Cadmium bromide

Every other substance in this family of pages is something done to a picture — a bleach, an intensifier, a toner, a fixer. Cadmium bromide was inside the sensitive layer, one of the ingredients that decided how fast a plate was and what colours it could see. You cannot describe the wet collodion process honestly without naming it.

In photography

It was a halide source dissolved in the collodion. A collodion plate is not coated with silver bromide; it is coated with a solution of soluble halides in nitrocellulose, alcohol and ether, and the silver halide is formed inside the film when the plate is dipped in the silver bath.

⚗The chemistry

The salting reaction is a double decomposition that happens in the coating rather than in a beaker:

CdBr₂ + 2 Ag⁺ → 2 AgBr + Cd²⁺

Silver bromide formed inside the collodion film in the silver bath

Because the crystal grows in place, everything about it — its size, its distribution, how the nitrocellulose holds it — depends on what the halide was dissolved in and what came with it. That is why the choice of cation mattered so much to collodion workers, when in a simple precipitation it would have washed away as a spectator ion. The cadmium stayed in the film’s chemistry.

Cadmium was chosen for stability and body. Towler’s 1864 manual gives the reasoning for the iodide, and it transfers: a thin alcohol collodion iodised with an alkaline iodide is unstable and not permanent in its sensitiveness, whereas one iodised with a cadmium salt is more glutinous and much more stable; it takes longer to ripen, but once ripe it keeps its sensitiveness far longer. Photographers therefore combined the two, cadmium in excess, to get the alkaline salt’s quick sensitiveness with the cadmium salt’s permanence — and then added a bromide, Towler says, to give the collodion a greater capacity for colours.

What more or less of it does is set by that last clause. Eder quotes Barreswil and Davanne’s Chimie photographique of 1851: some photographers added bromine salts — cadmium, ammonium or potassium bromide — because silver bromide reproduces the green better, in the usual proportion of four parts iodide to one part bromide. Abney records that silver iodide in its pure state is sensitive only to the ultraviolet, violet and blue rays, so the bromide addition is what lets the plate use light the iodide cannot; Eder notes that the trade settled on ratios between about three and six parts iodide to one of bromide, and that pure bromide collodions were not thought advantageous until emulsion photography arrived.

And it was the bromide of the first gelatin emulsion. Eder’s account of Maddox’s 1871 work records that Maddox himself asked for study of which soluble bromides could replace the cadmium bromide he had used: the substitution question was open from the first day of the gelatin era. Eder also notes W. H. Harrison’s earlier, unsuccessful gelatin experiments with cadmium iodide and bromide in 1868, and his own 1876 investigation of the cadmium double salts formed with alkaline iodides and bromides, which he found gave a maximum of permanence and sensitivity. Abney’s 1885 book on emulsions carries cadmium bromide in Canon Beechey’s unwashed emulsion and a double bromide of cadmium and ammonium in Chardon’s.

The salt has not left the field. Bostick & Sullivan’s wet-plate collodion kit still ships a bromo-iodiser whose stated composition is cadmium bromide, ammonium bromide and potassium iodide in a half-and-half alcohol and ether solution.

Properties

An odourless white or pearly solid, highly hygroscopic, and very soluble in water at 115 g per 100 g at 25 °C. Its behaviour in organic solvents is the property the collodion era cared about, and it is the one the record is least sure of: one HSDB entry calls it freely soluble in alcohol, moderately soluble in acetone and slightly soluble in ether, and another calls it slightly soluble in acetone and ethanol. The course reports both.

The hydrate question is real and the historical texts knew it. The research record identifies the anhydrous salt and a tetrahydrate as separate substances with separate CAS numbers, and Abney’s recipes specify “anhydrous” or “dried” cadmium bromide where they mean it. A hygroscopic salt weighed from an opened bottle is not the salt the formula assumed.

CAMEO’s reactivity note is unusually mild for this family: cadmium bromide has weak oxidising or reducing powers and belongs to the non-redox-active inorganic compounds. Nothing here is dangerous for what it does in a beaker.

Handling

Read the aggregated notification headline alone and cadmium bromide looks like the mildest thing on these pages: Warning rather than Danger, harmful if swallowed, in contact with skin and if inhaled, and very toxic to aquatic life — five statements at 100 per cent of 45 reports across four notifications. That is not the hazard picture, and this page exists partly to show why an aggregated headline must be read against something else.

The something else is the occupational limits, which treat cadmium as a carcinogen. NIOSH’s cadmium entry states explicitly that its recommended limit and the OSHA permissible limit apply to all cadmium compounds measured as cadmium, sets the OSHA limit at 0.005 mg/m³, marks the NIOSH limit as a carcinogen entry, and gives an immediately-dangerous-to-life level of 9 mg/m³. Its symptoms run from pulmonary oedema, breathlessness and chest tightness to loss of the sense of smell, emphysema, proteinuria and mild anaemia, with the respiratory system, kidneys, prostate and blood as target organs and prostatic and lung cancer noted. HSE’s EH40 gives cadmium and its compounds 0.025 mg/m³ as cadmium over eight hours with a carcinogen notation — and the same PubChem record that gives the mild ECHA headline carries a NITE-CMC block classifying the substance Danger for germ-cell mutagenicity.

⚠Why Level D

The classification rubric names cadmium compounds in its Level D criteria, alongside mercury vapour development, cyanide fixing and uranium. The control it requires is a page that describes chemistry, workflow in outline, image characteristics and hazards and states explicitly that it is not a procedure — which is what this is. No collodion formula, no salting proportion in weight and no coating instruction appears above; the iodide-to-bromide ratios are given as the historical ratios they are, not as a recipe, and there is no silver bath here to use them in.

Two hazards compound in the historical use and neither is visible in the GHS headline. The salt was weighed as a dry powder, so the exposure route was inhalation of a cadmium dust at a carcinogen limit; and it was dissolved in alcohol and ether, so a spill or a fire produced, in CAMEO’s words, toxic cadmium oxide fumes. Modern wet-plate practitioners who use a cadmium bromo-iodiser are working at Level C at best, with extraction and a waste contract.

Waste

Cadmium’s waste problem is environmental before it is anything else: very toxic to aquatic life, with long lasting effects, in every one of the reports behind its classification. Nothing a darkroom could do would change that, because the cadmium ion does not decompose.

The Environment Agency’s WM3 chapter on wastes from the photographic industry is a good illustration of a gap. Its absolute hazardous entries cover developer, fixer and bleach solutions — the baths a processing room produces — and none of them describes an unused collodion salting solution, which is a flammable alcohol and ether mixture with a heavy-metal salt dissolved in it. A waste that does not fit a listed code still has to be assessed and consigned, and this one goes to a licensed contractor. Check your local regulations; they govern.

History

Cadmium arrived in collodion within two years of the process itself. Eder credits Laborde with introducing cadmium salts into negative collodion in 1853, especially in mixtures with alkaline iodides, which gave lasting and sensitive collodions; the recognition that cadmium and the alkali metals had a favourable affinity for each other was, he says, entirely empirical at first, and the explanation — that the mixture forms double salts — came from his own work in 1876.

Its disappearance was not dramatic. Gelatin emulsions moved to potassium and ammonium bromide, which are cheaper, less hygroscopic and easier to purify, and the industry’s chemistry was reorganised around the ripening of a precipitated emulsion rather than the behaviour of a salted collodion in a dish. By the time cadmium’s occupational toxicity was established, the photographic use had already narrowed to a specialist corner — the collodion revival, where a bottle of bromo-iodiser can still be bought and where the honest thing to say is that the recipe is authentic and the substance is a carcinogen at the limit.