Residual hypo and residual silver tests

Purpose

To answer two different questions about one print with two different tests: residual thiosulfate, which says whether the wash worked, and residual silver, which says whether the fixing did.

When to use it

On the unexposed control sheet carried through the batch, after the final wash and while it is still wet. Also on any single print whose history you have reason to doubt.

What this procedure runs, and what it is not. The classical designations are HT-2 and ST-1, and both are published: HT-2 and ST-1, read from the J-1 page images after a text search of the corpus had found neither designation anywhere. The residual-silver step below is ST-1’s composition, reached through ILFORD’s sheet rather than Kodak’s. The residual-hypo step below is not HT-2: it is Kodak Limited’s 1949 one per cent silver nitrate immersion, where HT-2 is an acidified silver nitrate drop read against a printed tint card this course does not hold. Name in your notes the one you actually mixed; “HT-2” against a test that was not HT-2 is borrowed authority.

Before you start

This is Level B work throughout, because silver nitrate is named in the rubric’s Level B criteria by name. Chemical splash goggles that seal against the face, not spectacles; nitrile gloves, changed the moment either solution touches them — the glove table records no entry for silver nitrate in the permeation guide the course read, so these are splash protection with no breakthrough time behind them. Eyewash within reach before the first bottle is opened; first aid read beforehand rather than during.

One step of this procedure is above Level B, and it is not performed here. Weighing solid sodium sulfide is Level C work on its acute toxicity by ingestion and skin contact. The stock — 2 g in 125 mL — is therefore made once, as a separate operation, under the controls its encyclopaedia entry requires. Inside this procedure the reagent exists only as a solution dosed in drops. If you do not have those controls, run the thiosulfate test alone and write in the record that the fixing was not tested, which is a decision rather than a blank.

No acid comes to this bench. Sulfide and any acid give hydrogen sulfide, which the incompatibility matrix lists; silver nitrate and sulfide destroy each other on contact. The control is layout, not care.

Procedure

1. Make 100 mL of 1 per cent silver nitrate: 1 g weighed over a tray, made up to 100 mL, by the silver nitrate procedure. Label it and stand it in a tray.
2. Dilute the sulfide stock 1+9 for use. Nothing is weighed at this bench.
3. Set the bench out with the two reagents at opposite ends, one dedicated dropper or pair of tongs each, of visibly different appearance, and a separate waste container for each.
4. Take the control sheet out of the final wash and keep it wet. Both tests read a wet sample; a dried one is a different material.
5. Cut the sheet in two across its width. One half is the test half; the other is the untreated half and is the control for this specific sample.
6. Immerse the test half in the silver nitrate for about three minutes, using its own tongs. Keep your fingers out of the dish: the solution stains skin black, and the stain is the visible proof that a control failed.
7. Rinse it briefly in the second dish.
8. Lay it beside its own untreated half, both wet, and compare in subdued daylight or artificial light. No colour difference means the hypo has been removed; a yellow-brown tint means it is present.
9. For the silver test, confirm first that the print is well washed. The test is not effective on a print straight from the fixer, and a reading taken there is no reading at all rather than a weak one.
10. Make the reference spot first: one drop of the diluted sulfide on a white area of a print known to be well fixed by the two-bath method and thoroughly washed.
11. Blot the excess off with clean blotting paper after a few seconds. A pool left to dry gives a dark ring that is not a reading. What remains should be a barely visible cream tint — that is the reference for this paper.
12. Put one drop on a white area of each sample, blot it the same way, and compare with the reference. Yellowing means the print is not properly fixed.
13. Write every reading down in words, at the bench, against the reference. A remembered tint is not data.
14. Where the sulfide test yellows, soak the prints in water for five minutes and give them the recommended fixing and washing sequence again in fresh fixer.
15. Route the waste: the silver nitrate solution, its rinse and the treated strips to silver-bearing waste; the sulfide to its own container and never into one holding a stop bath or a fixing bath; both under the disposal caveat.

How you know it worked

• Each comparison was between two halves of one sheet, which removes paper, developer, fixer and wash from it in a single stroke.
• The reference spot was made on the same paper, on the same day.
• Both strips were still wet when read, in even, subdued light.
• The record describes every sample in words, including the ones that passed.
• A sample that could not be tested is recorded as not testable, with the reason.

If a step fails

Step 1, you have no balance you trust for a 1 g weighing. Run the bench check first. A test solution of unknown strength produces a colour of unknown meaning.

Step 4, the sheet has dried. Re-wet it thoroughly and note that you did. A tint read on a re-wetted sheet is weaker evidence than one on a sheet that never dried.

Step 6, the strip darkens all over at once. It is carrying free fixer rather than traces of it, which usually means the batch never had a first wash. Record it and rewash.

Step 8, you cannot decide whether there is a difference. Improve the viewing rather than the guess: one steady light, a white background, and the two halves touching along the cut edge. If you still cannot decide, record “no difference detectable by eye” and say what the light was.

Step 9, the sample came straight from the fixer. Record the row as not testable and say the reason is a property of the test. Free thiosulfate holds the silver the sulfide is looking for.

Step 11, the spot dries into a dark ring. The blot was late. Do it again on a fresh white area, blotting within a few seconds.

Step 12, the sample yellows and the reference does too. The reference print is not the known-good sheet you thought it was. Make a new one on a sheet fixed by the two-bath rotation and washed by the full sequence before any reading is trusted.

What to record

One row per sample: the date; the print or control sheet and its paper; where it was in the batch; the wash route it had; the thiosulfate reading in words; the silver reading against the reference; and, where either failed, what was done about it. Add one line naming which two published tests were run, so a reader in ten years is not left to assume they were the ones whose names everybody uses.