Incendiary Films

The Public Library of Cincinnati and Hamilton County has a wonderful collection of materials about the use of the Ohio and Mississippi rivers known as the Inland Rivers Collection. Recently a group of photographs with accompanying film negatives from this collection came to the Lab.

What for? Well, film has been made using different chemicals over the years, some of which are very unstable. The Lab’s task was to divide and house the entire grouping according to film type. Special attention was given to identifying any cellulose nitrate negatives and isolating them in separate storage housing with the recommendation they be digitized and then disposed of in accordance with Ohio’s guidelines for discarding hazardous materials.

Cellulose nitrate film was the first widely used flexible plastic film. In the late nineteenth century it supplanted heavy, fragile glass plates. Great! Except nitrate is also a chemical component in gunpowder. As cellulose nitrate film degrades it goes through several distinct stages, beginning with silver mirroring and yellowing. Then it may become sticky and smelly (nitric acid odor). Gradually the negative changes to an amber color with the image beginning to fade. Eventually the negative can soften to a point where it may stick to adjacent pictures or its enclosure. In the final stage it turns to a brown acidic powder. As deterioration progresses it accelerates and in the last stages the film may begin to generate its own heat and ignite. A cellulose nitrate fire doesn’t need oxygen to burn so most of the usual firefighting methods won’t put it out. Any grouping of cellulose nitrate film increases the risk. Reels of motion picture film contained in metal canisters concentrate the off-gassing and accelerate the deterioration process.

silver mirroring on nitrate

Cellulose acetate film came along in 1912. It was clearly marked “Safety Film” to indicate to users it was less hazardous (less combustible) than cellulose nitrate. It has issues too though. Over time it degrades and develops a strong vinegar smell, sometimes shrinking and becoming brittle or developing channeling which makes the image indecipherable.

In new condition cellulose nitrate and cellulose acetate look very similar but as they age they can develop distinct differences. The most obvious is color. Aging nitrate film turns a yellowish color, which turns more amber and then brownish the older it gets. Aging acetate film also changes color but it leans toward more pink or blue shades. This visual clue helped us identify most of the film we were given to look at. There were still some question marks though. Even the words “safety film” weren’t a sure indication that we were looking at cellulose acetate film as many of the negatives seemed as if they could be copies.

three films

Turns out quite a few of the negatives we couldn’t positively identify immediately were actually a type of polyester film. Polyester film started to be used in 1955. In this particular collection a polyester film called Kodak professional copy film 4125 popped up often. How did we find this out? Well, first we identified the notch code, a sequence of distinctive notches film manufacturers put on the edge of their film. There were negatives we weren’t sure about and which didn’t have notch codes though. To be more certain of our conclusions we built and used a film viewer to conduct a polarization test. In a film viewer for this purpose two polarizing filter sheets are crossed so they seem black when you look through them. The film is placed between them. When viewed through this viewer cellulose acetate or cellulose nitrate film appears black. Polyester film appears to show the image more clearly and with a sort of pink and green rainbow effect, like you would see on a soap bubble, or the surface of a puddle oil has dripped in.

film viewer

After using the notch code identifiers and polarization tests as well as degradation markers of identification there were still a handful or two of negatives we just weren’t sure about. A few things that might be nitrate but we had doubts. At this point our only options were destructive tests. There are chemical tests but they involve highly toxic chemicals which we prefer not to work with in the Lab if we can help it. The simplest way to find out if a negative is cellulose acetate or cellulose nitrate is to burn it. Not ALL of it, just a tiny, tiny piece no more than 3mm wide. Burning a piece of nitrate film even an inch square is considered extremely dangerous! We asked the department head in charge of the collection if we could conduct the burn test, explaining that we would only be using a thin strip from the border of the negatives in question and that no portion of the images themselves would be lost.

Permission granted, we set up the test. We assigned a number to each negative, placed the fragments of film in numbered petri dishes and went to an outdoor location. Wearing gloves and eye protection Ashleigh held the fragment to be identified in a pair of tweezers and ignited it using a pocket lighter. Cellulose acetate negatives smoked and glowed a bit but the fire fizzled out to a trail of smoke as soon as the lighter was withdrawn. The cellulose nitrate film ignited quickly and burned brightly and completely with the flame burning out in all directions, much like a sparkler, even when the lighter was withdrawn. As each sample was burned we noted “nitrate” or “not nitrate” on its petri dish.

not nitrate

nitrate burns

So now the collection has been divided up according to film type and will be housed separately. Along with the plastic films we discovered some glass slides and a few glass plate negatives. As none of these are as volatile as the cellulose nitrate film or as quick to degrade as the cellulose acetate film there is less urgency in the push to reformat them. Eventually all the film will be digitized and will need appropriate enclosures to keep them safe in storage. Let Phase 2 of this project begin!


Want to learn more? You can find out more about film identification and the various tests here.

Veronica Sorcher (PLCH) — Conservation Technician, author

Jessica Ebert (UCL) — Conservation Technician, photographer

Leave a Reply