[Becky]

Becky asked the Naked Scientists:

It's in the show intro, but I haven't heard the question answered.  

I have always been told to keep my receipts, but even stored away from sunlight, they still always fade.

I want to know why! This makes keeping them pretty much pointless because by the time you have to do your taxes, the receipts are useless.  

thanks for your time,
Becky in Missouri, US

What do you think?

[lyner]

They do it on purpose - so you throw them away instead of cluttering up your desk drawer with them.
If you want to retain the info, photocopy them in batches. You'd be doing the tax inspector a favour.
They aren't printed with conventional ink; afaik they are produced by infra red on special paper (at least, they used to be).

[techmind]

Becky asked the Naked Scientists:

I have always been told to keep my receipts, but even stored away from sunlight, they still always fade.

I can't help with *why* they fade, but I can clarify that it's primarily the receipts printed on "thermal paper" which do fade. The thermal paper turns black (actually dark brown) when it's heated - but gradually returns to white over many months. This is exactly the same technology that used to be used for faxes... and has become popular for store receipts over the past decade.
The only way to keep permanent copies is to photocopy the receipts soon after you get them.

Kitchen science experiment: hold an (unwanted) receipt or bus ticket somewhere hot** and see it turn black.
** Hot = momentarily against a 25 or 40W lightbulb, in the steam from a kettle, or against a particularly hot household radiator.

I don't know why the process is reversible. Perhaps a chemist can help?

[Bored chemist]

What you need is a chemist with not a lot to do. Oh, sorry, wasn't paying attention there. That's me isn't it.
OK
The property that makes them change colour is thermochromism. There's some stuff aboout it here
http://en.wikipedia.org/wiki/Thermochromism
Unfortunately, it doesn't really tell you why they fade so here's a bit of speculation on my part.
The first reason they might fade is the same as a lot of other dyes- they just aren't very stable in air- particularly in the presence of light.
This won't be helped by the fact that only the dyes that work in this rather unusal process (ie forming leuco dyes etc) can be chosen. Whereas, if I want a stable picture or ink, I can use any colourant I like- Carbon black is especially stable for example.
Another possibility is that the process is simply reversible. Given time, the mixture of chemicals reverts to the original white form.
Now here's a challenge for anyone with an enquiring miind and a long memory.
Find a reciept or something that you don't need. Heat part of it against a light bulb or something until part of it blackens. With a pencil (which is prtty permanent) mark out the blackened area. Leave it lying about until it fades. Finaly re heat it. If the dyes have just faded by decomposition it will stay faded white. If it has reverted to the original form it will blacken again.

[techmind]

I'm fairly sure that an old faded receipt can be blackened again by heating, ie the process reverses - as opposed to a decomposition.

It's interesting that if you hold the receipt in the steam from a kettle (I did more experiements at the weekend) the blackened bit momentarily gets lighter again while in the steam (the hottest part???), but then settles to dark-brown as it cools. The dark/light swirls around a bit with the air/steam flow. It's rather fun to watch.

[Karen W.]

What a cool experiment! Thanks guys.. Question once you have reheated it... does it fade back eventually or does it remain more visible on a permanent basis?

[techmind]

Bonus experiment, following a random discovery elsewhere on the internet:


Apply a piece of Scotch tape to a thermally-printed receipt, and the printing underneath the tape will fade within a few days.
I tested this with genuine Scotch(tm) tape (the slightly cloudy/translucent tape) and verified the tip-off. Tested with Sellotape(tm) (ordinary clear tape) and no such accelerated fading was observed.

You might experiment with different brands/types of sticky-tape and see which work.

[Karen W.]

I will try it sounds fun and I am bored! LOL will be interesting!!

[Don_1]

This was a problem found some years ago by businesses having a VAT inspection.

It was particularly prevalent with fuel receipts issued by 'pay-at-the-pump' systems. The receipts had faded, leaving a blank piece of paper, which pleased the VAT inspectors no end.

HMC&E came up with an excellent solution to the problem. Don't use these systems or any garage which only issues receipts on thermal paper.

Ah! The science and understanding of the VAT man knows no bounds.

[lyner]

My God, tm. You are as sad as me. My family is always taking the mick when I do little experiments like that.