Invisible Ink Pen, MALEDEN Spy Pen with UV Light Magic Marker Kid Pens for Secret Message and Party Goody Bag Stuffer (6)
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Lemons were also used as organic inks by Arabs around 600 AD, and during the 16th century in Europe. 
Invisible Ink Pen, MALEDEN Spy Pen with UV Light Magic Marker Invisible Ink Pen, MALEDEN Spy Pen with UV Light Magic Marker
John A. Nagy (2009). Invisible Ink: Spycraft of the American Revolution. Westholme Publishing. ISBN 978-1594161414. Some commercially available invisible inks glow very brightly, in a variety of colors, under ultraviolet light. This makes them suitable for use in readmission such as hand stamping.Some inks glow faintly or fluoresce when under an ultra-violet lamp. This is a property of many substances, particularly organic substances and body fluids.
Invisible Ink Pen - Etsy UK Invisible Ink Pen - Etsy UK
Kristie Macrakis (2015). Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. Yale University Press. ISBN 978-0300212600.Suggest not using. Ammonia gas is TOXIC. Ammonia concentrate is CORROSIVE & DANGEROUS FOR THE ENVIRONMENT. This would need a very well ventilated room and student behaviour assessment also CLEAPSS advises a maximum of 2M for Year 7. It is 410BCE and the Ionian (or Decelean) war, between Athens and Sparta has been raging for four years. You have been tasked by King Agis of Sparta to develop a way of delivering secret messages to his armies, which would be unreadable if they fell into the enemies’ hands. You wonder if there is a way of writing with an ink that is only visible under certain conditions. Like all good science-artists, you decide to investigate further… Heat the paper over a light bulb or candle to reveal the hidden message. Hold your piece of paper a few inches above a light bulb or lit candle for a few minutes. The heat from the bulb or flame causes a chemical reaction that darkens the lemon juice so you’re able to read the secret message clearly.  X Research source
Invisible Ink - ThoughtCo How to Make Your Own Invisible Ink - ThoughtCo
Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultra-violet component in light from the photocopier scanning head. Teacher and Technician SheetThe ink can be later made visible by different methods according to the type of invisible ink used. The ink may be revealed by heat or by application of an appropriate chemical, or it may be made visible by viewing under ultraviolet light. Inks which are developed by a chemical reaction may depend on an acid-base reaction (like litmus paper), reactions similar to the blueprint process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapor, e.g. ammonia fumes used to develop phenolphthalein ink. Any of the following can serve as the acidic solution for the invisible ink: milk, vinegar, or lemon juice.