|MadSci Network: Chemistry|
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.
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