PMG Cased Photographs: Daguerreotype
Cased Photographs. Including daguerreotypes, ambrotypes and tintypes was published as Chapter 2 of the The Photographic Materials Conservation Catalog in 1998. This printed information has served as basis for this wiki page, and current information has being added as necessary.
Daguerreotypes are direct positive photographs on metal supports. The supports are copper plates clad with a polished layer of metallic silver. The image particles, which lie on the surface of the polished silver, constitute the image highlights; they are primarily composed of silver but may contain small amounts of gold and mercury. In highlight areas, the silver image particles scatter incident light and appear to be white, while in the shadow areas the polished silver reflects light like a mirror; when the plate is positioned to reflect a dark surface, the image appears as a positive. Image particle size and spacing are in the same order of magnitude as the wavelengths of light; this may result in the appearance of physical (non-colorant) colors in the image through diffraction effects, particularly in highlight (high particle density) areas. Fine parallel polishing lines are typically observed on daguerreotype plates. They are generally horizontal across the image, since the plates were designed to be looked at using a light source positioned to the side, rather than above, the image. These marks should not be confused with damage. During plate preparation, a silver electroplating step may have been employed; this will usually be signalled by the presence of a dull silver layer on the plate verso. From 1842, daguerreotypes were routinely "gilded" -- treated with a solution of gold chloride. This procedure, which heightened the image contrast, also seems to have improved the resistance of the plate to tarnishing and hardened the image particles against physical disruption. Daguerreotypes were sometimes hand-colored by the careful application of dry pigments mixed with gum arabic; colored areas may have transparent overcoats applied. Most daguerreotype images are laterally reversed; some late daguerreotype cameras included a mirror system to correct the image reversal. Refer to PMCC Process page Daguerreotype also.
Daguerreotype plate size designations
The sixth plate is the most common studio portrait format. Because of the approximate way in which the silvered plates were cut down by daguerreotypists to make smaller formats, the confusion between metric and imperial units, and the existence of at least two sets of standards, there are some discrepancies in the published dimensions of daguerreotype full plates and fractions. Formats are not simple fractions.
- The regular American "whole-plate" (or "full-plate" or "4/4") is 8½ x 6½ inches (21.6 x 16.5 cm).
- The half-plate (6½ x 4¼ inches, 16.5 x 10.8 cm),
- The quarter-plate (4¼ x 3¼ inches, 10.8 x 8.3 cm) and
- The eighth-plate (3¼ x 2-1/8 inches, 8.3 x 5.4 cm) sizes are simple divisions of the full plate.
- The sixth-plate (3¼ x 2¾ inches, 8.3 x 7 cm),
- The ninth-plate (2½ x 2 inches, 6.3 x 5 cm) and
- The sixteenth-plate (1-5/8 x 1-3/8 x inches, 4.1 x 3.5 cm)
Another set of plate formats derives from the "Imperial" (or "Mammoth") plate. See references in Buerger, Kempe and Rinhart (1975) on these formats.
- Imperial/Mammoth plate measures 14 x 11 inches (35.5 x 28 cm).
- A 16½ x 14½ inch (42 x 36.8 cm) plate is also reported.
- Plates exist which are as large as 24 x 16 inches (61 x 40.6 cm).
- Panorama format plates also exist.
Note that plates may have been precut by the plate manufacturer, in which case they will often show the manufacturer's name and a plate grade as blind-stamps in one corner. (Higher grade numbers indicate thinner silver layers on the copper plate.) Alternatively, the daguerreotypist may have done the cutting. These plates usually lack the manufacturer's identification; they may be irregular in shape and may deviate from any standard dimensions.
Stereographic images were sometimes made in multiple image (nonstereograph) cameras, then cut and realigned to create a stereographic effect. When removing these from a package for treatment, ensure that the plates can be unambiguously identified for return to the package in their proper orientation. A carefully recorded diagram of the relative spacings of the plates within the package should be included in the pre-treatment documentation. The stereo effect may be checked by observing the image through a stereoviewer.
