The complexity, variety, and extreme fragility of early photographic materials are often underestimated by archivists and collectors. Their stability problems have both technical and human dimensions; to preserve them it is essential to know the objects and their problems, and to continually monitor how they are being handled and stored.
To assess stability problems, it is useful to consider each photographic material in the light of a structural analysis in which the following "generic" components appear:
A. IMAGE-FORMING SUBSTANCE
D. CASE/PACKAGING/SECONDARY SUPPORT
E. INTERACTIONS OF THE ABOVE
The purpose of this "generic" structural analysis is to make it easier to identify possible stability problems for a given photographic object, not only with its constituent materials, but also with interactions or incompatibilities between its components. The exercise of fully characterizing a photographic object in light of this analysis method is surprisingly informative, and it leads to a more well-developed sense of stability problems than simply listing the known problems with each photographic process (though that too, is useful).
A definition of the terms used in the analysis follows:
In every photographic material something creates an image by absorbing or scattering light. The substance which actually comprises the image, in the finished, processed photograph is known as the image-forming substance. The most commonly encountered image-forming substance is metallic silver, but there are other possibilities. For example, in a modern color transparency there are three image-forming substances, all organic dyes; in a platinum print the image-forming substance is metallic platinum. If nothing else were known about a photographic material but its image-forming substance, a general assessment of its relative permanence and many of its specific stability problems could be predicted.
For example, knowing that an image consisted wholly or partially of metallic platinum would suggest that such a photograph would be quite resistant to fading, because platinum is a very stable chemical substance which does not oxidize or tarnish easily. On the other hand, knowing that organic dyes are the image-forming substances in a color transparency might cause concern because of the tendency of photographically generated dyes to fade in light or in the dark under moist, warm conditions. Further, each of the three dyes might fade at a. different rate--it is important to be specific when considering an image-forming substance.
In discussing silver as an image-forming substance, the form of the metallic silver and the relative size of the individual image particles should be considered. Examination of photographic materials in the electron microscope shows that silver is deposited in two general forms: as twisted, intertwined strands known as filamentary silver, and as spherical or rounded lumps known as physically developed silver. Whether the image deposit is made up of filamentary silver or physically developed silver depends on what kind of developer solution was used in the original processing. Nearly all black and white prints and films made after 1880 have filamentary silver as the image-forming substance. Examples of historical processes with physically developed silver image deposits are calotype negatives, ambrotypes, and tintypes.
Another important aspect of silver as an image-forming substance is the relative size of the individual image particles. Larger particles of silver tend to lead to more image stability and impart a blacker image color. In general, filamentary silver image particles are larger, appear blacker, and have more image stability than physically developed image particles. The smaller the individual particles, the less stable the image will be and the less neutral black the image will appear.
A special case of physically developed silver is found in nearly all 19th-century prints. Most prints of that time were not developed at all, but "printed-out", meaning that the image was produced by prolonged exposure to daylight. In this event physically developed particles of extremely small size comprise the image. In accord with the general principle stated above, such images are very unstable and have a warm, reddish image color (unless modified by toning). Examples of this type of print are salted paper prints, albumen prints, gelatin printing-out papers and collodion printing-out papers.
It is useful in most cases for the image-forming substance to be dispersed in a transparent layer which will keep it suspended to maximize its light absorption, and protected from physical and chemical attack at the same time. The material which suspends and protects the image-forming substance is known as the binder, and the most commonly encountered binder is gelatin. Two other binder materials which have been used extensively in historical materials are albumen and collodion. Just as a knowledge of the general attributes of the various image-forming substances is helpful in assessing stability problems, an awareness of the properties of the common binder materials is also helpful.
Gelatin is a protein material made from the bones and hides of cattle. It is a chemically stable substance; most of the stability problems associated with gelatin result from its physical properties (see INTERACTIONS below). Gelatin becomes brittle and contracts in dry conditions, and softens, becomes more permeable and swells under humid conditions. Gelatin is easily abraded unless adequately hardened. Hardening of gelatin (accomplished with various substances or by adjusting pH) affects its water solubility and permeability to gases which would attack the image. Some kinds of deterioration affect the hardness or solubility of gelatin layers, sometimes making them very soluble in localized areas and complicating conservation treatment.
