| 1916 (1936/1956) |  |
| 1917 (1937/1957) |  |
| 1918 (1938/1958) |  |
| 1919 (1939/1959) |  |
| 1920 (1940/1960) |  |
INTRODUCTION TO FILM STOCKS
Copyright Ó 2000 Regents of the University of California, UCLA Film and Television Archive
Film Stocks
Motion picture film consists of three elements: the base, the emulsion, and the binder, which holds the emulsion to the base. Film stocks can be defined as the specific relationship between these three elements. Changing the character or chemical make-up of one element changes the relationship of the elements to each other and therefore influences the quality of the film stock. Different film stocks are used for different purposes. Some are used in the camera to produce the original negative image (or positive image in the case of reversal film); others are used in the reproduction process to make prints or printing elements. The manufacturers of film stocks also calibrate the emulsions for different light sensitivities, allowing film makers to shoot under diverse light conditions and produce a more or less sharp, more or less grainy image in the printing process. This lecture will discuss various film stocks and their physical and chemical characteristics.
For over one hundred years, since the invention of cinema in 1895, images have been fixed on a clear plastic strip which when run through a projector renders those images in motion on a screen. Throughout the eighteenth and nineteenth century still images on glass slides were projected for scientific and educational purposes, such "magic lantern" presentations also being a very popular mass entertainment. Before the invention of photography, images were painted on glass. After about 1850 projected photographic images were in circulation. However, projecting a series of images that created the illusion of movement presented special problems. Early attempts to rapidly project glass slides failed to create natural movement. What was needed was a base material that was clear, flexible, durable, maintained its shape, and able to withstand the stress of repeated projections through a mechanical apparatus.
With the invention of celluloid in 1855 by the Englishman Alexander Parkes, by combining nitrocellulose (gun cotton) with camphor and alcohol, a solution came within reach. However, this material was still too inflexible, until Hannibal Goodwin improved on the formula in 1887 by adding fusel and banana oils. A year later, Henry Reichenbach, working for George Eastman at the Eastman Kodak Company, developed a similar formula for commercial use. Nevertheless, Eastman was sued over patent rights and eventually paid five million dollars in damages to the estate of Goodwin. In 1888 Eastman introduced a cellulose film for his Kodak Brownie camera. W. K. L. Dickson, who was working for Thomas Edison on the development of motion pictures, realized immediately that the Kodak material would be suitable for motion pictures.
Base
In 1889 Eastman Kodak began commercial production of nitrate base film, which was delivered to Edison in 1892 as cellulose nitrate. It was a proxylin plastic, made up of organic material. Cotton or wood fiber was treated with a mixture of nitric and sulfuric acids. It was then processed with the addition of solvents, plasticizers, and flame retardants. The result in chemical terms was a nitrate ester of cellulose with the following chemical formula:
[ C6H9O5(NO2) ] n
This base material is then mixed into a viscous solution with solvent and a plasticizer and spread on a slow moving, heated, chromium-plated drum. Heat then causes the solvent to evaporate, leaving a thin layer of film. The technique of solvent casting must be very precise, so that the film has a uniform thickness. Unfortunately, the solvent cannot be completely removed in this process, and film continues to shrink over a period of years as the solvent continues to evaporate.
This first motion picture base met all the criteria needed for projecting moving images. It was strong, flexible, maintained its shape under varying conditions, and almost perfectly transparent. Eastman originally delivered the film in 70mm rolls, which Dickson then sliced into 35mm rolls, creating a standard that has lasted over one-hundred years. In another innovation no less important, Dickson then punched perforation holes on both sides of the film material, so that it could be drawn through a camera and projector. Only material of superior strength could hold up in the apparatus, given the millimeter separations between perforations.
The problem with nitrate base motion picture film was that it was chemically unstable. In particular, it was highly flammable, having a very low flash point. According to A.S.A. testing, nitrate film can self-ignite at a temperature of 300° F., and decomposing nitrate in unventilated conditions has been known to ignite at temperatures as low as 125° F. Much later it was realized that the nitrate was also subject to extreme deterioration. (See "Nitrate".)
