The Christmas treasure hunt for 2011 required the players to swab the castle map in the blood of the murderer. Doing so revealed a secret message. Here’s how to set it up.
Preparing the map
Ordinary baking soda is what we use to write the message. Dissolve a little bit in water, and dip a calligraphy pen or paint brush into the solution.
I must recommend using a calligraphy pen (the kind that you dip in an ink well) and wiping or blotting it on a damp sponge before pressing it against your paper. Any extra water will bead up and slowly soak into the paper, resulting in expansion marks, which will make it obvious that the map holds more than first glance suggests.
After your message dries completely, the surface of your paper may feel a bit grainy. It is entirely safe to wipe these invisible grains clean until your paper is smooth. The hidden message will still show up fine later.
If you see expansion marks after your message dries, all is not lost. Just roll or crinkle up your paper so that it looks old and mistreated.
Preparing the heart
After Halloween, I purchased a cheap fake heart and crammed it into an empty glass jar. I recommend Adams peanut butter, not just for the jar but also for the peanut butter. …But the jar is about as good as you’ll find also. The bright blue or green lid rather betrays the spirit of its contents, however, so I painted mine.
You’ll need a watertight seal, which the jar can no longer provide on its own but which you can achieve by laying a sheet of good plastic wrap across the opening and then screwing the lid down on top.
The reason for watertight seal is that the “blood” which will reveal the secret message is going in the jar as well. It’s grape juice concentrate. You don’t need a lot, only enough for effect; the treasure hunters will only need a tablespoon or so to cover an entire page.
I found that my fake heart absorbed fluid, so I was obliged to put in more than a minimal amount of blood. I noticed, furthermore, that the heart was only permeable through its valves, so I angled them upward, away from the blood.
The paper turns purple, the blood message turns green. It should take a few seconds for the message to show up, varying depending on temperatures. For a few seconds after it shows up, the green will continue to intensify.
My final result was not nearly so bold as my test messages. In fact, I prepared a test message at the same time as the real message, using the same materials, and left the test message alone until a few days after the treasure hunt (which took place a couple of weeks after the two messages were prepared). When I developed the real message (i.e. applied the grape juice) it was just as bold as expected, but when the treasure hunters developed the real message, it was not so good. Perhaps it was due to using unrefrigerated grape juice which had sat in a bottle with the fake heart for two weeks? Write in your own results if you have this problem unlocked