Daguerreotype Metal Plate
Daguerreotype plates are manufactured by the "Sheffield plate process" in which a thin layer of silver is fused to a copper ingot by heat then rolled to the desired plate thickness. Exfoliation may occur; in this condition the topmost silver layer carrying the image particles separates from the underlying silver layer which remains bonded to the copper support. All types of metal supports are subject to mechanical distortion from bending or other types of mechanical working. Mechanical distortion frequently results in damage to image layers. All types of metal support materials used for photographs are subject to corrosion or degradation caused by the oxidation of the metal itself. In daguerreotypes, this may be seen as films, spots and blister-like growths on the plate surface. The daguerreotype's silver surface is susceptible to tarnishing. Tarnish products, primarily silver oxides, may cover the entire plate or be concentrated at the edge of the mat window opening. Thin, evenly deposited tarnish films are characterized by a series of interference colors. The most brilliant series of colors occur where the tarnish layer is thinnest. Thicker, more uneven tarnish films appear pale grey or black, the color of bulk silver oxides. The alkaline gels and silicate crystals formed by weathering corrosion on the inner surface of the cover glass may spall off onto the daguerreotype surface, where they can initiate various types of metal corrosion. One form of such corrosion are the mold-like masses noted in the discussion of cover glass corrosion (see general cased objects previous and entry below). Barger; Smith; White (1989) report deep dendritic fissures in the daguerreotype surface where these masses have grown. Other forms of corrosion on the plate surface may result from the transfer of glass corrosion products. These may show as variable-size grey spots on the plate that may be nucleated and may have associated accretions. Note that grey spots may also derive from splashes of mercury deposited on the plate during processing.
Glass used for both glazing and support materials for cased photographic images was usually commercially available pane glass. There were some companies that sold "photographic" glass destined for use as supports for photographic plates. This glass was free of physical flaws and was relatively colorless, but did not have superior durability or corrosion resistance. All glass is subject to physical breakage. Broken cover glass on cased photographs may cause immediate physical damage to the image underneath. In addition, it may lead to chemical deterioration of the image in the localized area under the breakage by allowing the direct ingress of air; this is particularly common with daguerreotypes with cracked cover glasses. Some glass formulations are chemically unstable due to a high flux content, especially those with a high ratio of sodium flux in proportion to the alkaline earth flux (calcium and magnesium oxide). Note that glass with a high sodium content produces a yellow-orange fluorescence under UV illumination, whereas glass with a higher alkaline earth content tends to produce orange-magenta fluorescence. Glass corrosion is caused by inherent instability linked to unfavorable environmental conditions. Glass corrosion, leading to the accumulation of alkaline materials and silicates on the surface of the glass, is a major cause of deterioration in cased photographs. Unfortunately, the design of the cased photograph package encourages the type of corrosion in cover glass called "static weathering." In this process, the inner surface of the cover glass is exposed to a small enclosed air volume; this air is subject to transient periods of high or cycling relative humidity. The glass used in photographic packages should be checked regularly for signs of weathering corrosion. It may be easier to detect corrosion films by examining the glass by specular reflection. Look for:
- early signs -- a faint surface haze or clouding;
- more advanced states of "weeping" glass, including the presence of tiny droplets that lend a "greasy" or "soapy" feel to the surface; these are primarily amorphous sodium silicates (water glass) and have a high pH -- in the range of 10-14 (the presence of alkaline corrosion products may be detected by burnishing a pH indicator strip on to the affected surface);
- needle-like crystals and incrustations on the inner surface of the glass - they may also have spalled off, leaving the crystals on the surface of the photograph below;
- blister-like crystals that appear to have a small darker-colored core;
- mold-like masses on the surface of the daguerreotype plate below, sometimes obviously associated with a spalled-off crystal corrosion product from the cover glass above; these have a "bead-and-thread" morphology reminiscent of Candida spp. but are entirely inorganic.
Refer to Barger; Smith; White (1989) for more information on cover glass deterioration. See Katherine Whitman's section Glass in Photographic Materials for more details regarding glass history, manufacture, and repair. Also refer to the general Cased Photographs glass section for more information regarding the condition of nonadhered cover glass.
Some daguerreotypes develop green-colored blister-like growths. These are due to copper corrosion formed at tiny holes in the silver layer of the plate. Bronze powder particles may produce similar corrosion products.
Various chemical cleaning treatments may have produced microscopic pitting of the silver surface. This may be manifested as a lowered overall image contrast or by the appearance of a white, cloudy "veil" over the image.
Daffner; Kushel; Messinger (1996) have noted a bright fluorescence on some daguerreotype plates under short-wave UV illumination (UVC; 200-280 nm.).
The significance of these observations is not yet entirely clear, but the authors associate the fluorescence with previous chemical cleaning treatments, prolonged exposure to the environment, general mishandling and the presence of cyanide compounds.
