The Magic Mug
Then I knew. This was a promotional mug that used thermochromic ink to change color based on temperature. (Apparently these are all the rage because the inks are no longer toxic, and the changeability they afford to the marketing message is a show-stopper.) They really grab the viewer, so they can “capture client share and ensure brand loyalty.”
In this particular case the use of thermochromic inks was ideal (that is, appropriate for the ultra-deep-sea, miles-below-the-surface, no-light-anywhere ambiance of the subject matter). On the bottom of the mug there was a “cheat sheet,” a drawing of about eight deep sea fish. One of them I recognized from the art therapy project: the angler fish. I knew I was onto something.
So I turned on the tap and put some water in the mug. Then I put the mug in the microwave. Thirty seconds later the same drawing I had seen on the bottom of the mug was visible on the side of the mug (only larger and more colorful). However, the top half of the mug was still black. Apparently, this was because the top half of the mug had not yet reached the temperature needed for the inks to change (or more specifically for the black ink to turn clear and reveal the image printed below it). Way cool. Fortunately I was smart enough to lift the mug by its handle, which was still cool to the touch.
Why This Works
So I went to school on thermochromatic inks. The “thermo” part means temperature. The “chromatic” part means color. (I had Latin in high school, but not physics, and this is why I found this mug so unique. It’s also why I missed the note on the bottom of the mug about not washing it in the dishwasher.)
To simplify all of the technical, scientific information, there are two ways to achieve this color-changing effect with heat: by using thermochromatic liquid crystals (TLCs) and by using leuco dyes.
Quoting from Wikipedia, “At lower temperatures, these liquid crystals are mostly in a solid, crystalline form. In this low temperature state, TLCs may not reflect much light at all, thus, appearing black.” Heat applied to the TLCs changes the spacing between them, and this changes the way they reflect light (thus changing the color of the substrate). Mostly TLCs are used for things like thermometers, since they are harder to use successfully than their alternative, leuco dyes.