Whether you're a secret agent planning a clandestine mission or you simply want to send a private message to a friend, you’ll need a way to keep your correspondence from being intercepted by prying eyes. From ancient civilizations to modern-day enthusiasts, people have turned to the hidden power of invisible ink to protect their secret messages. From decoder pens to secret scientific formulas, there are countless ways to create hidden messages that can only be revealed by those who know how to unmask the secret.
There are a few different techniques for writing with invisible ink, but the ideal method depends on the intended use and how easily detectible you want your message to be.
Method #1
When it comes to securing valuable items or allowing readmission at events, fluorescent inks are a simple solution. Marking an item with an invisible ink pen (aka security marker) or using a hand stamp with fluorescent ink allows anyone with a UV light or blacklight to illuminate the ink. This method has been used by banks, night clubs and even the United States Postal Service.
Recipes
Tonic Water (Quinine)
- Ingredients: Tonic water (contains quinine), cotton swab or paintbrush.
- Method: Dip a cotton swab or brush into tonic water and use it to write your message on paper. Once dry, the writing will be invisible under normal light.
- Revealing: Shine a UV light over the paper, and the quinine in the tonic water will fluoresce with a bright blue glow, revealing your message.
Highlighter Pen Ink
- Ingredients: Yellow or green highlighter, water.
- Method: Extract the ink from the highlighter by removing the ink cartridge and soaking it in a small amount of water. Write your message using a cotton swab or paintbrush dipped in the diluted highlighter ink.
- Revealing: When dry, shine a UV light over the writing. The fluorescent dye in the highlighter ink will glow, revealing your hidden text.
Laundry Detergent
- Ingredients: Liquid laundry detergent (with optical brighteners), cotton swab or paintbrush.
- Method: Dip a cotton swab or brush into the detergent and write your message on paper. Once dry, the writing will be invisible under regular light.
- Revealing: Shine a UV light over the paper. Most liquid laundry detergents contain optical brighteners that glow a bright blue or white under UV light, making the message visible.
Vitamin B-12 (Riboflavin)
- Ingredients: Vitamin B-12 tablets or B-12 liquid, water.
- Method: Crush a Vitamin B-12 tablet and dissolve it in water or use liquid B-12 directly. Write your message with a brush or swab using this solution.
- Revealing: Shine a UV light over the paper, and the riboflavin in the Vitamin B-12 will fluoresce with a yellow-green glow.
Petroleum Jelly (Vaseline)
- Ingredients: Petroleum jelly (Vaseline), cotton swab or brush.
- Method: Apply a thin layer of petroleum jelly to paper using a cotton swab or paintbrush. The writing will be nearly invisible under normal light.
- Revealing: When exposed to UV light, the petroleum jelly will glow with a bluish tint, making your message visible.
Cerium Oxalate
- Write with a cerium oxalate solution.
- When dry, the writing will be invisible.
- To reveal the message, expose the paper to ultraviolet light. The cerium oxalate will fluoresce, showing the hidden text.
Fluorescent ink is easily revealed, so if you’re looking for an invisible ink that’s harder to detect, you’ll need to use ink that remains hidden until heated or modified by a chemical reaction.
Method #2
Acidic fluids like lemon juice, milk and vinegar can be applied to paper using a pen, brush, cotton swab or even a finger. Once dry, the “ink” becomes invisible. Simply hold near a heat source like a light bulb, hair dryer or candle and the ink will turn brown, revealing the hidden message.
Recipes
Lemon Juice
One of the most well-known methods involves using lemon juice. Simply squeeze some lemon juice into a bowl and use a cotton swab or a fine paintbrush to write your message on paper. Once the juice dries, the writing becomes invisible. To reveal the hidden message, gently heat the paper by holding it near a light bulb, a hairdryer, or over a candle (with caution). The heat oxidizes the lemon juice, turning it brown and making your secret message visible.
Sugar Solution
- Write your message with a solution of sugar dissolved in water.
- Once dry, the writing will be invisible.