As early as 1911 Kodak developed the first "Safety" base film, which was not flammable and it was hoped that would replace the dangerous nitrate film. Cellulose diacetate is produced by treating cotton or wood fibers with acetic anhydride, glacial acetic acid and sulfuric acid. The result was:
[ C6H7O2(OCOCH3)3 ] n.
The problem with cellulose diacetate was that it lacked the geometric stability, tensile strength, and flexibility of nitrate cellulose. The industry thus preferred to continue using 35mm nitrate, despite occasional nitrate fires. In 1923, Eastman Kodak introduced a modified diacetate that was less brittle and more flexible. This diacetate became a standard for the amateur film market (28mm, 17.7mm and 16mm), but the industry continued to prefer nitrate for 35mm production.
Chemists continued to experiment with various acetate compounds, but it was not until the early 1940s that a stable cellulose triacetate was developed that could stand up to heavy wear and retain its shape and flexibility. After 1948 the industry began rapidly switching to triacetate base film stocks. However, in Europe nitrate base films continued in use until at least the mid 1950s.
By the 1940s, chemists had also developed a polyester based film stock. Polyethylene terephthalate was invented in 1941, and was thought superior to other cellulose esters, because it was based on petroleum bi-products, rather than wood or cotton. The compound was formed by reacting ethylene glycol with dimethylterphalate or:
(OH-CH2CH2-OH) + (CH3-O CO-(O)-COOCH3)
The initial problem with this polyester base was that it lacked solubility, i.e. film could not be spliced together with conventional glues. This made the base unusable for film makers. Secondly, its extreme durability made it susceptible to static electricity which in turn turned the film into a magnet for dust and dirt particles. Neither properties were desirable, either in the film making or film printing process.
In recent years, both problems have been solved, making polyester base film stocks the standard for the industry. Ultra-sonic splicers which literally weld two pieces of film together as well as new more robust hot splicers and glues make it now possible to edit polyester based film stocks. Secondly, new coatings on the base make it less susceptible to static electricity.
Any base will shrink with age. The degree of shrinkage is measured with a micrometer gauge, which can then be translated into percentages. If a film's base shrinks radically, the film will no longer run smoothly through either a projector or a printing machine, since the distance between perforations has changed too much, causing the pull down mechanism in the projector or printer to tear the perforations. In such cases, a new print can only be generated in an optical printer in which the film is moved forward manually, frame by frame.
For well over fifty years, nitrate film was the standard for the motion picture industry. As a result, all standard gauge 35mm film produced between 1892 and 1950 is on nitrate base film stock. For many years, nitrate film was given a bad rap, because of its chemical instability and flammability. In the 1970s, when film preservation efforts in the United States first gained wide recognition, the battle cry of archivists was, "Nitrate Can't Wait." Since then, film archivists have seen acetate base films decompose just as quickly, and realized that nitrate, when handled carefully and under certain climate control conditions, will survive for many more years than originally thought.
From the cinema's earliest days, nitrate fires were a reality. Nitrate, once ignited, can burn under water, since the compound contained enough oxygen to continue the oxidation process without any outside air. For all practical purposes, this meant that it was impossible to put out a nitrate film fire. While strict fire laws were passed after the Ferguson Building fire in Pittsburgh in 1909, which caused the deaths of thirteen persons, fires occurred regularly in theatres and in the vaults of the motion picture companies. In 1910, the National Board of Film Underwriters formulated a series of regulations, concerning the handling and storage of nitrate stock. Revised in 1919, these regulations called for the installation of automatic sprinkler systems and small separation vaults. Still, as late as 1978, two disastrous fires at the National Archives in Suitland, Maryland, and at George Eastman House, destroyed several million feet of nitrate film. The fumes given off by a nitrate fire are also extremely deadly, since oxidation releases nitric oxide (NO), a gas which needs an additional oxygen atom to become a stable compound, nitrogen dioxide (NO2). In certain circumstances, e.g. the simultaneous exposure to nitrous oxides and carbon monoxides, the body is deprived of oxygen, leading any person caught in the vicinity of a nitrate fire to suffocate.
