Potassium sodium tartrate tetrahydrate

Rochelle salt is where a kallitype gets its colour. The same exposed sheet, developed in a tartrate bath, comes out sepia; developed in borax, it comes out black; and the printer moves between them by changing the ratio of two stock solutions, one of which is this salt.

In photography

The kallitype developer. Photographers’ Formulary’s kit makes Stock Developer Solution A from 100 g of Rochelle salts in 200 mL of distilled water at 52 °C, and Stock Solution B from 75 g of borax in 400 mL at the same temperature. The three published working developers, each made to 500 mL, are:

• Black tone — 72 mL of A with 128 mL of B; develop 5 minutes, and the bath “works best if it is warm, around 38 °C/100 °F or higher”.
• Brown tone — 96 mL of A with 64 mL of B; 5 minutes, likewise warm.
• Sepia tone — 48 mL of A and no borax at all; 10 minutes at room temperature.

The kit states the rule that connects them: more of solution A increases the sepia tone of the print, more of solution B increases its blackness. The tartrate developer is also the slow, cool one — twice the time, no heating — which is worth knowing before deciding that warmth is always an advantage.

The kit offers one further control the course does not pass on. It suggests increasing contrast by adding 5 to 20 drops of 10 per cent potassium dichromate per 500 mL of mixed developer. That is chromium(VI), which this course does not use at any level, in any process; see the chromium policy. The tartrate-to-borax ratio remains available and is what the course teaches with.

⚗The chemistry

Tartrate is a chelating ligand: two carboxyl groups and two hydroxyls on a four-carbon backbone, arranged to wrap around a metal ion. In a kallitype the exposed sensitiser holds iron(II), and the developer’s job is to bring that iron into solution fast enough for the silver beside it to be reduced where it stands. A tartrate does that, and so does a citrate, and so does an oxalate; the three developers differ in how strongly and how fast they hold the iron, and that difference is what shows up as image colour.

The course does not have a sourced mechanism for the colour change itself. Neither the kit instructions nor any source read for this page explains why a tartrate bath gives sepia and a borate bath gives black. The observation is well attested — the kit states it as a working rule and Jarman found a 12 per cent Rochelle salt developer gave a hard, contrasty image in his gold printing trials of the same era — but the explanation is not given, and this page does not supply one.

It gets cold as it dissolves. The Formulary’s mixing note is unusually candid and worth quoting: “Rochelle salts dissolve with absorption of heat: therefore, the solution will cool and it may be necessary to warm the bowl slightly. Upon stirring, the mixture turns cloudy but, in time, will clear.” That is why the stock is specified at 52 °C — not because the salt needs heat to dissolve, but because dissolving it takes heat out of the water. A cold-water stock will look like a failure long before it becomes a solution.

Wall’s silvering process. Outside printing, Wall’s 1924 formulary gives a “Rochelle salts process” for silvering glass: a silver nitrate solution, and a second solution of 2 g of silver nitrate with 1.66 g of Rochelle salts in a litre of boiling distilled water, boiled about 25 minutes until a grey powder collects, then filtered repeatedly until clear. The tartrate is the reducing agent there, and it is the same chemistry as the Tollens mirror.

Properties

Translucent or white odourless crystals, slightly efflorescent in warm air. Relative molecular mass 282.22 for the tetrahydrate — and Hardwich, writing in 1864 in equivalent notation, gives the same number, 282, for a salt he writes as “+ 8 Aq”. The anhydrous salt is a different article, CAS 304-59-6, relative molecular mass 210.16; a gram of it carries 34 per cent more tartrate.

The record has two gaps and this page names both. PubChem carries no solubility section for the tetrahydrate and no GHS classification for it either. The solubility figures quoted here — one gram in one millilitre of water, 47.4 g per 100 mL at 6 °C, very soluble in hot water, insoluble in ethanol — are from the anhydrous record and are labelled as such. What can be said about the tetrahydrate directly is empirical: Photographers’ Formulary dissolves 100 g of it in 200 mL of water, so a stock of about 50 per cent w/v is achievable, which is far beyond anything a developer needs.

Handling

Three ECHA entries are filed against the anhydrous salt and all three say the same thing: not classified. 234 of 235 reports under one entry, 42 of 42 under a second, 3 of 3 under a third, with a single notification in the whole set carrying any hazard code. There is no entry at all for the tetrahydrate. There is no ILO-WHO Chemical Safety Card. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit, and EH40 states that absence from its list does not indicate that a substance is safe. The salt is the food additive E337 and a pharmacopoeial laxative, which describes regulated uses at their own concentrations and is not a hazard assessment.

This is the cleanest hazard record on any page in this family, and the course still declines to turn it into a positive statement of safety. Only 0.4 per cent of companies supplied information under the main entry; an absent classification from a small number of notifiers is weak evidence, not a clean bill.

Why Level A. Against the course rubric it meets Level A on every criterion: no classification approaching the irritant ceiling; no heating demanded by the salt itself; no mains work; and a waste stream that is a collectable developer. Gloves, eye protection and ordinary powder discipline are the whole of the control.

⚠The one operation that takes this out of Level A

Mixing Stock Solution A. Photographers’ Formulary specifies distilled water at 52 °C, which is two degrees above the rubric’s Level A ceiling of 50 °C, and it specifies it for a reason — the salt absorbs heat as it dissolves, so the bowl needs warming again partway through. Hot water plus a bowl being stirred plus a solution that has to be poured while warm is a scald hazard, and the assessment for that step is Level B: goggles, an apron, a water bath or a kettle rather than a hotplate, and the vessel on the bench rather than in your hands. The developer itself, once cool, is Level A again — and the sepia-tone formula, the only one this salt makes alone, is used at room temperature.

Waste

A spent kallitype developer carries dissolved iron from the sensitiser, silver from the sheet, and the tartrate itself, which is a food additive. The metals decide the routing. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most often set and a tartrate developer sits inside it, which again makes pH the wrong test. Collect the developer, the clearing baths and the first wash together as a silver-bearing stream, label the container, and follow the disposal caveat and your local authority; ILFORD’s route for domestic users in the United Kingdom is a household waste and recycling centre.

History

Rochelle salt reached photography from the apothecary — Hardwich’s 1864 manual files it under “Potash and soda, tartrate of” and notes that it “is often sold in commerce under the name of Rochelle Salt”, prepared by adding soda to cream of tartar — and it arrived early enough to be one of the substances Herschel tested for photographic sensitivity in its own right.

His 1840 paper on the chemical action of the spectrum devotes two sections to it, and they are among the most rueful passages in early photographic literature. “Of all the salts I have examined, this presents, perhaps, the most singular peculiarities,” he writes: a paper washed with silver nitrate, then saturated Rochelle salt, then silver nitrate again gave a photographic spectrum that began as a delicate lead colour and then, on longer exposure, developed a second and much more intense darkening confined to the blue and violet and to the middle of the sun’s image. Then the admission — “the quality of its papers however varies much with the doses of the ingredients, and, in fact, I have never succeeded in hitting again that precise proportion” — and the reason he published anyway, which is that he still had the sheets in front of him.

A tartrate developer that gives an image colour nobody has explained is, in that light, exactly in character.