- To reveal the message, gently heat the paper. The sugar will caramelize, making the message visible.
Milk Writing
Milk can also be used as an invisible ink. Dip a cotton swab or a brush into milk and write your message on paper. Once the milk dries, the writing will be invisible. To reveal the message, gently heat the paper, just as you would with lemon juice. The heat will cause the proteins in the milk to react, darkening the writing and bringing your message to light.
Bodily Fluids
- Historically used in espionage, write with urine or other bodily fluids and let it dry.
- The writing will be invisible when dry.
- To reveal the message, heat the paper. The urea in urine will decompose and darken, revealing the text.
Aspartame
- Write with a solution of aspartame (a sugar substitute).
- After it dries, the writing will be invisible.
- Heat the paper gently to reveal the message. The aspartame will decompose and darken, making the writing visible.
Method #3
The chemical reaction between an acid and a base may be the most difficult type of invisible ink to detect. When certain acids are applied to a surface and dry, they become invisible to the naked eye. But when a base is applied, a chemical reaction occurs, causing the acidic solution to reappear.
Recipes
Baking Soda Solution
Mix equal parts baking soda and water to create a solution. Use this mixture to write your message on paper with a cotton swab or brush. Once dry, the writing remains invisible. To uncover the hidden text, brush grape juice over the paper. The acid in the grape juice reacts with the baking soda, revealing your secret message in a contrasting color.
Phenolphthalein and Ammonia
For those interested in a more scientific method, phenolphthalein and ammonia offer an intriguing option. Phenolphthalein is a pH indicator that is colorless in neutral or acidic solutions but turns pink in basic solutions. Write your message using a phenolphthalein solution and let it dry. To reveal the hidden text, lightly spray or brush the paper with a dilute ammonia solution, or expose the paper to ammonia fumes (use caution and do this in a well-ventilated area). The ammonia will react with the phenolphthalein, turning the writing pink.
Starch and Iodine Solution
- Mix a small amount of starch (like cornstarch) with water to create your ink.
- Write your message with this solution and let it dry.
- To reveal the message, paint over the paper with an iodine solution. The iodine will react with the starch, turning it dark blue or purple.
Vinegar and Red Cabbage Juice
- Write your message using white vinegar.
- Once dry, the writing will be invisible.
- To reveal the message, paint over the paper with red cabbage juice. The vinegar will react with the red cabbage juice, changing color to reveal the message.
Soap Solution and Lemon Juice
- Write your message with a diluted soap solution.
- After it dries, brush over the paper with lemon juice.
- The acidic lemon juice will react with the alkaline soap solution, revealing the hidden message.
Sulfuric Acid and Potassium Ferrocyanide
- Write your message with a dilute solution of sulfuric acid.
- Once dry, the writing will be invisible.
- To reveal the message, brush over the paper with potassium ferrocyanide solution. The sulfuric acid will react with potassium ferrocyanide to produce a blue color, revealing the message.
How to avoid detection
If you’re using invisible ink to stay under the radar, be sure to choose a method that provides adequate protection from prying eyes. To avoid detection, be sure to follow these simple guidelines:
- Avoid inks that are easily detected, such as fluorescent ink.
- Apply gentle pressure to prevent indents on the paper's surface, as impressions can be seen at certain angles or by placing another sheet of paper over the message and using a pencil to create a rubbing, revealing the hidden message.
- Avoid using substances that cause a shiny surface on a dull paper.
- Avoid inks that have a detectible smell.
How do you know if you’ve been discovered? If your message is visible upon arrival, the recipient will know your message has been intercepted.
Invisible ink methods can be as simple or as complex as you desire. Whether you're engaging in a fun craft project, teaching kids about chemical reactions, or just enjoying the art of secret communication, invisible ink offers endless possibilities. So, gather your materials and let the spy games begin!
Safety Note: While most of these methods use harmless household items, some chemicals can be irritants. Always use protective gear when handling chemical substances, and ensure you are in a well-ventilated space. When using UV light, avoid looking directly into the light source to prevent eye damage.
Posted August 22, 2024