The most serious problem for film archivists is the deterioration of nitrate. While some nitrate will decompose after only a few months, other nitrate materials remain in pristine conditions after one hundred years. Until very recently it was not known what factors contributed to nitrate decomposition. Generally, though, five different stages of nitrate decomposition are visible, with partial preservation still possible in the first three phases:
1. Image fades, discoloring of emulsion, edges become sticky.
2. Film base becomes sticky at center, emulsion dissolves from base.
3. Gooey, foul smelling bubbles appear when gasses cannot escape.
4. Bubbles spread over entire reel, which solidifies into "hockey puck."
5. Brownish powder is all that remains of nitrate reel.
Accelerated aging tests by the Image Permanence Institute at Rochester Institute of Technology in the early 1990 proved conclusively that nitrate decomposition was attributable to two factors: temperature and relative humidity. In fact, it was demonstrated that for every ten degrees colder a film could be stored and every 10% lower humidity, the life-span of nitrate film could be doubled. Humidity was found to be the greatest culprit, since water reacts chemically with the nitrate ester to produce nitric acid gasses.
This process begins slowly, but the production of oxides accelerates over time. Given the fact that acid gasses are produced as a byproduct of the decomposition process, well-ventilated film cans are thus a necessity. Tightly sealed cans can speed up the process of decomposition, since the film literally stews in its own juices.
The IPI of Rochester found the ideal storage temperature to be 32° F., at 20% RH. Nitrate film stored under such conditions from the moment of its manufacture could theoretically last 1000 years. Unfortunately, nitrate was never stored under ideal conditions, and all film materials inevitably begin decomposing. The process cannot be stopped, even if film is later stored under ideal conditions, it can only be retarded. The decomposition process also leads to shrinkage of nitrate film. Such shrinkage changes the distance between the perforations, and in some cases causes the film to buckle, making any optical reproduction of the film difficult or impossible.
As a result of the testing in Rochester, film archivists have come to realize that storage conditions, and not reproduction onto acetate or polyester base film, is the first line of defense for film preservation. Film preservation priorities have thus shifted away from costly, short term reproduction to long-term storage solutions, while valuable resources for preservation are reserved for those materials in most immediate danger of decomposition.
On the screen, nitrate film has a quality all its own, which is markedly different from later acetate and polyester films. In part, this has to do with the high silver content of nitrate film, since the silver influences the way light is refracted through the image. The term "silver screen" may have been coined in relation to the quality of light when it is projected through a nitrate image. The high silver content, however, also meant that its was economically feasible to reclaim the silver from old nitrate prints and negatives, a practice that resulted in the wanton destruction of countless films by producers and distributors, attempting to recoup their investments.
Acetate/Safety film
From 1951 to the early 1990s, triacetate film base was the industry standard for 35mm film production, distribution, and film preservation. Cellulose triacetate was thought to have the advantage that it demonstrated the same physical properties as nitrate, in terms of its durability, flexibility, tensile strength, while being chemically stable and fire resistant. It was called "safety" film, because it was truly slow burning, as defined by the International Organization for Standardization (ISO). Unfortunately, time has proven acetate, whether di- or triacetate, to be as chemically unstable as nitrate, even if the decomposition process looks different.
Diacetate films were first used for amateur formats. George Eastman and other manufacturers were very concerned about safety in the home movie market, since amateurs not only shot their own films, but also projected them. Due to the heat of the light sources, it was during projection that fires were most likely to occur. With the development of triacetate base film for amateur and professional use, it was assumed that not only the safety issue, but also the matter of chemical instability had been solved.
By the 1980s, film archivists realized that their older acetate film collections were giving off an acidic "vinegar" odor and that films which smelled were subject to shrinkage and sometimes extreme buckling. It was realized that, like nitrate, acetate bases are subject to accelerated decomposition. In aging tests it was demonstrated that triacetate film shrank as much as 0.02% in six months at 70° F. and 40% RH. Likewise, decomposing acetate gives off acidic gasses as a byproduct of the decomposition process, leading to the coining of the term "Vinegar Syndrome" in the 1980s. In point of fact, the vinegar odor was a product of acidic gasses being released by decomposing acetate base films. As in the case of nitrate, acetate decomposes more rapidly at high temperatures and humidity, while lower temperatures and relative humidity can retard, if not completely stop, the decomposition process, as all film bases are subject to decomposition from the moment of their manufacture. It was also realized that films with vinegar syndrome could attack "healthy" films, if they are not segregated, since the acidic gasses released accelerate the decomposition process in all films.
