PMG Humidification, Drying and Flattening

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Photographic Materials Conservation Catalog
Humidification, Drying, and Flattening of Paper-based Photographic Material

Dates of Compiler History: by ? (1998?- 2000?); [split into Humidification] Erin Murphy (2000-2005?), [and Flattening] Jiuan-Jiuan Chen (2000-2005) [rejoined] Jiuan-Jiuan Chen (2006-2008), PMCC chair, Stephanie Watkins, posted on trial WIKI (January 2009), moved to AIC WIKI site (fall 2009), continued development 2015-2016


Contributors (alphabetical): Gary Albright, Brenda Bernier, Luisa Casella, Jiuan-Jiuan Chen, Lee Ann Daffner, Thomas M. Edmondson, Monique Fischer, Martin Jürgens, Nora W. Kennedy, John McElhone, Jennifer McGlinchey Sexton, Amanda Maloney, Paul Messier, Erin Murphy, Jenelle Norris, Stephanie Watkins


The Photographic Materials Conservation Catalog is produced by the Photographic Materials Group of the American Institute for Conservation of Historic and Artistic Works as a convenience for the membership. The treatments, methods, or techniques described herein are provided for informational purposes. The reader assumes responsibility for any application results.


PURPOSE: The considerations when humidifying, drying, and flattening paper, and paper-composite original photographic materials are addressed. The aim is to provide insight along with a listing of materials and techniques utilized in these treatment techniques. Mounting techniques are not covered here, found instead in the Backing, Lining, and Mounting entry.

TABLE OF CONTENTS:
Introduction
Factors causing planar distortion
Purpose of humidification, drying, and flattening
Factors to consider
Method
Treatment Variations
Process-specific methods and considerations
Bibliography

Introduction

  • Humidification, drying, and flattening can be localized or overall treatments.
  • Humidification is defined as the introduction of moisture directly or indirectly into the paper support or components of the photograph (Murphy). Humidification alters organic materials by swelling fibers and softening emulsions. If excessive moisture remains in organic materials, chemical and physical damage can occur, so controlled removal of the moisture (drying) is required.
  • Drying processes have the potential to create distortions within organic materials. Material can expand and contract. Dimensions of a support can be different after drying.
  • Flattening during drying is desired for photographic materials as the images are more easily appreciated, understood, handled, and stored when planar. Photographic materials on paper supports are typically considered to be two-dimensional or flat" because of the inherently thin cross-section ("z" direction) of the substrate. Paper and paper-plastic supports require planarity for mounting and tipping to secondary rigid support boards or when supported in album formats.
  • Some exceptions include convex prints such as crayon portrait enlargements, stereoviews that are on a concave secondary support, and objects such as fotosculturas that are paper prints adhered onto a three-dimensional wood support in the shape of the portrayed subject.

Nomenclature of planar distortion

Some common terms for forms of planar distortion are (See the AIC's Lexicon Terms for more information):

  • Blind stamps (intentional distortion)
  • Buckling
  • Bulge/ bulging
  • Cockling
  • Crease/ creasing
  • Curling
  • Dent/ denting
  • Distortion
  • Indentation
  • Rippling/Frilling (small, tight edge distortions)
  • Undulation
  • Wavy/ wave/ wave-like distortions

Common types of planar distortion

Localized distortions can include creases, bulges and depressions. Cockling -- parallel or radiating series of wave-like distortions -- are often localized but over a larger area of the photograph. When the cockling is more subtle, it could be described as undulations in the paper support. Overall curl is usually along two parallel edges but can be along all four edges.

ISO description of curl in photographs

ISO 18910:2000 specifically deals with determining the curl of photographic materials and suggests a standard terminology for the description of curl direction.

  • T curl: direction of curl is transverse or cross-grain.
  • L curl: direction of curl is longitudinal or parallel to grain.
  • D curl: diagonal or two opposing corners.
  • C curl: cupping or all four corners.
  • A plus (+) designation is for curl towards the emulsion while a minus (-) designation describes curl towards the paper side.

For example, a +T curl might be used to describe an unmounted albumen print that has curled across the grain, with the albumen side in. (Bernier)

Factors causing planar distortion

  • The composite structure of photographic material
  • Improper handling
  • Inadequate housing
  • Environmental factors: cockling of mounted prints is common for photographs stored in an environment with fluctuating relative humidity, particularly as the print and the mount expand and contract differently.
  • Consequence of treatment process


When subjected to repeated cycling of humidity, some photographs will start to curl. Unmounted albumen prints are often found tightly curled, parallel to the fiber orientation direction of the paper (aka "grain" direction). Most planar distortion is caused by human intervention -- poor handling techniques, improper physical support when stored over a long period of time, folding, use of fasteners, such as paper clips, rubberbands, etc. Panoramas and other large unmounted prints (of any process) can be found intentionally rolled, usually for ease of storage. The rolling can be either parallel or perpendicular to the paper fiber orientation direction, with the emulsion in or out, although emulsion-side in is the more common configuration. (Bernier)

Purpose of humidification, drying, and flattening

  • Humidification: Relaxes binders, supports, and swells residual adhesives in preparation for further treatment such as removal of adhered papers, hinges, backings or linings and in preparation for washing, suction-table work, and stretch-drying among other procedures (Murphy)
  • Humidification: Revive aqueous-based binders and help with the consolidation of media and pigments (Murphy)
  • Correct planar distortions such as cockling, strain, or rolling (Murphy) occurring over time or imparted during treatment
  • Correct planar distortions that pose potential risk for further damage
  • Realign deformations such as creases, folds, and ridges occurring over time (Murphy)
  • Restore the viewing aesthetic intended by the artist and the photograph's original configuration, such as flatness, embossings, or surface texture (Murphy)
  • Improve access or storage: Over time, as rolled or severely distorted photographs become less pliable, humidification and flattening is necessary for the photographs to be safely handled by researchers. Distorted prints that are part of a large set may safely nest together for storage if the distortions are uniform throughout the set. However flattening may be necessary if distorted prints are to be safely stored with adjacent flat material (prints or documents). (Bernier)
  • Reduce potential risk for further damage due to planar distortion: If left untreated, some distortions can result in localized image abrasion, creases, and tears. Also some distortions, particularly on mounted prints, can create areas of ingress for dirt or pollutants, resulting in small micro-climates that can speed localized deterioration of the image. (Bernier)

Factors to consider

  • Unlike most paper artifacts, photographs have greater complexity in structure and materials. The basic structure of binder, image materials and support has important roles for effective and safe humidification, drying, and flattening. Before proceeding, take into consideration:
    • The sensitivity of the support, binder, media, coatings, inscriptions, signatures, and any mixed media components to water and pressure (Murphy).
    • The overall condition of the binder, baryta layer (if one exists), and paper support (Murphy).
    • The tendency for photographic prints to curl, both naturally and during treatment, because of the composite nature of organic materials with differing responses to the environment (McElhone)
    • The tendency for components of the photograph to migrate and stain the photograph, such as adhesive residues, deterioration products in the paper or emulsion layer, and presence of a baryta layer (Murphy).
    • The degree of desired humidification necessary to achieve treatment goals. Also, whether fast or slow introduction of moisture is most advantageous to the photograph in treatment (Murphy).
    • The affects on expansion and compression rates of multiple composite materials within the photograph and potential multiple mounts/supports (Murphy).
    • The ability to accomplish humidification procedures given available equipment, space, expertise, manpower, and drying options (Murphy).
  • Overall flattening is generally more successful than localized flattening as distortions tend to move around the photographic structure.
  • Multiple humidification and flattening methods may need to be used in succession or conjunction to achieve desired level of planarity or flatness for an acceptable result.
  • Multiple attempts with long (several weeks to several months) dwell times in a press or stack may be necessary to achieve the desired level of planarity or flatness.

Inherent nature/ Inherent vice

  • Composite nature of photographic materials
  • Binder type (albumen, gelatin, collodion)
  • Response to moisture (binder in relation to its support and support type and condition)
  • Processing variables - some photographic prints are bound to cockle because of distortions that were set in them at the time of original processing such as prints that were washed or just left in water too long (McElhone).
  • Tipped attachments to a secondary support
  • Mounted photographs
  • Composite photographic objects
  • Additions potentially influencing or limiting treatment approaches: hand coloration, surface retouching, inscriptions on front or back (e.g. signatures, photographer processing notes, exhibition and estate marks)

Sensitivity of image layer components

  • The degree of tackiness and softness of the image layer after moistening will vary by photograph and degree of either depends on the initial degree of curl, original hardness and current rigidity of the binder layer. In extreme cases, the image layer will need to be tacky to relax the photograph enough to successfully flatten (Daffner).

