The root of the problem: light carrying energy
Everything grows by the sun, because the sun contains energy. In fact, all the light is like this, and it is this energy that has become one of the chief culprits of the aging of cultural relics. The most deadly possibility is the photochemical reaction: under the action of these energies, the molecules on the surface of the cultural object either decompose or react with other substances, thus losing the original characteristics.
However, in the case of light, energy is not equal. Light transfer energy is not continuous, but is divided into small energy packs, each of which corresponds to a "photon". The bluer the light, the greater the energy of each photon, and the more the photochemical damage is usually caused; even if the total energy is the same, the redder light will cause less photochemical damage. Not strictly speaking, it is like having nothing to do with ordinary tennis, and being beaten by a hundred times of quality super tennis may be going to happen.
Therefore, paying attention to the influence of light on cultural relics requires two things to pay attention to: one is the total energy carried by light, and the other is how many photons are high-energy and how low-energy. When discussing the display of artifacts, the former can be approximated by "illuminance", while the latter can be approximated by "color temperature".
Strictly speaking, to measure the energy of light, the radiant power should be used. But the most important instrument we receive light in our daily environment is our eyes. The most common criterion is the brightness that the eyes feel. So when discussing visible light, we often use "illuminance" - the light intensity is converted into the human eye. The brightness to reach.
Similarly, to measure the photon energy distribution, strictly speaking, spectral information should be used. But museums and photography generally don't use strange light sources, and many ordinary light sources can be approximated by ideal black bodies. So here we use the corresponding temperature of the black body - "color temperature" to approximate the energy state of the photon: in each case the light source will emit various photons of different energy sizes, but the higher the color temperature, the more high-energy photons, the photochemical destruction The greater the force.
It is of course ideal to preserve cultural relics in pure darkness, but this loses the educational and aesthetic significance of cultural relics. A good museum will strictly control the light source in the museum, which will allow visitors to see important details and maximize the life of the cultural relics. But with the best control, the external flash will be ruined. So, what kind of light will the flash light when shooting? Does it exceed the tolerance of the exhibits?
The light of the flash, and the durability of the exhibitTaking the most commonly used xenon flash lamp as an example, in order to understand its luminescent properties in more detail, we discuss the emission spectrum of a xenon flash lamp. As can be seen, in addition to the visible light region (400 nm - 700 nm), the xenon flash lamp has two distinct emission regions, respectively, in the shorter wavelength, higher energy UV region (200 nm - 400 nm). And an infrared region (700 nm – 1200 nm) with a longer thermal wavelength than red light.
Xenon flash emission spectrum: the abscissa is the wavelength range and the ordinate is the intensity [1]
So does the xenon flash meet the requirements? First look at the color temperature. As an excellent substitute for sunlight, the color temperature of xenon lamps is similar to that of 6200K, which exceeds the requirements for collections that are sensitive to light. Although the illuminating time of the xenon lamp as a flash is very short, the instantaneous illuminance can reach tens of thousands of lux when it is 2 meters away from the object [2] - this is obviously far greater than the illuminance value that the collection can withstand.
Recommended value of exhibit illuminance. [3]
Colorful fabrics depend on a variety of dyes. The so-called "Cheng Xiao Xiao He, defeat also Xiao He", the fragility of the dye itself, also makes color fabrics more difficult to preserve.
There are many reasons why dyes are so "sweet", and "photobleaching" is one of the culprit. As the name suggests, photobleaching of dyes refers to the fading of dyes under the influence of light. The mechanism is quite complicated, but most studies have shown that dye photobleaching can be divided into two ways: direct decomposition and oxidative decomposition of dyes. [4,5] Direct decomposition generally requires high-energy ultraviolet light, and the conditions are slightly harsh; and the oxidative decomposition pathway, or light-promoting oxidation pathway, because of the low light requirements, coupled with ubiquitous oxygen In which "for the tiger", it is easy to happen under normal conditions.
According to how the dye molecules react with oxygen after being activated by light, the light-promoting oxidation pathway can be further divided into two types.