Nevertheless, film archivists were reluctant to switch to the alternative of commercially available polyester base films, because it was known that these could only be repaired with tape splices which were not considered archival.
Polyester
As noted above, polyethylene terephthalate or polyester was invented in the 1940s, but was not commercially accepted as a substitute for triacetate until the 1970s. Even then, film archivists hesitated to use the material. Still, it must be noted that polyester base film is more transparent than acetate, allowing for light to pass through it more easily, is more durable, and dimensionally more stable.
Unlike nitrate and acetate films, polyester is not manufactured through the process of solvent casting, but rather through the process of melt casting. Molten polyester is forced through a narrow slot in a casting cylinder. The material is then quickly cooled and at a specific temperature it is stretched to a required length and width. It is then reheated to "set" the shape. Stretching the polyester in this manner gives it added strength and flexibility. The base layer is also thinner than nitrate or acetate bases. Given the new ultrasonic splicers and other equipment specifically designed for polyester bases, polyester has become the industry standard. However, while it is assumed that polyester will last for hundreds of years, its actual longevity and therefore its usefulness as an archival medium is still open to debate.
Binders
Photograph emulsion will not stick to a clean film base. In order to adhere emulsion to the base, a binder must be utilized. The photographic emulsion of silver halide crystals is suspended in a gelatin layer that holds them onto the film base. This state of suspension of evenly divided matter is called a colloid. The protective colloid for photographic materials is gelatin. Gelatin is an organic material obtained by the partial hydrolysis of collagen derived from skins, animal tissue and bones.
The process of preparing the base with a binder for photographic coating is called subbing. The process involved putting a mixture of water, gelatin, and solvents onto the nitrate or acetate film base. The solvents dissolve the top layer of the base, allowing the gelatin to stick to the base. Once the solvents and water have evaporated, the gelatin layer is ready for coating. Polyester is handled slightly differently, because of its increased durability and the increased difficulty of dissolving the top layer of the base. In the melt casting process, a special chemical layer is applied, before the polyester is stretched into its final form. This chemical layer makes it easier to then attach a gelatin layer in the manner described above.
Gelatin binders have the following advantages: 1. They provide a protective support for the photographic emulsion; 2. Gelatin is transparent, 3. It is chemical stable, 4. It is water permeable, allowing for processing, 5. It is easily coated and provides good adhesion; 6. It is thermally reversible, changing from a liquid to a solid to a liquid, depending on the degree of heat. Acceptable and commercially viable synthetic substitutes for gelatin have yet to be developed.
Emulsion
According to the American National Standards Institute (ANSI), "a photographic emulsion consists of dispersions of light-sensitive materials in a colloidal medium, usually gelatin, carried as a thin layer on film base." The light sensitive material consists of silver salts, which are made by dissolving pure silver nitric acid to produce silver nitrate crystals. These crystals are then mixed with gelatin and other chemicals to form silver halide grains. The size of the grain and the light sensitivity of the silver material determines the photographic quality or speed of the film, i.e. the amount of light needed to produce an image on the film. The more grainy the film, the more visible the grain will be in projection, decreasing the film image's sharpness. The sensitivity or speed of an emulsion to light is calibrated according to a number. In the United states that number is assigned by the American Standards Association (ASA), while in Europe and elsewhere DIN values (Deutsche Industrie Norm) are assigned. ASA numbers are arithmetic, so that doubling the ASA index value, doubles the speed of the film, making it twice as sensitive to light. Films with speeds of ASA 50 or lower are considered slow, while ASA 400 and higher are considered fast.
Negative
Negative film is what is usually used in the motion picture camera to produce the camera original, unless reversal film (see below) is used. Negative film must be kept in the dark, until it is exposed in the camera or laboratory, which means that the camera or printer must be loaded in the dark. Negative film used in the camera to shoot a scene is called the camera negative. After negative film is developed in the laboratory, it can be viewed under normal lighting conditions. The image on a negative film is reversed, i.e. blacks appear as white (or clear), and whites and grays appear as black. With color negative, color dyes have replaced the metallic silver halide grains. After development, the dyes are visible as complimentary colors to the film's actual subject, i.e. reds appear on the negative film as a shade of green, blue as orange, and purple as yellow. In the case of the Technicolor or other color separation processes, the camera exposes three black and white negatives, simultaneously, with each negative sensitive to a particular portion of the color spectrum.
