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Invisible Writing

Updated on April 18, 2011
Dr. Jose Rizal's portrait c/o alexmoi04
Dr. Jose Rizal's portrait c/o alexmoi04
UV pen at
UV pen at
vlnky at
vlnky at

Onion paper and salt are the keys

How can we do it? Using onion paper and salt are the main ingredients in invisible writing. We used to do it when I was child, if some secret words are to be relayed to my sisters and brothers, without the knowledge of our parents. Just for the heck of it.

I've read it in the autobiography of our national hero, Dr. Jose Rizal, the pride of the Malay race (and of couse, the Filipinos). He used it to convey love messages to his girlfriend or secrets to the propagandista during the Spanish regime.

To start with, we need:

a) onion paper ( 1 or 2 pcs), b) small amount of table salt (iodized or raw), c) drinking glass with water, d)pointed wooden stick (resembling a pen), e)candle and f)matches

The procedures are:

1) Melt the salt in the drinking glass with water. The water should equal the level of melting salt.

2) Using the wooden stick, dip it in the liquified salt mixture, then start writing on the onion paper the topic you desire (whether it's a love letter, a secret code of something and the like). Don't let the liquid drip on the pointed edge of the stick. (I'm sure, you'll enjoy doing this).

3) Let your finished writings dry on the onion paper for a few minutes. You will see that all the words you've written are disappearing one by one. Once totally dried up, carefully blow the excess of salt that are visible on the onion paper.

4) To retrieve the writings on paper, light the candle. Slowly view the paper near the lighted candle (very near but avoid burning it) and you can see the content of paper.

Amidst the modern technology that we are enjoying right now, it's also important that we knew the things that our ancestors did, especially those who wrote our world history.


Even kids are into invisible writing!

I thought this best kept secret among the elite society or the revolutionists in our ancestors' era is still hidden in an old treasure's chest. I was wrong. Kids already knew it!

Ordinary white bond paper and lemon juice will do. With the help of stick or cotton buds and a flat iron, you can easily read what they have written. Provided the lemon juice is not yet dry on the paper. Just plug in the electric flat iron an press the paper against another bond paper. Presto, you can read the "cryptic" message of your kids! You can also use milk instead of lemon juice.

How about just using white crayola as writing medium on the white bond paper or just ordinary paper? To read it, burn a page of your old newspaper. Spread the carbonized paper evenly on the paper with writings and you can now read its content or message.

How about you, fellow hubber, do you have a method of invisible writing?

Understanding invisible writing

During World War II, secret messages were passed through this kind of writing using liquids or chemicals that will not be traced on a piece of paper, without fumes or any chemical remains on the surface. Here's how they determine what substance to use.

The World War II SOE training manual identified the following properties of an "ideal" invisible ink:

  1. Very water soluble, i.e. non-greasy.
  2. Non-volatile, i.e. no pronounced smell.
  3. Not depositing crystals on paper, i.e. not easily seen in glancing light.
  4. Invisible under ultraviolet light.
  5. Does not decompose or discolour the paper e.g. not silver nitrate.
  6. Unreactive with iodine, or with any of the other usual developers.
  7. Potential developers for the ink should be as few as possible.
  8. Should not develop under heat.
  9. Easily obtainable and has at least one plausible innocent use by the holder.
  10. Not a compound of several chemicals, as this would violate "7".

Organic inks

Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of 'heat fixed' ink, any acidic fluid will work. The most secure way to use any of the following substances for invisible ink is by dilution, usually with water, close to the point when they become difficult to develop.

  • Cola drink
  • Honey solution
  • Lemon, apple, orange or onion juice
  • Milk
  • Bodily fluids such as urine, semen, saliva or blood serum.
  • Soap water
  • Sugar solution
  • Wine, or vinegar
  • Cobalt chloride, which turns blue when heated and becomes invisible again after a while (if not overly heated)

The writing is rendered visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100-Watt light bulb is less likely to damage the paper.

Inks visible under ultraviolet light

Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a property of many substances, particularly organic substances and body fluids.

Absorbing ultraviolet light without flouresceing is a special property of inks with a yellow tint.

Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.

Examples of inks visible through an ultraviolet light are:

(Source: Wikipedia)


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    • travel_man1971 profile imageAUTHOR

      Ireno Alcala 

      7 years ago from Bicol, Philippines

      @AmaTainted: Thanks for the appreciation. Kids are really amazed by the way this 'top secret' discovery is on the loose here on HubPages (LOL). :D

    • AmaTainted profile image


      7 years ago from Texas

      nice post! use to amuse the children with this...

    • travel_man1971 profile imageAUTHOR

      Ireno Alcala 

      7 years ago from Bicol, Philippines

      Thanks for dropping by, sailor. This technique is still practiced by people who want to be different, in a way that sending messages can be this exciting!

    • thesailor profile image


      7 years ago from Seven Seas

      Great hub. In this era of technological advances, we can still use the old conventional way to pass sensitive facts aside from cryptic messages online. No password needed.

    • travel_man1971 profile imageAUTHOR

      Ireno Alcala 

      9 years ago from Bicol, Philippines

      Thanks, Cosette. After writing this hub, my niece, Jana Kris, told me that they were doing it at school, but instead of salt they're using lemon (spanish-the yellow ones or mexican-green ones) juice and cotton buds. They can heat the back of ordinary bond paper using gas stove or just pressing another bond paper with hot electric flat iron. How ignorant of me! Really, I'am such an old-fashioned uncle.

    • profile image


      9 years ago

      :) that is so cool. we didn't have onion paper but we still found a way to conduct espionage...we would write our secret message on a pad of paper, pressing down hard, then give the recipient the paper directly underneath the original piece of paper, then destroy the original. the recipient would then swipe a lead pencil over the paper in sideways strokes and the message would magically appear. great hub, that takes me back...


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