Overview: Experiments on Development

Part VIII told you what a developer is. Nothing in it was measured by you. Every pH, every gram, every mechanism came from a manufacturer’s sheet, a 1928 primer or a paper — and that is the right way to learn chemistry, but it is not the same as knowing it.

This part is where the reading stops and the strips start. Three experiments, one break/fix and one report, all built on a single set of test negatives you expose yourself. The chemistry does not change. What changes is who produced the number.

One variable at a time, against a control

Development has at least a dozen inputs that all move density: exposure, the agent, the pH, the sulfite, the bromide, the dilution, the temperature, the time, the agitation, the age of the bath, the volume of it per unit of film, and the film itself. Change two and any difference you see belongs to neither of them.

So every experiment here holds eleven of those still and moves one. That is not a formality. It is the entire reason a home darkroom can produce evidence rather than anecdote, and the first lesson of the part exists to make it concrete rather than pious.

The sample: step-wedge strips from one exposure batch

The unit of evidence is a strip of film about 135 mm long, contact-exposed through a 21-step transmission wedge under a fixed lamp, and then developed in one cell of one experiment. A Stouffer T2115 carries 21 steps of 0.15 density each, so one strip records the film’s response across three log units — ten stops — in a single exposure.

What the part makes, and where each piece of it goes

1. 1One exposure batch — one emulsion batch, one lamp, one distance, one time, one sitting
2. 2Five practical sessions — each spends strips from that batch, and each runs one shared reference strip from it
3. 3The archive — fixed, washed, dried, labelled and sleeved — the deliverable of the whole part
4. 4Part XXVII — measures these strips rather than developing new ones

The arrow that matters is the long one on the right. Everything the part asks about labelling and archiving is there because a measurement made in Part XXVII on an unidentifiable strip is not a measurement.

Reading it, and what the reading cannot do

There is no densitometer in this part. Part XIII builds the theory of measurement and Part XIV builds the instrument; until then every reading here is made by eye against the strip beside it, and by flatbed scanner, whose numbers are relative and uncalibrated.

That buys three quantities honestly: the threshold step, the first step carrying any density above the unexposed patch; the step count over which tones stay separable; and base plus fog, read on a masked patch every strip carries. It does not buy a slope, because a slope needs densities and the eye returns positions. Knowing exactly where that line falls is the subject of the first lesson.

What it costs

About 41 strips of film, allowing for spoilage — ten or eleven come from a 36-exposure cassette, so four cassettes of one emulsion batch, or a single 30.5 m bulk tin, which is cheaper per strip and removes cassette-to-cassette variation at the same time. Roughly two litres of D-76 stock and a litre of the metol-sulfite base, both mixed in Part VIII’s lab, plus small quantities of borax, sodium carbonate, boric acid and potassium bromide. A 21-step transmission wedge, bought once. And 1,525 minutes of time on task across the seven pages, of which 1,110 are the three experiments.

⚠What is new on the bench, and what is not

Nothing here is more hazardous than Part VIII’s mixing lab, and most of it is the same chemistry handled again. Two substances are new. Sodium carbonate is classified H319, causes serious eye irritation, in 99.8 per cent of the ECHA reports that classify it, with Japan’s NITE-CMC and Safe Work Australia both going further to H318; the course takes the union of those views, which means eye protection whenever it is open. Potassium bromide is classified H319 by 91.4 per cent of reporting notifiers, with H315 and H335 as minority views.

Benzotriazole appears only in one optional extension and is a Level B substance in this course’s encyclopaedia — H302, H319 and, from a quarter to two fifths of notifiers, H332 and an aquatic hazard. That extension is worked at Level B controls inside an otherwise Level A session, and the page says so rather than averaging it away.

Everything else is Part VIII’s list, with Part VIII’s controls: gloves, eye protection, ventilation above five air changes an hour, weighing over a tray, and separate labelled containers for the alkaline developer waste and the silver-bearing fixer. The classification rubric governs the letters and the disposal page governs why the waste answer is jurisdictional.

What you need first

From Part II, measurement and uncertainty and concentration and dilution: this part uses both constantly and re-derives neither. From Part III, measuring pH and buffers. From Part VIII, all of it, and in particular alkalis, buffers and pH, restrainers and antifoggants and the mixing lab, whose two bottles are the controls for everything that follows.

The pages of this part

Part9 of 28Level3 — AdvancedPages7Estimated time25.3 hoursHighest safety levelLevel A

✱5 of 7 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 / 7 lessons in this part completed

• LessonExperimental Design for the Darkroom75 min
• LabLab: Preparing Standard Test NegativesA✱ Darkroom180 min
• ExperimentExperiment: The Activity Series - Alkali and RestrainerA✱ Darkroom300 min
• ExperimentExperiment: The Solvent Series - Sulfite, Dilution and AgitationA✱ Darkroom330 min
• ExperimentExperiment: Comparing Developing AgentsA✱ Darkroom300 min
• Break/fixBreak/Fix: The Developer That Went WrongA✱ Darkroom150 min
• AssignmentAssignment: The Developer Laboratory Report180 min