Albumen is an image binder which was used extensively for prints during the 19th century. Albumen (chicken egg white) is a protein substance which differs. significantly in structure and properties from gelatin. Both the chemical and physical properties of albumen lead to stability problems. Because of its chemical makeup, albumen tends to yellow unless kept cool and dry. As a consequence, most albumen prints have yellowed highlights. The physical properties of albumen resemble gelatin somewhat in that albumen softens and swells in moist conditions and becomes brittle and contracts under dry conditions. However, albumen tends to fracture into a network of cracks and fissures, which in albumen prints often have a parallel orientation along the machine direction of the paper support. Albumen does not tend to become soluble upon deterioration as gelatin does, and also is less susceptible to mechanical damage such as abrasion.
Collodion is produced when guncotton (cellulose nitrate) is dissolved in a mixture of alcohol and ether. Collodion was used as an image binder for the "wet collodion" family of photographic processes (wet plate negatives, ambrotypes and tintypes) and for photographic papers in the era 1885-1930. Collodion as a binder material does not have the notorious instability of cellulose nitrate as a support material. Unlike gelatin which must be hardened, collodion is naturally brittle and must be made pliable and permeable with plasticizing additives for use on prints. Collodion does not absorb water and is very easily abraded. It is readily soluble in alcohol and a number of other solvents. Although it does not tend to fissure as extensively as albumen does, collodion layers on prints often display numerous hairline cracks. All collodion prints are of the printing-out variety and are on baryta coated stock.
The component of the photographic material upon which the binder rests is known as the support. Binder layers are by necessity very thin and do not have the mechanical strength or optical properties be a complete photographic object. The most common support materials are paper, glass, and film. In most cases the support materials do not contribute major stability problems in their own right with the notable exception is cellulose nitrate film base, which destroys the photograph as a result of its own inherent chemical instability. Supports are involved in stability problems mostly through interactions -with other components, and mostly because their physical properties do not harmonize with those of other components bonded to them.
The most frequently encountered support material is paper, which has been used as a support for both negatives and prints over the course of photographic history. The most essential requirement in paper supports is their purity, or chemical inertness toward the photographic coatings. It was learned very early that only the finest and purest papers were suitable for use in photography. In the 19th and early 20th centuries, this meant that paper had to be made from linen and cotton, which contained the least impurities to start with and required a minimum of chemical treatment during papermaking. Modern paper supports are made entirely from wood pulp.
Paper has found its primary application as a support for reflection prints. To increase the whiteness and smoothness of the paper surface, after 1885, the practice of baryta coating was adopted, in which the surface was given a smooth top layer consisting of barium sulfate (a white pigment) and gelatin. The photographic emulsion was then applied to the baryta coated stock. The physical properties of paper are largely determined by the fact that it is composed of cellulose fibers aligned mostly in one direction by the papermaking machinery. This means that when the paper absorbs moisture it expands more in one direction (across the long axis of the fibers) than the other. There is no such orientation on the expansion of gelatin or albumen. Paper also contracts and becomes embrittled at low humidities, and may tear or break easily under those conditions. Paper supports for photographs require much more careful handling than the casual handling given paper objects in everyday life, and the photographic layers coated on them are even more delicate. Secondary supports should be provided for unmounted photographic prints.
Another stability problem with paper supports, especially thick ones, is the tendency of paper to retain thiosulfate from fixing solutions. The residual "hypo" may be adsorbed to the paper sizing or baryta layer, or trapped in small capillaries between and among the paper fibers. This residual thiosulfate is difficult or impossible to remove, even by prolonged washing. Film or glass supports do not share this undesirable characteristic. Thin paper supports with no baryta layer (such as are found in albumen prints) are washed free of thiosulfate relatively easily. Photographic paper supports are also deteriorated by the same forces that affect other paper objects, namely UV and visible radiation, high humidity, acidity and oxidants. The high purity of photographic paper supports makes them more resistant to these forces than most other types of paper, however.
