good reference material.The way that black ink works, it’s a black dye which is mixed with crystals of a salt which is either very acidic or very alkaline, and that paint is actually pH sensitive. It’s sensitive to whether the environment is acid or alkaline. As the temperature changes, that material melts and it mixes with the dye. That actually changes the optical properties of that dye and changes it from being very opaque black paint to being completely transparent. So, it’s all governed by the melting temperature of these crystals that you have mixed in with that dye that change the pH of the environment very radically to a particularly temperature. They mean that when the mug is hot, the dye is black, you can't see the picture and then as you drink your tea, you get down the mug, the mug cools down and the picture is revealed from the top as you get down your mug of tea.