Invisible ink and reveals

A handout that keeps something back until the players do the right thing to it. There are four ways in — household chemistry, ultraviolet light, a colour filter, and water or heat — and all of them are cheap. None of them works if nobody at the table suspects there is anything to find.

Which is the rule that matters more than any recipe below: put the method into the players' hands before the handout that needs it. A lamp that was already in the room two scenes ago, a candle somebody lit for another reason, a glass of water on the table. If the only ultraviolet light in the fiction appears the moment an ultraviolet message does, you have not written a puzzle, you have written an instruction.

Second rule: never hide a message on an otherwise blank sheet. A blank page handed to a player is a puzzle announcing itself, and the table will hold it to every light source in the house until something happens. Hide the secret among ordinary writing — in the margins of a letter that says something else, between the lines of a form — so that finding it is a discovery rather than a chore.

Sympathetic inks

Lemon juice

The classic, and still the best first attempt. Squeeze a lemon, dip a fine brush or a cocktail stick, write, and let it dry completely — what you have then is a sheet with a faint gloss on it and nothing else. Heat brings it up: a hairdryer on full, an iron on a low setting through a cloth, or the paper held a few inches above a hot bulb. The acid caramelises before the paper does, so the writing browns while the sheet stays pale.

Write with the brush barely loaded or the liquid spreads along the fibres and the letters close up; capitals and short words survive that better than a sentence in cursive. And practise the reveal before the session, because the gap between not yet visible and scorched is about ten seconds of hairdryer, and that is a poor thing to learn with four people watching.

Milk

Milk behaves like lemon juice but slower and softer: it needs more heat to appear and it comes up in a warm brown-grey rather than a sharp brown. Two advantages over lemon — it does not smell, and it will not eat into the paper if you overdo the drying — which makes it the version to use on a handout you have already aged and cannot replace.

Baking soda and grape juice

Write with baking soda stirred into warm water until no more dissolves, and let it dry to nothing. The reveal needs no heat at all: wipe the sheet with dark grape juice on a sponge and the writing appears in a different shade from the paper around it, purple against brown-purple. Because the players do the revealing themselves with an ordinary liquid, this is the one to use when you want the discovery to happen at the table rather than at your ironing board.

Soda, turmeric and alcohol

The same soda writing, revealed by a different liquid: turmeric stirred into rubbing alcohol, brushed over the sheet, turns bright red wherever the alkali is and stays yellow everywhere else. It is the strongest colour change in this section and the most theatrical — a red message rising out of a yellow page — at the cost of turning the whole handout yellow forever.

Vinegar over soda writing

The gentlest of the group: vinegar on a sponge over dried soda gives a faint yellowish reaction, legible but quiet. Use it when the message should look like it was always faintly there rather than conjured, and when you want the reveal to be a slow wipe rather than a bang.

Ultraviolet

UV pens and a blacklight

A UV marker costs about $5 to $10 and writes an invisible line that glows blue-white under a 395 nm lamp. It is the most reliable reveal here — no heat, no liquid, no practice run, and it works on aged paper, on card, on the back of a photograph and on a wall. That reliability is why it has become the standard trick, and also why it is worth disguising.

Disguise it by putting the UV writing over or beside ordinary writing, so the page has a reason to exist before the lamp arrives. A ledger with an extra column nobody can see, a letter with a second letter written between its lines, a map with one more road on it. And give the lamp a reason too — an evidence kit, a forgery-detector at a border post, a cracked crystal that hums.

A room dark enough to see it

Ultraviolet writing is invisible in a lit room, so the reveal needs the lights down, and a group that has to turn the lights off on your instruction is a group being told a trick is coming. Build the darkness into the scene instead: a power cut, a cellar, a night watch, a ritual that must happen in the dark. Then the lamp is the only light and the reveal happens inside the fiction.

UV on terrain and models

The same pen works on painted surfaces, so a dungeon tile can carry a rune, a model's base can carry a name, and a door in a three-dimensional set can carry the word that opens it. Players who have learned that the lamp reveals things on paper will start pointing it at everything else, which is exactly the behaviour you want from a party in a haunted house.

A lamp hidden inside a prop

Put a UV LED and a coin cell inside a crystal, a ring or the head of a staff, and the magic item does the revealing. Mechanically it is the same trick; dramatically it is a different one, because the players are not using a torch, they are using the artefact they found in the tomb — and they will try it on everything they own.

Colour filters

The red-filter decoder

Print the hidden message in red, then bury it under a dense scribble of cyan and green. To the eye it is noise; through a sheet of red cellophane the red lines vanish into the background and the noise goes dark, so the message stands out as clearly as if nothing else were on the page. It is the only reveal here that works in full light, at a table, with no smell, heat or liquid.

The filter itself should be an object in the world: a pair of spectacles from a dead man's case, a ruby lens, a piece of coloured glass in a church window. Hand it over separately and a page of nonsense becomes a page the party carries around looking for the thing that reads it.

Designing the layers

Build the page in two layers in any editor that has them: the message in the colour that will vanish, the camouflage in colours that will darken. Then test it through the actual gel before printing a hundred copies, because screen colour and printed colour disagree and a filter is unforgiving about the difference. Red is the most forgiving filter and the easiest to source, which is why almost every version of this trick is red.

Water and heat

The warmth of a hand

Thermochromic ink and till-roll paper both change colour at about body temperature, which means the player is the mechanism. Hand over a slip of paper, say nothing, and watch what happens when somebody holds it long enough.

Immersion in water

Some papers go translucent when wet, so a message written on a second sheet behind the first becomes readable the moment the handout is dipped or painted over with a wet brush. The fiction writes itself: a letter that must be read in the river, a page that gives up its secret in the rain, a document the enemy soaked to destroy and accidentally opened.

Steam

Holding a page over a kettle or a pan does two jobs: it develops a heat-sensitive ink evenly, without the scorch marks a hairdryer leaves, and it softens old glue, which is how a sealed envelope gets opened without tearing. Steam is safer than a flame and gentler than an iron, and it is the method to use on anything irreplaceable.