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I am planning on running a game (not sure what system yet) with pre-made characters for a few weeks while one of my friends is back home for Christmas.

I had an idea that one of the characters has blocked out a portion of his past, which he will remember once he visits a particular scene. Once this character remembers this, I would remind him about an item he once stuffed into a hidden pocket in his coat, which is still there now.

What I would like to do is use some kind of invisible ink to add the item in the hidden pocket to the characters inventory, and upon him discovering it have the player write over the invisible ink with something to make the item visible on the character sheet. That way it really was there all along!

Is there a way to achieve what I'm looking for? Can anybody recommend either a product or a home solution? A solution that works for multiple weeks would be preferable, but I can work with a solution that lasts for only a few hours if I need to.

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    \$\begingroup\$ be careful the location where you write this information, as your player overwrite it as they update their gear list. You might want to put it at the end of the list, so it is in the final slotand unlikely to be overwritten. \$\endgroup\$ – atk Nov 25 '14 at 20:59
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    \$\begingroup\$ I am imagining all the other players trying to heat up their sheets to look for hidden items, and one of them accidentally setting their sheet on fire! :-D \$\endgroup\$ – Dronz Nov 25 '14 at 20:59
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I don't off hand know a place to find them but I have a UV pen. The ink lasts for a long time on paper and you can't see it until you use a UV light of some kind. I think you can try making your own but it shouldn't be too hard to find. The one I have came with a UV light attached just write on the paper and when it's time use the UV light to show the message. Also feels very spy like and makes a cool glowing effect when you end up showing the message. Here are a bunch of pens on amazon so I'm sure you can find some in a store if you need it sooner than the shipping.Search on amazon for UV Pens

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If you have traditional writing utensils, you can use milk or lemon juice as ink. Both work pretty well with simple nib. I prefer lemon juice, because there is less chance for it to rot.

  1. Get some juice
  2. Filter it to obtain clear liquid
  3. Use nib pen to write what you want
  4. When the time comes, tell your player to heat his character sheet over a candle. Gently! Writing will turn brown faster than paper around.

Clarifications

Traditional light-bulb or any heat source over about 70 °C / 160 °F may be safer. These temperatures came from my personal experience. Lower ones will work, eventually, but your players may get bored waiting.

If stored in relatively cool, dry place, writing can last undeveloped for months. One month was max I tried personally. Citrates needs thermal energy to form, and creation is easier in water, that's why you want to keep it dry and don't put it in direct sunlight for long periods of time. Other than that, citric acid (important ingredient) have no limit on it's life (not one we need to care about in this situation, anyway). And it sinks into paper structure, so there is no real risk you will rub it off accidentally.

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    \$\begingroup\$ @JeorMattan I live in EU - light bulbs are mostly banned :( Will add that to my answer for all people living in places without such stupid regulations. \$\endgroup\$ – Mołot Nov 25 '14 at 11:05
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    \$\begingroup\$ CFL or poor quality LED probably heats enough. \$\endgroup\$ – Jeor Mattan Nov 25 '14 at 11:08
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    \$\begingroup\$ @JeorMattan Nope. Or I have too good ones at my place. I mean yes, it will work eventually, but players would manage to go to the store and buy some beer before text is visible ;) Definitely too slow for in-game use. Only my experience, mileage may vary. \$\endgroup\$ – Mołot Nov 25 '14 at 11:09
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    \$\begingroup\$ @Mołot Do you know how long this might last? Will it last a few weeks, or some shorter time frame? Also, if it works, you could also use an electrical or gas heater to reveal the ink! (Unless you're from a location that predominantly uses woodfire heating, I guess.) \$\endgroup\$ – doppelgreener Nov 25 '14 at 11:41
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    \$\begingroup\$ @LordScree Actually I believe sugar may play some role in getting that nice brown colour. Pure sugar solutions works, too, after all, slower and less reliable but they do. It's a topic open to experiments ;) If OP has time, i hope he'll try on his own. Of he don't, I can only suggest what I actually tried. \$\endgroup\$ – Mołot Nov 25 '14 at 17:09
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Ink:

I've always used a Simple mix of Sugar and Water (About 3 teaspoons to a glass of lukewarm water, mixed well). It keeps for a while too, if you want to keep a small bottle around for future escapades.

