Ammonium dichromate

Two dichromates did the same photographic job, and photographers chose between them on solubility. Ammonium dichromate dissolves about three times as freely as the potassium salt at room temperature, which lets a coating carry more sensitiser in the same volume of gum or gelatin — and that is most of why Wall’s 1912 dictionary says it is often used in preference to the potash salt in photo-mechanical printing and for the gum-bichromate process.

In photography

Its main use was as the sensitiser of a pigmented colloid. The dichromate ion is the light-sensitive part and behaves as it does in the potassium salt: the Encyclopaedia Britannica’s 1911 survey records that a dichromate is altered only by the rays it absorbs, in the ultraviolet and blue-violet, and only where organic matter such as gelatin or albumen is present to be attacked. The Getty Conservation Institute’s atlas of the carbon process traces the discovery through Ponton in 1839 and Talbot in 1852 to Poitevin’s carbon process of 1855; by the time gum printing became a pictorialist medium at the end of the century, the ammonium salt was the one the manuals reached for.

⚗The chemistry

The photochemistry is the reduction of chromium(VI) to chromium(III) at the expense of the colloid, which is left insoluble where the light fell. The ammonium ion contributes nothing to that, and the difference between the two salts is physical rather than photochemical: at 20 °C the ammonium salt gives about 36 g per 100 mL against about 12 g for the potassium salt, so a saturated solution holds three times the dichromate. A colloid mixed with a saturated ammonium dichromate solution therefore starts more sensitive and reaches a higher chromium loading than the same colloid mixed with the potassium salt, without adding water that would thin the gum.

What more or less of it does. Wall’s gum-bichromate entry gives the trade-off directly, and the direction is worth understanding even though this course will not let you use it: raising the proportion of dichromate to pigmented gum buys sensitiveness and the rendering of the fainter gradations, and spends contrast and the hardness of the deep shadows. It is the same exchange a modern printer makes by changing paper grade, made here by changing the chemistry of the coating.

The proportions Wall prints are deliberately not reproduced here. They are a working sensitiser recipe, and under the chromium policy chromium(VI) is never used at any level in this course, so a page that gave the ratios would be handing over the one thing the policy withholds. The mechanism is the teachable part; the recipe is not.

It survives commercially as a contrast agent. At least one alternative-process supplier still ships a dilute ammonium dichromate solution as the strongest contrast additive for an iron-based printing-out process, describing it as raising contrast about twice as much as gold chloride, cooling the print colour, and increasing grain and exposure time. The same instruction sheet says in the next breath that it is a known carcinogen and that gloves are always worn with it. That pairing — a substance still sold, still effective, and still carcinogenic — is the whole of the modern position, and it is why this course teaches the ion and reaches for a different contrast agent.

Properties

Bright orange-red crystals, odourless, freely soluble in water. It is the thermal behaviour that separates it from every other salt on these shelves. CAMEO’s fire entry records decomposition beginning at about 180 °C and becoming self-sustaining at about 225 °C, with spectacular swelling and evolution of heat and nitrogen, leaving a chromic oxide residue; the explosion entry adds that direct exposure to heat or shock will explode it, and that gases confined in a closed container can burst it. The green residue that CAMEO calls voluminous is chromium(III) oxide.

(NH₄)₂Cr₂O₇ → Cr₂O₃ + N₂ + 4 H₂O

Self-sustaining decomposition on heating

CAMEO names the heat, the nitrogen and the green chromic oxide; the water completes the balance. The chemistry is worth seeing because it is the same redox step as the photographic one — chromium(VI) falling to chromium(III) — with the ammonium ion, rather than gelatin, supplying the electrons. The oxidant and the reductant sit in the same crystal, which is why heat and shock matter here in a way they do not for the potassium salt.

Handling

The aggregated ECHA notifications classify it Danger under the oxidiser, corrosive, acute-toxicity, irritant, health-hazard and environmental pictograms, and the agreement between notifiers is complete on almost every entry: toxic if swallowed, harmful in contact with skin, causes severe skin burns and eye damage, may cause an allergic skin reaction, fatal if inhaled, may cause allergy or asthma symptoms if inhaled, may cause genetic defects, may cause cancer, causes damage to organs through prolonged or repeated exposure, and very toxic to aquatic life with long lasting effects, each in 100 per cent of 194 reports across 10 notifications. NIOSH’s chromates entry, which covers this salt, is a carcinogen entry with a recommended limit of 0.001 mg/m³ as chromium; HSE’s EH40 gives chromium(VI) compounds 0.01 mg/m³ over eight hours with carcinogen and sensitiser notations. CAMEO’s own health paragraph adds the classic chromate picture: irritation and asthmatic symptoms from inhalation, nasal septum perforation with continued exposure, conjunctivitis, and skin irritation, ulceration and rash.

⚠Why Level D

The classification rubric puts chromium(VI) compounds at Level C in any quantity, because a fume cupboard and a specialist waste route are the recognised controls. This course teaches in a domestic darkroom and can supply neither, so the substance is studied and not handled, and the page is written to the Level D pattern: chemistry, workflow in outline, image characteristics and hazards, and no procedure. No quantity, dilution, coating or exposure appears above, and no protective equipment is listed, because listing it would imply a procedure that this page deliberately does not give.

One hazard here is not shared with the potassium salt and deserves naming on its own. This is an explosive solid by CAMEO’s classification, sensitive to heat and shock, whose decomposition once started sustains itself. Gum printers who do work with it store small quantities, cool and unconfined — and that is a laboratory judgement, not a darkroom one.

Waste

The chromium problem is the dichromate problem, and it is answered the same way: reduce the chromium(VI), then take the acid away so the chromium(III) precipitates, filter or settle the solid, and send the solid to a licensed waste-disposal service. Kodak’s guidance is explicit that dichromate solutions must not reach a septic tank, where they are hazardous to the bacteria on which the tank depends.

The ammonium ion adds a second, quieter problem. The same Kodak document explains that a treatment works chlorinates its effluent at the end of the process, and that chemicals which react with chlorine — it names ammonium — consume that chlorine before it can do its job, so an effluent with a high ammonium load may need pre-treatment. A dichromate waste stream carries both halves of that sentence at once. Under the Environment Agency’s WM3 guidance, entry 09 01 05* covers bleach solutions and bleach fixer solutions as absolutely hazardous. Check your local regulations; they govern.

History

Ammonium dichromate belongs to the second half of the bichromate era rather than to its discovery. Wall’s 1912 entry defines it in three lines — made by neutralising chromic acid with ammonia, often used in preference to the potash salt in photo-mechanical printing and for the gum-bichromate process — and then sends the reader to Carbon and Oil Processes. That is the shape of its career: it was the working printer’s dichromate, chosen for solubility, while the potassium salt stayed the laboratory’s and the intensifier’s.

Its disappearance from ordinary practice was not a change of technique but a change of who is allowed to handle chromium(VI), and where. The gum print did not become a worse picture; it became a picture made by people with extraction and a waste contract. What reaches an ordinary darkroom today is a 25 mL bottle at 1 per cent, sold to add contrast to a palladium print — a hundredfold dilution of the thing that once coated the paper.