Going to the movies in the first half of the 20th century was dangerous. The movie theatre projection room was a hazardous place to work. And while the source of the danger was well understood, it took several decades for filmmakers to finally offer moviegoers and projectionists a safer environment to enjoy Hollywood’s finest works.
Before looking at the dangerous chemistry of early film, let us remember how motion pictures work. The video portion of a movie is a series of pictures, projected at a usual rate of 24 per second, although that can range from 12 to 48 frames per second – television video in North America is usually broadcast at 30 frames per second. At these speeds, our eyes do not see the individual pictures, but rather perceive movement on the screen. In the mid-19th century, instruments such as zoetropes were used to create illusions of motion from still photographs. These instruments were limited as only a small number of photographs could be used, restricting the length of any “video” to a few seconds – and of course, with no attached audio.
The technology did not exist to capture continuous photographs for a significant length of time. Étienne-Jules Marey’s chronophotographic gun could capture 12 frames per second, but all images were superimposed onto the same picture. How to capture each image onto a separate surface, and then replayed later?
Different sources credit Thomas Edison or William Friese-Greene as developing the first motion camera. Friese-Greene’s apparatus, patented in 1889, was able to capture 10 photographs per second, with the images captured on a celluloid film. At the same time, the Eastman Kodak Company developed a celluloid film base – cotton was dissolved in nitric acid, in a reaction catalyzed by sulfuric acid. The cellulose in the cotton would react to form nitrocellulose:
C6H10O5 + 2 HNO3 → C6H8(NO2)2O5 + 2 H2O
A plasticizer, often camphor (the ingredient that gives Vicks VapoRub its distinctive odour) was added to the mixture, allowing the nitrocellulose to stretch into thin celluloid strips, which were then coated with a light-sensitive emulsion that would capture the image. The celluloid film had the right combination for a film base: it was transparent to light, and tough but flexible, allowing it to be cut into strips and perforated so it the strips could be guided through the camera, spooled for storage and then fed into the projector. It was also cheap and relatively easy to produce.
Both cellulose and nitrocellulose are polymers. Each repeating entity of a polymer is called a monomer. The figure to the right shows (A) a cellulose monomer and (B) a nitrocellulose monomer – note that three OH groups in cellulose become ONO2 groups in nitrocellulose. The nitrogen content of the nitrocellulose determines whether it will explode spontaneously. Fully nitrated nitrocellulose (with three nitro groups, as shown in B) contains 14.2% nitrogen. This was originally formulated as a weapon, but this was quickly abandoned when it was realized that it was too easily set off. Any nitrocellulose with over 12.6% nitrogen is considered explosive [ref].
The nitrocellulose used for motion picture films had two nitro groups per monomer instead of three, and its nitrogen content was closer to 11% – no longer prone to explosion, but still highly combustible. (Flash paper, used by magicians who need to burn something quickly, is another form of nitrocellulose with similar nitrogen content.) The motion picture film was sensitive to moisture, static electricity, friction, light and heat. This made the nitrate film, and therefore the projectionist, highly vulnerable to sparks and heat from the lamp of a film projector in a small, cramped room. The exact mechanism and products of the decomposition of nitrocellulose appears to be unknown; recent publications point to the formation of nitrogen monoxide (NO), nitrogen dioxide (NO2), oxygen (O2) and nitric acid (HNO3). The formation of oxygen as a product of decomposition is problematic in fighting a film fire, since smothering it won’t work – the reaction generates enough oxidant to keep it going until all of the film is consumed. A celluloid film also cannot be extinguished by immersion in water.
As movie houses became more popular in the 1910s, and the length of movies (in time, and therefore in physical length of film) increased, so did the risk of fire and, unfortunately, the occurrences of fatal accidents. The projectionist was much busier in that era, since a single roll of film contained no more than 20 minutes of video. Two projectors were used to ensure a seamless transition from the end of one roll to the start of the next roll. Unfortunately, film did catch on fire in the projection rooms, sometimes with disastrous consequences, including the Dromcolliher cinema tragedy in Ireland in 1926, the Laurier Palace Cinema tragedy in Canada in 1927 and the 1929 Glen Cinema Disaster in Scotland, each killing over 50 people, mostly children. (The front page of the Montreal Gazette on the morning after the Laurier Palace tragedy pointed out that children under 16 were not allowed in movie theatres in the province – a reaction from the Quebec government meant to improve safety for children.)
Despite the hazards of nitrate film, its advantages still outweighed its drawbacks, and nitrate film was used through much of the Golden Age of Cinema. It wasn’t until the 1948 that Eastman Kodak finally offered a viable alternative – cellulose acetate film (or “safety film”), which was much less flammable. In the final decades of nitrate film, projection rooms were stripped of any wooden furniture and insulated with – wait for it – asbestos, to isolate any fires within the room. As far as I know, projectionists weren’t entitled to danger pay.
Most nitrate films from this era are now lost or in poor condition. Even under proper storage, the decomposition of nitrocellulose will, to quote Kodak,
yellow the film base, yellow and soften gelatin, and oxidize the silver image. Later, the base cockles, becoming very brittle and then sticky. Finally, it disintegrates completely. This inevitable deterioration is usually gradual, but elevated temperatures and humidity speed it greatly.
The sharp, pungent smell of nitric acid becomes strong, and the buildup of gases in a well-sealed container causes heat to build, making conditions rife for combustion. This information sheet on Kodak’s website lists precautions for handling nitrate films that rival what chemists have to follow when handling dangerous compounds in the laboratory.
Some nitrate films have been preserved by transferring them onto acetate film, but even those films are beginning to decay, so there is a movement to transfer all film onto digital platforms. Unfortunately, fire has already consumed the original nitrate film negatives of many classic films. A 1937 fire at the vault of the 20th Century Fox studios and a 1967 fire at the vault of the MGM studios resulted in losses of many silent films that were recorded on nitrate films.
Anyone who watched the excellent 2011 film The Artist, which was set in the late 1920s and early 1930s, will remember this scene:
George Valentin, shaken as the invention of talkies (films with recorded sound) would mean the premature end of his acting career, angrily unspooled the film from his collection onto the floor, and set the pile on fire with a single match. He initially laughed as the flames quickly grew, but the intense smoke caused him to lose consciousness. His ever-loyal dog was able to draw the attention of a police officer who reluctantly followed him to the burning house, where George was found badly burned and barely alive, clutching a film canister that remained intact in the blaze. A symbolic act for sure, but also a reminder that in the first half of the 20th century, the most dangerous part of attending the movies was not the content projected on the screen, but the actual film that contained those pictures.
[Note: this article was originally published on Atoms and Numbers in 2013. Some edits were made for this publication, and some links were updated.]