Surface texture of photograph

  • Manufacturer's Inherent surface of the photograph
  • Ferrotyped surface
  • Presence of coatings applied post-processing
  • Differential surface sheen within photograph

Materials, Supplies, and Equipment

Humidification Materials, Supplies, and Equipment List (alphabetical)

  • Baker's Racks (for batch work)
  • Brushes, soft haired, flats and rounds
  • Domed Chambers or poly(ethylene) tents
  • High-Density Poly(ethylene) plastic sheeting
  • Nebulizers
  • Platens: glass, plastic
  • Poly(ester) webbing
  • Poly(ester) terephthalate film (Mylar Type D, Melinex, etc.)
  • Screens: Pecap, silkscreening fabric, window screening mesh, rigid plastic "eggcrating"
  • Sponges, small micropore
  • Spray equipment: Airbrush, spray gun, hand-pump (trigger-style) sprayers, Dahlia brand, finger pump (nozzle) sprayers, atomizers
  • Steamers
  • Swabs, cotton
  • Synthetic, felted and unfelted membranes (Gore-Tex, Sympatex)
  • Trays: glass, plastic, metal or sinks that can be stoppered and covered
  • Ultrasonic humidifiers
  • Water (purified, filtered, distilled, deionized)

Drying and Flattening Materials, Supplies, and Equipment List (alphabetical)

  • Blotter
  • Corrugated archival boards
  • Boards (smooth, thin)
  • Felts
  • Platens (glass, plastic, wood, wood composite)
  • Poly(ester) webbing or Nylon fabric (Bondina, Hollytex, Reemay, Cerex)
  • Presses (letter, copy, book, dry mount)
  • Silicone Release paper
  • Weights

Equipment, Materials, and Supply Specifications

  • Baker's Racks
    • Commercial baker's racks can be modified into a large humidity chamber for batch humidification work by covering with plastic sheeting held closed with clips, and screens replacing the solid drawer-style trays.
  • Brushes, soft haired, flats and rounds
  • Domed Chambers or poly(ethylene) tents
    • Domed "incubator" structures often sold with vacuum-suction tables can be utiltized as humidity chambers.
    • Poly(ethylene) sheets draped over PVC piping or bendable tent supports and clipped makes an easily affordable tent for humidification.
  • High-Density Poly(ethylene) plastic sheeting
    • HDPE is found in hardware stores
  • Nebulizers
  • Screens: Pecap, silkscreening fabric, window screening mesh, rigid plastic "eggcrating"
  • Sponges, small micropore
  • Spray equipment: Airbrush, spray gun, hand-pump (trigger-style) sprayers, Dahlia brand, finger pump (nozzle) sprayers, atomizers
  • Steamers
    • Both handheld and stand alone units with an attached hose are available. Periodically, hoses may need replacement on individual units as they can get moldy inside. Brands: Oscrow Seamstress, Seamstress II, Steam Buddy Garment Steamer
  • Swabs, cotton
    • Cotton swabs can be bought in various sizes and shapes. The medical profession also manufactures rayon swabs.
    • Cotton swabs can be made from roll cotton on skewers, toothpicks, and even chopsticks as desired for shape and size. Roll cotton manufactured without small nubs is ideal.
  • Synthetic, felted and unfelted membranes (Gore-Tex, Sympatex)
    • Gore-Tex,
    • Sympatex, a 100% polyester non-porous membrane applied to a polyester non-woven (Talas website, 2016)
  • Trays: glass, plastic, metal or sinks that can be stoppered and covered
    • For very small photographs, Pyrex glass or ceramic cooking and backing trays can be used.
    • Photographic developing white plastic, flat bottomed trays can withstand chemicals and alkaline solutions are ideal for treatment situations. Cescolite is one brand still manufacturing new, heavy duty plastic trays (2016). Trays can be obtained from the second-hand market very inexpensively, however, what chemicals used in what trays is likely to be unknown: Chemical residues from plastic trays used by a previous owner could cause unexpected quirks during treatment.
    • Stainless steel trays are manufactured for the restaurant business. They can be manufactured by any metal welder to specification, also.
  • Ultrasonic humidifiers
  • Water (purified, filtered, distilled, deionized)

Stack and Interleaving Materials

  • Synthetic supports
Four types of amorphous poly(ester) webbing
Image: Stephanie Watkins
    • Poly(ester) webbing (sometimes shorted to "poly-web" in the vernacular) is used as a releasing, interleaving substrate because it does not adhere to blotters and most photographic materials (Murphy). It is also used as a secondary support during treatment. Poly(ester) webbing varies in thinness and thickness, and from an open pore to an almost continuous closed structure. Textures vary from very smooth to very rough. Choose carefully based on the many varieties available (Kennedy) and the needs of the photographic material. Support types include, alphabetically by brand name: Bondina, Hollytex (also spelled "Holytex" within the literature), and Reemay. Nylon fabric, Cerex, is used in similar situations as poly(ester) webbing.
      • Bondina is a non-woven 100% poly(ester) webbing that is thin and very smooth; as such, it is well suited for pressing prints with glossy surfaces, since there is less danger of impressing the surface texture of the webbing into the emulsion (Jürgens).
      • Cerex is a spun-bond, non-woven nylon fabric [[1]] (McElhone).
      • Hollytex/Holytex is a calendared, non-woven poly(ester) webbing. It is manufactured with varying "openness" to the amorphous webbing and comes in a lightweight and heavyweight, smooth, continuous sheet (Daffner, Watkins). It can withstand heat up to 350°F (176.7°C).
      • Reemay is a spun-bong, non-woven poly(ester) webbing available in a variety of thicknesses.
    • Using a fresh, pristine sheet of webbing will reduce potential embossing occurring during the flattening procedure. Poly(ester) webbing can leave an unwanted texture or impression on photographic surfaces because of softness from humidification and high pressure (Murphy). It is advisable to avoid any poly(ester) webbing with rough textures and protruding fibers. In addition to introducing an unwanted texture to the photograph, protruding fibers can catch on sheet edges and cause physical damage (Kennedy).
    • More openness within the webbing is believed to let more moisture pass than the ultra smooth, more continuous sheets. Retouching or other media on the surface might be altered or transferred by a "slick" poly(ester) surface.
    • Using thicker poly(ester) webbing can help slow moisture absorption.
    • Photographs are also placed directly against smooth paper, blotter, or boards without using a releasing substrate (e.g. a poly(ester) webbing material) (Murphy, McGlinchey Sexton). If the binder layer gets too soft, it can potentially stick to the pressing materials.
  • Silicone release paper
    Silicone release paper can be used on the front of a damp photograph during a weighted method. It helps protect the damp photograph from Hollytex or blotter paper impressions. However, the dampness of the photograph can ripple the release paper and cause the print to cockle. The blotters should be changed and the silicone release paper removed shortly after the photograph has dried (Fischer).
  • Blotters
    Blotters come in a variety of manufacture and thickness. Levels of blotter purity include: Diagnostic/Science lab grade, Photographic grade (American National Standards: ANSI PH4.10-1977), and Industrial grade. Contemporary photographic grade blotters made of unsized 100% cotton linters are often preferred. Thicker blotters that absorb moisture readily are often preferred (Daffner). While traditionally held that blotters are "unsized", blotters can contain optical brightening agents (OBA), dyes (traditionally pink), fillers, furnish, or sizing (McElhone, Watkins). Any water-soluble additions within blotters can potentially transfer to a print during a pressing process (Robb).
    Commercially made, machine-made blotters have inherent air (luft) and screen sides that are discernible in raking light. The texture of the screen side is smoother, more continuous than the more amorphous, bumpy air side. The higher the amount of sizing in the blotter, the harder, more smooth the blotter surface can become, but smoothness in blotters can also be achieved during manufacture using a heated roller towards the end of production. Most photographs will flatten more uniformly next to the screen-side of a clean, pristine blotter. (Watkins)
  • Smooth Paperboards and Matboards
    Pristine highly calendared, smooth paperboards and mat boards, such as "Mirage", Bristol weight, and 2-ply matboards are used as pressing boards or wicking layers within a dry mount press and pressing stacks (Fischer, McGlinchey Sexton). Some brands of 100% cotton boards include (listed alphabetically): Crescent Conservation Matboard or Crescent RagMat Matboard, Rising Museum Boards, and Strathmore Museum Mounting Board.
  • Corrugated Archival Boards
    Corrugated Archival boards (both blue and tan) are sometimes used at the top and bottom of a stack or at intervals within the stack to allow more air circulation within the stack. The boards should be cut for the maximum air circulation through the flutes to be most effective. The corrugation pattern can transfer to the photograph if dried in direct or nearby contact, especially under high pressure. For that reason and for efficiency and stability, some conservators adhere a same-size matboard to one side of the corrugated boards intended to stack next to the blotter. Some believe the addition of the adhesive layer reduces the efficacy of moisture removal, negating the advantage of the corrugated board within the stack.
  • Felts
    Traditional matted felts are made of wool (and traditional papermaking felts were woven). Contemporary felted material can be made of non-woven synthetics, such as acrylic and poly(ester). Evenness of the felt surface is important when used with photographic materials. Thicker matted felts can also add a desirable amount of pressure to a stack. Felts are sometimes used in conjunction with a single blotter on either side of the photograph. Felts slow down the drying process, so are at more risk for growing fungi (mold, conidia).
  • Platens
    Rigid, smooth platens are used on top of a drying or pressing stack to provide uniform planarity within the stack and as a support for adding additional, often shaped, weights as desired. If the table-top the stack sits upon is not completely level or flat, a secondary platen of the same material placed under the stack is advised for the best result (e.g. sandwich the stack between two platens). Various materials can function as rigid platens:
    • Glass
      • Heaviness and translucency or transparency can be an advantage.
      • Tempered and thick versions available reduce hazards if shattered or chipped.
      • Polishing the edges and rounding the corners is best for the comfort of the conservator. Adding tape inevitably causes problems as the tapes get caught, provide resistance to sliding platens on and off the stack, and inevitably dry out and fail.
    • Plastics
      • Examples include: Plexiglas, Acrylite, Lucite, Perspex (PMMA- polymethylmethacrylate) and Sintra (closed-cell polyvinyl chloride extruded sheet)
      • Costs are higher than glass, yet "free scraps" can be obtained from discarded frames.
      • Can be less weight than other platen material types. Because of the inherent lighter weight, multiple layers and uniform weighting around the perimeter are often necessary to achieve optimal results.
      • Polishing the edges and rounding the corners is best for the comfort of the conservator. Adding tape inevitably causes problems as the tapes get caught, provide resistance to sliding platens on and off the stack, and inevitably dry out and fail.
      • Thinner versions can bow and curl within a few hours with moisture from a stack.
    • Wood and Table-tops (e.g. Melamine covered, Medium-density fiberboard or MDF, plywood, particle boards, Masonite).
      • Wood is easily available in hardware stores: Compare types. Sanding the edges and rounding the corners is best for the comfort of the conservator.
      • Apply several coats of topical sealer (e.g. polyurethane or acrylic), penetrating sealer (e.g. (siliconates, fluoro-polymers and siloxanes), or Impregnating sealer (e.g. silanes or modified silanes) as appropriate before use to reduce any deleterious off-gassing unless the wood or wood-composite is well aged.
      • Melamine, ready-made table tops can be purchased individually (e.g. IKEA, Elfa system, etc.) or see a local kitchen supply center for counter-top materials.
    • Metal sheets
      • Dry Mount dwelling/cooling metal platens can be used when flattening smaller photographs, or as weights on top of a larger platen. To be most useful, the underside must be kept clean and free of scratches and dents.
      • Local blacksmiths and metal workers can fabricate metal platens and weights to individual specifications.
    • Weights (individual)
      • Multiple types of materials that are heavy can be used as weight on top of a platen, even letter, copy, and small book presses if available. Lead is traditionally used and preferred as weight material, yet lead dust is hazardous to human health. See Sourcing Studio Lab Weights for more information on obtaining weights and methods to contain lead dust within lab/studio weights.