Daguerreotype Image Layer
Daguerreotypes do not have a binder layer; instead, the image particles of the daguerreotype are formed directly on the surface of the daguerreotype plate. The daguerreotype surface has a micro-scale roughness due to the topography of the image particles.
Daguerreotype Silver Image
The image particles found on a daguerreotype plate are 0.1 μm to 50 μm in diameter (roughly 10 to 100 times larger than those found in other types of photographs). Daguerreotype image particles are primarily composed of metallic silver but may include small amounts of gold and mercury. Image particles are susceptible to mechanical abrasion.
Daguerreotype image particles are susceptible to oxidative corrosion, as described above in the silvered metal plate entry.
Residues from thiourea cleaning may cause spots, or "measles," on daguerreotype surfaces. As the plate is repeatedly cleaned with cyanide- or thiourea-based chemical cleaning solutions, the image particles are reduced in size and their spacing increases, resulting in reduced image contrast.
Residues from cyanide and thiourea cleaning also leave corrosive films on the entire plate surface.
The daguerreotype image does not fade due to light exposure.
A variety of protective coatings were initially proposed for daguerreotypes before the introduction of gilding. These included varnishes of copal and other resins and glues. This was not widely practiced, and coated plates are extremely rare. Some modern materials have been tested for use as protective coatings for daguerreotypes and are not recommended.
Daguerreotype Paint/Pigment components
To color the surfaces of daguerreotypes, pigments were ground into a fine powder with gum arabic and applied dry to the surface of the images. In one application method, the colorist breathed lightly over the area to warm and humidify the gum, thereby activating the adhesion of the powder to the plate. Colorants are sensitive to light, moisture and abrasion.
"Shell gold" (metal powder in oil or gum arabic solution) was used to highlight selected image areas, such as buttons and jewelry. A sharp tool may have been used to scratch into the image surface to create highlights and to reinforce or create image elements.
Reprography of daguerreotypes is complicated by the appearance of reflected images of the copy camera, copy photographer, etc., in the daguerreotype plate, as well as by misleading exposure readings from "through-the-lens" (TTL) camera light meters.
The plate should be oriented with the direction of the final polishing marks -- usually horizontal across the image -- parallel to the direction of illumination from the photographic lights to minimize their appearance in the copy photograph. The lights should be positioned at approximately 45-degree angles from the image plane, as with other reprography. Polarizing filters on the light sources can be used to suppress physical damage on the plate surface, so they may be useful for making publication-quality reproductions where the intent is to give the best possible representation of the image. (Fully polarized light reprography, with parallel filters on both sources and lens, results in a distorting increase in image contrast.) Similarly, photographing the plate while it is immersed in an inert solvent, such as heptane, may suppress reflections and physical damage. All such image modification techniques are inappropriate for photo-documentation of conservation treatments.
Colored tarnish films may be reproduced only poorly, or not at all, by color photographic materials. Color filters could be used to enhance the photographic reproduction of daguerreotype corrosion films, but no work has been published on this subject. Optimizing the specular reflection off the plate surface may allow tarnish films to be recorded more clearly.
A low-reflective black mask or shroud should be fitted to the camera lens barrel to reduce unwanted reflections in the plate. Use something like black felt mounted on cardboard or use an empty black photographic paper box -- 8x 10 x 3 inches (20.3 x 25.4 x 7.6 cm). A filter holder may be useful to arrange the mounting of the mask on the lens barrel. All ambient illumination in the room should be turned off to avoid unwanted reflections off the lens into the plate.
The mirror-like reflection from the plate will result in TTL light meters indicating exposure readings that are too large; reduce exposure by approximately 1½ stops from the indicated reading. (Incident light measurements are not subject to this adjustment.) Record the pretreatment exposure used and manually set the same exposure for mid- and post-treatment photographs; this is necessary, since exposure measurements taken from the plate may change as treatment proceeds.
See Daffner; Kushel; Messinger (1996) for details regarding the photographic recording of fluorescent patterns on daguerreotype plates.
Daguerreotypes, like all cased photographs, are composite artifacts having complex reactions to environmental conditions. Conditions that are optimal or harmless for one component may be damaging to another. Environmentally induced deterioration processes in one component may cause reaction products harmful to another component to be released.
