Thermocromic Paper (vermilion)

This translucent fibrous rectangle piece of paper was created as part of a research project aiming to make heat sensitive paper react to different ranges of temperature. It was made in a series of mouth-watering colours: this one is tangerine, flaring to a saffron-yellow when exposed to heat. If you rest your hand on it for three seconds, a faint imprint of your palm will appear in yellow. 

The paper is impregnated with a solution of micro-capsules containing a colour-changing substance called a leuco dye. They also contain acids which, when exposed to heat, react and become temporarily colourless, reverting to colour when cooled. This dye was memorably demonstrated in the 1990s range of Global Hypercolour T-shirts, which looked great until you started sweating and revealed armpits of bright contrasting colours. 

Thermochromic paper is more familiar to us as fax paper or till roll for receipts. Thin and waxy feeling, this white paper is especially impregnated or coated with the leuco dye and a different type of acid, and is passed through a printer with a special thermo print head. The resulting print should then be stable at any temperature, but in reality the dye eventually prefers its colourless state. These thermochromic papers have improved, but if you’ve kept an important fax, or receipts from the 1990s that are now completely blank, you’ll appreciate the effect.