Humidification Methods

  • Humidification is the most effective method for relaxing paper-based photographs, not including objects with plastic incorporated into the support (e.g. resin coated supports), cloth, or skins, such as parchment or leather and many residual adhesives on photographic prints.
  • Humidification can be done locally or overall.
  • When done in combination with aqueous treatment, humidification is sometimes referred to as a "one-step" method. When air-drying follows any wet treatment with final flattening after re-humidification, the process is sometimes referred to as a "two-step" approach.
  • Preparing for the drying process before starting the humidification process is advised.
  • The photograph is humidified, commonly overall, prior to flattening under pressure or with tension, and thus retains planarity when dry. During the application of pressure, the photograph is commonly sandwiched between interleaving materials such as polyester webbing, silicone release papers, blotters, smooth boards, and thick, smooth (not napped) wool felts while under or between platens. This common method is known sometimes interchangeably as a drying stack, pressing stack, or blotter stack.

Humidity

  • Ambient humidity
    • The higher the ambient humidity in the work space, the faster the humidification process proceeds.
    • Likewise, the lower the ambient humidity, the faster the drying process proceeds.
    • Internal localized air flows within a work environment can have an impact on the rate of drying and the evenness of drying larger items. Consider the location of heat and air-conditioning vents, fans, windows, etc. when choosing locations for photographs to dry.

Ways to Introduce moisture

  • Humidity Chamber or Humidity Room (water as gas)
    One of the safest ways to introduce an even absorption of moisture into a photograph is by using a traditional humidity chamber.
  • Mylar sandwich
  • Gore-Tex (felted membrane)/Damp Blotter Pack (water as vapor)
    With Gore-Tex, and similar synthetics, such as Sympatex, water passes in small molecules when in direct contact with paper. Prior to the introduction of Gore-Tex membranes to the conservation profession, a damp blotter pack was used by conservators to introduce moisture. This method can be local or overall.
  • Ultrasonic humidifiers/Nebulizer (water as vapor/cold "steam")
    These machines produce moisture vapor. They can be locally used or used overall in conjunction with domed chambers and plastic tents.
  • Steamers (hot water gas)
    Useful when extremely fast and aggressive humidification is needed or for softening attachment and residual adhesives, however, more components of photographic prints can be adversely affected.
  • Misting/Water spray (water as liquid)
    Another traditional and faster method of dispensing water. Can deliver water too fast without sufficient absorption of the support.
  • Brush/sponge (water as liquid)
    Localized treatment to introduce moisture carefully, with much control.

Humidification Treatment Variations

Humidity Chamber or Humidity Room (water as gas)

  • Simple chambers are made of plastic, glass, or metal trays and covered with plastic or glass platens.
  • Sinks can also be converted into a chamber if the drain can be plugged.
  • Large chambers can be made by encasing baker's racks.
    Baker's Racks adapted for humidification or drying
    Image: Stephanie Watkins
  • In some institutions and businesses doing high volume conservation projects, an entire closet or room is built specifically for safe humidification procedures and to deter fungal growth.
    Large scale humidification - Georgia State Archives
    Image: Stephanie Watkins
  • Basic Configuration
    • To create a basic humidity chamber, first a shallow amount of clean, purified (often distilled or deionized) water goes into a tray or sink. Sometimes (scrap, but clean) blotter is placed in the bottom of the tray to reduce potential water splashing if the tray is accidentally bumped, although not an issue if a stationery sink is used. Cool temperature water is generally safer, however, warm water can be used if the situation necessitates it.
    • Window screens, silkscreen screens, or plastic commercial fluorescent light fixtures "eggcrating", are commonly used to create a support for "suspending" the photograph during humidification.
    • The screen is separated from the water in the bottom of the tray by setting on top of other eggcrating (make a stack), or placed on water-inert spacers placed in the corners and around the perimeter (e.g. small plastic or glass weights, glass custard cups, plastic piping or tubing connectors, scrap Ethafoam blocks, etc.)
    • A sheet of poly(ester) webbing or silkscreen fabric (sometimes both), larger than the photograph, but smaller than the overall size of the screen, is sometimes placed on top of the screen to facilitate safer transfer of the photograph while damp and limp from the chamber to the press.
    • Once the photograph is placed face-up on the screen support, the tray is covered with a glass or plastic platen (transparent) so progress can be easily monitored.
    • Numerous variations to the method described here are commonly done.
  • Pros:
    • Creates slow, controlled, and even moisture absorption by a paper-based photograph.
    • Keeps swelling of fibers and movement of inscriptions and retouching to a minimum if room temperature water is used.
  • Cons:
    • Chambers can be slow to become saturated. Chamber container (e.g. plastic, metal, glass, wood) can influence maximum concentration of humidity attained.
    • If chambers are left assembled for stretches of time, fungal growth can occur within the unit.
    • Empirical judgement is needed to determine when the photograph is dampened enough for removal to the press. The length of time to humidify a photograph will vary depending on the type of photograph, condition of the photograph, the ambient humidity in the work environment, and the time at which the chamber was set up.
  • Tips:
    • Cut poly(ester) sheets to sizes smaller than the screens used in a chamber. Sheets larger and unsupported by the screens can drop into the water as the poly(ester) absorbs moisture. If this occurs, water at the bottom of the tray (in liquid form vs. gas in the chamber) will wick to the photograph.
    • If the tray is large, light weights at the corners of the tray can help keep plastic platens flush with the tray top. If plastic platens are pieced or abutted to create the top, packing tape along the seam can keep a tighter seal, and prevent edges from bowing too much.
    • Silkscreening fabric comes in a variety of tight weaves and weights. Heavier screening materials are more helpful for humidifying larger items due to weight of the item and potential stretch of the silkscreening fabric. Synthetic materials used for lining paintings can also be used as screening materials.

