Re: Why do pictures fade over time?

Question: Why do pictures fade over time?

Carolina, thank you for your question.  Over the years I have spent a lot 
of time on this problem, both trying to understand why photographs fade 
and trying to improve their fade performance.  

You asked about “pictures” which is a little more general.  Some pictures, 
for instance, oil paintings last a very long time. Also, electro-
photographic images like the ones produced by colour photocopiers can last 
well.  Others, like some ink-jet prints, don’t last long as long as people 
expect.  Putting your question another way, that is, “why don’t pictures 
like photographs and ink-jet prints last as long as oil paintings ?”, 
enables us to explain why fading occurs by comparing a stable picture with 
one which fades.

There are two big differences between the most stable and least stable 
pictures.  One is that the most stable materials use what we often call 
pigments whereas the less stable ones use dyes.  When I use the 
term “pigment” I mean some coloured materials which is insoluble and when 
these are used as colorants in paints, inks or electro-photographic 
toners, they are in the form of very small particles.  Imaging dyes on the 
other hand are usually soluble and used as solutions in water or other 
solvents.

The fact that a pigment is in small particles makes it less efficient as a 
colorant.  That is we need more colorant to give us the same “strength” 
or “density” of colour  ( We use the word “density” here as a short form 
of “optical density” which has a well defined technical meaning giving us 
a useful numerical measure. )  The larger the particles the less efficient 
they tend to be.  A lot of the colorant is effectively hidden behind other 
colorant.  This is a good thing as far as fading is concerned.  Losing 
some colorant is not important because the hidden colorant takes over the 
job of absorbing light. In oil paintings of course, there may be quite a 
thick layer of paint, so losing the top-most part is often not noticeable. 

There is a disadvantage of using low-efficiency pigments though.  The 
colours tend to be less pure and darker.  The smaller the particle size, 
the better the purity of colour.  This is important for being able to 
reproduce some colours accurately.  Also, the smaller the particle, the 
more efficient the colorant is.  The trouble then is that when we lose 
some colorant it has a bigger effect on the density so fading is faster.

Pigments can be made from inorganic materials which are very stable 
chemically.  Dyes though are predominantly organic, that is made of carbon 
compounds.  These can be quite stable, especially when “metallised” by 
combining them with metal ions.  However, the most stable dyes are not 
always the best for other purposes.  In photography in particular, the 
dyes usually have to be produced in the chemical processing which forms 
the image so there is a limited choice of dyes.  Also, the colour of the 
dyes is very important for image quality.  The major manufacturers of 
colour papers have worked hard to find dyes which are both stable and give 
high quality images. Modern photographs last well if they are not exposed 
to bright lights for a long time.

The three main dye-based imaging print systems, photographic paper ( 
silver halide based ), ink-jet and thermal dye-transfer ( sometimes 
called, a little carelessly, “dye sublimation” ) all use the dyes 
efficiently.  Any dye loss therefore results in a measurable density loss.

So how do the dyes fade and what can we do about it ?  It is useful to 
separate fade due to light from “dark” fade when the pictures are not 
being exposed to light.  In light fade, when the dyes absorb light, they 
become what is known as “electronically excited”.  This means that the 
electrons are rearranged in the orbitals they occupy with one being 
promoted to a higher energy state.  In practice, particularly with light-
stable dyes, this situation only lasts a very short time but once in a 
while a molecule of dye will be lost.

There are two main chemical processes which can cause dyes to be destroyed 
in this way.  One way is for the excited dye to react with something near 
it resulting in the addition of hydrogen to the molecule.  This is called 
a “reductive” reaction and in this case a “photo-reductive” reaction since 
it is caused by light.  

More commonly though, the dyes are destroyed by oxidative processes 
involving attack by oxygen.  The process is a little more complicated.  
When the dye absorb light it becomes excited but then before losing the 
electronic energy settles into a slightly lower energy state called 
a “triplet”.  ( I won’t go into the reasons for these names here ).  
Oxygen is an unusual molecule in that, unlike most molecules which are 
in “singlet” states, its normal state is a triplet.  The rules governing 
the transfer of electronic energy from one molecule to another allow 
triplets to transfer readily to triplets and singlets to singlets but not 
between singlets and triplets.  The dye in its triplet state can pass its 
energy to oxygen which is then excited in a singlet state while the dye 
returns to the original state before absorption.  

If the dye is back to normal, why should it fade ?  Well, the problem is 
that the excited singlet oxygen is a highly reactive molecule and will 
react with almost anything around given the chance.  It is produced by 
contact with dyes and the dyes are vulnerable to attack.  The resulting 
oxidized forms are often essentially colourless so fading will occur.
The most stable dyes tend to produce less singlet oxygen and react slowly 
with it if it is produced.  Also, some molecules are known to shorten the 
lifetime of singlet oxygen so that it has less time to react.  These are 
usually used in photographic products.

Because the fade of the best dyes is normally very slow, it is difficult 
to keep oxygen away.  However some polymers can provide effective barriers 
and when photographs are laminated between thin sheets of these polymers 
fading can be reduced.

We havn’t talked about dark fading so much.  Fading in the dark is slower 
of course since the light fade occurs in addition to any dark fade which 
would occur in any case in the light.  It is less of a problem but again 
care has been taken to make dyes stable in the dark.  The mechanisms are 
less well understood because they tend to depend on the individual dyes a 
lot more and are less well studied.  However, most fade seems to involve 
hydrolysis so keeping water away helps.  In all chemical reactions, 
temperature increases the reaction rates so cool dry conditions are good 
for dye fade.

There’s lots more to say but I expect you already have more than you 
bargained for !  I hope you enjoy your pictures in the future.