Innovations and Payment Systems

Licking, plastic bagging and other credit card secrets


It’s gross, but licking a credit card’s magnetic stripe can make it scan in a reader. We explain why.

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Here’s a sticky question: Why would a cashier lick your credit card?

A quick scan of the blogosphere turns up several posts from shocked consumers wondering why their grocery clerk suddenly went all Gene Simmons on their plastic like it was raspberry flavored.

Other equally baffled cardholders beg to know why applying Scotch tape to the magnetic (mag) stripe, encasing the card in a plastic baggie, rubbing the card on clothing, or wrapping their plastic in a dollar bill or a register receipt magically enabled the cashier to complete their transaction.

As any cashier who has ever been saddled with an older or balky point-of-sale (POS) terminal will attest, some or all of these techniques can turn a rejected card into an accepted card. They just can’t tell you why.

We licked this mystery by going to the source: The folks who design, produce and fine-tune card readers.

Product labs don’t include lick, bag tests
Don’t go wagging your tongue at Norman Castner, countertop product business unit manager of Hypercom, a major POS terminal manufacturer.

“That’s disgusting!” Castner chuckles. “In most cases, when you see a bad ‘card-read,’ it’s usually that the card has a flawed mag stripe or dirt or something on it to where licking cleans it off, as disgusting and unhealthy as that may sound. Obviously, electronic devices don’t handle liquid very well in most cases, so it’s not recommended. We have not done any licking tests in our lab.”

To fully understand what’s going on with all these hinky (not to mention unhygienic) credit card workarounds, let’s start by getting small. Really small.

According to a white paper by Anadigm, a Cupertino, California, maker of programmable analog signal processors (don’t ask), information on your mag stripe is stored on microscopic bar magnets rather like railroad cars. These cars are arranged with like poles abutting (north against north, south against south). At regular intervals where these poles collide, a magnetic flux or “tick” is produced.

The arrangement of “cars” between these ticks (known to you engineers as Aiken Biphase encoding) is what carries the binary information that a POS terminal needs to identify and approve your card.

“It’s basically similar to an antenna,” says Castner. “The mag stripe is putting off a very low frequency which the reader picks up.”

One more techie note: Because the coding is time dependent, there is a series of zeros (think empty cars) called “clocking bits” at the beginning and end of each mag stripe train that tells the reader the pace at which to read the swipe. That’s why POS terminals often instruct you to insert and remove your card quickly. It also enables you to insert your card in either direction on most modern terminals.

“If you’re going extremely slow, it may not read as well,” Castner says. “I’ve seen where people try to go too slow and it just doesn’t accept the read.”

Cut to your purse or wallet, which, let’s face it, is not exactly an ideal environment for mag stripes in general. Dirt, abrasion, temperature extremes and exposure to other magnetic fields all corrode that mag stripe signal. Scratches or other wear can create unintended “ticks” or remove intended ones.

“The card reader in the terminal is expecting data in a certain format,” says Castner. “If there are anomalies in there or just missing data, it cannot resolve and accept the ‘read’ because it’s not getting the data in the format it needs.”

Old cards, machines respond better to bagging, sticking, licking
The various improvised solutions work in two different ways.

If you lick the mag stripe, apply and remove Scotch tape, or rub it on your clothing, you’re essentially removing dirt and debris that may be preventing the reader from accepting the card.

If you slip a baggy, dollar bill or register receipt over the card, or leave Scotch tape on the mag stripe before swiping, you’re increasing the distance between the mag stripe and the reader head. That essentially blurs or softens the crispness of the data signal, which may prevent the reader from rejecting the card due to anomalies.

“In lab testing, it has been shown that doing that does reduce some of the noise that the card interaction creates, and sometimes that can result in a good swipe,” says Hypercom spokesman Pete Schuddekopf. “The serious downside to that approach is that the material that is being used can become lodged in the terminal swipe channel and even damage the reader.”

Card reader software is designed to correct for errors due to the wear and tear of extended card use. Terminals, after all, are in the business of saying yes, not no. But a combination of an aging POS terminal and a card on its last legs forces some merchants to nurse their costly systems along with the tricks mentioned here.

Castner says Hypercom terminals are tested to more than 300,000 mag stripe transactions, roughly the equivalent of five-plus years of service. On average, credit cards are built to last half that. He says when a merchant frequently has to resort to creative solutions like these to get a green light, it’s time to either clean the terminal or replace it.

“Just as changing the oil in your car will extend the life of your car, minimal regular care, whether it’s a quick squirt with compressed air or a couple swipes with a special cleaning card, will extend the life of your terminal,” he says.

If you have a card in your wallet that is particularly problematic, contact your card issuer; they’ll be happy to replace it for free. To get longer life out of your card, consider using the plastic card sleeves that many issuers provide.

“In my wallet, my card is right up against the leather with another card behind it, so it’s always getting some sort of friction somewhere,” says Castner. “I think those holders would definitely help to protect the card. I think that’s why issuers offer them. It’s that many fewer cards they have to replace.”

But for the sanitary sake of your fellow shoppers, please don’t lick the mag stripe, OK?

See related: The short, unhappy life of a credit card

Editorial Disclaimer

The editorial content on this page is based solely on the objective assessment of our writers and is not driven by advertising dollars. It has not been provided or commissioned by the credit card issuers. However, we may receive compensation when you click on links to products from our partners.

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