{"id":3434,"date":"2021-05-14T13:37:09","date_gmt":"2021-05-14T13:37:09","guid":{"rendered":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/?p=3434"},"modified":"2021-05-14T13:38:29","modified_gmt":"2021-05-14T13:38:29","slug":"photo-chemical-fascinations-part-3-the-gurney-mott-mechanism-or-heartbreak-and-silver-mirroring","status":"publish","type":"post","link":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/2021\/05\/photo-chemical-fascinations-part-3-the-gurney-mott-mechanism-or-heartbreak-and-silver-mirroring","title":{"rendered":"Photo-Chemical Fascinations, Part 3: The Gurney-Mott Mechanism, or Heartbreak and Silver Mirroring"},"content":{"rendered":"\n

If you\u2019ve 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\u2019ve likely seen a\u00a0pretty common\u00a0phenomenon known as silver mirroring. Areas of the photograph will have taken on a shiny surface\u00a0tarnish, a bit like a dull mirror.\u00a0\u00a0<\/p>\n\n\n\n

\"Image
Pictured: A \u201cbefore\u00a0treatment\u201d image from a 2019\u00a0treatment to reduce silver mirroring. Note that the edges are more heavily mirrored than the center. Mirroring most often begins at the edges of a photograph.<\/em><\/figcaption><\/figure>\n\n\n\n

There is a fun bit of chemistry involved in this, and we\u2019re going to talk about that today.\u00a0Note that this is just a quick overview of a LOT of information, so if you\u2019re interested in a deeper dive, the information I\u2019ll be discussing is drawn from the\u00a0AIC Photographic Chemistry Series<\/a>, particularly unit two,\u00a0The Latent Image<\/a>. The videos discussing the Gurney-Mott Mechanism, Photolytic Silver, and Silver Ion Traps\u00a0can\u00a0help provide an even\u00a0greater\u00a0understanding\u00a0of these concepts.\u00a0<\/p>\n\n\n\n

Remember\u00a0last month<\/a>, when\u00a0we went into\u00a0valence bands and movement of electrons from the ground state to the excited state\u00a0in order to\u00a0become available for chemical reactions? Well, now we\u2019re going to look at a bit of what happens once we have that excited electron, and how\u00a0the reduction of silver ion becomes a silver metal. This process is the basis of photographic image formation, as well as the process\u00a0that\u00a0eventually\u00a0leads to the\u00a0aforementioned silver\u00a0mirroring.\u00a0This is known as the Gurney-Mott Theory.\u00a0<\/p>\n\n\n\n

\"Photo-Chemical
Pictured:\u00a0the\u00a0Gurney-Mott Mechanism. Silver ion (Ag+<\/sup>) gains 1 electron and becomes stable silver metal (Ag0<\/sup>).<\/em><\/figcaption><\/figure>\n\n\n\n

So, the light has struck our silver halide\u00a0ion, and an excited electron has been generated. Now what? Well, first, we\u00a0have to\u00a0look at where that electron originates, the halide. Halide\u00a0atoms, in a vacuum, have only 7 valence electrons. In that state, it\u2019s uncharged, expressed as X0. It\u2019s 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\u00a0to create\u00a0that stability. When that happens, it becomes an X–<\/sup>\u00a0ion and has a\u00a0closed valence shell<\/em>\u00a0(it has maximum allowable electrons \u2013 it\u2019s\u00a0closed<\/em>\u00a0to additions.)\u00a0\u00a0<\/p>\n\n\n\n

\"Photo-Chemical
Pictured: The halide ion, in chemical terms. A stable ion (X–<\/sup>) is struck by light (hv), leading to the escape of an electron (e–<\/sup>). A recaptured electron will return the atom to the X–<\/sup>\u00a0state.<\/em><\/figcaption><\/figure>\n\n\n\n

Once that electron is stimulated enough to reach the conductance band and break free from the halide, it will roam\u00a0and fall into a lower energy level, where\u00a0it 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.\u00a0The electron is then sensed by a positive silver ion, which then comes seeking to bond with that free electron.\u00a0The energy trap\u00a0is the site of\u00a0all of\u00a0the reactions within our\u00a0AgX\u00a0crystal. These traps can be shallow or deep; deeper traps have higher energy and are more stable reaction sites.\u00a0\u00a0<\/p>\n\n\n\n

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\u2019s noted as S2-<\/sup>). A sensitizer creates attractive, consistently deep traps for electrons to congregate. It\u2019s a lot like digging a pit to trap a wild animal. Dig a deep pit, and once you lure the animal, it\u2019s a lot harder for it to escape. Here, it\u2019s electrons and the silver ions that will come looking for them in the world\u2019s tiniest single\u2019s bar.\u00a0\u00a0<\/p>\n\n\n\n

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\u2019t be very sensitive at all, which is no good. They also wouldn\u2019t be particularly stable, as once you have silver ions and electrons in a trap, you want them to stay there\u00a0as long as\u00a0possible.\u00a0\u00a0<\/p>\n\n\n\n

\"Photo-Chemical<\/figure><\/div>\n\n\n\n

Now we have our energy trap, our electron, and our silver ion (Ag+<\/sup>). We\u2019re all set for chemical magic. They meet, get friendly, and\u00a0form\u00a0silver metal (Ag0<\/sup>). This is the Gurney-Mott Mechanism in a nutshell.\u00a0<\/p>\n\n\n\n

If\u00a0we can get four of these silver metal atoms to congregate in the trap,\u00a0they will form\u00a0a silver speck. Now we\u2019re getting places! This is the key to photosensitivity. The\u00a0more deep\u00a0traps we can generate, the more sites of silver speck formation we have, the more sensitivity and better image formation we achieve.\u00a0\u00a0<\/p>\n\n\n\n

Alas, time takes its toll on all things, including our Ag0<\/sup>\u00a0coupling. Silver, you see, isn\u2019t the most stable of partners. It likes being Ag+<\/sup>, and it will work to get back to that state. Eventually, our Ag0<\/sup>\u00a0union will\u00a0dissolve\u00a0and the Ag+<\/sup>\u00a0ion will wander off. When this happens, the freed silver ions will migrate to the surface of a photograph and reduce to become silver sulfide.\u00a0And that, friends, is how silver mirroring happens.\u00a0\u00a0<\/p>\n\n\n\n

Hyacinth Tucker (UCL) — Bindery and Conservation Technician<\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"

If you\u2019ve 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\u2019ve likely seen a\u00a0pretty common\u00a0phenomenon known as silver mirroring. Areas of the photograph will have taken on a shiny surface\u00a0tarnish, a bit like […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-3434","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/posts\/3434","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/comments?post=3434"}],"version-history":[{"count":5,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/posts\/3434\/revisions"}],"predecessor-version":[{"id":3445,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/posts\/3434\/revisions\/3445"}],"wp:attachment":[{"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/media?parent=3434"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/categories?post=3434"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/libapps.libraries.uc.edu\/thepreservationlab\/wp-json\/wp\/v2\/tags?post=3434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}