. The trickster soaked a double piece of paper in the so-called “vivifying ink,” made out of quicklime and orpiment, and placed it in the hidden case at the end of the book. Then an audience member selected a question she wanted answered.

The trickster placed the question on a piece of paper on top of the one written in the book, closed the book, pounded it shut, and placed a weight on it. When he opened Öre book, the “vivifying ink” had developed the answer. This feat relied on sympathetic inks developed by liquids or the vapors from liquids.

Books and Cabinets

By the end of the 18th century there was a resurgence of interest in the subject of natural magic; invisible ink, with its use of natural substances, fit right in. Some practitioners were conjurors and stage magicians who manipulated reality through sleight of hand. Others were natural scientists enchanted by the trend of packaging magic tricks for science education, and many wrote books to advance this goal.

These new natural magic books contained less on experimental physics and more on spectacular physical effects, including sympathetic ink. One monumental series was Johann Christian Wiegleb’s Natural Magic Consisting of All Kinds of Amusements and Useful Tricks, first published in 1779. Wiegleb (1732-1800) was one of Enlightenment Germany’s most knowledgeable and respected apothecary-chemists. Wiegleb’s publisher wanted to impregnate several pages of his book with sympathetic ink to help sell copies. Unfortunately, Wiegleb was too busy to procure the sometimes hard-to-find cobalt chloride.

The explosion of interest in natural magic coincided with the invention of chemical cabinets, chests full of supplies for the avocational experimenter. In 1791 the German chemist and pharmacist Johann Friedrich August Göttling (1753-1809) built one of the earliest “portable chests of chemistry,” as he called it. Interestingly, test tubes are not included in the kit; Göttling used a wine glass to mix chemicals. The natural magic tradition and these new portable chemistry cabinets eventually led to the design and marketing of the modem chemistry set.

British author Jane Marcet’s Conversations in Chemistry (1805) contributed to chemistry’s popularity at the time because it was written as a discussion and illustrated concepts using everyday examples. And in his wildly popular Chemical Recreations, published in the mid-1800s, the British chemist John J. Griffin included descriptions of Hellot’s sympathetic ink and some of the stories surrounding it. By the middle of that century, his company J. J. Griffin and Sons also started to manufacture and sell 11 different kinds of chemical cabinets, dominating the market for some 50 years until the outbreak of World War I.

This popularity of chemistry as an everyman’s pursuit coincided with a fad in the Victorian era, in the middle of the 19th century, of entertaining guests in one’s parlor with magic acts. Chemists at that time encouraged conjurors to incorporate chemistry experiments, often involving compounds that would turn invisible in their tricks. Chemical cabinets provided the raw materials for these pursuits, including appearing messages or images. Manuals such as The Magicians’ Own Book, issued by Dick & Fitzgerald Publishers in 1857, also taught amateur magicians about optical, mechanical, or “magnetical” illusions.

In the early 20th century many magicians had become “scientific conjurors,” and “Professor” Ellis Stanyon offered courses on “Fire and Chemical Magic” at his “School of Magic” in London. Although these classes emphasized scientific wizardry and operated without witchcraft, Stanyon thought that they had achieved “real magic at last” with dramatic color changes, from chemicals including cobalt chloride, that key ingredient of some sympathetic inks.

In 1909, the magician William Linnett, a member of the Society of American Magicians, declared that chemistry was uniquely adaptable to magic and recommended that all magicians become familiar with it. Invisible inks provided a bridge for these two fields: Many magicians took Linnett’s advice to heart, whereas many chemists became enchanted with color changes. John Lippy, a chemist and amateur magician, wrote a manual for magicians called Chemical Magic in 1930. By 1959, teachers were quoted in the updated version of the book, calling it a “very fine reference for ambitious teachers who want to popularize scientific information by presenting dramatic experiments.”

Harry Blackstone, the world-famous magician, endorsed the book in the introduction and described the alchemists’ quest, but noted that the world is now built on “physical principles,” not the “black arts.” Still, he argued, “magic stays with us-not black magic, but a magic that entertains … magic that, at its best, thrills”; the magician “can unlock the secrets of chemistry.”