Poly(ester) terephtalate film "sandwich" (Mylar, Melinex)

  • Basic configuration
  • Variation: Open face Mylar Sandwich
  • Pros:
  • Cons:
  • Tips:

Gore-Tex (felted membrane)/Damp Blotter pack (water as contact vapor or gas)

Goretex Pack Face-up.jpg
Gore-tex stack
Image: Stephanie Watkins
  • Gore-Tex and damp blotter packs can be assembled so the photograph is either face-up or face-down.
  • Humidification generally occurs from the paper side (e.g. back in most cases) only, and for a few minutes (15-20) although exact duration must be determined by the conservator depending on the individual photograph, dampness of the blotters used (or direct water spray on the felt), and overall room humidity.
  • Once humidified, placement in a stack proceeds quickly as the vapor can dissipate quickly.
  • Basic Configuration (Gore-Tex, Sympatex)
    • For assembling a simple Gore-Tex damp pack, a layer of water-impermeable material (e.g. sheet of smooth Mylar, plastic film, or rigid plastic) is placed on a table.
    • Dampened blotters (increasingly dry as they are stacked) are placed next, with the smooth side of the last, driest damp blotter face-up.
    • Place the photograph face-up (back to the damp blotter), directly against the Gore-tex. Some like to insert an ultra-thin and smooth poly (ester) webbing. However, this method requires direct contact, so inserting barrier materials can cause the process to slow down.
    • A smooth, clean sheet of plastic or Mylar much larger than the stack is draped over the top.
    • A clear, lightweight platen is placed on top for even, direct contact.
    • Placing small platens or weight around the perimeter edges of the larger, flexible draping plastic or Mylar, can hold the moisture in more effectively, generally reducing the time needed for this procedure.
    • Other variations to the method described here are commonly done, including inverting this stacking process.
  • Basic Configuration (Damp Blotter Pack)
    • For assembling a simple damp blotter pack, follow the procedure for assembling a Gore-Tex damp pack. Generally, multiple layers of open weave poly(ester) webbing replaces the Gore-Tex level next to the back of the photograph. As with Gore-Tex membrane damp packs, progress is dependent on how much moisture is used, condition of the photograph, type of photograph and ambient humidity in room. Variations to the method described here are commonly done, including inverting this stacking process.
  • Pros:
    • Small molecule of water introduced reduces swelling of the support fibers and movement of inscriptions, signatures, and retouching to a minimum.
    • Curled and tightly rolled items can sometimes be gently uncurled a little at a time if a deep chamber is not available. Or, loosened enough to allow for chamber humidification.
    • Large, and very large items (beyond the size of trays and sinks, but within the width of a Gore-Tex fabric or table-top) can be easily, sufficiently humidified overall at one time.
    • Face-up configuration has the advantage of being able to observe the photograph during the process.
    • Face-down configuration, with the stack on top, has the advantage of easily adding additional moisture as necessary and sometimes the procedure can go faster, but the disadvantage is the image cannot be seen during the process.
  • Cons:
    • The photograph can ferrotype against the top plastic drape or be impressed by a thin poly(ester) webbing as the image layer softens.
    • This is a contact method, so it is not suitable for photographs that cannot withstand any physical pressure on their surface.
    • Thinner photographic papers can become temporarily "attached" to the smooth Gore-Tex membrane.
    • If uneven contact is maintained in the pack, humidification will be uneven.
    • Empirical knowledge and skill are needed to create the perfect balance of humidification within the packet for photographic materials. Too little water in the system and the photograph needs long periods of time that may be detrimental to it. Too much water in the system and the photograph can become too saturated or wet with water, beyond dampness.
    • Checking the system too frequently for progress can interrupt the humidification process.
    • Checking the system too infrequently for progress can leave a photographic too moist or wet.
    • Gore-Tex, Sympatex, and other synthetic laminate membrane surfaces are effective if their coatings are continuous. The surface needs to be without creases, cuts, or tears to evenly humidify. Water can penetrate more rapidly in areas where the membrane is physically compromised or when two pieces are abutted to one another causing differential wetting.
  • Tips:
    • Whenever possible, use continuous sheets of Gore-Tex for humidifying overall.
    • Small areas can be humidified when the surrounding area is covered with smooth paper or blotter with thin plastic on top to protect the photograph from moisture. Exact shaped Mylar templates can be made, also.
    • Plumber's Teflon thread tape can be used to locally isolate smaller areas from humidifying when the surrounding area needs to be humidified.

Ultrasonic Humidifier/Nebulizer (water as cold vapor)

  • Basic configuration:
    • Photograph is placed flat on a non-absorbent surface with or without a secondary support materials (e.g. poly(ester) webbing, screening material, Mylar).
    • The photograph is weighted.
    • Work with purpose to place photographs in the press immediately after humidification as Ultrasonic and Nebulizing machines introduce very small molecules of moisture that can dissipate quickly.
  • Pros:
    • Small molecule that keep fibers and potential additions from swelling too fast, or too much.
    • With wand or hose use, vapor can be directed if areas that require more humidification (based on condition of the photographic layer or support).
    • Cold vapor can be very effective in locally softening residual adhesives.
  • Cons:
    • Water drips can form on the edges of the dome or tent, and potentially on the photograph face.
    • Can take a long time for material to become damp enough to relax sufficiently.
  • Tips:
    • When working locally, Mylar templates can be cut to mask off areas from unwanted moisture.
    • For overall humidification, create a domed or tented area on a table covered with plastic (where the photograph will go) and place the unit outside the dome or tent with vapor fed in with a hose.
    • When creating a dome or tent, put the hose at the bottom rather than near the top of the structure. This will reduce some of the dripping onto the photograph. Likewise, if the machine is placed on another table or floor nearby, any potential drips from the machine are isolated from the treatment area.

Steamers (hot water gas)

  • Basic Configuration:
    Osrow steamers
    Image: Stephanie Watkins
    • Place the photograph on a blotter and weight it. This can free up both hands during treatment if tweezers, swabs, knives, or spatulas are needed in conjunction with the steam.
    • Use the steamer locally by gently circling the area to treat.
  • Pros:
    • Useful for localized humidification such as softening attachments and residual adhesives
    • Useful when extremely fast humidification is needed to limit moisture exposure
  • Cons:
    • More components of photographic prints can be adversely affected by steam and higher water temperatures. Stains and discoloration are more likely to move.
    • Steamers generally have a warm-up period, so it is best to keep them on (vs. turning them off and on) when in use.
  • Tips:
    • If small areas are needed to be treated, masking off other areas with blotters or boards with Mylar, on top or with Mylar templates can help reduce humidification in unwanted areas.
    • Be attentive to potential dripping. Wiping the unit with a soft cloth, periodically can help reduce dripping.
    • Have a metal tray with a towel in it, to store the unit during treatment, while it is on.
    • Steamers that can release steam while in a horizontal position (like an iron) are most helpful for treating smaller items. Conversely, steamers that must be upright to function can be less helpful for bench work on smaller items, yet more helpful for large scale pieces that can remain vertical during treatment.
    • Conservators who wear glasses or magnifiers while treating may find their optical lenses steaming and fogging over during treatment. Lens material permitting, over the counter, temporary-acting, commercial "anti-fog" spray liquids or wipes are available (e.g. Fog-Buster, Cat Crap). In addition, cleaning both sides of the lenses with a thin layer of soap or shaving cream can retard fogging.

Misting/Water spray (water as liquid droplets)

  • Basic Configuration:
    • Support the photograph on a secondary support before beginning. Often photographs are on screening or poly(ester) webbing material to facilitate ease of handling into a pressing stack. If air-misting to introduce moisture, choose a secondary support that is not absorbent and will retain rigidity.
    • Spray lightly and evenly. Let the paper absorb the moisture.
    • Once relaxed sufficiently, transfer to press.
    • Numerous variations on this procedure are done.
  • Pros:
    • Fast, quick method. Time efficient for large batches of materials that can withstand a spray or misting approach.
  • Cons:
    • Can leave irregular-sized water drops on the surface
    • Can unevenly and incompletely humidify all layers of a photographic
  • Tips:
    • A dispenser with the finest, even spray is recommended. Water can be sprayed via an air-brush, a spray gun, a Dahlia (brand) sprayer, a hand-pump, a finger pump, atomizers, brushes, and sponges (Murphy). Dahlia sprayers remain a favorite tool amongst conservators, however, some have concerns about potential metal from it's internal construction leaching into the water it holds. In this case, pump plastic containers are often chosen instead.
    • Spray head nozzle sizes can be adjusted on many types of sprayers. A wide spray (as opposed to a linear stream) is generally preferred.
    • Holding the sprayer more vertically (perpendicular) to the photograph will allow less direct spray force to deliver water to the photograph and is considered a gentler approach than "shooting" the water spray directly at the photograph.
    • At the beginning of each spraying, direct the spray away from the photograph as sprayers usually create larger droplets when they are started.
    • Working in a deliberate, fluid, grid pattern will generally provide an even application of water.
    • Depending on the photographic type, sometimes only the back is misted.
    • If both sides of the photograph are to be sprayed, begin with the back first, then flip over using a poly(ester) webbing or film support. This will leave the photograph face-up. Switch the order if it is desired to press the photograph face-down.