The negative is used to make positive prints, which when projected reveal a black and white or color image, as captured by the camera. The original camera negative is spliced together from individual shots, conversely negatives that reveal a large number of splices can usually be assumed to camera negatives. In order to make the splices invisible, negatives are "A and B rolled": the first shot is spliced on the A roll followed by black leader the exact length of the second shot, while the second shot is spliced on the B roll behind black leader the exact length of a first shot, the third shot following on the A roll, etc.
If a large number of prints have to be generated from the negative, then a printing negative is made from an interpositive, which in turn has been produced from the camera negative. In other words, unless it is reversal film, a positive can only be printed from a negative, and an inter- negative only from a positive. An internegative or printing negative is produced, because a negative cannot be used repeatedly to make prints, since the mechanical wear and tear of the printing process will eventually wear out the film. It is one of the ironies of film history, that it is the most popular films, whose negatives have suffered the most, e.g. Casablanca, whose original negative wore out decades ago through overuse.
In the silent period, a film company would usually produce two camera negatives, placing two cameras next to each other to shoot a scene. Since the angle would be ever so slightly different for each camera, editors had to make sure they consistently used material from either a "A" or the "B" negative. One negative was usually used for domestic print release, the second often inferior (since the camera position was not perfect) negative for foreign distribution prints.
Since the sound period (after 1930), the negative has consisted of two parts, a picture negative and a track negative. The picture negative captures the visual information, while the track negative captures the sound.
Positive
The positive film is in most cases the print which is sent through a projector to produce a positive image on the screen. It is usually produced by exposing positive film stock to light that has passed through a negative, i.e. a negative and positive film stock are sent simultaneously through a printer. The further the pre-print material is from the original camera negative, the grainier a film image will become, since the grain is reproduced in each succeeding generation as ever lager particles of silver halide. Other factors influencing grain, include the temperature of the developer, and the length a film is left in the developing solution. The longer a film develops, the more time silver halide particles have to convert to larger clumps of silver grain.
Any defects in the negative will also appear in the positive, generated from that negative. Some factors to look for are contrast, gradation, resolution, and acutance:
1. Contrast defines the relative density of the image in light and dark areas. High contrast images tend towards stark black and white, while low contrast images tend towards a richer assortment of gray values.
2. Gradation refers to the entire range of tonal values that a given film stock is capable of achieving. Films that record many tones faithful to the original subject are said to have good gradation.
3. Resolution or the sharpness of the image is dependent on the film's ability to reproduce details, and is often measured by its resolving power. A resolution test is used to measure sharpness. A given film stock is used to shoot an image with test lines on it. The greatest number of lines per millimeter from a test image that a film is able to produce sharply is its resolving power. The more fine grained a film stock is, the higher will be its resolution.
4. Acutance is a measure of sharpness that has a high correlation to an observer's subjective perception. Films with a high acutance have an abrupt change of density between light and dark areas, while a gradual change in density results in low acutance.
Fine Grain Master
A fine grain master, known in Europe as a lavender print, is a low contrast, positive film stock, which is not used for making projection prints. Rather fine grains are printed from original negatives or dupe negatives for the sole purpose of making another dupe negative from the fine grain. A low contrast, extremely fine grained emulsion is used, since as much visual information as possible from the original must be rescued, and the more contrast and grain an emulsion allows for, the thinner will be the actual emulsion on the copied material. Fine grains (FGM) have a slightly purple color to them, and are therefore sometimes called lavender prints. They should not be projected, even if they are positive prints, because the image quality will be poor, compared to a timed projection print, and because the perforations have a slightly different shape than projection prints. They usually have also not been waxed, as are projection prints, so they will not run smoothly through a projector. Nevertheless, many fine grain masters show signs of wear from having been projected, because their owners treated them as anything but projection prints, once its original purpose - namely to generate a dupe negative - had been fulfilled.
