Nitrocellulose

This is a study entry. Nitrocellulose is the material of the wet plate and of the first flexible film, so a course that skipped it would have no account of the second half of the nineteenth century or of the first half of the twentieth. What follows is the chemistry, the hazard data and the historical role. There is no quantity, no solvent ratio, no pouring sequence and no apparatus, because the course classifies collodion work at Level D and a Level D page is not a procedure.

In photography

Collodion is nitrocellulose in solution, and that is all it is. The AIC conservation wiki’s page on the collodion negative defines it in one line — collodion is produced by dissolving pyroxylin, a form of nitrocellulose, in alcohol and ether, and pyroxylin is made by treating cellulose from cotton or wood with a mixture of nitric and sulfuric acids. Pour that solution on glass, let the solvent begin to leave, and what remains is a tough, transparent film that holds silver halide against the plate. The binder does the job gelatin does in every later process, and it does it without needing to be an emulsion: the halide is precipitated in the layer itself.

Its virtue was also its constraint. The AIC states the mechanism plainly: once the alcohol and ether have dried, the collodion becomes impervious to the processing solution. So the plate had to be sensitised, exposed and developed before it dried, and the whole of the wet-plate workflow — the portable darkroom, the field tent, the roughly 250 kilograms of equipment the AIC records a photographer needing to take a negative away from the studio — follows from a property of this polymer.

Then it became the support rather than the binder. The Image Permanence Institute traces the line: cellulose nitrate combined with a plasticiser such as camphor is celluloid, first made by Schönbein in 1846, trademarked by Hyatt in 1873 and sold in blocks for cutting into sheets. John Carbutt made the first plastic film negatives from those sheets and showed them in 1887; by August 1889 Eastman Kodak was selling gelatin emulsion roll film on a cellulose nitrate support, and celluloid soon became the only commercially important film base. Kodak last manufactured nitrate film in 1951.

⚗One name, a family of substances, and why the difference decides everything

Nitrocellulose is not a compound. The FDA’s registry classes it as a polymer — homopolymer, linear, substituted — and carries the registry’s own note that the entry is incomplete. What varies is how many of cellulose’s hydroxyl groups have been converted to nitrate esters, and that one variable sets both the solubility and the hazard.

Hardwich’s 1864 account is the clearest statement of it the course has read. He describes pyroxyline as a series of compounds obtained by the action of mixed nitric and sulphuric acids on vegetable fibres, “all more or less explosive, the strongest being Gun-cotton”, and lists four varieties in order of substitution. The most substituted, A, is true gun-cotton and is insoluble in ether and alcohol. B and C, less substituted, dissolve in ether and alcohol — and those are the ones photography used, which is why the photographic grade is called collodion wool rather than gun-cotton.

The safety card draws the same line from the other end, as a threshold: the formulation with more than 12.6 per cent nitrogen is used exclusively for the manufacture of explosives. The photographic material sits below that figure. Below it, the substance is a flammable solid; above it, it is something else entirely. The course states the threshold and gives no route to either side of it.

Properties

A white solid, harsh and crackling between the fingers when dry, somewhat hygroscopic, insoluble in water and soluble — in the useful grades — in a mixture of ether and alcohol. Archer noted in 1854 that gun-cotton is not soluble in pure ether, that is, ether free from alcohol, which is why collodion needs both solvents and not one.

The property that governs the rest is that the molecule carries its own oxygen. The safety card lists under physical dangers that the substance is very sensitive to heat, shock, friction, naked flames, sparks and electrostatic discharges, and under chemical dangers that it decomposes rapidly on burning, producing nitrogen oxides. The Image Permanence Institute puts the consequence in a sentence a photographic archivist never forgets: cellulose nitrate is very flammable and capable of sustaining combustion even in the absence of oxygen and underwater.

The second property is that it does not sit still. IPI describes nitrate decay as irreversible and autocatalytic, running through six stages — yellowing and silver mirroring, then stickiness and a strong smell of nitric acid, then an amber support and a fading image, then softening and adhesion, and finally a brown acid powder. The off-gassed nitrogen oxides form nitric acid in the presence of moisture, which fades the silver image and destroys the gelatin, which produces more acid.

Handling

There is no exposure limit and no health classification to quote, and the page says so. HSE’s EH40 carries no entry for nitrocellulose, and EH40 states in its own introduction that absence from the list does not indicate that a substance is without risk. The safety card is equally direct: it prints “See Notes” against routes of exposure, effects of short-term exposure and effects of long-term exposure, and its note reads that insufficient data are available on the effect of this substance on human health, therefore utmost care must be taken. That is an absence of evidence and not evidence of absence, and nothing on this page should be read as saying the material is benign to a body.

What is documented is fire, and the documentation is unusually specific. The card’s fire-fighting line is water in large amounts, no foam and no carbon dioxide — because smothering does not work on a fuel that supplies its own oxidant. Its storage line requires the material to be kept damped; its spillage line requires a spill to be kept wet and not allowed to dry out.

☠Why Level D

The Level D policy covers wet collodion under “the dangerous solvents” and makes the argument in full; this page cites it rather than restating it. What is specific to nitrocellulose is that the hazard is not toxicological and is not managed by ventilation, gloves or dose. It is ignition, and the ignition sources the card names — friction, shock, a spark, a static discharge — are ordinary events in a domestic room. The material also has to be handled alongside diethyl ether, whose vapour is heavier than air and whose flash point is far below room temperature, so the solid and the solvent raise the same hazard from two directions at once.

The one operation that would take this entry out of Level D is drying it. That is the wrong way round, and deliberately so: everything the course has read says the material is kept damped, and the step that makes it usable in a darkroom is the step that makes it dangerous.

Waste

Nothing here makes waste, because nothing here is used. The practical question a reader is far more likely to meet is a shoebox of old negatives, and the identification is worth knowing: IPI records that films made between the 1930s and the 1950s were often edge-printed NITRATE, that cellulose acetate films from 1930 onwards were edge-printed SAFETY precisely to distinguish them, and that early amateur nitrate roll film can be recognised by its tendency to curl into very tight scrolls. Not every manufacturer marked every format. Treat an unmarked, curling, vinegar-sour or acrid-smelling old negative as a fire load, not as rubbish, and check your local regulations; they govern.

History

The wet collodion process arrived in about 1851 and its authorship is still argued over — the AIC records the claim for Gustave Le Gray between 1850 and 1851 alongside the conventional attribution to Frederick Scott Archer, whose own 1854 account is one of the sources for this page. What is not argued over is the effect: a glass negative sharper than a calotype and cheaper than a daguerreotype, printable many times, which is why the second half of the nineteenth century looks the way it does.

The material then outlived the process by seventy years in a different job, and the ending is the part worth carrying forward. Nitrate film was the cause of many fires and, in IPI’s phrase, innumerable losses of archival records. It was not replaced because a better picture was found; it was replaced by a safety base that made a worse one for a while. That is the shape of most of this course’s Level D arguments, and nitrocellulose is the case where the industry itself reached the same conclusion first.