Brushes and Sponges (water as liquid)

  • Large soft brushes, sponge brushes, and micropore sponges can be used to deliver and remove controlled amounts of water locally. Mizubake (Japanese water brushes) can be used to quickly introduce moisture overall to the back of a photograph.
  • Pros:
    • Method can be local or overall.
    • Very fast method.
    • Nuanced control of moisture content is possible.
  • Cons:
    • Like spraying, moisture intake into the support may not be even.
    • Physical manipulation of the fibers can occur while wet, causing fiber disruption (pilling).
  • Tips:
    • Non-latex finely pored make-up removal sponges come in handy wedge shapes and are easily cut and shaped to sizes needed.
    • Brushes and sponges can deliver moisture and remove unwanted material from a photograph.
    • Use the best quality brushes.
    • Store Mizubake brushes in a moderately humidified environment. In a dry environment, the brush sheds hairs more readily and those remaining are more easily broken.

Immersion in water

  • When a washed, saturated photograph is prepared for immediate drying and flattening without an intermediate stage of air-drying, the process is sometimes referred to as a "one-step" process. (A "two-step" process is when the photograph is allowed to dry, then is re-moistened before flattening).
  • Basic approach
    • After removal from the bath, remove the bulk of the water by lightly padding the photograph with clean blotters front and back (Chen).
    • Dry the photograph further, face-up for gelatin and face-down for albumen (McElhone), on a clean drying screen until the gelatin emulsion is gelled and not tacky. When the material goes from saturated to damp and before distortions begin in the paper substrate, transfer the photograph from the screens to whichever flattening procedure deemed best.
  • Pros:

Photographs are saved an expansion and contraction cycle.

  • Cons:
    • Empirical judgement is needed to determine when in the air-drying cycle the photograph would benefit from placement in a press.
  • Tips:

Drying and Flattening Methods

Pressure application

  • For most cases, pressure applied during the flattening process needs to be equally distributed throughout the photograph.
  • Determining the amount of pressure to apply can depend on the amount and degree of the curl. Often the emulsion needs to become tacky for the photograph to relax enough for flattening (Daffner).
  • Determining the amount of weight needed for even pressure in pressing individual photographs or of specific photographic processes is usually empirically learned.
  • Generally, placing weights around a perimeter of a rigid platen will provide more even pressure overall than placing weights in the center of the platen alone. This is especially true if thin plastic platens are used. Placing weights around the perimeter and evenly spaced over the entire surface of the platen will provide more overall pressure.
  • Dry mount presses are useful for providing smooth, conductive surfaces designed for photographic materials. They provide even, consistent pressure overall and the amount of pressure can be adjusted somewhat through building up support boards within the press and degree of clamping done.


Conditions when flattening or applying, even, overall pressure are exceptions:

  • oversized objects where it is practically impossible to apply even pressure
  • objects with differential levels of damage
  • objects where inherent vice precludes overall flattening (e.g. objects composed of multiple fragments that were never entirely flat)
  • objects where planar distortion has caused irreversible creases that would be intensified by overall application of pressure

Position of photograph during drying

  • Image orientation: Face-up or Face-down?
    For any treatment, the judgement of the conservator based on knowledge of photographic materials, experience, and the individual situation at hand will dictate orientation.
    • When a paper-based photograph is placed face-down in a blotting stack, it is believed that any inherent discoloration in the paper will more likely settle to the back (or top side) of the photograph.
    • For photographs that tend to curl in toward the image side, placing the photograph face-down reduces physical damage during handling (Murphy). It can facilitate easier, faster placement within the stack for these types of photographs with the benefit of potentially saving time when pressing large batches of similarly sized photographs in similar physical condition.
    • For photographs that tend to curl outward, placing the photograph face-up reduces physical damage during handling (Murphy). It can facilitate easier, faster placement within the stack for these types of photographs with the benefit of potentially saving time when pressing large batches of similarly sized photographs in similar physical condition.
    • Photographs can go from a saturated state in a wash bath into a controlled drying situation (sometimes referred to as "one-step" treatment). In between immersion and flattening, temporarily air dry on a screen to take the photograph from wet to damp. Place gelatin prints face-up (to reduce damage to the swelled gelatin) and albumen prints face-down (to reduce curling) (McElhone).
    • It is has been observed in batches of materials that had the same origin and storage and were treated the same way, that photographs dried face-up may suffer more physical damage, such as crizzling, than those dried face down. (Edmondson)
    • Conservators may wish to place photographic materials face-up for their own comfort (seeing it is okay until the last possible moment) in ensuring nothing gets jarred or moved during the assembly of the pressing stack.
      If a photograph is going into a dry mount press (with or without heat), often the photograph will go face-up to be nearest the smooth metal platen. Some may choose to put the photograph face-down to diffuse heat through any boards and other materials chosen if a heated-dry mount press is used.
  • Does the distance to the edge of a pressing stack matter?
    • The closer the photograph is in overall size to the drying or pressing stack (and therefore closer to the edges of the stack), the more likely edge distortions will occur.
    • Two to three inches spacing of the extension of any pressing materials beyond the size of the photograph being flattened, can sometimes help alleviate edge distortions.
    • If the thickness, or "z direction", is high, creating a template to fit around the photograph from the similar acting moisture-wicking material chosen (blotters, board) of the same thickness as the thickness of the photograph, can also help alleviate edge distortions by artificially "extending" the edges of the photograph and creating a continuous, level stack.

Drying and Pressing Treatment Variations

Blotter Rotation

The initial frequency of blotter changes and over the course of the drying period will depend on the treatment processes chosen, the type of photograph treated, as well as the philosophy of the individual conservator (Kennedy).

  • If a photograph has been immersed, conservators will sometimes blot it on both sides, then place it in a stack. Sometimes the blotters will then be changed frequently at the start, with ever increasing time increments between changes (Kennedy). Even if photographs were not placed in stacks when wet (vs. damp), changing blotters is a chance for the photographs to relax when exposed to air during changing and achieve better flatness. The idea is to flatten, not to stretch (Daffner). More blotters are potentially used with this approach.
  • Other conservators prefer to leave the blotters in place for longer periods feeling that the drying process should be slow, though not so slow that mold has a chance to grow (Kennedy) within the stack. It is also felt that opening up the stack interrupts a complete drying cycle, drying papers faster for the few moments while blotters or position in the stack is changed, potentially introducing distortions into paper. Stack blotters can potentially be reused with this approach.
  • The inclusion of corrugated boards in drying stacks (between the blotters-polyester web-photograph-polyester web-blotters groupings) is seen as a way that air can circulate through the stack, reducing the need for blotter changes (Kennedy).
  • Use of desiccated blotters is another alternative method (Kennedy) to remove water more quickly from the stack and thus, the photograph.

Letter, Copy, or Book Press

Large bookpress
Image: Amanda Maloney
Detail of stack in bookpress
Image: Amanda Maloney
  • Basic configuration:
    The basic configuration for a drying stack is (bottom to top):

- Bottom of press - Rigid board - Mat board or Blotter(s) 1-2 (-3 or more if multiples in a stack) to mirror number on either side of the photograph - Smooth polyester web - Object - Smooth polyester web - Mat board or Blotter(s) 1-2 (-3 or more if multiples in a stack) to mirror number on either side of the photograph - (repeat with multiple objects as needed) - Rigid board - Top of press

  • Variations:
    • Poly(ester) webbing is optional as it can impart a texture or impression into the emulsion surface. It is used to prevent potentially tacky emulsions from adhering to the blotters and boards. If poly(ester) webbing is not used, an extra smooth, place a high quality board of paper again the emulsion surface. Paper is easier than boards to remove should it adhere.
    • Press boards traditionally used are wood or compressed fiber-board (book boards such as Davey Board), but thick plastic sheets can also be used.
  • Pros:
    • Ease of use.
    • Very even pressure.
    • Numerous items can be stacked within one press at a time.
  • Cons:
    • Method is limited by dimensions of the press. Letter, copy, and nipping book press platen sizes are generally limited to less than 11" x 17" (28 x 43 cm) (for a legal sheet of paper), whereas free standing book presses can have platen sizes as large as 28" x 40" (71 x 101.5 cm). Larger photographs may not fit into any size press (Murphy), even with extenders.
    • Older, established labs/studios may have these machines, yet new(er) labs/studios may not be able to find nor afford such equipment. New presses are still manufactured with a cost of several hundred dollars. Once relatively inexpensive and easy to find, old presses are rare(r) from second-hand resale options and can also be expensive if found (2016).
  • Tips:
    • Using rigid plywood or Masonite boards (smooth sides toward stack) on the top and bottom of a large book press can extend the pressing range of the press. The limitation will be the longest width between the press bars, that becomes the shortest dimension of the boards. With thinner wood boards, fall-off of even pressure can occur at the protruding ends of the extender boards, in which case, attaching large "C" and bar clamps and similar fasteners can rectify the loss of pressure at the ends, somewhat, as desired. A board can be attached or not to the top of the press. If not, boards with both sides smooth will be easier to move in and out of the press.
    • Insert boards as needed (e.g. perhaps every third photograph) to retain even pressure and planarity throughout the stack.
    • Corrugated boards can be inserted throughout the stack but in this case apply less pressure to avoid creating an impression on the photograph.
    • Using Mirage board or a highly calendared board placed between the Hollytex and blotter paper during drying helps reduce the possibility of impressions in the gelatin emulsion layer when using any weighted flattening methods. (Fischer)

