Overview: Permanence and Responsibility

The fixer comes off and the negative looks finished. It is not. Two things are still in the gelatin and one of them is invisible: the thiosulfate the bath left behind, and whatever silver the bath dissolved and did not carry away. Neither does anything today. Both are the reason a print made in 1885 can be a clean purple-brown and the print beside it, from the same studio in the same month, a yellow ghost.

The four questions this part answers

What has to leave, and how fast can it. Washing is diffusion outward and nothing else, so it obeys the arithmetic of gradients rather than the arithmetic of time. That is why n changes of water beat a trickle of the same total volume by orders of magnitude, why a fibre-base print takes an hour where a resin-coated one takes thirty seconds, and why a hypo clearing agent shortens the job by exchanging one ion for another rather than by dissolving anything.

What happens if it does not leave. Retained thiosulfate supplies sulfur to the image silver; retained silver complexes supply silver to the whites; and the air supplies oxidants to both. The three failures look different — a general yellow, a stained highlight, a blue metallic sheen at the edges — and they have different causes, which means a print can tell you which mistake was made.

How you prove it. Not by looking at the print. By two spot tests on materials carried through the same session, read against controls, which is the lab in the middle of this part.

Where the silver goes. A working darkroom’s fixer is the most concentrated silver stream most people will ever handle, and the second half of this part is about recovering it and about the rules that govern the rest of the waste.

What gets measured, and with what

Two tests, both published, both at drop scale.

Residual thiosulfate, by the test Kodak Limited printed in 1949: process an unexposed white sheet of the same paper alongside the batch, cut a strip after the final wash, immerse it for about three minutes in a 1 per cent silver nitrate solution, rinse it, and compare it wet against the wet, untreated part of the same sheet. No colour difference means the hypo has gone; a yellow-brown tint means it has not.

Residual silver, by the test ILFORD publishes for its rapid fixer: 2 g of sodium sulfide in 125 mL of water, diluted 1+9 for use, one drop on a white area of a washed print, blotted, and read against a reference spot placed on a print known to be well fixed. A barely visible cream tint is the reference; anything yellower is a print that was not properly fixed.

Both answer a question the bath-side instruments cannot. Kodak is blunt that silver estimator strips work above about 1 g/L and that on-site techniques “cannot typically be used to demonstrate regulatory compliance”. A test that reads the fixer tells you about the fixer; only a test on the material tells you about the material.

⚠What this part asks you to be careful with

Silver nitrate, at Level B, because that is where the classification rubric puts it and because the encyclopaedia entry records H314 in almost every notification on the ECHA inventory. It also stains skin black, which Kodak’s own 1949 caution says in those words, and the stain is the visible sign that a control failed. The silver nitrate procedure governs every time the bottle is opened.

Sodium sulfide, because it releases hydrogen sulfide on contact with any acid and because Kodak’s toning sheet gives the photographic form of the same rule: sulfide solutions are never discarded with stop baths or fixing baths. The two test solutions in this part are also chemically incompatible with each other, and a drop of one in the bottle of the other destroys it.

The waste, all of it, under the disposal caveat and the silver-bearing waste procedure. The jurisdictional sentence on every waste section of this course is not a formality here; it is the whole subject of the part’s second half.

What you need first

From Part III, diffusion and osmosis, which owns Fick’s law, the halving law and the swollen gel this part applies without re-deriving, and complex formation, which numbers the stepwise thiosulfate complexes.

From Part XI, how fixer works and fixer capacity, exhaustion and residual silver. The second of those is the page this part continues: it named the two failures, stated the course’s own fixing criterion, and said plainly that the residual-silver test was a Part XII lab because it needs reagents Part XI deliberately did without.

From Part II, chemical waste and silver waste, which set up the containers, and laboratory safety and PPE.

Where this part stops

It does not tone. Protective toning is named here as the intervention that changes image silver into a less reactive compound, and Kodak’s own toner publication is quoted for the claim, but the chemistry, the choices and the practice belong to Part XX. It does not print the international standards on image stability, because the course does not hold them; it names them by number, states its own criterion in public where you can check it, and labels every figure it derives as its own. And it does not refine silver: recovery here stops at a sludge in a jar and a container with a label on it.

The pages of this part

Part12 of 28Level2 — PractitionerPages6Estimated time11.0 hoursHighest safety levelLevel B

✱2 of 6 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 / 6 lessons in this part completed

• LessonThe Physics of Washing60 min
• LessonPermanence and Image Deterioration60 min
• LabLab: Testing for Residual Thiosulfate and Residual SilverB✱ Darkroom180 min
• LessonSilver Recovery and the Responsible Darkroom60 min
• ExperimentExperiment: Small-Scale Metallic ReplacementA120 min
• AssignmentAssignment: Your Archival Processing Sequence✱ Darkroom180 min