Heat alone is not a primary determinant of deterioration for daguerreotypes, although sustained elevated temperature may accelerate the corrosion of thiourea- or cyanide-cleaned plates. High temperatures may cause desiccation of case components. Very high temperatures can cause dimensional fluctuations that may contribute to image exfoliation on plates that have been excessively treated with gold.
Cycling ambient temperature produces the relative humidity cycles in the microclimate of the sealed package that are a major driving force behind the weathering corrosion of cover glass. The glass corrosion, in turn, causes metal corrosion on the plate.
Lower storage temperatures will retard deteriorative processes of paper and leather components. There is no reason why daguerreotypes cannot be housed in RH-controlled low-temperature storage areas. (These facilities must be equipped with appropriate transition climates to avoid transitory high humidity conditions when items are removed to normal temperature areas.)
In all types of cased objects, high relative humidity will contribute to deterioration of unstable glass, as well as corrosion of metal supports, mats and preservers. The leather, paper and wood components of cases will swell and shrink with fluctuating humidity, and they may become embrittled at low humidity. Leather can be structurally weakened at high humidity. Relative humidity above 60% can sustain mold growth.
High humidity can contribute to tarnishing and other corrosion processes of daguerreotype plates.
The major driving forces behind glass corrosion are moisture and fluctuations in relative humidity. Water (from moisture in the air) interacts on an atomic level with the glass surfaces, initiating glass corrosion cycles. Glass is best maintained at moderate relative humidity (40-50%) without excursions to higher or lower humidity.
For a mixed collection of daguerreotypes, ambrotypes and tintypes, 40% RH seems the best compromise condition. If possible, minimize the difference between the humidity conditions in the storage area and the areas where the objects are used for reference and display.
Atmospheric pollutants can cause image deterioration in all types of cased objects. Sulfur dioxide, a common pollutant, can combine with oxygen and moisture to form sulfuric acid, which is particularly damaging to leather and paper. The silver image material of all cased photographs may be damaged by the ammonia, ozone, sulfur dioxide and nitrogen oxides generated in industrial environments.
The silver images of daguerreotypes react with airborne oxidizing gases.
A daguerreotype that is well sealed and boxed will be relatively free from harm by airborne pollutants. However, the source of corrosion-inducing pollutants may be the materials composing the package and case. These should be removed or isolated from the daguerreotype plate. Some preliminary investigations were done into the use of an activated charcoal "scavenger" within the daguerreotype package (See Mustardo).
Although daguerreotype images are not sensitive to light, the pigments used for hand-coloring are frequently quite fugitive. The paper and textile components of the packages may also be light sensitive. Daguerreotypes with broken glass or seals may have an increased potential to tarnish if they are strongly lit and kept in a polluted atmosphere because light accelerates the formation of silver sulfide in the presence of unreduced sulfur gases.
Daguerreotype Storage Containers
Packaged daguerreotypes may be stored flat inside Solander boxes equipped with Ethafoam layers cut out to create cavities in which the cases fit snugly. The interior surface of the cavity is lined with a fine unbleached cotton textile to provide a nonabrasive surface. In boxes containing daguerreotypes, the interior of the Solander box lid may be lined with Pacific Silver Cloth (or equivalent) to provide a scavenger for oxidative gases entering the box (also, see "zeolites" below); the brown cloth also provides a dark surface to reflect on the daguerreotype images for optimal viewing.
Alternately, custom-made individual matboard boxes of the clamshell or slipcase variety may be constructed to house daguerreotypes. A four-flap enclosure adapted for housing cased photographs is used at the Harry Ransom Humanities Research Center, University of Texas at Austin, USA. (Brown, 1996) This method is well-adapted for storing cased photographs on their edges, a measure recommended by some conservators to reduce glass corrosion-induced damage on the plate surface.
Padded plastic wrappings should be used only with caution. One instance has been noted of severe corrosion of daguerreotypes caused by the off-gassing of plasticizers used in plastic wrapping, even though these were described as "archival." However, food-grade polyethylene "zip-lock" bags have been successfully been used to store polished silver objects. As long as appropriate tests are carried out on the specific product used, these may provide inexpensive and effective protection for stored daguerreotypes.
Conditioned sheet-form silica gel might be included in the bags as a humidity buffer. Likewise, boards and papers containing molecular sieves (zeolites) may be included in closed packages to sequester oxidizing gases.