Blotter stack for pressing and drying with release/barrier layer

  • Basic configuration:
    The basic configuration for a drying stack is (top to bottom):
    Standard Blotter Stack.jpg
    blotter pressing stack
    Image: Stephanie Watkins

- Weights (adjust to object, object condition, dampness) - Flat surface (e.g. thick acrylic) - Blotters (two to four) - Smooth poly(ester) webbing - Photograph - Smooth poly(ester) webbing - Blotters (two to four) - Flat surface (e.g. thick acrylic - optional) - Table top

  • Pros:
    • Numerous photographs can be stacked and pressed at a time. If size permits, multiple items can be placed on each blotter, and within the same drying stack.
    • Packs can be stacked directly on top of each other for large batch treatments. Inserting rigid boards periodically within the stack helps keep even pressure. Typical configuration might be ten packs high with Plexiglas sheets at every three to five packs.
  • Cons:
    • Photographs at the bottom of the stack can be cumbersome to reach or remove.
    • Photographs on the bottom of the stack will receive the greatest amount of pressure from the added weight above, so some limitations may need to be set regarding height of stack created.
    • Conversely, the thicker the blotters (more air) and the taller the stack created, the more pressure can be needed to replicate the results from a single photograph flattened in a single pack or "short-stack".
    • Moving of stacks with photographs inside can cause unintentionally folding or creasing of poly(ester) webbing that can be transferred to the photograph.
    • Blotter stacks can take up valuable table-space in small work-spaces.
  • Tips:
    • Using Mirage board or a highly calendared board placed between the Hollytex and blotter paper during drying helps reduce the possibility of impressions in the gelatin emulsion layer when using weighted flattening method. (Fischer)
    • Use desiccated blotters to speed up the drying time. Desiccate blotters by placing on a hot plate for a short period of time or by placing in the microwave and heating on low heat for short periods of time (microwave powers and time will vary).
    • A tall rolling baker's cart is good for holding blotter and interleaving supplies during batch treatments. It can be stored out of the way and rolled to the workstation when needed. One shelf labeled for interleaving, one for dry blotters, and one for slightly damp blotters -- the stack usually dries nicely within a week. Photographs are not dried on the cart; it is for supplies only.

Variation #1: Pressing Stacks without Poly(ester)

  • Basic configuration for a drying stack is (top to bottom):
    Stack w Smooth Boards .jpg
    Pressing with smooth boards without poly(ester) webbing
    Image: Stephanie Watkins

- Weights (adjust to object, object condition, dampness) - Flat surface (e.g. thick acrylic) - Very smooth Blotter or extra smooth, calendared board - Photograph - Very smooth Blotter or extra smooth, calendared board - Flat surface (e.g. thick acrylic - optional) - Table top
Similar pros and cons as with a blotter stack method cited above.

  • Tips:
    • Very smooth blotter, Mirage board, 2-ply matboard, or a highly calendared board is placed next to the photograph and the release/barrier material is eliminated to reduce the possibility of impressions in the image layer when using weighted flattening methods.

Variation #2: Blotter stack with inserted corrugated boards

  • Basic configuration:
    Stack w Corrugated board.jpg
    Pressing stack with corrugated boards
    Image: Stephanie Watkins

-Platen -corrugated board -blotter (1-2) - poly(ester) webbing (optional) -photograph - poly(ester) webbing (optional) -blotter (1-2) corrugated board
Prepare a stack of sets of blotters and corrugated boards. Corrugated boards serve as air channel to dissipate moisture from the stack and thus, the photographs held within. Cutting the boards so the corrugations are on the short sides theoretically increases air potential. Limit the number of blotters to one or two placed on either side of the photograph, as multiple blotters defeats the purpose of having an air channel. Can have more than one blotter/photograph set between in each corrugated board set in the stack. Much pressure is not needed, but even pressure is very important (Edmondson). The configuration is illustrated (top down):

  • Pros:
    • The use of corrugated board that allow more air circulation through the cores, helps to speed up the drying time.
    • Fewer blotters are needed. Having too many blotters will defeat the purpose of the air channels (Murphy).
    • The minimal pressure required for this method could be beneficial for gelatin emulsions (Brown).
  • Cons:
    • The use of corrugated boards limits the amount of pressure possible to exert as the board can impart a corrugated impression into the photographs.
  • Tip:
    • Faster, uniform drying can be achieved by placing a fan (or conversely, a vacuum) in the direction of the open corrugated air holes to push or pull air through the flute-holes.

Variation #3: Blotter Stacks with Felts

Stack w Felts.jpg
Pressing stack with felts and blotter
Image: Stephanie Watkins
  • Basic configuration:
    Felt - blotter - poly(ester) webbing - photograph - poly(ester) webbing - blotter - felt.

Thick felts may be used as a cushioning measure to prevent exerting too much pressure on the photograph, such as when the image layer has softened or when the support has severe creases that were not completely relaxed with the humidification (Casella).

  • Pros:
    Low pressure on the image layer (Casella).
  • Cons:
    Cushioning and low pressure may not result in complete flattening as with other methods.
  • Tips:

Variation #4: Hard/Soft Sandwich in a weighted stack

  • Basic Configuration:
    • A felt on one side of the photograph (often the face) with smooth absorbant blotter or rigid matboard on the other. Polyester webbing interfaces can also be used.
  • Pros:
    **Flattening between two rigid surfaces can cause minor deformations, mended tears, or paper disturbances on the back of a photograph to "get pushed" to the image side. This configuration seems to allow the material to move as needed through the drying cycle, while being "encouraged" into planarity by the rigid planar surface on one side.
    • Method works best for sensitive media layers or where severe distortions have occurred.
    • Method can be repeated, or used as part of a series of flattening steps.
  • Cons:
    • May not get materials as planar as when dried between two rigid surfaces.
    • Traditional wool felts can develop mold issues.
  • Tips:


Variation #5: Friction Mounting package in a blotter stack

  • Basic Configuration:
Stack w Friction Mount.jpg
  • Pros:
    **Method works best for thin papers as it was originally designed to flatten long-fibered oriental papers (e.g. Burmese, Chinese, Korean, Japanese, etc.).
  • Cons:
  • Tips:
    **While a Uchibake (pounding brush) is traditionally used to create the fiber bonding in the border area, a thick or larger Nadebake (smoothing brush) with stiff brushes can be used in a similar up-right drop motion. The thicker and heavier the Nadebake the less work with better results can be achieved. Philippine hemp varieties make better Uchibake substitutes as Japanese hemp Nadebake tend to be softer and more pliable.
    • Spray or soak Japanese brushes with water to reduce wear and breakage of the brushes.

Drying on the Vacuum-suction table

(Karl Buchberg & Lee Ann Daffner)
  • Basic configuration:

Procedure:? dried on suction table for 6 hours (pressure?) turned off the suction and weighted with blotter for 2 weeks. (polyester webbing?) Moved to stack or left on table?

  • Pros: Good for large photographs
  • Cons: Drying is from the edges, and the paper stays expanded during the process.
  • Tips:

Dry mount press without heat

  • Basic configuration:
    Basic configuration for using dry mount press without heat is (top to bottom):

- Top platen - Bristol board - Object (face down) - Bristol board - Press rubber

  • Variations:
    • Felts or blotters can be added on outside of the Bristol board to increase pressure.
    • The original rubber mat can be removed and replaced with felts, blotters, or boards to increase or decrease the pressure as necessary.
  • Pros:
    • The press provides uniform, consistent pressure, that when clamped, can be stronger than weights (Murphy). The pressure can be adjusted through how tight the clamp is kept. If lighter pressure is desired, the press platen can function as a heavy, uniform weight if it is not clamped at all.
    • Without heated moisture, there is less chance to change the characteristics of gelatin surface and silver degradation
  • Cons:
    • Without moisture to humidify and relax the photograph, the overall pressing may not be as effective.
    • Rippling at the edges of the photograph may result from the insufficient drying required of this method (Albright). However, rippling along the edges can be reduced if not altogether avoided if the photograph is transferred within 24 hours from a dry mount press to a blotter stack to continue flattening for a minimum of two weeks.
  • Tips:
    • With this method the object is in contact with Bristol weight press smooth board on both sides of the object. Make sure board surface is exceptionally clean as any small dust, debris, or hair will be impressed into the photographic surface.
    • Leave the photograph in the press for a minimum of 8 hours.
    • More blotters or felts can be added on outside of the Bristol board to increase pressure.
    • The use of Silicone release paper is not recommended as it absorbs moisture, cockling, that can then impart to the photograph (Fischer).

Pressure with heat

Pressure with heat can be obtained using a traditional dry mount press or a vacuum hot table followed by traditional pressing stack (Murphy). For overall flattening in conjunction with consolidation methods, see PMG Consolidation and Flattening of Cracks entry (Casella).

