A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society

This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Hidden Past of Invisible

Often synonymous with international , secret writing also had a long- forgotten heyday in stage magic and science demonstrations.

Kristie Macrakis

or many people, the words in- It was a heady time, an era when valleys. Until the 16th century, Schnee- visible ink readily conjure up popular science became intoxicating— berg was known as a a silver-mining images of childhood fun with low-tech entertainment in a low-key town, but after workers depleted the lemon juice and a candle, or spy world. But other historical and cultural known veins of that metal they turned Fmessages during wartime. And there is circumstances carried the to 20th- their attention to cobalt, which was in a good reason for this association, be- century America, and ultimately into high demand for coloring and cause secret writing has long been part modern popular culture. other goods. Miners removed hun- of the worlds of both magical entertain- dreds of thousands of kilograms of ment and international intrigue. But it Invisible Landscapes cobalt from the mountain. Streets such also has an overlooked long, colorful, I’ve been fascinated by the history of as Silver Road and Cobalt Road still and little-known history. invisible ink for a number of years, and snake through the Schneeberg land- The tale begins with the discovery of a have even worked with my colleague scape and define its past. Although sil- magical appearing and disappearing ink, Jason Lye, a color chemistry expert, to ver brought riches to the town, cobalt and crests with a convergence of trends re-create some traditional ink formulas. produced many more exotic marvels— during the end of the Enlightenment in That is why I was excited when, a few including invisible ink. the 18th century. During that era of in- years ago, I had the opportunity to visit The 16th century was the heyday of tellectual ferment, a rise of stage magic Schneeberg, Germany (below), one of “books of secrets,” compilations of tech- coincided with a revival of so-called the birthplaces of these magical sub- nical and medicinal recipes and magic © 2014 Museum of Fine Arts, Boston © 2014 Museum of Fine

natural magic—an occult tradition that stances, and a rich source of inspiration formulas. These books occasionally in- sought power from natural sources, such for modern chemistry. cluded directions for creating invisible as herbology and astrology—alongside I have fond memories of hiking up ink, written as a basic kitchen recipe an increased demand for public scien- and down the trails surrounding that using simple and readily available in- tific demonstrations. It was against the mountaintop village, where shop win- gredients. For example, a recipe would backdrop of this witches’ brew of cir- dows are filled with hand-carved nut- suggest dipping an -written let- cumstances that invisible ink flourished. crackers, smoking men, candelabras, ter in water, where the writing would angels, and pyramids. Villagers there turn from white to black; or it might ad- still use the miners’ greeting Glück vise rubbing powder on fig-tree milk Kristie Macrakis is a professor of history, technol- auf!—Godspeed—and the hammer to make it visible. Other directions ad- ogy, and society at the Georgia Institute of Technol- and pick symbol still decorates knick- vised the experimenter to paint, sponge, ogy. She received her Ph.D. in the history of science knacks, buildings, walls, and fences. or spray a liquid on the secret letter, or from Harvard University, and also specializes in the It is easy to imagine the music of wave it in the air, sprinkle powder on it, history of espionage. She is the author or editor of Bach wafting through the cobblestone hold it over fire, or dip it into water, de- five books, including Seduced by Secrets: Inside streets of this Baroque village. pending on what kind of ink was used the Stasi’s Spy-Tech World (Cambridge Universi- ty Press, 2008). Her latest book is Prisoners, Lov- Located in the Saxon Ore mountain to write the invisible message. ers, and Spies: The Story of Invisible Ink from range, the Schneeberg Mountains once Cobalt allowed invisible ink to be- Herodotus to al-Qaeda (Yale University Press, contained rich sources of silver, bis- come much more advanced. In 1705 a 2014), from which this article is adapted. Internet: muth, and cobalt hidden beneath the mysterious female German alchemist http://www.kristiemacrakis.com bucolic exterior of verdant hills and seems to have been the first person to