The first way is that light activates oxygen through the dye, which in turn destroys the dye. In order to better understand these two approaches, we need to introduce a concept - energy level. For the sake of simple understanding, we can think of the energy levels as floors of different heights. As the saying goes, the water flows down. Molecules actually like to stay at the bottom of stability. However, once there is light, the dye molecules will absorb the appropriate light energy and rush to the higher layers. On the other hand, if we are bathed in oxygen, we may feel that oxygen is very mild. In fact, this is because oxygen is generally triplet oxygen - oxygen in the bottom state. Usually, it is difficult for light to "snap" oxygen, and absorb the light energy, and the high-level dye molecules act as energy transmitters. They generously send light energy to oxygen, and they return. To the bottom. The oxygen that gets the energy goes into the sky one step at a time, turning into a more energy singlet oxygen, revealing the true face of the killer. This singlet oxygen is simply a white-eyed wolf, and when it comes back, it oxidizes the dye. [6]
The way in which singlet oxygen is produced.
Another light-promoting oxidation pathway is more straightforward. As we said before, molecules can be boarded on different floors. In fact, more microscopically, there are different floors inside the molecule, and the tenants are electronic one by one. The electronics have always lived in their own rooms from the lower level to the upper level. When the light comes, the situation is different. After absorbing the light energy, the electrons will jump to a higher floor. If this restless electron jumps back to the original room and releases the absorbed energy in other ways, such as light, then everything is fine; however, the appearance of oxygen makes the restless high-level electronics have new places to go - Light-activated dye molecules transfer electrons to oxygen, themselves are oxidized to free radical cations, and oxygen is reduced to free radical superoxide anions. Free radical superoxide anion can be said to combine the active and oxygen oxidative properties of free radicals, and is a demon who is pregnant. In front of this demon, the dye molecules lost their armor and were decomposed. [7]
The way in which superoxide anion is produced.
Despite the fact that there were not so many synthetic dyes in ancient times, people have obtained a wide variety of natural dyes from nature, such as indigo (anthraquinones), anthocyanins (flavonoids), shikonin (apes), and small mites. Alkali (alkaloids), etc., in which indigo dyes have a very long history of use. Ancient indigo dyeing relied on juices extracted from plants such as bluegrass. In the dyeing process, in addition to the formation of indigo, it is often caused by the temperature and pH changes during dyeing, which produces indigo red, a molecule similar to the indigo structure. However, some studies have found that UV lamps with a dominant wavelength of 365 nm have a significant degradation effect on indirubin in the dye [8].
Indigo dye.
Textiles made from indigo dyeing.
In addition, indigo carmine in the indigo dye (only more sulfonate than indigo, in addition to increasing water solubility, the basic structure and properties are similar to indigo), under the action of oxygen in the UV lamp, oxidative decomposition will occur very quickly. Eosin sulfonic acid. [9]
Indigo carmine red light oxidative decomposition into ruthenium sulfonic acid.
Fabrics use a variety of organic dyes to add color, while another color world - painting, also uses a variety of inorganic pigments, such as lead white, cinnabar and so on. So, can the collection of inorganic pigments, such as oil paintings, escape the flash?
Unfortunately, no. For example, bright yellow paints use a component called cadmium sulfide (CdS), which is popular among painters for its strong tinting strength, stability, and bright color. This paint is used extensively in the works of Monet, Van Gogh, Picasso [11-13] and others. However, under the action of visible light, sulfur in cadmium sulfide is gradually oxidized to sulfate. [13] This process can still be explained by the energy level model mentioned earlier: the electronic tenants who live in cadmium sulfide rush to a higher floor, and once they have room, they originally lived in sulfur. The tenants will come in. As a result, sulfur loses electrons, is oxidized to elemental sulfur, and is oxidized to sulfate.
Cadmium sulfide powder.
Cadmium sulfide (cadmium yellow) used in oil paintings.
A glimpse of the leopard in the tube is evident. The examples above are just for everyone to show that the light on the collection destroys a piece of pie on the leopard. And the damage of the light to the collection is more than this - infrared light, although low energy, but its remarkable thermal effect can accelerate the dehydration and cracking of cellulose-rich collections such as paper and wood; and organic collections, such as animal and plant specimens, bones The chromophores, such as carbonyl and aryl, which are rich in the same, can also be excited under light conditions, oxidized, or simply decomposed directly [15,16].
A small flash of flash light will certainly not be as harsh as the simulated conditions in the lab, but the accumulated damage is enough to produce a dripping effect. For the thickness of history can be passed down for thousands of years, please turn off the flash and carefully appreciate the precious collections!
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