Thermochromic inks are those that change colour in response to changes in temperature. These inks have serious applications such as in the food industry. They can be used to indicate when a packaged food has reached the correct temperature in an oven. They are also used in forehead thermometers.

They could also be applied to stamps. For instance, A thermochromic ink could be blue when cold and change to red when warm. Diagram ‘A’ shows a second class stamp when cold. When it warms up after delivery through the letter box, the base colour could change from blue to red (diagram ‘B’).
Alternatively, the crown on the Queens head could be designed to change colour when warmed up.

1. Colour /shade the commemorative stamp seen opposite.

2. Explain how thermochromic inks could be used on a stamp such as this to produce an usual effect.