|MadSci Network: Chemistry|
Julian, The class of inks that you are referring to are called "THERMOCHROMIC". These inks consist of dyes (sometimes encapsulated) whose molecular structure changes (usually from a colourless to a coloured form) with temperature. Some of these dyes are reversible and are often used on toys; Others do not necessarily revert back to their original colour after a certain temperature is reached; these dyes are used in ink jet printers or copiers. Typically, these ink jet dyes are encapsulated; once the printer or copier heats them to a certain temperature and the colour develops, the other ingredients in the ink will preserve the dye in the coloured form... For more information concerning these types of dyes, search the world wide web for "thermochromic inks (or dyes)".
Try the links in the MadSci Library for more information on Chemistry.