Why do receipt letters disappear in your pocket?

On ordinary paper, ink soaks in and leaves letters. Receipt paper is coated with a thin layer of a normally colorless dye plus a developer that turns that dye black when it melts. When the checkout printer's head heats up in the shape of each letter, the two chemicals melt and mix, and only those spots turn black. That's how numbers appear with no ink cartridge.

Scratch it hard with a fingernail and a dark line appears too; that's friction heat.

The color made this way doesn't last long. Long exposure to sunlight, getting wet with sweat or water, or touching hand oil, hand cream, or plastic sleeves breaks the color down again and the letters fade white. In very hot places, like a car in midsummer, the background itself can turn gray and bury the letters. A pocket combines body heat, sweat, and rubbing against coins and hands all at once, so receipts fade there especially fast.

For a long time, bisphenol A (BPA) was used as the developer in receipts. In a 2014 U.S. experiment, holding a receipt right after using hand sanitizer greatly increased the BPA absorbed through the skin. The sanitizer dissolves the chemical and helps carry it into the skin.

The European Union has banned BPA in thermal paper since 2020, but whether its substitute, bisphenol S, is safe is still debated. Handle receipts briefly with dry hands, and wash your hands afterward.

For a receipt you'll need for your budget or a refund, the surest move is to photograph it on your phone the day you get it. Keep the paper somewhere cool and dark, in a paper envelope rather than plastic.

There's something to know when you throw it out, too. Though it looks like paper, a receipt is chemically coated thermal paper, so it goes in general trash, not paper recycling. If you can choose an e-receipt, it won't fade and it's easier. If you paid by card, your card app keeps a record as well.