The fields of printing ink are vast, containing everything from solid ink to pigmented inks, water- or oil-based ones, and more. Some even serve special purposes, such as glow-in-the-dark ink or invisible ink, which will be introduced in this article. This type is often used not only as a fun design option but also to secure sensitive data and prevent forgery. The following article will dive into the topic of invisible inks, the different types and applications, as well as the pros and cons.
Definition: Invisible ink
Invisible ink, also called security ink or sympathetic ink, can only be seen under special circumstances, such as heat or UV light. It was often used in undercover missions during wartime or for security purposes in official documents or certificates.
After the ink has been applied to the surface, it dries completely clear, becoming invisible even with light reflections or texture. This, of course, varies with the quality of the ink, as cheaper ones might leave a shiny residue or feel soft to the touch.
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Applications and limitations
The applications of invisible ink are seemingly boundless. Apart from official documents and anti-forgery measurements, it can also be used in more private settings like children’s books, where a UV torch reveals hidden details. Other possibilities are markers to sign belongings and protect them against theft, fluorescent inks on product packaging to gain the consumer’s attention, and many more.
The limitations of invisible ink are often dependent on the quality. Low-quality inks frequently fade quickly and become visible without the UV light or other reactions, especially when exposed to sunlight. But even well-protected documents can fade and maybe even lose their ability to become visible at all. They are also sensitive to most external influences that can make them visible involuntarily because they have been exposed to their detector, such as high temperatures.
Invisible ink also does not stick well to glossy surfaces, especially in printing. The paper needs to have a rougher surface, which may, depending on the print job, raise suspicion as to why lower-quality paper was used.
One of the biggest limitations is how easily the print head can get clogged. This is because alcohol has a lower temperature at which it evaporates, leaving only the pigments behind. Therefore, the printer has to be cleaned regularly. Another limitation in application is that it will not stick properly to porous surfaces such as paper or natural cloth, making it inapplicable for most everyday printing jobs.
Quantum dots
Quantum dots are a new development in the field of invisible printing. They are tiny crystals, just a few nanometers big, which reflect different wavelengths or colors depending on their size. These crystals can be incorporated into the ink, providing a new layer of security to the printed works. This technique will most likely find application in money, passport, or other official document printing.
Types
There are several types of invisible ink, distinguished by the method by which they become visible. Some of them can even be made or found at home, working well for DIY projects.
These are usually acidic substances, most of them organic, which oxidize when heated and turn brown. It is recommended to dilute them with water until they are close to being unable to develop, to ensure maximum invisibility. But even slightly diluted substances can produce a satisfying result.
- Blood
- Soap solution
- Cobalt chloride
- Cola, honey, or sugar solution
- Lemon, onion, apple, or orange juice
- Milk, soybean juice, wine, or vinegar
Even though technically heating the substance is also a chemical reaction, these substances react with other substances to create visible text or images, often depending on acidity or basicity or other chemical reactions, such as redox.
Base
| Ink | Developer |
| Phenolphtalein | Ammonia, Na₂CO₃ |
| Ammonia | Red cabbage water |
| Copper sulfate | NaI, Na₂CO₃, NH₄OH, K₃[Fe(CN)₆] |
| Iron(II) sulfate | Na₂CO₃ or K₃[Fe(CN)₆] |
| Iron(III) sulfate | Na₂S |
Redox reactions
| Ink | Developer |
| Cobalt(II) chloride | K₃[Fe(CN)₆] |
| Starch | Iodine solution |
| Cerium oxalate | MnSO₄ + H₂O₂ |
Acid
| Ink | Developer |
| Vinegar | Red cabbage water |
| Semen | Amino acids |
Others
| Ink | Developer |
| Lead(II) nitrate | NaI |
| Starch | Iodine solution |
| Sodium chloride | AgNO₃ |
One of the most common types of invisible ink is that which is activated by ultraviolet light. These include the following:
- Sunscreen
- Lemon juice
- Body fluids, blood plasma, saliva
- Soap and laundry detergents with optical brighteners
Some substances, such as distilled water, will alter the fibers and sizing of the surface on a barely visible level. If the paper is then exposed to the fumes of heated iodine, the written parts will turn brown. This process can be reversed by exposing the paper to direct sunlight, making the ink invisible once more.
Advantages and disadvantages
Invisible ink has many advantages and disadvantages to consider before deciding on using it.
Advantages
- Versatile applications
- Easy to make at home
- Enhanced design for product packaging
- Safety against forgery and counterfeiting
Disadvantages
- Effect fades over time
- Vulnerable to external influences
- Chemical types are not sustainable
- Need for developers to reveal the print
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FAQs
There are three main types of reactions:
- Heat
- Chemicals
- UV light
Some types of ink react to more than one of those, while others require specific reactions to become visible.
It can last from a few minutes up to a few years, depending on the quality, type, as well as storage of the paper.
Yes, there are a few ingredients, such as lemon juice or soap, that you can mix with a bit of water to gain invisible ink. Lemon juice works best by holding the paper above a candle (without burning it), while soap water even reacts to UV light.