In the following section, it is assumed that all treatments will be carried out by qualified conservators familiar with the materials and characteristics of the objects they are treating. No treatment indications given here can be considered safe for any object without suitable testing, careful observation and skilled, judicious application.
Note that plates that were not gilded have particularly fragile image surfaces. Nongilded plates sometimes show lower apparent image contrast, or they may show more areas of physical image loss than gilded plates. However, it is usually necessary to resort to instrumental analysis, such as X-ray fluorescence spectroscopy, to determine the presence or absence of gold.
Note that some connoisseurs consider the presence of coherent tarnish layers on the periphery of a daguerreotype image to be an enhancement of the aesthetic qualities of the object, particularly when the tarnish shows a series of smoothly gradated interference colors. The decision to remove such tarnish should be made only by a fully informed owner/custodian. In many cases, such tarnish films should not be removed. The presence of original and intact sealing materials should also weigh heavily against a decision to open a package in order to perform cosmetic treatments.
Dusting of Daguerreotypes
Plates that do not exhibit exfoliation can be dusted to remove loose debris. The preferred method is to use a gentle stream of air from a rubber bulb syringe. Alternately, a controlled low-pressure stream of air can be had from the airbrush attachment on a suction table. A clean airbrush should be reserved exclusively for cleaning daguerreotypes, and its air hose should be equipped with a moisture trap.
Washing and Drying of Daguerreotypes
To remove adhered accretions and some of the transferred products of glass corrosion, uncolored daguerreotypes may be safely washed in distilled/deionized water or in a pH 8.5-10.0 solution of ammonium hydroxide in distilled/deionized water. Five to fifteen minutes of immersion accompanied by gentle agitation of the wash tray will be sufficient to remove many water soluble accretions. The alkaline wash should be followed by a brief rinse in a bath of distilled/deionized water. This is followed by two or three rinses or an immersion in very clean absolute ethanol. (Some conservators use acetone instead of ethanol or follow the ethanol rinse with an acetone rinse.) Set the plate upright at a slight angle with a blotter underneath the bottom edge to drain. Note that there is a tendency for water to condense in tiny droplets on the plate surface during the alcohol/acetone evaporation when the ambient relative humidity is high enough; if this happens, use a gentle stream of warm air from a hair dryer to warm the plate while the solvent evaporates. Always apply the warm air stream to the verso surface of the plate rather than risk blowing dust particles onto the image surface. The daguerreotype surface can be permanently marred by drying marks, so drying must be carried out very carefully.
Colored daguerreotype plates can sometimes be immersion washed in heptane to dislodge the surface particles not dislodged by dusting; careful color testing is required. Before undertaking this procedure, consider whether it is likely to provide useful cleaning.
Chemical Cleaning Methods of Daguerreotypes
The most common method used to remove tarnish from daguerreotypes has been the application of a "silver-dip," a solution designed to remove tarnish from ornate silver objects. These solutions were originally made of cyanide compounds and, after the early 1950s, were made of thiourea in a mineral acid. Both cyanide cleaners and thiourea cleaners etch the daguerreotype plate, causing irreparable damage to the plate's surface, and leave behind insoluble compounds; these residues initiate new corrosion on the daguerreotype surface. Such methods should never be used. See Edmondson; Barger (1993).
Aluminum Tray Electrolytic Cleaning of Daguerreotypes
Silver corrosion products may be removed by electrolytic cleaning in an aluminum tray filled with an ammonia solution. Use an uncoated aluminum baking container. The most concentrated solution of ammonia that can be used in the procedure is prepared by adding one part concentrated ammonium hydroxide (approximately 30%) to two parts of distilled/deionized water. (Note that a fume hood is necessary.) Slower cleaning actions will be obtained by using weaker concentrations of ammonium hydroxide. The daguerreotype plate is placed face up in the aluminum tray; the copper surface of the daguerreotype must be close to the aluminum tray for the electrolytic cell to be established. The ammonia electrolyte is poured into the tray to cover the daguerreotype. The corrosion product removal can be monitored visually as it proceeds. Hydrogen bubbles are evolved from the aluminum as cleaning proceeds. As grey oxides build up on the aluminum surface underneath the plate, the rate of cleaning will decrease; it may be necessary to move the plate to another position in the tray to allow cleaning to proceed. The reaction rate can be slowed by using a more dilute electrolyte or by using a smaller aluminum tray. Note that the cleaning action progresses from the edge of the plate inwards towards the center of the plate. This may mean that a full-plate daguerreotype cannot be completely cleaned using this method. Electrolytic cleaning might be used as a preliminary step before considering a decision to proceed with electro-cleaning (see below). Electrolytic aluminum tray cleaning should not be used on ungilded plates.