Glass supports are chemically inert unless kept in very moist conditions, and their main features of interest lie in their physical properties. The attraction of glass as a support originally came from its planarity and transparency, and it was in use extensively for negatives and transparencies from the late 1840's to the 1920's. Glass is extremely dimensionally stable and does not shrink or swell with changes in ambient humidity. Its stability problems originate from its weight, fragility and smoothness. Glass has considerable weight which generates numerous storage problems and creates breakage. The fragility of glass is obvious from everyday experience and the sharp edges of hand cut or broken glass also are a problem in the storage of glass photographic objects. The smooth surface of glass causes many problems of separation of binder materials carrying the photographic image.
Film supports are a complex field of study in their own right. The term film is used to designate a wide variety of flexible, transparent materials used as photographic supports. Early film supports present a number of stability problems, but modern film bases are remarkably stable. The earliest flexible films were made of cellulose nitrate, either cut from a solid block or cast from a solvent solution poured onto a smooth table or drum. Cellulose nitrate films slowly decompose and release oxidants and acidic gases which destroy the image. Materials with cellulose nitrate supports should never be stored together with other types of photographs. Cellulose nitrate shrinks as it decomposes, and this is often the first serious manifestation of its instability. The overriding concern with cellulose nitrate film is always the serious fire hazard which it poses.
Thin film supports of all types can have severe curling problems at low humidities, but the modern film bases cellulose triacetate and polyester (polyethylene terephthalate) present few chemical or physical problems. The dimensional stability and overall retention of physical properties in... accelerated aging tests of polyester films exceeds that of cellulose triacetate materials.
Many photographs are part of an integral package which is designed to support or protect the photograph. Examples of this are the cases used to enclose daguerreotypes and the mounts of cartes de visite. Such accessory materials may be designated as integral secondary supports. Since they are regarded as part of the photograph, integral secondary supports contribute a wide variety of potential stability problems.
Original mounts for prints are the most common form of integral secondary support. A mount for a print involves both an adhesive and the mount itself. Starch and gelatin were widely used as adhesives and usually did not cause serious damage. Sulfur-containing adhesives such as rubber cement are very harmful to silver images. Use of poor quality mount board is the most frequent source of stability problems from integral secondary supports. Many 19th-century mounts were composed of a laminate of good quality top and bottom sheets with a center which contained groundwood or highly lignified fibers. Discoloration and fading of photographs can result from the use of such mounts. The extent to which such defective mounts deteriorate and affect photographs is controlled in large measure by the ambient relative humidity.
Many albumen prints, for example, are mounted on very poor quality mount board which should be isolated from other photographs by interleaving. Deterioration causes embrittlement of such mounts and they should also be provided with additional secondary supports to prevent breaking of the mount (and the photograph) during handling. The variety of integral secondary supports is enormous, since to a degree such constructions reflect individual taste and expression. Elaborate or simple, both their physical and their chemical properties should be considered in light of the susceptibilities of the photographic object itself.
The component materials of a photographic object may be entirely stable in and of themselves, but the photograph which they constitute may not. It is important to consider not only the individual constituents of a photograph, but also how the various components interact. The largest class of harmful interactions arises from the laminate structure of photographic materials, where the physical properties of the support do not harmonize with those of the binder layer. An example is the separation and tearing of the gelatin emulsion layer often seen at the edges of gelatin dry plates. The glass support does not swell in moist conditions or contract in dry conditions, as the gelatin does. The stresses which can develop are more than enough to tear or delaminate the emulsion layer. Unequal stresses of this sort also frequently cause cracking and tearing of binder layers on paper supports. The contraction of albumen layers at moderate and low humidities is so strong that unrestrained prints curl into tight, cigarette-like rolls. These rolls are very difficult to relax without harming the print, especially when a number of prints have been rolled up together for many years.
