Part IV Overview: Silver and Light

A photograph is the record of one improbable sequence, repeated a few thousand million times over the area of a negative. A photon is absorbed inside a crystal of silver bromide about a fifth of a micrometre across. It frees an electron. The electron is caught at a fault in the lattice. A silver ion, which in this solid is not fixed in place, drifts to the same fault and takes the electron, becoming a silver atom. That happens again, and again, until a cluster of a very few atoms sits on the crystal. Nothing about the crystal has visibly changed. Then a developer arrives, finds that cluster, and converts the whole crystal — every one of the silver ions in it — to metallic silver.

That is the chain. Everything practical in the rest of this course is a variation on it: what the crystal is made of, how big it is, what shape, what is on its surface, what light it can absorb, and what the developer does when it gets there. This part is where the chain is taken apart.

The chain, and which page owns each link

From absorbed photon to developed silver, with the page that owns each step

1. 1Photon absorbed — the-silver-halides — only light above the absorption edge counts
2. 2Electron freed, hole left behind — the-latent-image
3. 3The hole reaches the surface — the-latent-image — a halogen acceptor must take the bromine, or the reaction reverses
4. 4Electron trapped — the-latent-image — at a lattice fault, or at a sulfide or gold speck put there on purpose
5. 5Interstitial silver ion arrives — the-silver-halides supplies the mobile ion
6. 6A cluster of a few atoms: the latent image — latent-image-behaviour asks how long it survives
7. 7Development: the whole crystal reduced — development-as-amplification
8. 8A grain you can see — grain-speed-and-resolution

Read left to right. Steps 2 to 6 happen in something under a millisecond and leave nothing visible behind; step 7 happens minutes later in a tray and is the only one you can watch.

What is settled, and what is not

Three parts of that chain are not in doubt. Light is absorbed by the crystal and frees an electron; silver ions inside a silver halide crystal are mobile, which no ordinary ionic solid manages; and a developer distinguishes an exposed crystal from an unexposed one by the presence of a small silver cluster. Those are the load-bearing facts, and every page here rests on them.

The mechanism joining them is a different matter. The account this course teaches is the one R. W. Gurney and N. F. Mott published in 1938 — electronic and ionic steps alternating at a trap — as the chemist Mike Ware sets it out. It was not accepted quietly: J. W. Mitchell, with Mott himself as co-author, published a differing treatment in 1957, and the smallest developable cluster has been given as different small numbers by different authors. The latent image page names those disagreements, names who held which position, and says which papers this course has read and which it has only verified the existence of.

What you will have made by the end

Four things, all of them measurements rather than demonstrations.

• A precipitation and solubility table for the three halides, with a balanced equation for every change observed and a shielded control for every exposed tube.
• A measured ratio between the exposure that prints an image out on ordinary paper and the exposure that develops one: your own estimate of what amplification is worth.
• A spectral response map of the paper you own, and a safelight check written as a test your room passes or fails.
• A composed photogram, fixed and washed, for the portfolio.

Precipitating the Silver Halides is the first time you weigh and pour silver nitrate; the darkroom session The Latent Image Made Visible is the first time you develop, stop, fix and wash anything.

What you need first

From Part II: the dilution arithmetic of concentration and dilution, which the experiment uses and does not restate; the protective equipment habits of laboratory safety and PPE; the standing rules of silver nitrate handling; and the container discipline of chemical waste and silver waste.

From Part III: solubility and precipitation, which owns the solubility product and the common-ion effect; oxidation and reduction, which owns the half-equation that development is; complex formation, which owns fixing; and rates and temperature, which owns the difference between “will not develop” and “has not developed yet”. None of those is re-derived here.

⚠From here on, silver nitrate is on the bench

The experiment in this part is the course’s first at Level B: chemical splash goggles that seal against the face, nitrile gloves, an apron, eyewash within reach, and work near an open window with a fan. The rubric puts silver nitrate at Level B by name. Two rules from Part II apply without exception from now on: every silver-bearing liquid, rinse and wiped paper goes to the silver waste container, and no ammoniacal silver solution is ever stored or allowed to dry.

!Equipment used before it is built

From this part onward the course assumes a room that can be made dark, an amber or red safelight, and a set of trays — none of which has been engineered yet. The room (making-a-room-dark), the LED safelight (build-an-led-safelight) and the fog test (the-safelight-fog-test) are designed and proved in Part XVI, and the safelight build depends on the electronics of Part XIV, so neither can be brought forward. Until then, the acceptance test for whatever room and lamp you have is Station 4 of the darkroom session in this part: half a sheet covered, increasing times under the safelight, developed and read against a fog control. A changing bag plus a windowless bathroom is an accepted route.

▷Before you read on

Write in your notebook, now, in your own words, what you believe happens inside a silver bromide crystal when light strikes it. Do not look anything up. The same question is set at the end of the part, and the interesting result is not whether you were right but which words you did not have.

The pages of this part

Part4 of 28Level2 — PractitionerPages9Estimated time10.3 hoursHighest safety levelLevel B

✱1 of 9 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 9 lessons in this part completed

• LessonSilver and the Silver Ion45 min
• LessonThe Silver Halides60 min
• ExperimentExperiment: Precipitating the Silver HalidesB90 min
• LessonThe Latent Image75 min
• LessonReciprocity Failure and the Life of the Latent Image50 min
• LessonDevelopment as Amplification60 min
• LessonGrain, Speed and Resolution60 min
• LessonSpectral Sensitivity and Colour Response60 min
• ExperimentDarkroom Session: The Latent Image Made VisibleA✱ Darkroom · Mains120 min