Electro-cleaning, as outlined in Barger; Giri; White; Edmondson (1986), is based on a well-tested method for cleaning metal that has been adapted for the special requirements of daguerreotypes. In this process, the daguerreotype plate is made to be one electrode of a direct current electrical circuit; the second electrode is a silver wand used to direct the cleaning action. The circuit is completed by placing the daguerreotype and wand in a solution of ammonium hydroxide. The wand and plate do not directly contact each another. When a reversible DC current is applied through the circuit, cleaning is effected by forming and dissolving layers of silver oxide in a controlled manner.
Electro-cleaning process was re-examined in 2010 by the Netherlands Institute of Cultural Heritage (ICN) (Wei, Gerritsen, von Waldthausen). The research concluded that electrochemically daguerreotypes was feasible, but recommended that "cathodic polarisation at a constant potential controlled using a reference electrode, a procedure based on that which is used in industry". This adaption was considered a “safer” procedure .... as "small surface changes can still be seen at a microscopic level for the current method."
The method leaves no chemical residues on the plate surface. The method produces a micro-polishing effect, in which tiny irregularities in the silver surface are evened out by a combination of silver removal and redeposition on a very small scale. This leaves the plate less susceptible to future corrosion. Some conservators have reported the appearance of translucent white "veils" overlying previously tarnished image areas after the application of electro-cleaning. The nature of this phenomenon is unclear. It has been suggested that copper ions, released into the electrolyte solution during cleaning, might be redeposited on the image surface. (Heller, 1988) While this seems unlikely to happen, given the electrochemistry of the system, the suggestion has been made that the copper surface of the plate be "stopped out" with silicon rubber before treatment. (Note, however, that most silicon sealants evolve acetic acid as they cure.) The significance of silver redeposition is also the subject of some debate. The procedure is carried out (in a fume hood) using a glass tray filled with an electrolyte solution made of one part concentrated ammonium hydroxide (approximately 30%) added to two parts of distilled/deionized water. A DC power source should produce between 2-5 DC volts. Cleaning action should be controlled by monitoring the current read on an ammeter in the circuit. The current is determined by the distance between the end of the wand and the daguerreotype plate surface; in both anodic and cathodic phases, the current should be maintained in the range of 8-25 milliamperes (mA). Regular switching of the current direction is nescessary for the cleaning action to take place. If some mechanical action is required to move the loosened tarnish products off the plate surface and into the solution, this should be done with a soft brush or by using an rubber bulb syringe to force a stream of electrolyte over the surface while cleaning. The direction of the brushing action should be the same as that of the final polishing marks. Heavily tarnished areas may be susceptible to abrasion. Contact between the silver wand and the plate during cleaning will produce sudden increase in current and will damage the image. Such current surges can be prevented by using a constant-current power source or by adding appropriate fuses or resistors to the device. Electro-cleaning cannot be used on hand-colored daguerreotypes or on plates that have not been gilded.
Hydrogen Plasma Reduction Cleaning/ Physical Sputter Cleaning of Daguerreotypes
Two variations of cleaning daguerreotypes using plasmas have been used for a number of years in Europe. Hydrogen plasma reduction cleaning results in oxidized silver being chemically reduced to silver metal. Physical sputter cleaning uses a chemically inert gas plasma, such as argon, to physically remove corrosion from the daguerreotype surface. These methods may be applied to hand-colored daguerreotypes. Both require sophisticated equipment to contain the plasma, and to create and maintain a high vacuum environment, as well as specialized operating technicians.
While several groups researching both these methods have observed that some daguerreotypes treated in plasmas develop white surface films, it seems that the hydrogen plasma cleaning procedures do not produce microetching in the silvered surface. The advantage of hydrogen plasma reduction cleaning over other cleaning methods is that oxidized silver is reconverted to silver, not removed from the plate.
