Copper(II) sulfate

Copper is the third metal this course meets as an image substance, after silver and gold, and the only one that arrives as something a gardener would recognise. It has two jobs in a darkroom — making a red print and bleaching a negative — and one historical use the course will describe and never perform.

In photography

It makes a red toner, and the mechanism is replacement rather than conversion. Kodak’s 1928 primer sets out the general pattern for this family: the silver image is bleached to a silver ferrocyanide and an insoluble coloured ferrocyanide of another metal takes its place — iron citrate for blue, uranium nitrate for reddish brown, copper citrate for red. The Getty Conservation Institute’s atlas confirms it on real prints, reporting that X-ray fluorescence of a copper-toned developing-out photograph shows both copper and iron, from the copper ferricyanide precipitate that is responsible for the red colour.

The formula that carried the practice is W. B. Ferguson’s, published in 1900 and recorded by Abney. Three 10 per cent stock solutions are kept — neutral potassium citrate, copper sulfate and potassium ferricyanide — and the working bath is made from them. Ferguson’s original proportions were one ounce of the citrate solution with 70 minims of the copper sulfate and 60 minims of the ferricyanide; the editor of Photography proposed a modification, which Ferguson approved, of 1000 parts citrate to 75 copper sulfate and 66 ferricyanide, to avoid the pink stains the original sometimes gave. Tones run from purple-brown to bright red with the time of treatment.

⁇Is a copper-toned print more permanent?

Abney reports the period’s view in a form that gives itself away: “There is reason to suppose that the tones so obtained are permanent.” There is reason to suppose is not a measurement, and no source this course holds provides one.

Against it stands a manufacturer’s own caution: ILFORD warns that metal replacement toners such as blue (iron) and red (copper) may give no extra protection, and that the image might fade. The course takes copper toning as a colour decision and not an archival one, and says so wherever it appears. That is the opposite of the position it takes on sulfide and selenium toning, where the manufacturers’ claim of protection is explicit and the conservation literature agrees with it.

It bleaches, and that is how the intensifiers used it. The British Journal Photographic Almanac for 1906 carries several cupric bleaches. A print or plate soaked soft is immersed in copper sulfate with potassium bromide until completely bleached, washed, and then either exposed to daylight for intensification or toned. A stronger version for silver intensification adds concentrated sulfuric acid to the copper sulfate and bromide, and after bleaching the plate is washed in four changes of dilute nitric acid and re-formed in silver nitrate with nitric acid, giving a dark brown image. Copper(II) is doing the same job a ferricyanide does — taking an electron away from metallic silver — and the bromide is there to give the resulting silver(I) something insoluble to become.

And it appears in one process the course will not reproduce. Robert Hunt’s chromatype of 1844 dissolves a drachm of copper sulfate in an ounce of distilled water and adds half an ounce of saturated potassium dichromate; the paper is coated, dried, and prints out in sunshine. Ware records that Hunt went on to combine dichromate chemistry with the ferrocyanides in his chromo-cyanotype.

☠Chromium(VI) is not used at any level

The chromatype is a dichromate process, and the course’s chromium policy settles what happens to it: chromium(VI) compounds are never used, at any level, anywhere in this course, and are taught as chemistry and history with no procedure. That ruling is cited, not restated, and it applies to this page in one direction only — it says nothing about copper, which is a different element with a different classification. What it forbids is the other half of Hunt’s sensitiser.

Properties

The two grades are the practical problem. Anhydrous CuSO₄ is 159.61; the pentahydrate CuSO₄·5H₂O is 249.69; a formula written for one and weighed as the other is out by more than half. Almost every photographic formula in the historical literature means the pentahydrate, because that is what “copper sulphate, crystals” was, and CAMEO’s description settles the identification instantly: blue crystalline granules, white when dehydrated. If the jar is white, it is not what the formula assumed.

The solubility figures disagree across sources by about fifty per cent at room temperature, and the page above lists them rather than averaging them. Nothing this course mixes approaches saturation, so the disagreement is a caution about reading data rather than a practical difficulty.

Handling

Four classifications sit in one record and they diverge. The harmonised European entry — law rather than notification — gives Warning, with the irritant and environmental pictograms: harmful if swallowed, skin irritation, serious eye irritation, and very toxic to aquatic life with long lasting effects. The aggregated ECHA notifications give Danger and put serious eye damage at 19.7 per cent of 1,268 reports. Japan’s NITE-CMC goes further again. And the pentahydrate — the article the formulas mean — carries its own harmonised entry at Danger with H318.

HSE’s EH40 lists copper and compounds, dust and mists, as Cu, at 1 mg/m³ long-term and 2 mg/m³ short-term, with copper fume separately at 0.2 mg/m³. The fume entry is a welding number and does not arise here; the dust entry does, at the moment of weighing.

⚠Why Level B, and the one operation that is not

The rubric admits to Level A substances classified “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. Read against the harmonised entry for the anhydrous salt alone, copper sulfate would clear that bar. It is placed at Level B because of what the reader actually buys: the pentahydrate’s harmonised classification is Danger with causes serious eye damage, and Level B’s criterion about procedures whose failure mode is a splash of something harmful rather than a spoiled negative follows directly. Splash goggles and a dust mask for weighing are the controls; no source the course holds names a fume cupboard, so it does not reach Level C.

The one operation that takes it out of Level B takes it out of the course entirely. Combining it with a dichromate, as Hunt’s chromatype does, brings in chromium(VI), and the chromium policy places that at Level D — historical study, no procedure, at any concentration. Copper sulfate is a Level B reagent right up to the moment a dichromate joins it.

Waste

The environmental pictogram is on almost every classification in the record, and the harmonised entry is unequivocal: very toxic to aquatic life with long lasting effects. A copper bath is therefore bottled rather than poured, and it carries silver from the bleached image as a second reason. Kodak’s J-52 lists the effluent parameters municipalities most often regulate — BOD, COD, suspended solids, chlorine demand, pH between 5.6 and 9.4, silver at 1.2 mg/L and iron at 17 mg/L — and copper is not among them, which tells you what that particular table covers and nothing about what your own authority permits. ILFORD’s advice to domestic users in the United Kingdom is a household waste and recycling centre. Check your local regulations; they govern, and they differ.

History

Copper’s photographic career is a long tail rather than a peak. Hunt had it in a sensitiser in 1844, paired with dichromate, in the years when every soluble metal salt was being tried against light. Ferguson had it in a toner in 1900, at the end of the period when a printer chose a colour by choosing a metal. By the time Kodak’s 1928 primer summarised the field, copper appears as one line in a list of mordants and replacement toners, ranked behind sulfide for permanence and behind gold for colour. What kept it in the darkroom longest was not toning at all but bleaching, where an inexpensive oxidising salt that leaves an insoluble bromide behind is exactly what an intensifier needs.