How thermal printers work, and why receipts fade

A thermal receipt printer has no ink, no toner and no ribbon. It has a row of tiny heating elements and a roll of paper that is coated to darken when it gets hot. That is the whole mechanism, and it explains both why receipt printing is so fast and so cheap — and why the receipt in your wallet may be blank by the time you need it.

Heat instead of ink

Thermal paper carries a coating of a colourless dye and a mild acid, held apart in a solid binder. When a heating element touches the paper, the binder melts, the two chemicals meet and react, and that spot turns black. The print head fires hundreds of these elements in a line, the roller advances the paper a fraction of a millimetre, and the next line prints. With no ink to move and no impact, the process is nearly silent and fast enough to produce the familiar stepped ratcheting sound of paper feeding out.

Why the image disappears

The reaction that darkened the paper is reversible, and anything that supplies energy can undo it — or set it off everywhere at once. Heat is the obvious enemy: a receipt left on a car dashboard can go completely black in an afternoon. Sunlight fades it to blank. So do oils and solvents, which is why a receipt kept loose in a bag with hand cream or petrol on the hands goes patchy. Even ordinary plastic wallets can leach plasticisers into the coating and wipe the print over a few months.

Keeping a receipt you actually need

For anything that matters — warranties, expense claims, large purchases — photograph the receipt the day you get it, and keep the photo, not the paper. If you must keep the original, store it flat, dark, cool and away from PVC sleeves and other receipts printed face-to-face. And if you are the one issuing receipts for high-value items, print them on plain paper with ordinary ink instead; a receipt that can be read in five years is worth more than one that printed a second faster.