Liquid crystal-based TLCs are a temperamental bunch and rather difficult to incorporate into labels, clothes or other goods. Leuco dye inks, though, feature more durable chemistry that lets product designers employ these inks for all sorts of fun applications.
One of the most famous applications of leuco dyes is on cans of Coors Light beer. These cans feature a graphic of a mountain landscape next to the company's logo. At room temperature, the mountains appear white. Cool the can to drinking temperature (about 45 degrees Fahrenheit or 7 Celsius), though, and those same mountains turn a vivid, bright blue. As the beer warms in your hand, the graphic again shifts to its original white. This color change can happen over and over again.
As with TLCs, leuco dyes are also microencapsulated into tiny droplets that are only about 3 to 5 microns in size, which prevents them from reacting with or being damaged by other chemicals.
Usually, leuco dyes are colored when they're at a cool temperature. Then, as heat rises, they become translucent, which lets them reveal any colors, patterns or words that may be printed on an underlying layer of ink. In other products, leuco dyes can be blended with another color so that as temperatures change, a two-tone effect occurs. Mix blue with yellow, for example, and you have an ink that looks green at lower temperatures and yellow when heat rises.
It sounds a bit magical, but there's some basic science behind the way the inks work. The teensy capsules contain a colorant, an organic acid and a solvent. At lower temperatures the solvent remains in a solid state, keeping the colorant and acid in close proximity to each other -- and as a result, they reflect light and create color. As the solvent warms, the colorant and the acid separate and there's no visible color, which in turn exposes underlying inks.
Take your maple syrup bottle as an example. At room temperature, the bottle shows a picture of a tiny black microwave; when warmed, the black area fades, temporarily revealing the word "hot."
When it comes to temperature accuracy, leuco dyes are more ham-handed than TLCs, so you can't depend on them for applications where you really need a precise temperature reading. But leuco dyes can be integrated into all sorts of fascinating and amusing products.