Robert of Christchurch asks :-
To remind car-owners of when their car should be next serviced I have been using a marker pen to write the details onto car window stickers. By the time of the next service this writing has nearly faded to be invisible. What causes this?
Alan Happer, an organic chemist at the University of Canterbury, responded.
If this happens for untinted windows but less so for tinted ones then we are most likely dealing with photobleaching, and you can't do much about that other than fit each car with tinted windows, which should slow the process down until the next service is due.
Photobleaching is strong light (ie high energy photons) causing the breakdown of the organic molecules used to colour the ink. You will often see this in the blueing of colour posters with age. The red parts of the poster have ink molecules which absorb blue light, the highest energy photons of light, so these molecules are usually the more easily damaged. Hence the red fades, leaving the blue molecules which absorb red light (low energy photons). So the poster ages from full colour to a blueish hue and finally this may also fade.
In your case there may be another reason. We have trouble with marker pens and vinyl labels. In this case I think the problem may be migration into the label of whatever gunk holds the dye to the label, so that it disappears, and the dye simply slowly falls off or is more exposed to photobleaching.
The sample label appears to be paper-backed vinyl. We have also found that if you use paper labels with ballpoints and cover the writing with sellotape the dye fades with time, which suggests that the sellotape is doing something similar.
The answer, of course it to throw away that particular marker pen. A 2B pencil never fades, because the colour is due to graphite (just like traditional printing inks.) However if some tamperproof record is needed this may not work, and in this case the sort of pen used to mark clothing would be needed.