198 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Although invisible ink came into its modern usage largely would heat the card to show the hidden image, now turned via Europe, other cultures have also delighted in its magical brown (a method called aburidashi in Japanese). This print is properties. Woodcut prints like this one from the Edo pe- likely inspired by the legend of Jiraiya, a samurai who could riod (1603–1867) in Japan were sold at festivals; the purchaser magically morph into a giant toad. identify bismuth-cobalt as a valuable color changed from rosy red to grassy rable by eyes that sparkled with liveli- substance from which to make invisible green to sky blue when heat was ap- ness. It is easy to imagine a twinkle in his ink. This alchemist was also the anony- plied. When the cobalt was prepared eyes when he started playing with this mous author of three books, including and turned into a solution with which unusual new substance. one with the alluring title On the Key to to write, it was clear, but it produced a In the summer of 1736, the anony- the Cabinet of the Secret Treasure Room of fabulous blue-green color when heated. mous German artist mentioned by Hel- Nature, which included a discussion of The writing disappeared when cooled. lot showed some members of the Royal the changing bismuth-cobalt colors. The Jean Hellot (1685–1766), the French Academy of Sciences in Paris the magi- author’s name was listed only by the ini- scientist who did more than anyone else cal cobalt phenomenon. The artist had tials DJW. Historical research has shown to investigate, promote, and advance the climbed the hills of Saxony in search of that she might have been Dorothea Juli- subject of invisible ink during the 18th the source rock. He dubbed it Minera ana Walchin (or some other form of the century, referred to another person—an Marchassitae, based on a general term name, such as Wallich or Wallichin), but artist, name also unknown—when al- for ore including bismuth, cobalt, iron, little else is known about her. locating credit for the discovery of the and other minerals. The curious prop- This obscure woman alchemist magical qualities of cobalt. Hellot lived erties of this material enchanted Hellot, seems to have discovered a cobalt most of his professional life as an indus- who experimented assiduously during mineral that appeared red in a solu- trial chemist during the early period his leisure time. By the next summer he tion and was supposed to be the “first of the French Enlightenment. He was had written a paper on the subject for matter” from which all other elements a pioneer in the technical chemistry of the Academy’s journal. formed—the Holy Grail for the al- dyeing, mining, and assaying. Contem- Soon after Hellot’s pathbreaking pa- chemists. The cobalt mineral also dis- porary accounts refer to him as a short per was published, German scientists played remarkable visual qualities: Its and rather chubby man, made memo- objected that a Frenchman got credit www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 May–June 199 with permission only. Contact [email protected]. Johannes Balthasar Friderici’s treatise on cryptography, pub- lished in Germany in 1684, in- cludes instructions for the prepa- ration and use of invisible ink. The frontispiece features Mars, the god of messaging, flying through scenes of codes and in- visible writing.

