The conservation and restoration of film is the physical care and treatment of film-based materials (plastic supports). These include photographic film and motion picture film stock.
Film supports
Cellulose nitrate Cellulose nitrate () is the first of film supports. It can be found as roll film, motion picture film, and sheet film. It is difficult to determine the dates when all nitrate film was discontinued, however, Eastman Kodak last manufactured nitrate film in 1951. Cellulose nitrate is cellulose fibers modified with nitric acid to create a nitrocellulose solid. Cellulose nitrate is extremely flammable and capable of burning even in the absence of oxygen and underwater. It has been the cause of many fires and losses of motion picture films, photographic materials, and archival records.
Cellulose acetate Cellulose acetate is also known as "safety" film and started to replace nitrate film in still photography in the 1920s. There are several types of acetate that were produced after 1925, which include diacetate (), acetate propionate (1927 – ), acetate butyrate (1936–present), and triacetate ( – present). The earliest, diacetate, was found to be inferior to nitrate because it had high shrinkage rates. Eastman Kodak then worked with mixed esters of cellulose to create acetate butyrate and acetate propionate, which were used for sheet film, X-rays, amateur roll film, and aerial maps, but they were still found to be inferior to nitrate by the motion picture industry. Triacetate was then produced and is still in production for roll film today.
Polyester Polyester (1955–present) started to replace acetate as a film support for many types of sheet film; however, due to its rigidity, it cannot be used as roll film. Polyester can also be identified as "safety" film. It is inherently more stable than both cellulose nitrate and cellulose acetate, because it is a synthetic polymer that is inert (chemically inactive). It shows exceptional chemical stability and physical performance. A polarization test can be used to identify polyester from cellulose acetate or nitrate. When viewed between cross-polarized filters, polyester will exhibit red and green interference colors, similar to a rainbow moiré pattern. Nitrate and acetate films will not show these interference colors.
Film formats
Motion picture Perforated roll films create for sale or theatrical distribution typically found in 35 mm, but also 65 mm, 70 mm, and other gauges. Seen on reels, rolls, strips or frames. Can be nitrate, acetate, or polyester.
Small gauge motion picture Found in short rolls and may be cut after processing into strips or single images. Includes 35 mm, still-camera negatives, 120 and 220 size roll films and slide (transparency) films. Can be nitrate, acetate, or polyester.
Still photo (roll) Found in short rolls and may be cut after processing into strips or single images. Includes 35 mm, still-camera negatives, 120 and 220 size roll films and slide (transparency) films. Can be nitrate, acetate, or polyester.
Magnetic soundtrack Roll film with a ferromagnetic coating. Seen on rolls or reels and a variety of gauges. Can be acetate or polyester.
Aerial Sheets or roll film in sizes of 4x5, 8x10, and 10x10; can splice together single sheets into larger rolls. Can be nitrate, acetate, or polyester.
Microfilm Roll film in 35mm and 16mm widths, and 105mm microfiche. Can be reels, rolls, cards, or sheets. Can be acetate or polyester.
Sheet Single-sheets of film commonly found in sizes of 4x5, 5x7, and 8x10 inches. Also known as flat film. Can be nitrate, acetate, or polyester.
X-Ray Large format sheet film, 8x10 or larger. Can be nitrate, acetate, or polyester.
Agents of deterioration
Temperature and humidity The deterioration of both cellulose nitrate and cellulose acetate negatives is highly dependent on temperature and relative humidity. Storing materials at room temperature or warmer, and with high humidity, will inevitably cause deterioration. Extreme fluctuations in temperature and relative humidity will rapidly increase the rate of deterioration.
Exposure to light Permanent display of photographs is not recommended; most photographic materials are vulnerable in varying degrees to deterioration caused by light. Damage caused by light is cumulative and depends on the intensity, length of exposure, and the wavelength of the radiation. Visible light in blue part of the spectrum (400 to 500 nanometers), and ultraviolet (UV) radiation (300 to 400 nanometer region) are especially damaging. Sunlight and standard fluorescent light are both strong sources of UV. Color slides are particularly susceptible to fading when exposed to both visible and UV light.
Pollution Air pollution attacks photographs in the form of oxidant gases, particulate matter, acidic and sulfiding gases, and environmental fumes. Oxidant gases are composed primarily of pollution created by burning fossil fuels such as coal and oil. This can cause photographic images to fade. Particulate matter, such as soot and ash particles from manufacturing processes, exists in abundance outdoors and can enter the library or archives through heating and cooling ducts, doors, and windows. Particulates, which may be greasy, abrasive, and chemically or biologically active, settle on shelves and on collection materials and create dust that is spread to other materials when they are handled. The by-products of combustion combined with moisture in the atmosphere pose another risk to photographic materials. These acids attack all components of photographs and cause silver images to fade and paper and board supports to discolor and become brittle. Environmental fumes can be especially damaging to photographic images even in small quantities.