Other kitchen-ready options include:

  • Lemon/Lime Juice - can be bought in small bottles for use with fish etc
  • Fruit Juice
  • Bi-carb soda

Writing:

A Cotton Bud/Swab (below) works well for writing.

cotton buds

Revealing the text:

Heat the paper. This will cause the sugar to brown, and reveal the text.

  • Hair dryers work, but aren't great and tend to be noisy.
  • A simple 60-80w lamp (with a filament bulb) should be enough for most small amounts of text, so long as the globe has warmed first - leave it on in the corner/on a table (where most lamps would be) until needed.
  • A toaster works really well (It should go without saying, but just hold the paper above it! Don't put it in the toaster)

Side Notes:

  • Wet paper tends to warp slightly, let it dry at room temp, and flatten it between a couple of books or other heavy flat surfaces for a couple of hours/days.
    • As Molot mentions in the comments, using watercolour-ready paper will help negate the warping effect
  • When mixing stuff, test the solution first, using a different piece of paper. Go through the whole process, including browning. You'll know if you need to add more solvent (Sugar/Bi-carb etc) to the solution.
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    \$\begingroup\$ Cotton bud leaves broad mark. It can wrinkle - in exactly the places you put lines of your letters - but you already addressed that. I just wanted to add that if paper is watercolour-ready kind, it is not an issue at all. Modern watercolour papers happen to be really good :) \$\endgroup\$ – Mołot Nov 26 '14 at 7:45
  • \$\begingroup\$ @Mołot - Yeah good point! proper water-ready paper makes a ton of difference (although you may have to bluff past the players as to why one side of their character sheets are now shiny. "Um, bought the wrong type of paper lel" \$\endgroup\$ – Robotnik Nov 26 '14 at 8:09
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    \$\begingroup\$ @Robotnik I only print them on one side, so players can spread and shuffle them. This gives perfect explanation of "because I wanted them to be pretty, that's why". Given that I sometimes use colorful papers, textured ones, once even handmade sheets at handmade papers, I have no problems with sneaking in whatever I want. Besides, I genuinely want them sheets to be pretty! My players can bring a lie detector on me if they want ;p \$\endgroup\$ – Mołot Nov 26 '14 at 8:15
  • \$\begingroup\$ I also print one-sided, but I like the idea of making proper sheets with colours & textures, haha maybe I should start doing that. That way when 'shiny paper' makes it's way in they won't even notice :P \$\endgroup\$ – Robotnik Nov 26 '14 at 8:18
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Get an ink eraser pen. Write the note with it in big widely spaced letters as the paper will suck up the liquid and you don't see anything while writing. Cut up an ink capsule and pour some drops on a flat surface. Dip a paper towel into some of the ink and smear over the hidden message before the ink dried up in the towel. Even dried up the chemical from the ink eraser works on small amounts of liquid ink. The result is a negative reading (white on blue background) of the message written with the ink eraser.

I never tried this with a message hidden over time longer than a couple of hours but the ink eraser liquid dries really fast. I don't see why it should stop working. Plus, it won't become visible accidentally, unless your player writes on the character sheet with ink on the same spot as you left your note. However there is a risk of blurring the writing as the dried up ink eraser liquid becomes fluid again while "absorbing" the actual ink. I know it's not really "absorbing" but I don't know exactly how it works and it looks like that.

Test run the process and be careful not to spill ink everywhere. Have the ink eraser handy in case you did. ;)

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  • \$\begingroup\$ This just reminded me so much of primary school. :) \$\endgroup\$ – fgysin reinstate Monica Jan 5 '17 at 6:50
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You can get invisible ink / decoder pens which work in a similar way to the lemon juice tactics in that the writing is initially invisible but later becomes visible under normal light by a process of developing the ink.

One end of the pen writes the invisible ink, the other develops the ink via a chemical reaction making the text visible.

This is different to the UV/blacklight pens in that with the UV pens you require the UV light source to read the text - when the light is taken away, the text reverts to being invisible.

http://en.wikipedia.org/wiki/Decoder_pen

http://www.ebay.co.uk/itm/Undercover-Spy-Gear-Invisible-Pen-Decoder-Pad-Write-Top-Secret-Notes-/271321056456

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