We hold these truths to be self- evident, that all movies are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of chemical accuracy. Sound familiar?
Admittedly, the above is a somewhat altered excerpt from the one and only Declaration of Independence, and although it might have been tweaked just a tad, the message conveyed is clear nevertheless. We’ve managed to come across a particular scene in the film National Treasure – one that will make experts cringe and bring any respectable curator to shudder at this seemingly harmless mistake.
In the scene, actor Nicolas Cage and his partner in crime, actress Diane Kruger, bend apprehensively over the Declaration of Independence. They are trying to decode what they think is a 300-year-old hidden message written in invisible ink. Their way of going about this is critical- at least in the realistic sense. In the movie, Kruger simply rubs a q- tip across the citric surface of a sliced lemon and slides it across the paper, almost magically revealing words. Disposed of their duty to scientific precision, the film then quickly moves on to take care of more pressing matters. However, what is not disclosed along with the decoded message is the fact that the utilized technique would most likely have permanently damaged the priceless document with its crude detail. Why? Just what is this key idea that can make or break such a scene? Our answer lies in the chemical process of oxidation and caramelization.
For those not in the know, invisible ink, also known as security ink, is a unique substance used for writing. Although invisible upon application, visibility can be induced by a specific set of physical changes or chemical reactions, most of which are simple enough to be performed using common household materials. Its purpose makes invisible ink a common target of espionage, property marking, hand stamping, anti-counterfeiting, and, in this case, common identification.
In general, the types of invisible inks can be categorized into three kinds: those developed by heat, those developed by light, and those developed by chemical reactions. Lemon juice, as used in the scene, turns paper light blue when sprayed with iodine and turns the paper brown when exposed to heat. Because there are too many indicators that would reveal the lemon juice, lemon juice is not often used as invisible ink in real life by spies.
Lemon juice is initially not visible to the naked eye because it is composed of sugar, water, and citric acid. None of these components contain much color, and thus they appear “invisible” after the lemon juice dries on paper. The citric acid also inhibits the oxidation of the other components, like lemon, thus preventing the lemon bits from browning. However, the lemon juice can be made visible by the process of heating, which causes the citric acid to decompose and the lemon bits to consequently become oxidized. Oxidation reactions involve the loss of electrons from the species, or the increase in oxidation state by a molecule, atom, or ion.
The following diagrams illustrate the degradation of cellulose fiber, found in paper, by an acid. The glucose units in the cellulose chain are broken down by hydrogen ions from the citric acid, leaving one side of the unit stable while the other seeks to bind with a water molecule. This sets off a repeated cycle that continuously weakens the fibers in the paper. This breaks down said fibers into sugars, allowing the weakened part of the paper to burn faster and with more ease when exposed to heat, therefore allowing for the process of caramelization to occur.
The process of caramelization, which is the oxidation of sugar, is also implicated in the ‘secret’ behind invisible lemon juice ink. It begins, once again, with the heating of the paper on which the message is written. Paper, which contains cellulose fibers, burns with ease and browns quickly. Lemon juice, however, contains acid that weakens the components of paper and breaks down the cellulose fibers in the paper into sugars, which accelerates the reaction of burning and browning.
When heat caramelizes the carbon-based sugars, they turn brown because when the carbon comes into contact with the air, it oxidizes. As a result, the part of the paper with lemon juice on it burns and browns more quickly than the rest of the paper, revealing the invisible message. In fact, if a piece of paper saturated with lemon juice were to be left alone, it would eventually become brown; heating simply speeds up the process.
What was wrong with National Treasure?
In this scene, lemon juice is utilized as the indicator, or the developer, that would reveal the message. In some cases, invisible inks can be made visible using acids or bases. The most common example of this would be phenolphthalein, an indicator that turns a bright pink hue when exposed to ammonia fumes. However, organic acids such as lemon juice are primarily the invisible inks themselves, made visible by the oxidation reaction initiated by the introduction of heat to the acid. National Treasure’s mistake lies in its choice of technique; rather than using lemon juice as either an indicator or an ink, it mixed and matched various parts of different methods to form one big conglomeration. In the film, the team spreads lemon juice over the document, and uses their own breath to supply the heat to reveal the words. Needless to say, this would not have sufficed to produce the subsequent reaction that would turn the juice brown. Later they use a hair dryer instead, which is better, but still not ideal. Still, the main issue lies within the usage of the juice as a developer to reveal the hidden message and then heating- all that would have achieved in reality would be to oxidize the juice covering up all the secret writing, thus rendering the code irretrievable and damaging the document.