Thermal receipts are a time bomb, and most of them have already gone off

Thermal paper works by heat-darkening a coating rather than by depositing ink. That coating is unstable. Heat, light, humidity and contact with plastics all accelerate its return to blank - and unlike ink, it does not fade evenly across years. It holds up, and holds up, and then over a few months it goes. By the time someone notices, the receipt they needed for a warranty claim, an expense report or a tax record is a grey rectangle.

This matters more than it sounds like it should, because an enormous amount of financial evidence lives on thermal paper: till receipts, card terminal slips, boarding passes, parking tickets, lab and pharmacy printouts, fuel receipts. Most of it is treated as disposable right up until the moment it is the only proof of something.

What is actually left on a blank receipt

More than you would think. Thermal print heads apply pressure as well as heat, and that pressure leaves a physical impression in the paper that outlasts the visible darkening. Run a fingertip across an old receipt in raking light and you can often feel and see where the text was, even when the darkening has vanished entirely.

There is also, usually, a residual differential in the coating itself - the printed areas remain very slightly different from the blank ones, far below the threshold a scanner's contrast curve will pick up but well within reach of a process that is looking for structure rather than brightness.

How the recovery actually works

Faded thermal recovery reads the page the way a conservator would read a pressure impression, not the way a scanner reads brightness. The pressure relief and the sub-threshold coating differential are both forms of structure, and structure can be found by shape even when it cannot be found by tone. The surviving fragments of each digit and letter are located, their density is restored to the weight the print head laid down, and the result is read back as text so the numbers can be checked rather than trusted.

Practical advice, in order of usefulness

1. 1Photograph thermal receipts the day you get them if they matter. Nothing recovers better than not degrading in the first place - a phone photo of a fresh receipt is worth more than any amount of later restoration.
2. 2Do not store them in plastic wallets or sleeves. The plasticiser migrates into the coating and accelerates its decay; paper envelopes are far kinder.
3. 3Keep them out of direct light and away from heat - the top of a radiator, a sunny windowsill, a car dashboard and a laptop vent are all quiet killers.
4. 4If a receipt is already faint, capture it in raking light: a single lamp low and to one side throws a shadow into the pressure impression that flat, even light erases.
5. 5Flatten crumpled receipts under a heavy book overnight before capturing. Fold shadows break digit shapes and a broken digit is far harder to recover than a faint one.

For anyone holding receipts in bulk

Accounts departments, expense teams and anyone running a shoebox archive of till rolls should assume the clock is already running. The right move is a one-time capture pass - photograph or scan the lot now, at reasonable resolution, before another summer finishes them off. Recovery works on what has already faded; it works far better on what was captured before it did.

What happens when nothing survives

Sometimes the coating is genuinely gone and the pressure impression too shallow to read. In that case the field is withheld and marked, not guessed. On a financial document that distinction is the whole game: an invented figure that happens to be plausible will sail through an expense review or an audit and be wrong, and a wrong number that looks right is worse than an obvious blank.

On a receipt, a confident wrong total is a liability. An honest blank is just a receipt you photograph better next time.