Repackaging of Daguerreotype Plates
For daguerreotypes, two distinct approaches to repackaging exist. In one approach, the sealed daguerreotype package contains no hygroscopic material, thus avoiding the presence of a reservoir of moisture inside the package that could drive the various processes of metal corrosion. This practice reflects the original format of cased daguerreotypes where no paper products, except the sealing tape, were included in the package. The other approach is to intentionally include a quantity of high-quality unbuffered matboard inside the sealed package (but not in direct contact with the daguerreotype image surface). This acts as a humidity buffer inside the package, helping to dampen relative humidity fluctuations inside the package. This practice reflects the original format of passe-partout packages, which contain substantial quantities of paper, cardboard and adhesives. Whatever repackaging method is chosen, it is essential to ensure that all of the package components are well secured and will not slip or shift. Mechanical damage due to component slippage is a common phenomenon in cased photographs and avoiding future occurrences should be a major criterion in choosing a repackaging method.
This method uses a moisture-resistant barrier on the back of the package and does not introduce any hygroscopic material inside the package. A 5 mil Mylar backing is cut to be slightly smaller than the plate. If a barrier is required to prevent abrasion of the plate surface (see entry on Brass Preservers, Mats, Hinges, and Clasps), use a layer of a non-woven polyester web material. The package is closed with a moisture-resistant sealing tape (see entry on resealing below). This style of repackaging has the advantage of adding little or no extra thickness and allowing the plate verso to be seen without disassembling the package. Marvelseal, a nylon/aluminum/polyethylene laminate, has also been used as an impermeable backing, but it lacks the advantage of transparency. If the plate is much smaller than the package, this method is difficult to use.
Humidity-buffered package variant
Modify the package described above by including a rectangle of matboard cut to precisely fit between the verso of the daguerreotype plate and the Mylar. Use high-quality matboard that does not contain alkaline pH-buffering compounds and that has been conditioned to an appropriate moisture level. Use a matboard thickness that will not add significantly to the package thickness if the plate is to be returned to a case.
This variant is most useful in the repackaging of passe-partout style packages and stereo-daguerreotype packages. A multilayer matboard structure is made that holds the plate in correct register, provides maximum support for all edges and for the plate verso surface, and provides a separation between delicate surfaces and the glazing material. Use rectangles of high-quality, unbuffered 2-, 4- and 8-ply matboard cut larger than the finished package size to create a sink cavity for the plate. Use a layer of a nonwoven polyester web material to provide separation between the plate and any original package components that may cause abrasion or scratches. Adhere these layers with 3M Double-sided Film/Tape No. 415, starch paste adhesive, methylcellulose paste or PVA emulsion adhesive. It is essential to add only the minimum possible additional thickness to the package if it is to be returned to a case.
George Eastman House Housing
The Conservation Department at the George Eastman House has developed an elegant structure for housing unpackaged daguerreotype plates that combines some features of both approaches outlined above. This housing was inspired by a unique metal and paper housing associated with the American daguerreotypist Robert Cornelius. A matboard sink mat is made with cavities for the daguerreotype and for the cover glass. (See drawing below.) The daguerreotype cavity is slightly deeper than the total thickness of the plate, including its bevelled edges. The cover glass cavity should be the same depth as the glass thickness. A 5-mil Mylar cradle holds the edge of the plate and prevents it from moving inside the slightly oversized cavity or from contacting the inner surface of the cover glass. The cradle is made from two sheets of Mylar. One is cut to the width of the plate and folded at the top and bottom edges to form a Z-shaped spacer or spring; the other is cut to the height of the plate and is folded to form "Z" springs at the sides of the plate. The ends of the Mylar sheets should be trimmed to fit precisely into the spaces of the cover glass cavity. Once the cradle, the daguerreotype plate and the glass are precisely fitted into the matboard sink structure, the glass is sealed to the top surface of the matboard with paper tape. This system can be adapted to include an original brass mat behind the cover glass. The finished package can be easily overmatted and framed for display.
