Residual thiosulfate

Owned by Fixer Faults, Washing Faults and the Stains They Leave, which pairs this fault with retained silver and gives the two-by-two that separates them: this one is a washing failure, and a sample that passes the thiosulfate test while failing the silver test has a fixing failure instead, on which longer washing is explicitly no help. This entry was assembled from the wash testing lab and the physics of washing before that page existed.

What you see

Nothing, at the time. That is the defining property of this fault and the reason it needs a test rather than an inspection.

What appears later is a general yellowing and a loss of density across the image: image silver converted to silver sulfide by the sulfur the residue supplies, and seen by transmitted light.

Two neighbours have to be kept off it. It is not a stain in the whites, which comes from retained silver rather than from retained thiosulfate. And it is not silver mirroring, which is a different mechanism altogether — an oxidation that makes image silver mobile and deposits it at the surface, where it shows by reflection. The owning page sets those two against each other in exactly that way, and the test is free: look at the material, then look through it.

Likely causes

1. A wash too short.
2. A wash in which the water was not actually changing. Washing stops entirely when the water stops changing, whatever the tap is doing.
3. A fibre-base paper washed as though it were resin-coated. RC is waterproofed by its polythene coating; fibre base absorbs processing solutions into the paper itself and needs far longer.
4. Fixing carried far past what was needed, which drives solution into a fibre base.

What is happening: the chemistry and physics

Washing is diffusion outwards, and it obeys the same physics as everything else that moves through gelatin.

Kodak’s 1928 primer states the whole of it — the rate depends on the rate of diffusion of hypo out of the film, provided the water in contact is continuously removed; it has nothing to do with solubility; it is not helped by warm water, because swelling hinders diffusion in about the same proportion as the temperature rise accelerates it; and it proceeds by a constant halving time, about 15 seconds under a running tap and 30 seconds in most trays and tanks.

Two consequences follow and both are practical. Changes of water beat soaking, because a still bath comes to equilibrium and then does nothing. And a halving law never reaches zero, so a wash specification is a decision about how much is little enough rather than a claim of removal.

What the residue then does is chemistry. Thiosulfate is a source of labile sulfur, and sulfur converts image silver to silver sulfide — a general yellowing and a loss of density, over months to years.

Diagnostic questions

• The residual-hypo test, in full. Process an unexposed white sheet of the same paper with the batch. After the final wash, cut off a strip and immerse it in a 1 per cent silver nitrate solution for about three minutes, rinse, and compare it while wet, in subdued light, with the wet untreated portion. No colour difference means the hypo has been removed; a yellow-brown tint indicates its presence.
• The control that the test’s own footnote demands. Kodak note that the same yellow-brown tint can be caused if hydrogen sulphide or wood extracts are present in the water supply. That is a systematic error affecting every sample alike, and the control that removes it is a second arm rinsed in distilled water and compared against the first.
• Was the water actually changing? A tray with a tap running into one corner is not a wash if the water leaves from the same corner.
• Which material, and which wash? ILFORD’s figures are a minimum of 30 seconds in vigorous fresh running water for RC, and for fibre base an optimum-permanence sequence of a 5 minute first wash, 10 minutes in a washing aid at 1+4, and a 5 minute final wash.

⚠The test reagent is not a trivial one

Silver nitrate solution stains skin black, and the stain develops over hours and cannot be washed off. Kodak’s own instruction is that direct contact is to be avoided. Wear nitrile gloves and eye protection, work over a tray, and route the used test solution as silver-bearing waste. Local regulation governs disposal and differs between authorities.

Corrective action

Rewash, while the material is recent. Residual thiosulfate is in solution and diffusible, so a proper wash removes it — which is exactly what is not true of retained silver.

Once sulfiding has visibly begun, the silver has moved and there is nothing to redissolve.

A washing aid shortens the wash rather than replacing it, by ion exchange, and it is part of the published optimum-permanence sequence rather than an alternative to it.

Prevention

Use changes of water rather than soaking, and make sure each change is a change.

Match the wash to the material. Do not carry an RC time to a fibre-base print.

Test rather than assume. Run the residual-hypo test on a sacrificial sheet processed with any batch you intend to keep, with its distilled-water control, and record the result with the batch.

Do not over-fix. Prolonged fixing drives solution into a fibre base and makes the wash harder for no benefit.