Reversal
Reversal film is a camera film stock which when developed results in a positive, rather than a negative image. This saves the film maker the cost of making a positive print from an original camera negative, in order to see what he has shot. It was originally introduced as a medium for amateur movie makers, who were not necessarily interested in becoming accomplished film makers who cut and edited their films. Like many of today's amateur videographers, they were more interested in keeping a visual record of family events, and the growth of their children. Black and white and color reversal film was also manufactured in such a way as to result in good quality projection. Its provides good contrast and a rich array of colors. Thus, reversal film stock was initially available in non-standard gauge film formats: 16mm, 8mm, Super-8, etc.
Very quickly, however, reversal film stock became the medium of choice for many professional film makers. Television news, for example, relied for many years on 16mm reversal film to capture the days events, until it was replaced in the late 1970s by Betacam video. Avant-garde and experimental film makers also turned increasingly to 16mm reversal film, not only because of the cost, but also because of the quality of the material. Reversal film after processing is more scratch resistant than negative film, a big plus in the editing process when film material has to be handled repeatedly. Scratches that do occur are less visible on reversal than negative.
All black and white reversal films are panchromatic. Since its emulsion is very much like that of negative film, it can also be developed in the laboratory as negative. It is in the processing that a positive image is created with reversal film. Reversal film is not placed in a bath of fixer or hypo after being developed, thus that part of the emulsion which would have been dissolved in a fixer bath remains as a positive image. Instead, bleach is used to wash away the metallic silver negative image. The unexposed silver halide in the emulsion is then re-exposed to a controlled light source, and the film is then redeveloped, turning the silver halide into metallic crystals. Only then is it fixed, washed and dried.
Many technicians believe that reversal films are more fine grained than negative films with a similar sensitivity to light. Color reversal film, of course, produces after developing an image, whose colors approximate the original subject. It is also possible to make prints from reversal originals, using color or black and white reversal print stock. In some cases a low contrast reversal stock is used for prints, when the original reversal is normal contrast, in order to eliminate an unacceptable build up of contrast. Another system of printing and developing allows for normal contrast print stock to be used in conjunction with normal contrast camera reversal. Given the sometimes unacceptable increase in contrast, when color dupe negatives are generated from inter-positives, film producers have increasingly used a low contrast color reversal film stock, in order to generate dupe negatives directly from original negatives. Manufacturers produce as many emulsion types for reversal, as they do for negative film. Indeed, many labs, specially in non-standard formats, are much more experienced in dealing with reversal, than with negative.
Prints are, so to speak, the final product of the motion picture production process. They are the objects that are sent through the projector, casting a moving shadow on a nearly white screen, which the audience perceives as movies. In terms of archival material, many more prints have survived the ravages of time, than negatives or other pre-print material. Prints have almost always produced in multiples, but especially in the commercial film industry, so that the chances of a film surviving as a print is much greater. In point of fact, most restoration work in film archives is based on surviving prints, rather than negatives. Rare are the occasions when motion picture film companies have donated or placed on permanent loan their negatives or fine grain masters in film archives, as Warner Brothers did, placing nitrate collections at the Museum of Modern Art and the Library of Congress, respectively, or the Universal donation to the Library of Congress. Other larger collections of negatives survive, e.g. the MGM Technicolor negatives at Eastman House, or studio owned collections from the sound period, but most films from the silent period, most European and Third World films, and most American poverty-row and independent film productions survive only as used distribution prints.
Thus, projection prints are the general stock and trade of most film archives. Many of these prints remain as projection prints in the archive, and are used for screenings until they wear out. Other projection prints, which may be unique or only of one a handful of known surviving prints, are reclassified as preservation masters, from which eventually a new dupe negative and print material will be generated. This is done, since all prints are subjected to continuous damage, if they are projected.
One unprofessional projection can tear a print to shreds, but even expert projection cannot stop the film from being damaged. Projection prints, when they are damaged, are usually repaired by the projectionist or his assistant, but the damage remains. As a result of this wear and tear, no two projection prints are alike. Projectionists are also known to remove "their favorite" scenes from a given film, like the projectionist in the Italian classic, Cinema Paradiso. This is another factor contributing to the differentiation of prints. Beyond such technical and local influences, film prints are subject to another kind of manipulation which results, namely film censorship. Every country, and, up until the 1970s almost every American state, had its own set of regulations about what could be shown on the screen and what couldn't. Commercial and avant-garde films were censored for political, social, and sexual reasons. Thus the film prints shown in different countries often differed from each other, depending on the light or heavy hand of the local censors. This issue becomes a serious problem, when attempting to complete a reconstruction of a film from prints of different national origin. Even if an original script or the censorship records from a given country are available, it is not always clear what the film's original form may have been, making any reconstruction from prints a mere approximation of the original.
