Emperors, Spies and Seaweed: writing messages only you can read

Public summary: 

Demos for a Crash, Bang, Squelch! public talk featuring examples of steganography and cryptography.

Useful information
Kit List: 

Lemon Juice
Malt vinegar
Washing Powder/fluorescent ink
Suncream/antifluorescent ink
Writing implement: cocktail sticks, fountain pen or paint brush
Fire/heat: bunsen burner, lighter or hot steam from kettle
Red Cabbage Juice
UV light
Foam scytale (cylinder)
Paper scytale messages
White worklights
Water in transparent containers
Padlocked box


Invisible ink:
Lemon juice + heat
Malt vinegar + Red Cabbage Juice
Fluorescent/Antifluorescent ink + UV light

Can be enough for each audience member. Wrap paper message around appropriate scytale (they have different diameters) to reveal message

Padlocked Box:
Box locked by padlocks. To send message, Alice attaches her padlock and sends to Bob, who attaches his padlock and returns to Alice. Alice detaches her padlock and sends back to Bob, who takes off his padlock.

Key Exchange Demo:
Shine worklights through dyed water in darkened theatre to reveal colour. Alice and Bob have their own unique colour. There is one universal colour that all can access.
Alice and Bob perform tasks symmetrically: They both mix their own colour with the universal colour, send the result to the other and mix that with their own colour, resulting in both having the same colour; keys have been exchanged.

Risk Assessment
Date risk assesment last checked: 
Sun, 02/02/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Mon, 03/02/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Lemon juice/bicarb/washing powder/vinegar Lemon juice/vinegar and bicarbonate of soda irritant to eyes. 2 3 6 Only use small amounts. Do not squirt bottles, particularly in the direction of the audience/volunteer.
If in contact with eyes, wash with emergency eye wash provided if trained and confident to do so. Call first aider if necessary.
1 3 3
Lemon juice/vinegar/bicarb Children eating/drinking bicarbonate, lemon juice or vinegar. 3 1 3 Carefully supervise. Don't leave chemicals unattended.
If ingested, advise parents that reactants are all edible, but to seek medical attention if child is feeling unwell as reactants may have been in box for unknown amount of time.
2 1 2
Washing Powder and sun cream Children eating washing powder or sun cream. 2 4 8 Carefully supervise. Don't leave chemicals unattended.
Call first aider in event of injury, who may perform an eye wash if trained and confident to do so. Seek medical attention.
1 4 4
Cocktail sticks/writing implements Splinters/stabbing 2 3 6 Check for obvious splinters on cocktail sticks to be used. Supervise writing of secret message. 1 3 3
Scytales Restless children could fight with scytales 3 1 3 Request parents to keep hold of hand-outs when not in use. 2 1 2
Padlocked box Box could fall and cause injury if heavy. 2 2 4 Use a light box. Pick a volunteer large enough to carry it. 1 2 2
Box and padlocks Trapped fingers 2 2 4 Supervise box locking. Select volunteer old enough to be sensible. 1 2 2
Work lights Dazzling 2 2 4 Do not point lights towards audience. Advise audience beforehand in case of conditions such as epilepsy. Cover sections of lights not being used. 1 2 2
UV Skin/eye damage. 3 3 9 Use soft UV - the UV light was on sale to the general public so the wavelength is at the safer end of the UV spectrum. Point light away from audience. Warn users not to look directly at it. Avoid prolonged skin exposure. Read separate darkroom risk assessment.
In the event of an accident, call a first aider.
1 2 2
Water near work lights Electric shock 2 4 8 Only speakers to perform demo. Use as small quantities of water as necessary to observe effect. Use non slip containers. If spill occurs turn off at mains. In event of emergency call first aider (and turn off power if safe to do so). See electrical parts RA 1 4 4
This experiment contains mains electrical parts, see separate risk assessment.
This experiment is sometimes run in a darkroom, see separate risk assessment.