Heat press
Image: Luisa Casella

Variation #1: Pressure with moderate heat and short time; WITHOUT prior humidification

  • Basic configuration:

- Top platen - Bristol board - Silicone release paper (optional) - Photograph - Silicone release paper (optional) - Bristol board - Press rubber

  • Basic procedure:
    • Use moderate heat (recommended as 170°F to 200°F (76.7°C to 93°C) (Hoffman, Albright), but preferably not above 180°F (82°C) (Weaver)), and short time (approximately 45 seconds) (Hoffman, Albright)
    • Immediately after removal from the press, cool down facing a heavy, but smooth and clean surface (preferably conducting surface) for the photograph to come to equilibrium with the environment as long as it is needed. Quick check after the photograph has cooled down.
  • Tips:
    • Silicone release paper can be replaced with smooth matboards, folders, or blotter if the photograph is not humidified before pressing. Less heat can be used with fewer layers (Murphy).
    • Results can differ depending on whether the photograph is placed face-up nearer the metal platen or face-down away from the metal platen. Opinions vary as to which orientation is better.
    • A stock folder cut to the size of the press is useful for handling. The photograph is inserted into a folder made of silicone release paper, which in turn is inserted into the folder-stock folder. If used with non-humidified prints, the folder-stock can remain uncockled indefinitely (Bernier).
    • This method can be used in conjunction with dry mounting a photograph.

Variation #2: Pressure with moderate heat and short time; WITH prior humidification

  • Basic configuration: Basic configuration for using dry mount press with heat is (top to bottom):

- Top platen - Bristol board - Silicone release paper - Photograph - Silicone release paper - Bristol board - Press rubber

  • Basic procedure:
    • A silicone release paper is necessary to face the gelatin emulsion if the print is humidified prior to pressing. Any cockling or wrinkles in the silicone release paper will transfer to the photograph being flattened. Always use clean and smooth release paper. (Fischer)
    • Use moderate heat (recommended as 170°F to 200°F (Hoffman, Albright), but preferably not above 180°F (Weaver)) and short time (approximately 45 seconds) (Hoffman, Albright)
    • Rotate the photograph to a dry spot on the silicone release paper several times if the photograph is humidified prior to pressing.
    • Immediately after removal from the press, cool down facing a heavy, but smooth and clean surface (preferably conducting surface) for the photograph to come to equilibrium with the environment as long as it is needed. Quick check after the photograph is completely cooled down.
  • Pros:
    • Can be very effective to make a gelatin print flat.
    • The above method is “particularly effective for single weight, glossy gelatin photographs but good results can be achieved with double weight, matte and albumen photographs.” (Hoffman, p. 166).
  • Cons:
    • Some believe that historic resin-coated photographs cannot endure and should not be subjected to heat treatments as the surface characteristics might change (Murphy). Others believe that select RC photographs (chromogenic or gelatin silver) can be subjected to heat treatment in a dry mount press. Care must be taken to keep the temperature below 200°F, and preferably not higher than 180°F. Heat is often the only practical method of removing planar distortions from RC photographs. (Weaver)
    • This method can accelerate silver image degradation (Messier). It is very important to bear in mind that this treatment is similar to an accelerated aging—with high heat and high humidity, which can promote silver oxidation or silver mirroring. Changes and increase in silver mirroring have been observed with matte gelatin photographs using the above technique (Fischer). It has been observed that silver gelatin B&W photographs around 1970 could experience image color shift in this condition (Messier), also.
    • Salted paper print and platinum prints are not suitable for this method. In particular, the platinum prints have the velvet surface quality can be altered through this type of treatment. (Albright) Albumen layers can crack or crease under heat and pressure.
    • The long term effect of heat on a photograph is unclear.
  • Tips:
    • Photographs may need to be pressed twice with heat to avoid cockling of support and rippling of edges (Fischer).
    • If the print is longer than the dry mount press, such as with panoramas, half the print can be put in at a time. This requires that the support board, folder and silicone release paper extend past the edge of the press. The second side is placed in the press immediately following the first, before it has time to cool. Technique works well when used on dry photographs, that have been previously humidified and dried using other means but still retain some distortion (Bernier).
    • A stock folder cut to the size of the press is useful for handling. The photograph is inserted into a folder made of silicone release paper, which in turn is inserted into the folder-stock folder (Bernier).

Variation #3: Pressure with moderate heat and long time

Moist photographs are not suitable for this method.

  • Basic procedure:
  • Lower heat: let the press heat to no more than 170°F and turn off.
  • Put the photograph sandwiched between silicone release paper or smooth paper or blotter and leave the package in the heated press until the press cools down to room temperature. Cool down can take several hours.
  • Remove the photograph from heat press and press longer in regular pressing stack as necessary.

Variation #4: Vacuum pressure with heat

  • Oversized photographs may be flattened on a heat-suction (hot) table after overall humidification.
  • Basic Configuration (top to bottom):

Non-ferrotyping cover Photograph Table

  • Pros:
  • Cons:
  • Tips:
    • Poly(ester) webbing or other porous material should be in contact with image layer rather than a Mylar layer used for securing vacuum due to risk of ferrotyping.

Lateral Restraint Drying: Stretched or Tensioned Methods

  • Overall, stretched or tensioned drying can occur In combination with backing, lining or mounting techniques, such as Dacron mounting. See Backing, Lining, Mounting for additional procedures.

Drying Boards

A traditional Japanese drying board, a Karibari, is made of multiple layers of Japanese kozo paper over a wooden lattice structure. Traditionally, it is coated with permission juice that imparts a deep rust color to the paper.

  • Gatorboards and corrugated boards attached to wooden stretchers are modified drying board examples chosen for lower costs and sometimes, time restraints. While rigid and can be made large, they do not provide the same airflow for drying as traditional Karibari. However, they can be just as effective for use with photographic materials (Murphy).
  • Basic configuration:

- Japanese paper pocket - photograph - drying board

  • Basic Procedures:
    • Humidify photograph and a smooth, medium to heavyweight Japanese paper larger than the photograph.
    • When sufficiently relaxed, apply paste around the perimeter of the paper.
    • With the Karibari flat on a table, place the photograph face-in. With the adhesive side to the board, create a pocket for the photograph with the Japanese paper.
    • Use finger pressure to pull the paper taut and set the paste. Remember to place a small piece of paper in the paste border for a knife release after drying.
    • As the paper and photograph dries, tension will flatten the photograph. This configuration should likely be kept for several weeks if not months depending on the photograph.
    • Once the paper is taut and drier (so the photograph will not slip into the paste), usually after several hours or overnight, the Karibari can be moved into an upright position, face into the wall.
  • Variations:
    • If the photograph is backed with paper, the paper can be used to attach to the Karibari in the traditional vertical manner, not horizontal on a table. The photograph can also be attached face-out if desired. See Backing, Lining, Mounting.
  • Pro:
    • Keeps uniform tension across the entire surface if the paper is tensioned out evenly. This helps to control dimensional changes that occur during the humidification and drying processes.
    • Edges will not separate out.
    • As the photograph dries from the back, potential staining can migrate to the back, to keep staining away from the image (Murphy).
    • Multiple photographs can be tensioned this way at one time if the sizes permit given the size of the board.
    • Larger size photographs can be flattened as size is limited only by width and length of Japanese paper that can be obtained or Karibari that is made. If machined kozo paper is used, select papers can be as wide as 42 inches (106.7 cm) (Watkins).
  • Con:
    • Tensioning methods can create great stress on photographs
    • As drying occurs from the back, the front of the photograph cannot be accessed to observe any changes.
    • Staining from the board (from persimmon juice coating or previous attachments) can seep into the photograph (Murphy).
    • The photograph can slip into the paste if moved too quickly as it is not otherwise held.
    • Karibari technique was designed for flattening fiberous, organic materials in an humid environment (Japan). Be aware of dry ambient humidity within the work environment that can cause drying to proceed too quickly. As the photograph is not attached, it might not tear, yet could.

Edge Restraint

  • Without mounting, and for photographs with less reactive binders or well-hardened gelatin binders, the lightly humidified photograph can be restrained at the edges only with air circulation above and below the photograph. This is done with blotter strips and glass strips around the edges. (Kennedy) Lightweight earth magnets can also be used on the outside of the protective heavyweight blotters strips with a layer of thin board (for even pressure) to keep the packet taut. (Learn more about Magnets in Conservation on the Magnet Mounts page, and its Further Reading section.)
  • Pros:
    • Given a light humidification and the right photograph and conditions, this can be a gentle flattening technique with no contact with the surface of the work (Kennedy).
  • Cons:
    • Low ambient RH will cause the work to dry too rapidly. If the humidification is too strong the greatly expanded photograph can be placed under too much tension and in extreme cases may tear!! (Kennedy)
    • Likewise, this method is unsuitable for flattening photographs that were torn and mended alone, without the addition of a secondary support. (Watkins)

Perimeter stretch drying

  • If a photograph was backed with paper then air-dried, it can be humidified again and gently stretched into plane using the perimeter to pull the item flat (Murphy) while on a table. Restraint or manipulation is necessary until the photograph is dry or distortions can return.
  • Pros:
    • Less stressful tensioning method.
    • Possible to have more control in regulating moisture within the photograph.
    • Can be used in conjunction with edge restraint or a drying board.
  • Cons:
    • Not always as effective as other methods of flattening.