Edge Codes
It is not uncommon to find films in the vaults of film archives which have not yet been identified. This may have occurred, because the head titles are missing, the film is in an unknown foreign language, or because it is a fragment, anywhere from a few feet to several reels in length. One way that archivists have learned to identify such unclassified films is by looking for the edge codes. Some, though not all manufacturers, have placed marks on the edge of the film (outside or between the perforations), which allow the researcher to identify the film's manufacturer and in some cases date of the film stock's production. While this information may not be identical to the film's year of production, it usually falls very close to it, since film stock's usability expires relatively quickly (unless a very poor avant-garde film maker used out of date film). With that information and other clues it is sometimes possible to narrow down the possibilities, leading to a positive identification.
The most common edge coding comes from stock manufactured by the Eastman Kodak Company. The codes, usually found after the word Kodak on the edge of the film, identified the film stock's year of production. From 1916 onwards, Kodak used edge codes on film stock produced in Rochester. Beginning in 1917 Kodak's Harrow (Great Britain) plant did the same with a different edge code system, and in 1925 Kodak's Canadian plant introduced its own edge code. The Rochester edge codes for the first several years looked like this:
| 1916 (1936/1956) | |
| 1917 (1937/1957) | |
| 1918 (1938/1958) | |
| 1919 (1939/1959) | |
| 1920 (1940/1960) | |
Every twenty years the codes are repeated, but that is not a problem, since there is usually plent of other visual data to differentiate between say 1928 and 1948. Kodak supplies manuals which list all their edge codes. Since the code is printed photographically onto the film stock, one must be careful not to mistake an edge code that has been printed through from an older print with the edge code of the dupe print.
The only other film company to consistently use edge codes, at least in the silent period, was the Pathé Film Company of France. Pathé named the company differently in different years, then after 1921 started using the name of the company in conjunction with numbers. Gaumont, Dupont, and Agfa, among many others, also listed their company name on the edge code, but there are also many surviving film materials that cannot be identified by manufacturer or date. One can say, almost happily, that since Kodak had a virtual monopoly on film stock production for much of the first half of the century of motion pictures, and was still dominant for the second half, many films are more easily identified through Kodaks more specific edge code system.
Laboratory
Unlike artists, such as painters or writers, film makers are dependent on a large network of technicians to help them make a film. This dependence begins when the film cameraman must send his film to a laboratory for processing, in order to see what he has shot. It continues, after the film has been edited and a sound-track prepared, when again the film maker must send his edited film to the lab to make prints and his sound tapes to generate a track negative. Finally, in the film preservation process, laboratories work hand in hand with film archivists to generate new film preservation materials from old prints and negatives.
However, laboratories can also be responsible for making mistakes that damage the film, either immediately or long term. For example, if a film is not properly dried, water splotches (actual canyons in the emulsion) may remain on the emulsion that appear as white spots on the projected image. If the film is developed unevenly, it may have a mottled look; if it remains too long in the developer, it may become too contrasty. In the case of color film, the color balance may vary from one reel to the next, due to processing errors. A film's perforations may be damaged, or the emulsion may be scratched, due to mechanical errors in the printing process. Finally, if a film stock is not washed sufficiently, residual hypo may remain, which sooner or later will have an effect on the longevity of the film.
For film archivists, laboratories are important first to make new negatives and/or prints from surviving original materials. In that process, the original film and an unexposed film are both passed through a printer at the same time and exposed to light, resulting in the transfer of the image from the original to the copy. In making a copy, whether from negative to positive or positive to negative, several issues are important to consider:
1. Achieving optimal sharpness, i.e. making sure the copy is in focus on all parts of the framed image
2. Registration of the image, i.e. making sure the frame is copied consistently in the same place, so that the film image doesn't jump
3. Consistency in the print light, so that individual frames are not over or under exposed
4. The speed with which the film is copied, whether frame by frame in a step printer or more rapidly in a continuous printer
5. The mechanical properties of the printer, i.e. some printers are calibrated only to copy new film which can be rapidly generated, while other printers are able to deal with films that have shrunken significantly or are extremely brittle.
