Re: what has the properties of colour change with oxidation?

Hi John,

Sorry for the delay in answering your question, but I've found it hard to understand exactly what you're after. As I understand it you want a substance that can be applied to paper so that the paper's age (up to a month) can be determined by looking at the colour.

While oxidation would definitely achieve this effect (the fading of coloured molecules is usually caused by the oxidation, and hence breaking, of the chemical bonds that are vital for molecules to absorb visible light - if molecules stop absorbing visible light, then it will all be reflected and appear colourless or white) I'm confused why you need to focus on the mechanism involved if all you are after is a way of measuring time. Without being funny, can't you just use a clock?

That said, there are many dyes which will be capable of dyeing "normal" paper i.e. based on cellulose as this is the same polymer that makes up the cotton in clothes, so many cotton dyes could be put to use on paper. However, the aim of dye manufacturers is to stop dyes fading, i.e. make them inert to the environment once they are dyed onto fabric. Fading of dyed paper or cotton can occur due to the dyes being washed out e.g. by sweat, water or detergents in washing powders, or by chemical destruction (usually oxidation) caused by oxygen in the air, light (especially UV), chlorine bleach or the oxygen bleach systems in some washing powders.

Quite often fading is due to a combination of these mechanisms, so if you wanted to use your dyed paper, you would need to know whether the dye was susceptible to light, air, bleach etc. and, once the exposure began, have some control over the conditions.

So, where does this leave you? Well, if you want a way of measuring exposure of dyed paper to light, there is light fastness test (specifically for dyed cotton fabric, but you could adapt it) whereby you expose your test sample at the same time as exposing seven (I think) samples of blue-dyed cotton. Although the blues look identical to begin with, they are actually all different and have been developed to fade with exposure to light at different rates. These dyes have well-known fade rates and by using one or more of them to dye your paper you might be able to develop a "paper clock"

I notice you're in the UK, so if you want more info on the standard light fastness test, try the Society of Dyers and Colourists (www.sdc.org.uk) who develop the test. Also, there is a Textile Chemistry & Coloration Dept in the Paper and Textiles School at The University of Manchester (formerly UMIST) which have been involved in developing other fastness tests and have plenty of dyeing knowledge ( http://www.materials.manchester.ac.uk/content/tp/tphome.htm). Or, alternatively, approach the dye manufacturers directly - companies like Clariant, BASF, Hoechst (try Google or www.yell.co.uk).

Get back to me if I can help further.

Cheers,
David