from his scientific friends in different countries. Hellot became so en- chanted with cobalt’s varied color possibilities that he started thinking about painting landscapes with the inks or dyes. He dreamed of creating a win- ter landscape that would transform instantly into a springtime scene when it was heated. The idea caught on, and continued to captivate three common developer types as de- the public for the next two centuries. for “discovering” this substance as a scribed by della Porta. Hellot also added By around 1746, sympathetic ink was new phenomenon. They claimed that two new categories of his own. In the fashionable in Paris, and changeable a Professor Teichmeyer in Jena had, first new type, air developed some dyes, landscape fire screens became all the in fact, first demonstrated the same and in the second one, some inks simply rage. A barren winter landscape with cobalt ink to his students six years appeared and disappeared spontane- tree trunks and branches was painted earlier. Of course, there was also that ously. The common denominator of all on a fire screen with ordinary India mysterious German lady who had de- the old secret-writing pairings was that ink. Artists then painted a solution of scribed similar activity in 1705. Na- once developed, the writing stayed vis- cobalt chloride on the screen to create tionalistic pride became part of this ible even when cooled, unlike the new lush shrubs and greenery. They used priority dispute. The Germans surely sympathetic ink. acetate of cobalt to paint blue features were galled when cobalt ink became such as the sky. The cobalt was invisible known as “Hellot’s sympathetic ink” initially, but as soon as the heat from the in the centuries that followed, an fire reached the screen, the barren land- honor implying French discovery. (Ex- Jean-Jacques scape turned a verdant green. When the perimenters at the time described the Rousseau, the heat source was removed, the scene be- inks, especially those that involved came winter again. By the 19th century, multiple substances, as working with French philoso- women used similarly transforming each other by “sympathy.” The term handheld paper fire screens to shield soon led to sympathetic ink becoming pher, tried to repro- their faces from intense fireplace heat. a synonymous name for invisible ink.) Later in the 19th century chemists Historians now know that, even ear- duce an invisible began to realize that the color change lier, Italian scholar Giambattista della associated with cobalt compounds Porta (1535–1615), had written about ink experiment. It depended not only on heat but also invisible writing, and had observed on the humidity in the air. When the that some such inks developed by heat, burst in his face paper turned from blue or green to a others through application of a gluti- like a bomb, and rose color, it indicated high humid- nous material, and still others through ity. This discovery led to the creation application of a “liquor” that worked nearly killed him. of dolls and floral arrangements that only when paired with a certain chemi- doubled as weather indicators. The Lambert Photography/Science Photo Library Andrew cal. The latter ink that needs a specific doll’s dress or flowers’ petals were developer, known as a reagent, is the Hellot experimented with different impregnated with cobalt chloride so most important and secure kind of mineral and solution pairings to come that they changed color from blue to pairing. Robert Boyle, known as the up with his sympathetic ink. At the pink when there was an increase in the father of modern chemistry, used these time, a scientist could not obtain a solu- amount of moisture in the air and thus substances to make an ink “confess its tion of cobalt chloride at the druggist, a chance of rain. These devices were secrets” in the late 17th century. but had to make it. Hellot bought the often erroneously labeled barometers, Finally in the mid-1700s Hellot devel- mineral from various local apothecar- even though they measured humidity oped a full classification system for the ies or received samples of cobalt pieces rather than atmospheric pressure.

200 American Scientist, Volume 102101 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Decorative handheld screens were used by women in the 19th century to shield their faces from fireplace heat. These screens were sometimes painted with cobalt-based inks that turned green or blue when heated. In this way, barren winter scenes on the screens (a) transformed into verdant landscapes (b) when exposed to the fire’s heat. The inks would disappear again when cooled.

But there was still the question of how the inks worked. Back in Hellot’s time, a number of distinguished chem- ists, such as the Frenchman Pierre Jo- seph Macquer (1718–1784), described Science and Magic the dangerous solution of quicklime and promoted Hellot’s sympathetic Hellot’s discovery had a wide-reaching and orpiment. While a young man, in ink in newly created dictionaries and impact, far beyond this circle of chem- 1736, he tried to reproduce a sympa- textbooks for chemistry. In this way, ists. By the last decade of the Enlight- thetic ink experiment with directions such inks migrated beyond the world enment, science had spread from from a professor of physics and with of magic and became the focus of seri- private laboratories and elite royal so- the help of Jacques Ozanam’s Math- ous scientific attention. cieties to the popular press and urban ematical Recreations. After he had filled In the end, Hellot found the color culture. Philosophical salons, cafés, a bottle with the mixture, it began to changes inexplicable scientifically. Ear- and musées dotted the streets of Paris, “effervesce violently.” He ran to uncork ly experimenters simply attributed the and these new institutions for popu- the bottle, but it was too late. It burst in color changes to the effect of heat and lar science came alive with scientific his face “like a bomb.” cold. By the late 18th century a color lectures, experiments, and demonstra- Rousseau swallowed so much chalk chemist, Edward Hussey Delavel, pos- tions of science. Invisible ink fit right and orpiment that it nearly killed tulated that the salt attracted moisture in with these pastimes. him. He couldn’t see for more than six in the air when cold and expelled it The rise of popular science was not weeks. His health also declined after when heated. This theory was the clos- all harmless fun, however. Jean-Jacques this event. He felt “short of breath, had est early scientists came to matching Rousseau (1712–1778), the famous a feeling of oppression, sighed involun- modern explanations: When the salt is French philosopher, experimented with tarily, had palpitations of the heart, and heated it loses water—an anhydrous spat blood; a slow fever super- state—and when it is cooled the water vened,” from which he never returns to the salt—a hydrous state. fully recovered. Although early (See pages 202–203 for more on the chem- popular science books use the istry of invisible inks.) 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 al- ternative science, sometimes even what we would today call pseudoscience. Franz Mesmer mesmerized Pari- sians with his experiments on animal magnetism, Jean Nol- let electrified spectators with shows in which a charge pul- sated through a row of people, Cobalt chloride turns from blue to pink when and the public was blown exposed to water, as demonstrated by a paper away by the ballooning craze impregnated with the compound (above). The started by the Montgolfier color change occurs because of a hydration reaction; heat can drive off the water, causing brothers. Science’s power be- the cobalt chloride to return to blue. This reac- came intoxicating. tion was used in weather dolls (right) whose As science became part of blue clothing would turn pink in elevated public culture, it provided en- humidity, indicating a chance of rain. (Doll tertainment as well as general image courtesy of Julanne Ziccarelli.) education. Leading the charge www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 May–June 201 with permission only. Contact [email protected]. The Chemistry Behind the Color Change