Even though Blackstone rejected the alchemist and his rigmarole, he still thought alchemy had value because its “two heirs” are chemistry and magic. Invisible ink, with its early ties to alchemy, thus transformed over the centuries into a prominent tool for demonstrating the power of chemistry and for showcasing its entertainment value to a general audience.

Modem Toys

Through the 19th century, magic tricks kept interest alive in invisible ink and other chemical curiosities. In the beginning of the 20th, politics brought chemistry into the home in a whole new way. British company J. J. Griffin and Sons had dominated the chemical cabinet market until World War I, at which point Germany stopped exporting the necessary chemicals, and England and France redirected their limited chemical resources to industrial use. In 1914, as hostilities began in Europe, the American Porter Chemical Company stepped in and transformed Victorian chemistry cabinets into a toy-the modem chemistry set.

Harold and John J. Porter set up the Chemcraft line; earlier sets were made of wood, but they shared the side doors so characteristic of later metal sets. Even though the chemistry set started with a magical tradition, it soon developed the niche of an educational toy. And it was on this magical toy that invisible ink piggybacked into 20th-century America and enchanted scores of young children with magical color changes.

The Porter Company didn’t always get the historical details correct, however. In 1922, it advised new owners how to set up tricks and prepare a magic show to “amaze” their friends. Porter chemists suggested dressing up as alchemists, who at the time were seen as wizards. The chemical tricks that Porter described used simple chemical reactions to produce “beautiful color changes, thick clouds of smoke without fire, diabolical odors, and invisible inks.” Although alchemists brought about wonderful color changes in materials, they were not the ones responsible for propelling invisible ink into the world of stage magic.

The Chemcraft chemistry set featured the Oriental alchemist, incorporating a magical tradition while adding mystery. The orientalist theme was common among professional stage magicians at the turn of the 20th century. Even so, Chemcraft’s suggestion to use a madeup Ethiopian slave, “his face and arms … blackened with burnt cork,” and bearing a “fantastic” name such as Allah, Kola, or Rota as an assistant, sounds offensive to 21st-century ears.

By World Wars I and H, invisible ink had developed yet another popular identity: It had become synonymous with international intrigue and spies. During the last half of the 20th century, children’s games and books began to feature secret agents wielding secret ultraviolet pens and invisible inks (and indeed, there is quite a history of the use of invisible inks and other hidden message tactics in espionage). Some of the game books promised hours of “by-yourself enjoyment” as you rub the right answer to make the invisible image apparent.

Even if kids didn’t have this paraphernalia, what better and cheaper way was there to entertain oneself than looking in the refrigerator, grabbing a lemon, and writing a secret message to a friend with a toothpick or cotton swab? Then lo and behold, the gas oven, toaster, or a candle would show the message!

A Winding Journey

On its journey to becoming a childhood pastime, invisible ink has traversed through major historical periods and trends. The natural magic tradition, Victorian scientific and chemical recreations, and secular stage magic had all helped propel invisible ink into the world of magic during the 19th century. Innovation in invisible messaging still continues. For instance, in 2011 Manuel Palacios and David Walt of Tufts University, along with George Whitesides of Harvard University, and their colleagues developed a printed array of microorganism colonies that would release messages on demand through color changes.

But at its core, invisible ink conjures up timeless ideas of wonder, and this feature is no less important to humanity. The tricks of cobalt sympathetic ink have captivated visual artists and provided magical playthings for the home, such as the changeable fire screens and weather dolls. For scientists, curiosity and amazement are often the beginning of a passion for the natural world. The Nobel Prize-winning chemist Rudolph Marcus declared in his award biography: “My interest in the sciences started with mathematics in the very beginning, and later with chemistry in early high school and the proverbial home chemistry set.” Computing pioneer Gordon Moore was so inspired by them that his foundation recently issued a challenge for the creation of a modem equivalent.

Invisible ink in a chemistry set is a child’s toy, true. But for many inquisitive youngsters it is also a gateway to discovering the magic of chemistry and opening up their scientific imaginations-and probably those of their parents as well.