Illustration: George Eastman House
Reusing original tapes: This should be done when possible to preserve the original package components. It may be possible to change a deteriorated cover glass by releasing the tape from the front with minimal solvent application, removing the glass, fitting in the new glass and reactivating the original adhesive to secure the new glass. Damage on the original tape can be repaired using conventional methods. If original binding tapes must be removed and replaced with new materials, consider storing the removed tape fragments in the case tray, behind the package. Self-adhesive tapes: Many conservators prefer pressure-sensitive tapes for sealing cased photographs because they are easier to handle than paper tapes made with wet adhesives and they do not introduce moisture into the sealed package. Paper tapes such as Filmoplast P-90, Filmoplast P-91 or Filmoplast T (textile carrier) have been commonly used to reseal cased photographs. Polyester-backed or Mylar-backed tapes provide some moisture barrier. Notable among these are the 3M Polyester Film/Tape No. 850 (Silver), which is thin and flexible. It may be preferable to isolate the edge of the daguerreotype plate from contact with the adhesive. This can be done by laying a thin strip of nonadhering material down the centre of the sealing tape. Use 3-mil Mylar, Hollytex, Japanese paper or a thin strip of the sealing tape itself, turned so that its nonadhesive surface contacts the plate edge. See application instructions under "Plain paper tapes" below. Self-adhesive tapes can be used alone or be covered with the original paper sealing tape that has been removed and treated as described in Cased Photographs Treatment: Paper and Tapes and Mats entry.
- Alabastrine process -- a variant of the ambrotype process, using mercury chloride bleaching
- Ambrotype -- a collodion silver photograph on glass; a direct positive camera original
- Amphitype -- among other things, a process similar to ambrotype but using albumen
- Binding tape -- an alternate term for sealing tape
- Case -- a hinged box holding an ambrotype, or an American or British daguerreotype
- Clasp -- brass hardware on a case that secures the closure of the hinged case cover
- Collodion -- binder layer of ambrotypes, tintypes, and wet-plate negatives
- Collodion positive -- term used in Britain for ambrotype
- Cover glass -- protective glazing used on daguerreotypes, and some ambrotypes and tintypes
- Cushion -- cardboard, cotton fibre and textile pad fixed to the inside surface of the case cover
- Daguerreotype -- a photograph with silver image particles on a silvered copper plate
- Deliminator -- an alternate term for a metal mat
- Ferrotype -- an alternate term for tintype
- Gilding -- a step in daguerreotype processing in which gold is added to the image particles
- Hinge -- paper, leather or metal attachment between the case cover and tray
- Lacquer -- a pigmented paint layer, used on both ambrotypes and tintypes
- Lampratype -- a variant of the ambrotype process
- Mat -- paper or metal sheet with an aperture cut in its centre
- Melainotype, melanotype -- alternate terms for both ambrotype and tintype
- Package -- plate and housing components forming a sealed unit; may be cased or framed
- Pad -- an alternate term for cushion
- Pannotype -- a collodion positive, similar to a tintype, but on black waxed textile or leather
- Passe-partout (package) -- a daguerreotype housing format common in Europe
- Perfling -- an alternate term for retainer
- Pinchback -- an alternate term for preserver
- Pinch pad -- an alternate term for retainer
- Preserver -- for cased objects: brass foil that is folded around the edges of the plate package
- Relievo -- a variant of the ambrotype, in which the background image areas are scraped away
- Retainer -- velvet and cardboard strip lining the edge of the case tray
- Sealing tape -- strip adhered at the perimeter of a package and holding the components together
- Stereograph -- a two-image photograph which represents its subject three-dimensionally
- Tintype -- a collodion silver photograph on an iron plate; a direct positive camera original
- Tray -- the half of the case opposite the cover, which receives the package
- Union case -- a case made of an early thermoplastic material
Please refer to Conservation of Daguerreotypes. October 19-23, 2009. Weissman Preservation Center, Cambridge, MA for additional resources.
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- Smith, Brenda Lee. "Photographic Union Cases: The First Plastic Composite." Queen's University Art Conservation Program, unpublished student research report, 1994.
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Date initiated: September 1998
Contributors for wiki version: Luisa Casella, Amanda Maloney, Stephanie Watkins
Compiler for printed version (1998): John P. McElhone
Contributors for printed version (1998): Gary Albright, M. Susan Barger, Valerie Baas, Lee Ann Daffner, Deborah Derby, Tom Edmondson, Monique Fischer, Chris Foster, Lynne Gilliland, Marion L. Hunter, Jr., Wayne King, Barb Lemmen, Mark H. McCormick-Goodhart, Angela Moor, Ian Moor, Peter Mustardo, Debbie Hess Norris, Nancy Reinhold, Andrew Robb, Elena Bulat, and Annabelle Chabauty.
Edition copyright (1998): The Photographic Materials Conservation Catalog is a publication of the American Institute for Conservation of Historic and Artistic Works. The Photographic Materials Conservation Catalog is published as a convenience for the members of the Photographic Materials Group. Publication does not endorse nor recommend any treatments, methods, or techniques described within the chapter.
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