Invisible inks use a variety of chemical mechanisms to The structure then transitions to an anhydrous form (b), which perform their appearing and disappearing acts. Listed here has an intense blue color. When the substance is used on a fire are a few of the more common types. screen, heating it reveals the painted landscape. The reaction is reversible: When the complex cools back to room temperature, Cobalt Chloride moisture in the air slowly rehydrates the cobalt, converting it Changeable colors, such as in landscape drawings on some an- back to the pale pink hydrated form. tique fire screens, were made by painting a scene with a dilute solution of cobalt chloride. In aqueous solution, water displac- Lemon Juice es the chlorine and Like many fruit juic- forms a coordination es, lemon juice can 2+ H H complex with cobalt, be used on paper O H H creating a pale pink as an invisible ink, octahedral structure which is then devel- O O H H 2Cl– called hexaaqua- oped by heat to a Co cobaltate (II) (figure visible brown. The a; three-dimensional chemistry behind H H O O bonding is indicated the process is sur- H H by line type: bonds prisingly complex; O coming out of the page it amounts to an ac- H H are thicker; ones go- celerated scorching, a ing into the page are or thermal degrada- dashed). The pink tion, of the cellulose color of the cobalt chloride is not noticeable to the naked eye from which the pa- once dried on paper. However, when the complex is warmed per is made. in above room temperature, water molecules migrate out, allow- the lemon juice help ing the chloride ions to interact more strongly with the cobalt. to initiate reactions that remove wa- Kyle Chaplin ter and create free Cl Cl Cl Cl Cl radicals—highly reactive charged Co Co Co Co b molecules—and An invitation to an event at Burning Man these reactions has information stamped on it in lemon char the paper, pro- Cl Cl Cl Cl Cl juice. The recipient had to heat the paper ducing a complex to decipher where and when to go.

were scientist-magicians such as Henri swer in sympathetic ink. Nevertheless, had developed the answer. This feat Decremps (1746–1826), a self-styled magicians employed the compounds in relied on sympathetic inks developed “professor and demonstrator of amus- unexpected ways to appear as if they by liquids or the vapors from liquids. ing physics,” and Joseph Pinetti (1750– had extraordinary powers. 1800), known as the Professor of Natu- For instance, in one trick called a Books and Cabinets ral Magic. They both toured Paris and book of fate or fortune, the trickster was By the end of the 18th century there London performing what was billed as instructed to make a book of about 70 was a resurgence of interest in the sub- “amusing physics and various enter- or 80 pages, with a secret compartment ject of natural magic; invisible ink, with taining experiments.” But Pinetti was built into the back cover. The “magi- its use of natural substances, fit right really only a stage magician. Decremps cian” then wrote a question in regular in. Some practitioners were conjurors took it upon himself to unmask charla- ink, with an answer in sympathetic ink and stage magicians who manipulated tans; he exposed and explained Pinet- made from litharge of lead or bismuth. reality through sleight of hand. Oth- ti’s experiments as mere tricks. This up- The visible questions were listed in the ers were natural scientists enchanted heaval ruined the career of the leading table of contents. The trickster soaked by the trend of packaging magic tricks magician in Paris because his displays a double piece of paper in the so-called for science education, and many wrote were no longer a mystery. “vivifying ink,” made out of quicklime books to advance this goal. Decremps also included sympathetic and orpiment, and placed it in the hid- These new natural magic books con- inks among the secrets he exposed. By den case at the end of the book. Then an tained less on experimental physics now ordinary people at home could audience member selected a question and more on spectacular physical ef- choose five kinds of sympathetic inks she wanted answered. fects, including sympathetic ink. One developed by liquid, air, powder, or The trickster placed the question on a monumental series was Johann Chris- fire, and create amusements such as piece of paper on top of the one written tian Wiegleb’s Natural Magic Consisting the changeable landscapes or various in the book, closed the book, pounded of All Kinds of Amusements and Useful fortune-telling games that involved writ- it shut, and placed a weight on it. When Tricks, first published in 1779. Wiegleb ing a question in regular ink and an an- he opened the book, the “vivifying ink” (1732–1800) was one of Enlightenment