In today's highly technical, computerized laboratory, a lot of preparation goes into a film, before it is actually copied. Apart from repairing all mechanical defects (edge and perforation damage), and cleaning the film through various methods, the film original's emulsion density is measured, and the information is recorded and computed, so that the printing light may be changed from frame to frame or scene to scene. Apart from the brightness of the printing light, the light's color temperature may be changed, or filters added that eliminate certain portions of the light spectrum or correct the color temperature. When reproducing tinted and toned silent films, color filters are often utilized to reproduce certain tints on color film.
There are two kinds of printing: Contact printing and optical printing. In contact printing, the two film stocks (original and undeveloped) are literally laid over each other, emulsion to base, when light passes through them, exposing the second film stock. With a minimal loss of light, a copy very close to the original is achievable. In optical printing, light passes through the original material and a lens, before casting a shadow on the unexposed film. This is sometimes advantageous, when the film's format is being changed, e.g. when printing up from a 16mm original positive print to a 35mm dupe negative. When an original film is badly scratched, a so-called liquid gate, optical printer may be employed. In such a printer, the original film is held for a split second in the gate in a highly viscous liquid, which refracts the light in such a way as to "fill in" some of the scratches. However, this method also loses a lot of light, so that care must be taken not to underexpose the film.
Laboratories and archivists differentiate between various prints and negatives, depending on their relationship to the original material. The original material is called a master positive or original negative. The subsequent copy is called a fine grain master or dupe negative. The first print to be made from a processed negative is called an "answer print." Once it has been checked for quality, and changes made in the timing scheme, a second answer print or correction print is made. When everyone is happy with the results, serial prints can then be generated.
Processing
As noted above, film stock is sensitive to light until it has been processed in a laboratory. Processing film material, whether negative or positive, involves of several stages: 1) developing, 2) washing, 3) fixing, 4) washing, 5) drying. In the developing bath, the silver halide that has been exposed to light is chemically transformed into metallic silver crystals. The film is then washed in running water to stop the developing process, hence this stage is called a stop bath. Next, the film is placed in a fixer bath, which dissolves all remaining portions of the emulsion which have not been converted to metallic silver crystals. Next the film is thoroughly washed again in running water to remove all chemical residues. Finally, the film is dried on large drying racks, so that no moisture remains on the emulsion (causing splotches), before the film is rewound onto a reel.
Three factors influence film processing, the handling of which can either improve or ruin the final product. First, the chemical make up of the developer and the fixer can radically alter the look of the image, since processing is essentially chemistry, or the use of chemical compounds to cause a chemical reaction. Secondly, the length of time a film is developed changes the quality of the image, specifically its gamma values or contrast, because chemical reactions will have a greater or lesser effect depending on how long they are are allowed to occur. Thirdly, the temperature of the developing solution will increase or decrease the time needed for development, because increased heat speeds up the chemical reaction, therefore the warmer the temperature of the developer the quicker the film will be developed. As noted above, developing black and white reversal film stock involves several more steps. They include, first development, wash, bleaching, wash, clearing solution, wash, reexposure, second development, wash, fixing, wash, drying. Finally, processing color film negatives and reversals is an even trickier proposition. (See "Color.")
It is possible to manipulate these factors in processing to not only reproduce a film that has acceptable image quality, but to also correct for errors in the original. Thus, if the contrast in the original material is too low this can be corrected by overdeveloping (increasing the time of development). If it is too high this can be corrected by underdeveloping (decreasing the time of development) and therefore lowering contrast. If requested, laboratories will also "push" a film through forced development. This involves either increasing the temperature of the development solution or lengthening the development time, and will increase contrast and grain in black and white film and shift color values in color film. Film is usually pushed one or two stops, i.e. an ASA 100 film may be pushed to ASA 200 or 400, ASA 200 to ASA 400, etc., but under some circumstances certain film types can be pushed further.
While for many years errors in processing were thought to cause film decomposition, for example, because of residual hypo on the film, film archivists are now aware that the base of any film stock is doomed to decompose sooner or later.