Process-specific methods and considerations

One-layer processes

Photographic processes that are essentially bonded to one side of a paper support, can be humidified and flattened as with any other paper support. However, prolonged high humidity levels should be avoided as moisture might alter the image color or appearance. Experimental and early one-layer photographic images may be too fragile to flatten with conventional methods. Decisions to flatten should be made on an individual basis regarding each photograph in question.

Albumen

Collodion POP/matte collodion

(Ask the Moors for their special methods)

Gelatin POP/DOP

When a sheet of gelatin takes on moisture, then is dried, it will shrink fairly evenly in length and width. When a photograph is humidified and dried, the shrinkage of the gelatin emulsion is constrained by the paper base. The paper can contract more easily across the grain than along the grain. This means that as the gelatin shrinks, the photograph would have a tendency to curl towards the emulsion, in the direction of the grain (+L curl). (Bernier)

Carbon and Woodburytype

Carbon and Woodburytype prints can be humidified as with other paper-based printing processes in preparation for flattening. The introduction of too much moisture can cause curling during humidification and potential cracking of the heavier gelatin areas during the drying process. As with many gelatin-based photographs, longer pressing times (several weeks to months) create more uniform results. (Watkins)

Coated Photographs

Waxed Paper Negative

Resin-coated (RC) photographs

Flattening of resin coated (RC) photographs (applicable to processes with image layers that can withstand high temperatures, such as gelatin silver and chromogenic color) can be achieved with variation 2-Moderate Heat and Long Time, as detailed in 3.6.3. An initial temperature higher than 170°F may be required, but be careful above 190°F as damage to the RC support can be sustained. (Weaver)

Mounted photographs

Photographs on thin mounts, not on thick card, can more successfully be flattened. Photographs mounted to heavy papers or thin paperboards (not 4-ply or thicker mounts) can sometimes withstand moisture introduced in preparation for overall flattening. Vapor (Ultrasonic, nebulizer) or gas (chamber) methods offer the deepest even penetration of moisture and the slowest, controlled method of entry. The flattening configuration needs to be prepared beforehand so the mounted photograph is moved quickly from humidification to flattening.
If the photograph is mounted with an aqueous adhesive, the photograph can detach during humidification, causing wrinkling, cockling, and puckering of the photograph. Likewise, the mount can distort, such as bowing. Differential expansion between the photograph and board can also occur. If the photograph is dry-mounted, differential expansion by introducing moisture can occur. To modify this result, introduce minimal amounts of moisture from the back or try a lowly heated pressing process instead. Limited amounts of heat can release residual water in healthy paper mounts that can aid in pressing. (Watkins)

Film-based negatives

note to editors:  Andrea Pietsch? (sp) tip - Cambridge? here?

Bibliography

  • Banik, Gerhard, and Irene Brückle. 2011. Paper and Water: A Guide for Conservators. USA and UK: Butterworth-Heinemann, an imprint of Elsevier, Ltd.
  • Bernier, Brenda. 2004. “Issues in Humidification and Drying of Gelatin Silver Prints” Topics in Photographic Preservation, Vol.11. Compiled by Brenda Bernier. Washington: American Institute for Conservation, Photographic Materials Group. Pp. 6-16.
  • Caldararo, Niccolo. 1992. “Tests on the Effects of the Use of Ultrasound in the Humidification of Paper”, The Book and Paper Group Annual [[2]], (11): 1-20.
  • Clarkson, Christopher. 1992. “A Conditioning Chamber for Parchment and Other Materials”, The Paper Conservator: Journal of the Institute of Paper Conservation, (16): 27-30.
  • Daniels, Vincent. 1998. “The Effects of Water Treatments on Paper with Applied Pastel or Powdered Pigment.” The Paper Conservator. (22): 29-37.
  • Dobrusskin Sebastian, Hannah Singer and Gerhard Banik. 1991. “Humidification with Moisture Permeable Materials”, 7th Congress of IADA, Uppsala.
  • Fairbrass, Sheila. 1987. “Ultrasonic Humidifier”, Conservation News, (34): 9.
  • Fenn, Julia D. 1987. “The Planthouse Terrarium: A Collapsible Humidity Tent”, The Ethnographic Conservation Newsletter, Issue 3, March 1987.
  • Hendriks, Klaus B., Brian Lesser, Jon Steward and Doug Nishimura (1984) “Properties and Stability of Gelatin Layers in Photographic Materials” Preprints of Papers Presented at the Twelfth Annual Meeting Los Angeles, California 15-20 May 1984. American Institute for Conservation. [[3]]
  • Hoffmann, Anna. 1991. “Flattening Cracks in Photographs” Topics in Photographic Preservation, (4): 166-169.
  • Hofmann, Christa, Dianne Van Der Reyden, Mary Baker. 1992. “The Effect of Three Humidification, Flattening and Drying Techniques on the Optical and Mechanical Properties of New and Aged Modern Transparent Papers.” The Institute of Paper Conservation: Conference Papers, Manchester 1992.
  • ISO 18903:2002, Imaging Materials -- Film and paper -- Determination of dimensional change. International Organization for Standardization (ISO).
  • ISO 18910:2000, Imaging Materials -- Photographic film and paper -- Determination of curl. International Organization for Standardization (ISO).
  • Keefe, Laurence E., Dennis Inch. 1990. "Flattening Prints." The Life of a Photograph: Archival Processing, Matting, Framing, and Storage, Stoneham, MA: Focal Press.
  • Keyes, Keiko Mizushima. 1994. “Some Practical Methods for the Treatment with Moisture of Moisture-Sensitive Works on Paper.” Conservation of Historic and Artistic Works on Paper: Proceedings of a Conference, Ottawa, Canada, October 3-7, 1988, p. 99-107.
  • Keyes, Keiko Mizushima. 1984. "The Use of Friction Mounting as an Aid to Pressing Works on Paper". The American Institute for Conservation's Book and Paper Group Annual, vol.3: Washington, D.C.: American Institute for Conservation.
  • Lockshin, Nora, Kathy Ludwig, and Kristen St. John. 2002. "Archives Conservators Discussion Group 2002: Humidification and Flattening." Book and Paper Group Annual (21): 57-59. Washington, D.C.: American Institute for Conservation.
  • Maxson, Holly. 1987. “The Design of a Collapsible Humidification Chamber”, The Book and Paper Group Annual, (6): 85-88. Washington, D.C.: American Institute for Conservation. (unavailable on-line)
  • McCormick-Goodhart Mark H. (1995) “Moisture Content Isolines and the Glass Transition of Photographic Gelatin: their Significance to Cold Storage and Accelerated Aging” Research Techniques in Photographic Conservation. Copenhagen: The Royal Danish Academy of Fine Arts. Pp. 79-88.
  • Messier, Paul (1993) “Cracking in Albumen Photographs: An ESEM Investigation”, Microscopy Research and Technique Wiley-Liss, Inc. [[4]]
  • Norris, Debbie Hess. 1992. “The Conservation Treatment of Deteriorated Photographic Print Materials”, Papers Presented at the Centre for Photographic Conservation’s First International Conference at the Low Wood Conference Centre, Windermere, 6-10 April 1992.
  • Norris D. (1996) “Air-Drying of Water-Soaked Photographic Materials: Observations and Recommendations”, ICOM-CC 11th Triennial Meeting, James & James: London, United Kingdom. Pp. 601-08.
  • Purinton, Nancy; Susan Filter. 1992. “Gore- Tex: An Introduction to the Material and Treatments”, The Book and Paper Group Annual [[5]], vol. 11, p. 141-155, Washington, D.C.: American Institute for Conservation.
  • Robb, Andrew. note to editors: add tip about blotter transfer of OBA in TOPICS?
  • Sugarman, Jane E. and Timothy J. Vitale. 1992. “Observations on the Drying of Paper: Five Drying Methods and the Drying Process.” Journal of American Institute of Conservation 31: 175-197.
  • Thomasset, Annie. 1993 (Spring). “Conservation of a Hand-Colored Albumen Photograph”. Jose Orraca Conservation Studio.
  • Vischi, Christophe and Greg Hill. 2005. “The Study of Two Humidification and Flattening Methods for Albumen Prints to Determine Their Impact on the Evolution of Cracks in the Albumen Layer (Abstract) Topics in Photographic Preservation, Vol. 11. Compiled by Brenda Bernier. Washington: American Institute for Conservation, Photographic Materials Group. p. 95-96.
  • Watkins, Stephanie. 2002. "Practical Considerations for Humidifying and Flattening Paper." The Book and Paper Group Annual, 21: 61-76, Washington, D.C.: American Institute for Conservation.
  • Weidner, Marilyn Kemp. 1986. “A Moisture Chamber/Suction Table Ultrasonic Humidification/Air Filtering System for Use in the Treatment of Art on Paper”, 10th Anniversary Conference of the Institute of Paper Conservation, 14-18 April 1986.


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