202 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. OH HO OHO

HO O O HO HO

Fe O O OH O OH HO OHO OH

mixture of brown-colored materials. Copper sulfate, another lic , a subcomponent HO HO OHOH ink that can be developed in the same way, chars the paper by of digallic acid. All of this HO O OHO helping catalyze similar degradation reactions. goes to show what a real HOHO O dog’s breakfast of com- O O O OH pounds might be found in HOHO HO HO HO OH OH old-style black ink. d HO HO When tannins are com- HO O OH bined with iron OH O O O salts, complexes O HO O O formed with O OH O O O Fe HO the iron atoms HO HO OH O O O change the elec- O HO O O tron energy lev- e OH OH O O O OH els of the pale HO O O O O O OH brown tannic acid, HO intensifying its light- OH OH Fe O OH absorption proper- Fe O ties to form a highly O O HO OH colored molecule. As O O OH HO iron can make up to OH c OH six bonds with other O O atoms, each iron atom OHOH Tannins and Iron Salts may form a complex HOHO OHOOHO Before the introduction of synthetic dyes, tannins, extracted with more than one f OHOH from growths on oak trees called galls, were combined with tannic acid molecule. iron salts to make black ink. Oak gall juice has also been used as Furthermore, each tannic acid molecule may also accept more an invisible ink, developed using iron salts. The major compo- than one iron atom. In this way, a complex mixture of colo- nents of the extract, tannic acid (c), is a complex molecule, con- rants made from polymers with relatively simple structure sisting of a carbon-based sugar called a saccharide unit, bonded but high molecular weight produce an intense black color, with up to five units of a substance called digallic acid (d). serving well as a color-fast ink once developed. —Jason Lye Within the tannic acid molecule, there are numerous loca- tions where iron can form a complex. One possible structure Jason Lye is president of Lyco Works Incorporated, a consultancy based in Atlanta. employs a (e); another uses the phenolic –OH He holds a Ph.D. in dye and pigment chemistry from North Carolina State Univer- groups (like the ones in f), which is shown here with gal- sity and a B.Sc. in color chemistry from Leeds University in England. OH OH O O O O O O O O OH Germany’s most knowledgeable and re- because it was written as a discussion ages. Manuals such as TheFe Magicians’ OH spected apothecary-chemists. Wiegleb’s and illustrated concepts using everyday Own Book, issued by Dick &Fe Fitzgerald O O O O publisher wanted to impregnate several examples. And in his wildly popular Publishers inOH 1857, also taught amaOH- pages of his book with sympathetic ink Chemical Recreations, published in the teur magicians aboutOH optical, mechaniOH- to help sell copies. Unfortunately, Wieg- mid-1800s, the British chemist John J. cal, or “magnetical” illusions. leb was too busy to procure the some- Griffin included descriptions of Hellot’s In the early 20th century many magi- times hard-to-find cobalt chloride. sympathetic ink and some of the stories cians had become “scientific conjurors,” The explosion of interest in natural surrounding it. By the middle of that and “Professor” Ellis Stanyon offered magic coincided with the invention of century, his company J. J. Griffin and courses on “Fire and Chemical Magic” chemical cabinets, chests full of sup- Sons also started to manufacture and at his “School of Magic” in London. plies for the avocational experiment- sell 11 different kinds of chemical cabi- Although these classes emphasized sci- er. In 1791 the German chemist and nets, dominating the market for some 50 entific wizardry and operated without pharmacist Johann Friedrich August years until the outbreak of World War I. witchcraft, Stanyon thought that they Göttling (1753–1809) built one of the This popularity of chemistry as an had achieved “real magic at last” with earliest “portable chests of chemistry,” everyman’s pursuit coincided with a dramatic color changes from chemicals as he called it. Interestingly, test tubes fad in the Victorian era, in the mid- including cobalt chloride, that key ingre- are not included in the kit; Göttling dle of the 19th century, of entertain- dient of some sympathetic inks. used a wine glass to mix chemicals. ing guests in one’s parlor with magic In 1909, the magician William Linnett, The natural magic tradition and these acts. Chemists at that time encouraged a member of the Society of American new portable chemistry cabinets even- conjurors to incorporate chemistry Magicians, declared that chemistry was tually led to the design and marketing experiments, often involving com- uniquely adaptable to magic and rec- of the modern chemistry set. pounds that would turn invisible in ommended that all magicians become British author Jane Marcet’s Conver- their tricks. Chemical cabinets provid- familiar with it. Invisible inks provided sations in Chemistry (1805) contributed ed the raw materials for these pursuits, a bridge for these two fields: Many ma- to chemistry’s popularity at the time including appearing messages or im- gicians took Linnett’s advice to heart, www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 May–June 203 with permission only. Contact [email protected]. Museo Galileo, Florence/photograph Franca Principe Museo Galileo, Florence/photograph