Interactions or incompatibilities between otherwise stable components of a photograph maybe chemical as well as physical. An example of this is the high catalytic activity of platinum or silver-gold-platinum image deposits in photographic prints. The platinum image often catalyzes the deterioration of paper (particularly lower quality paper) and causes a "transfer" positive image to appear in any paper material in contact with the print surface. This can sometimes include creating a "transfer" image in photographs on facing album pages. The catalytic activity may extend through the primary support of the platinum print (without affecting it) and cause imagewise discoloration of a lower quality secondary support. .
The utility of the analysis of stability problems through a "generic components" approach may be best demonstrated through an example of its use. Tintypes, also known as ferrotypes or melainotypes, are familiar photographic objects and will serve as an example. The first aspect to consider is the image-forming substance. In the case of tintypes the image-forming substance is physically developed silver. There are two facts supporting this conclusion; for reasons beyond the scope of this paper the conditions of development used with tintypes led to the formation of physically developed silver, and secondly, the fact that the image on a tintype is visible at all depends on the silver deposit being gray and reflective, not neural black like filamentary silver. The "darks" of a tintype are areas where little or no silver image exists, while the whites are areas where the most silver image exists. Therefore a gray, reflective silver deposit was highly desirable, and tintypists constantly adjusted their developing solutions in pursuit of higher image quality.
The stability of physically developed silver to oxidation in air is generally lower than that of filamentary silver, yet tintypes are not known for excessive image fading. In part this is due to the fact that oxidation would not be quite as detrimental to a silver deposit which reflects and scatters light in the way that the image deposit of tintypes does. The main reason for the apparent stability to oxidation of the tintype image deposit is the fact that tintypes were almost always varnished after processing, so the image is shielded from oxygen and moisture.
The binder material of a tintype is collodion; from this fact we might expect that the binder may tend to develop minute or even large cracks because of its low flexibility. Problems with adhesion to a smooth support might also be suspected. The support of a tintype consists of a thin sheet of iron with a dark brown or black lacquer coating. The support is the source of many of the stability problems afflicting tintypes. It is easily bent, has sharp edges, and because it is made of iron, has a tendency to rust. Corrosion of the support occurs vigorously at small pits or scratches in the lacquer coating on the back of the support, and likewise at any discontinuity in the coatings on the image side if they penetrate to the iron beneath. Because so many of the potential stability problems are associated with the support, housings which protect tintypes from flexing and from damaging each other are very important in preserving them. It is also important to minimize corrosion of the support by maintaining a low relative humidity.
The case / packaging / secondary support component of tintypes consist principally of the--varnish supercoat and various types of paper folders and mounts. The varnish contributes some stability problems of its own; one is a tendency to yellow, especially if irradiated for long periods. Prolonged display can both yellow and crack the varnish layer (and the collodion with it). Tintypes usually were supplied in a thin paper folder with a die cut window opening. Some were cased in pressed paper or leather cases of the style used for daguerreotypes and ambrotypes. The thin paper wrappers (now often missing) provided protection against abrasion. Stability problems due to interactions or incompatibilities are mainly ones of cracking, flaking and exfoliation of the binder and varnish layers from the support.
The above discussion of tintype stability problems is by no means complete, but it is a beginning. The analysis method for assessing stability problems suggested in this paper is simply one way to organize consideration of the attributes of a photograph's component materials and their possible interactions in the total object. It is useful because it tends to identify common themes in apparently different photographic materials. In dealing with all of the possible problems which affect photographs, the main priorities will necessarily remain providing environmental control for collections as a whole, and the devising of methods to minimize the damage done to photographs during handling.
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Rempel, S., "The Care of Black and White Photographic Collections: Cleaning and Stabilization", Technical Bulletin #9, Canadian Conservation Institute, Ottawa (1980)
Sturge, J., ed., Neblette's Handbook of Photography and Reprography, 7th ed., Van Nostrand, New York (1977)
Swan, A., "Conservation of Photographic Print Collections", Library Trends, Fall 1981