In daily life, receipts are everywhere, from supermarket checkouts to takeout orders. You may notice a gentle nail-scratch turns them black, which restaurant staff use to note served dishes. So, what's the science behind it? And how does receipt paper differ from regular printer paper?

Ever wondered why scratched receipts turn black?
Shopping receipts use thermal paper, which is heat-sensitive and shows black marks when warmed. A nail scratch generates heat and turns the paper black at once.
Unlike regular paper, thermal paper's coated with a film having a special dye. This dye, colorless at room temperature, changes when it meets acid. The coating holds both dye and acid; they're solid at normal temps and don't react, keeping the paper white. Heat makes them melt and react fast, turning the paper black.

Thermal printers work with thermal paper. Their simple principle involves a high-temp print head creating black prints on the paper. Since thermal paper self-colors, these printers need no ink and are lightweight, making them popular in supermarkets and stores, replacing bulky ink printers for receipt-printing.
Why do receipt words fade? Thermal paper, despite its fast-print and low-noise perks, sees printed content fade over time. The leuco-dye in it changes chemically at high temps to form an unstable colored structure, reacting with air and reverting to colorlessness.
Beyond natural fading, we can use thermal paper's traits to protect info. Instead of tearing or covering express labels, heat them with a lighter or dryer.
Heat isn't the only option. Organic solvents like summer toilet water or essential balm can dissolve the dye on thermal coatings, simply and eco-friendly, offering better privacy protection.

Can faded shopping receipts be re-colored?
Don't despair when holding a barely-legible, faded receipt. There's a simple fix.
Faded areas won't turn black again, but we can think differently. On thermal paper, the white parts with unreacted dye and acid remain intact. Heating these areas turns them black, making the faded words stand out as white.
All it takes is a safe heating tool like an iron or hair straightener to revive the receipt's readability.

To operate, use tweezers to pick up the faded thermal paper and press its back on the heating tool evenly. Soon, the paper darkens. The printed areas stand out lighter, making words visible. It may not ensure clarity, but often works well.
Safety first: avoid burns, no open flames. As the reappeared words may not last, snap a photo right away for future reference.

（来源：科普中国）(Source: Sci-China)