Chemical cabinets, designed for the hobby experimenter, arose in the 18th century. Some were the size of a desk (top left); smaller versions included Stratham’s stu- dent model (top right). In the early 1900s, the Porter Chemical Company brought chemistry sets to the United States (bottom right) and dominated the market for decades (bottom left). (Top right, courtesy of David Stanley Auctions/photograph by John James; bottom right, courtesy of the Chemical Heritage Foundation Col- lections; bottom left, courtesy of Kristi Eide.)

whereas many chemists became en- the “black arts.” Still, he argued, “mag- Modern Toys chanted with color changes. John Lippy, ic stays with us—not black magic, but Through the 19th century, magic tricks a chemist and amateur magician, wrote a magic that entertains ... magic that, kept interest alive in invisible ink and a manual for magicians called Chemical at its best, thrills”; the magician “can other chemical curiosities. In the be- Magic in 1930. By 1959, teachers were unlock the secrets of chemistry.” ginning of the 20th, politics brought quoted in the updated version of the Even though Blackstone rejected the chemistry into the home in a whole book, calling it a “very fine reference for alchemist and his rigmarole, he still new way. British company J. J. Griffin ambitious teachers who want to popu- thought alchemy had value because its and Sons had dominated the chemi- larize scientific information by present- “two heirs” are chemistry and magic. cal cabinet market until World War I, ing dramatic experiments.” Invisible ink, with its early ties to al- at which point Germany stopped ex- Harry Blackstone, the world-famous chemy, thus transformed over the cen- porting the necessary chemicals, and magician, endorsed the book in the turies into a prominent tool for dem- England and France redirected their introduction and described the alche- onstrating the power of chemistry and limited chemical resources to indus- mists’ quest, but noted that the world is for showcasing its entertainment value trial use. In 1914, as hostilities began in now built on “physical principles,” not to a general audience. Europe, the American Porter Chemical