Poor storage Poor storage, such as lack of enclosures, can lead to deterioration through exposure to light and pollution. Additionally, materials that are not carefully and safely stored can rub against each other, causing scratching and abrasions. Storing film materials in basements, attics, or garages can cause rapid deterioration through extreme temperature and humidity fluctuations, but also through exposure to insects and rodents. Storing materials on the floor can also increase the risk of damage by water leaks, insects and rodents.
Poor handling and use Film can be mishandled causing physical damage. Working at dirty workstations or using bad rollers for motion picture film can cause dirt, scratches, and abrasions. Tears can occur when the film is stressed during winding or projection, in addition to improper threading. Repeated use, including the repeated insertion and removal of film from sleeves can also cause scratches and abrasions. Ignorance, neglect, and carelessness account for a significant percentage of damage to photographs.
Improper chemical processing Major silver deterioration occurs when photographs are not correctly processed and washed. Over time, residual fixer left in the photograph causes the image, binder, and support to turn yellow or brown and the silver image to fade. High temperature and humidity speed this process. Photographs that were not well fixed remain light sensitive and may darken when exposed to light.
Nitrate deterioration Cellulose nitrate is chemically unstable; decay is irreversible and autocatalytic. When the film deteriorates, it turns yellow, brittle and sticky. The off-gassing of nitrate produces hazardous and corrosive products (nitrogen oxides form nitric acid) along with an acrid smell that cause fading of the silver image, decomposition of the gelatin binder, and the eventual destruction of the plastic support and image material. These products (nitric acid, nitric oxide and nitrogen dioxide) are destructive to other film-based negatives, and especially to photographic prints. Other artifacts, metals for instance, will be harmed by proximity to deteriorated nitrate material. Such gasses can cause skin, eye, and respiratory irritations. Allergic sensitivity has also been noted, as has dizziness and lightheadedness.
Types of deterioration
Embrittlement Caused by the loss of elasticity in the plastic film base over time. The film can snap or break under strain, and will no longer have flexibility.
Shrinkage The loss of the original dimensionality of the film and is irreversible.
Distortion Distortion is caused by uneven shrinkage across a film's dimension and starts to warp or curl. This can be due to the difference in shrinkage between the film and emulsion layers, or areas of the film that start to shrink more than other areas. Temporary distortion can be reversed, but permanent distortion cannot.
Gelatin and base decomposition As the film is exposed to moisture, heat, and acids, nitrogen oxides are produced, which creates nitric acid. These promote silver corrosion and decompose the gelatin emulsion layer. The chemical reactions that occur cause further nitrate decay. Softness and stickiness occur and the image starts to fade away. This is irreversible.
Silver mirroring and fading (black and white) When exposed to moisture in the environment and various pollutants, the image silver will corrode, creating a reflective metallic sheen on the emulsion side. Typically starts at the edges of the film. Poor quality enclosures and storage can hasten the process.
Redox blemishes These small pink, orange or yellow spots can found in the image area and are often found in microfilm. These are created by local oxidation of metallic silver due to moisture and pollutants in the environment.
Levels of deterioration Deterioration of nitrate film is often categorized in six stages, or levels:
Level 1: No deterioration Level 2: The film turns yellow. The silver gelatin can begin to mirror Level 3: The film becomes sticky and emits a strong noxious odor (nitric acid) Level 4: The film becomes an amber color and the image starts to fade Level 5: The film is soft, can adhere to adjacent film, enclosures and other materials Level 6: The film degenerates into a brownish acid powder
Acetate deterioration While considered "safety film" as it produces less hazards than nitrate, cellulose acetate still faces stability issues. The deterioration of acetate is autocatalytic just like nitrate: once the deterioration process starts, the degradation products induce further deterioration. If not properly cared for and preserved, cellulose acetate will become acidic, and face many of the same issues that nitrate faces. Deteriorated acetate can also emit a noticeable odor, and can cause the same skin, eye, and respiratory irritations as nitrate does, in addition to dizziness and lightheadedness.
Types of deterioration
Vinegar syndrome
Vinegar syndrome is the release of acetic acid, which produces a strong vinegary odor. AD strips are dye-coated paper strips that can detect and measure the severity of vinegar syndrome in film collections. The strips change color, shifting from a blue to green to level, based on the level of acidity found. If the AD strip level remains at blue, the film is in good condition and no deterioration is present. If the AD strip level turns yellow, the film condition is in critical condition, creating a possible handling hazard, and should be frozen immediately.
Embrittlement Caused by the loss of elasticity in the plastic film base over time. The film can snap or break under strain, and will no longer have flexibility.
Anti-halation The acetic acid that is released in cellulose acetate negatives can trigger the retrieval of the color dyes in the backing layer, known as the anti-halation layer, giving the negative a blue or pink appearance. This is irreversible.
Plasticizer exudation Plasticizer additives that move to the surface of the film and form crystals or bubbles as the acetate film deteriorates. This is irreversible.
Shrinkage