If you’ve ever come across an old silver-based print, perhaps in a box in the attic, or an old family photo album, or even in a stored collection in a library archive, you’ve likely seen a pretty common phenomenon known as silver mirroring. Areas of the photograph will have taken on a shiny surface tarnish, a bit like a dull mirror.  

There is a fun bit of chemistry involved in this, and we’re going to talk about that today. Note that this is just a quick overview of a LOT of information, so if you’re interested in a deeper dive, the information I’ll be discussing is drawn from the AIC Photographic Chemistry Series, particularly unit two, The Latent Image. The videos discussing the Gurney-Mott Mechanism, Photolytic Silver, and Silver Ion Traps can help provide an even greater understanding of these concepts. 

Remember last month, when we went into valence bands and movement of electrons from the ground state to the excited state in order to become available for chemical reactions? Well, now we’re going to look at a bit of what happens once we have that excited electron, and how the reduction of silver ion becomes a silver metal. This process is the basis of photographic image formation, as well as the process that eventually leads to the aforementioned silver mirroring. This is known as the Gurney-Mott Theory. 

So, the light has struck our silver halide ion, and an excited electron has been generated. Now what? Well, first, we have to look at where that electron originates, the halide. Halide atoms, in a vacuum, have only 7 valence electrons. In that state, it’s uncharged, expressed as X0. It’s incredibly reactive this way. It really does not like to be uncharged! Halide atoms follow what is known as the Octet Rule. That is, they prefer to have 8 electrons in its valence band, for maximum stability. An uncharged halide will seek an extra electron to create that stability. When that happens, it becomes an X^– ion and has a closed valence shell (it has maximum allowable electrons – it’s closed to additions.)  

Once that electron is stimulated enough to reach the conductance band and break free from the halide, it will roam and fall into a lower energy level, where it is drawn what is called a silver ion energy trap. This trap is a region of energy within the silver halide grain that pulls in and holds electrons. The electron is then sensed by a positive silver ion, which then comes seeking to bond with that free electron. The energy trap is the site of all of the reactions within our AgX crystal. These traps can be shallow or deep; deeper traps have higher energy and are more stable reaction sites.  

In the early days of silver-based photography, these traps were formed entirely randomly. As you can imagine, it made the reactions a lot harder to control, and created a lot of guesswork and trial and error. Once the process was better understood, chemical sensitizers were introduced into the process to make things more uniform. The most common sensitizer is sulfide (yes, from sulfur; it’s noted as S^2-). A sensitizer creates attractive, consistently deep traps for electrons to congregate. It’s a lot like digging a pit to trap a wild animal. Dig a deep pit, and once you lure the animal, it’s a lot harder for it to escape. Here, it’s electrons and the silver ions that will come looking for them in the world’s tiniest single’s bar.  

Energy traps are essential to latent image formation. Without them, the meetings between free electrons and silver ions would be totally random, and the resulting photographic materials wouldn’t be very sensitive at all, which is no good. They also wouldn’t be particularly stable, as once you have silver ions and electrons in a trap, you want them to stay there as long as possible.  

Now we have our energy trap, our electron, and our silver ion (Ag⁺). We’re all set for chemical magic. They meet, get friendly, and form silver metal (Ag⁰). This is the Gurney-Mott Mechanism in a nutshell. 

If we can get four of these silver metal atoms to congregate in the trap, they will form a silver speck. Now we’re getting places! This is the key to photosensitivity. The more deep traps we can generate, the more sites of silver speck formation we have, the more sensitivity and better image formation we achieve.  

Alas, time takes its toll on all things, including our Ag⁰ coupling. Silver, you see, isn’t the most stable of partners. It likes being Ag⁺, and it will work to get back to that state. Eventually, our Ag⁰ union will dissolve and the Ag⁺ ion will wander off. When this happens, the freed silver ions will migrate to the surface of a photograph and reduce to become silver sulfide. And that, friends, is how silver mirroring happens.  

Hyacinth Tucker (UCL) — Bindery and Conservation Technician