204 American Scientist, Volume 102 © 2014 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 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 human- ity. The tricks of cobalt sympathetic ink A heat-sensitive picture mug, shown cold (left) and hot (right), contains a modern cousin of invisible ink: temperature-sensitive powders encapsulated in the mug’s image. In response to have captivated visual artists and pro- heat, the powder turns transparent, revealing the hidden image printed beneath. (Photographs vided magical playthings for the home, courtesy of The Unemployed Philosophers Guild, www.philosophersguild.com.) such as the changeable fire screens and weather dolls. For scientists, curiosity Company stepped in and transformed identity: It had become synonymous and amazement are often the beginning Victorian chemistry cabinets into a with international intrigue and spies. of a passion for the natural world. The toy—the modern chemistry set. During the last half of the 20th century, Nobel Prize–winning chemist Rudolph Harold and John J. Porter set up children’s games and books began to Marcus declared in his award biogra- the Chemcraft line; earlier sets were feature secret agents wielding secret ul- phy: “My interest in the sciences started made of wood, but they shared the traviolet and invisible inks (and in- with mathematics in the very beginning, side doors so characteristic of later deed, there is quite a history of the use of and later with chemistry in early high metal sets. Even though the chemistry invisible inks and other hidden message school and the proverbial home chem- set started with a magical tradition, it tactics in espionage). Some of the game istry set.” Computing pioneer Gordon soon developed the niche of an educa- Moore was so inspired by them that his tional toy. And it was on this magical Invisible ink in a foundation recently issued a challenge toy that invisible ink piggybacked into for the creation of a modern equivalent. 20th-century America and enchanted chemistry set is a Invisible ink in a chemistry set is a scores of young children with magical child’s toy, true. But for many inquisi- color changes. child’s toy, but for tive youngsters it is also a gateway The Porter Company didn’t always to discovering the magic of chemis- get the historical details correct, how- many inquisitive try and opening up their scientific ever. In 1922, it advised new owners imaginations—and probably those of how to set up tricks and prepare a mag- youngsters it is their parents as well. ic show to “amaze” their friends. Por- ter chemists suggested dressing up as also a gateway to Bibliography alchemists, who at the time were seen discovering the al-Gailini, S. 2009. Magic, science, and masculin- as wizards. The chemical tricks that ity: Marketing toy chemistry sets. Studies in Porter described used simple chemical magic of chemistry. History and Philosophy of Science 40:372–81. reactions to produce “beautiful color During, S. 2002. Modern Enchantments: the Cul- tural Power of Secular Magic. Cambridge: Har- changes, thick clouds of smoke with- books promised hours of “by-yourself vard University Press. out fire, diabolical odors, and invisible enjoyment” as you rub the right answer Hellot, J. 1737. Sur une nouvelle Encre Simpa- inks.” Although alchemists brought to make the invisible image apparent. tique … Histoire de l’Académie royale des sciences. about wonderful color changes in ma- Even if kids didn’t have this para- Paris. Part I: pp. 101–120; Part II: pp. 228–247. terials, they were not the ones respon- phernalia, what better and cheaper way Lye, J., and H. S. Freeman. 1999. Azo and nitro- sible for propelling invisible ink into was there to entertain oneself than look- diphenylamine dye photochemistry. Advances in Colour Science and Technology the world of stage magic. ing in the refrigerator, grabbing a lemon, 2:124–141. Macrakis, K., E. K. Bell, D. L. Perry, and R. The Chemcraft chemistry set featured and writing a secret message to a friend D. Sweeder. 2012. Invisible ink revealed: the Oriental alchemist, incorporating with a toothpick or cotton swab? Then Concept, context and chemical principles a magical tradition while adding mys- lo and behold, the gas oven, toaster, or a of “Cold War” writing. Journal of Chemical tery. The orientalist theme was common candle would show the message! Education 89:529–532. among professional stage magicians at the turn of the 20th century. Even so, A Winding Journey Chemcraft’s suggestion to use a made- On its journey to becoming a childhood up Ethiopian slave, “his face and arms pastime, invisible ink has traversed For relevant Web links, consult this ... blackened with burnt cork,” and bear- through major historical periods and ­issue of American Scientist Online: ing a “fantastic” name such as Allah, trends. The natural magic tradition, Kola, or Rota as an assistant, sounds of- Victorian scientific and chemical rec- http://www.americanscientist.org/ fensive to 21st-century ears. reations, and secular stage magic had issues/id.108/past.aspx By World Wars I and II, invisible ink all helped propel invisible ink into the had developed yet another popular world of magic during the 19th century. www.americanscientist.org © 2014 Sigma Xi, The Scientific Research Society. Reproduction 2014 May–June 205 with permission only. Contact [email protected].