Lead(II) nitrate

Lead’s photographic career is short, unsuccessful and unexpectedly important. The intensifier built on it was, for a few years, the strongest anyone had; the toners built on it were, on the testimony of the handbook that printed them, not very good. What came out of studying it is why every sepia, blue and copper toner in the formulary works as it does.

In photography

Lead intensification, 1875. Eder records that he and Captain Victor Toth found that mixtures of potassium ferricyanide with lead salts deposit silver ferrocyanide and lead ferrocyanide on a silver image, producing a strong intensification — white, and made non-actinic by suitable after-treatment. They reported it to the Vienna Photographic Society on 14 December 1875 as Die Bleiverstärkung, eine neue Verstärkungsmethode, blackening the bleached negatives with dilute ammonium sulfide, and it was, Eder says, the strongest intensifier known at the time.

⚗The chemistry

The mechanism Eder and Toth stated in 1875, and which Eder proved by analysis the following year, is the one every ferricyanide bath still uses. Metallic silver reduces ferricyanide to ferrocyanide; the ferrocyanide then meets whatever metal ion the bath supplies and comes down as an insoluble compound, in the exact place the silver was:

2 Pb²⁺ + [Fe(CN)₆]⁴⁻ → Pb₂[Fe(CN)₆]

Lead ferrocyanide deposited where the silver image was

Eder went on to show that the same scheme gives a reddish brown with uranium salts and, with a ferric salt, a Prussian blue precipitate that colours the negative a strong distinct blue — which he identifies as the first description of blue toning. He is explicit that the basis of the method need not be a lead precipitate at all. Lead was the first metal tried, not the necessary one, and that is why this obsolete intensifier sits at the root of a family of toners the course does teach.

What more or less of it does, and what it costs. Wall’s 1924 handbook keeps the lead intensifier but hedges it: it gives very great intensification and is only suitable for black-and-white line work, which is a polite way of saying it destroys half-tone. The lead salts are tenaciously retained by the gelatin, so the plate needs a nitric acid soak and a wash before it can be trusted; and if the negative is subsequently developed, rather less intensification results. The colour of the final image is chosen in the second bath: chromic acid gives an orange image which is very non-actinic while the lines stay clear, and sodium sulfide gives the greatest increase of all.

Lead toning was the same chemistry aimed at a print, and Wall’s verdict is unusually direct: the results are not very satisfactory, there is a very great tendency for the whites to be stained, and great intensification is given whether it is wanted or not. What it offered was a palette no other process could match from one bleach. After bleaching in a lead salt with ferricyanide, the second bath decides the colour — ammonium sulfide for black, potassium chromate for yellow, ferric chloride or cobalt chloride for green, cupric chloride for red, uranium nitrate for red-brown, mercuric iodide for orange-yellow. Eder and Toth had already shown in 1876 that a chrome-yellow image turns a beautiful green under iron chloride, superimposing Prussian blue on the yellow.

A different lead compound, elsewhere. Lead turns up in early photographs for reasons unconnected with lead nitrate. The Getty Conservation Institute’s atlas records that lead was introduced into the platinotype by Willis in 1873 and later eliminated, that several later formulas added lead oxalate to make the reduction of the platinum salts more uniform, and that mounting boards and paper substrates may contain lead themselves. Lead oxalate is not lead nitrate, and the two uses should not be run together.

Properties

White or colourless crystals, freely soluble in water at about 52 to 57 g per 100 mL at 20 °C rising to 127 g at 100 °C, and very nearly insoluble in alcohol. Wall writes “lead acetate or nitrate” as interchangeable in a bleach, which is a fair summary of how the formulas treated the two.

It is also a nitrate, and so an oxidiser: CAMEO records that it is non-combustible in itself but accelerates the burning of combustible material, that toxic oxides of nitrogen form in a fire, and that an explosion may result if a large quantity is caught in one. Only 13.5 per cent of notifiers apply the oxidiser statement to it, which shows how much variation an aggregated classification can hide.

Handling

The aggregated ECHA notifications classify it Danger, but with the loosest consensus of any substance in this family: harmful if swallowed and if inhaled and very toxic to aquatic life at 88.7 per cent of 318 reports, serious eye damage at 78.9 per cent, the reproductive-toxicity statement H360Df at 61.6 per cent, suspicion of cancer at 48.1 per cent — and 11.3 per cent of reports stating that it meets no GHS criteria at all.

The occupational regimes are far less equivocal, because lead is regulated as lead rather than as a particular salt. NIOSH’s supplementary limits state that “lead” means metallic lead, lead oxides and lead salts, set a recommended limit of 0.050 mg/m³ over eight hours, and — the important part — add that air concentrations should be held so that a worker’s blood lead stays below 0.060 mg per 100 g of whole blood. The dose that matters is the accumulated one. HSE’s EH40 gives lead no workplace exposure limit at all: it notes that asbestos and lead are regulated separately and points to the Control of Lead at Work Regulations. CAMEO describes classic lead poisoning — gastrointestinal upset, weakness progressing to paralysis especially of the wrists and ankles, leg cramps and paraesthesia.

⚠Why Level D

On its own hazards this substance reaches Level C of the classification rubric: a reagent classified a reproductive toxicant and a suspected carcinogen, whose waste cannot lawfully enter a domestic drain and needs a licensed contractor. A home darkroom cannot supply those controls, so the course does not handle it and the page is written to the Level D pattern — chemistry, workflow in outline, image characteristics and hazards, and no procedure. No formula, concentration or bath sequence appears above.

The Level D criterion that a safer route exists and is taught instead is unusually easy to satisfy here, because the historical method was not good. Wall’s own summary — very great tendency for the whites to be stained, unwanted intensification, suitable only for line work — describes a process that a modern printer would reject on the picture alone. Selenium, gold, sulfide and iron toning give better prints from safer baths, and the ferricyanide chemistry they use is the chemistry this page describes, discovered on lead and then applied to something else.

Waste

There is no neutralisation to describe. An acid can be neutralised and a dichromate can be reduced, but the lead(II) ion is an element in a stable oxidation state: every treatment moves it out of solution into a solid, and the solid is lead waste in a filter instead of lead waste in a bottle.

A spent lead bleach falls under the Environment Agency’s WM3 entry 09 01 05*, bleach solutions and bleach fixer solutions, which is absolutely hazardous with no assessment needed. WM3 also notes that inorganic lead compounds are classified as carcinogenic by IARC and that the carcinogenic classification has to be determined when such a waste is assessed. GOV.UK’s household guidance sends hazardous waste from a home to a household hazardous waste collection point, never to the bin or the drain. Check your local regulations; they govern.

History

The lead intensifier’s real legacy is a piece of theory. Eder’s dissertation on the reaction of red prussiate on metallic silver, published in 1876, came directly out of the lead work, and Eder’s own account is that it determined the chemical basis for colouring and intensifying methods generally. Before it, photographers had a collection of recipes; after it, they had a mechanism they could aim at a chosen metal. Blue toning is a consequence of a lead experiment.

The lead recipes themselves faded slowly. Wall was still printing lead intensifiers and toners in 1924, with the reservations quoted above; by then sulfide, selenium and gold toning did the same jobs better, and photo-engraving, the one trade with a use for an extreme line intensifier, had moved on. What finished lead was not photography’s judgement but the twentieth century’s, in which a soluble lead salt on an open bench became something no ordinary workplace keeps.