PMG Chapters 5 & 6 - Humidification, Drying and Flattening

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This entry is a Draft

Photographic Materials Conservation Catalog
Chapters 5 and 6 - Humidification, Drying, and Flattening of Photographic Material

Previous 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)
Contributors (alphabetical): Gary Albright, Luisa Casella, Lee Ann Daffner, Thomas M. Edmondson, Monique Fischer, Martin Jürgens, Nora W. Kennedy, John McElhone, Paul Messier, Jenelle Norris, Stephanie Watkins

Copyright: ?. The Photographic Materials Conservation Catalog is a publication of the Photographic Materials Group 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 Croup. Publication does not endorse nor recommend any treatments, methods, or techniques described herein.

This outline addresses the considerations when humidifying, drying, and flattening primarily paper-based and plastic-based original photographic materials. The aim is to provide insight along with a listing of materials and techniques utilized in these treatment techniques. This outline does not cover mounting techniques.

TABLE OF CONTENTS:
5.1. Introduction
5.2. Factors to Consider
5.3. Treatment Variations
5.4. Other Methods Specific for Certain Photographic Processes
5.5. Bibliography

5.1 Introduction

5.1 The 2-dimensionality of photographic material

5.1.1 Give examples, mounted and tipped to rigid supports, in album form

5.2 Nomenclature of planar distortion

5.2.1 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.

5.2.2 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)

5.3 Factors causing planar distortion

5.3.1 Environmental factors

When subjected to repeated cycling of humidity, some photographs will start to curl. Unmounted albumen prints are often found tightly curled, parallel to the 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 paper clips, etc. Panoramas and other large unmounted prints (of any process) can be found having been intentionally rolled, usually for ease of storage. The rolling can be either parallel or perpendicular to the grain direction, with the emulsion in or out. (Bernier)

5.3.2 Part of necessary treatment process

5.3.3 The composite structure of a photograph

5.4 Purpose of drying and flattening

5.4.1 Correct planar distortion imparted during treatment

5.4.2 Improve the viewing aesthetic intended by the artist

5.4.3 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)

5.4.4 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)

5.5 Effective humidification methods to facilitate flattening Humidification is most effective for paper-based photographs, not including plastic incorporated into the support (e.g. resin coated supports), cloth, or skins, such as leather. Humidification can be done in combination with aqueous treatment. The structure of the main text…

Factors to consider, typical flattening methods (including tools and equipments), and special consideration for different photographic processes.

5.2. 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.

5.2.1 Inherent nature

5.2.1.1 With mounts

5.2.1.2 Tipped

5.2.1.3 Composite

5.2.1.4 Bound to be cockle

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).

5.2.2 Binder

5.2.2.1 Response to moisture, in relation to its support

5.2.2.1.1 Gelatin

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)

5.2.2.1.2 Albumen

5.2.2.1.3 Collodion

5.2.2.2 Tackiness and softness after moistened

5.2.2.2.1 How to prevent adhering to adjacent materials during drying process

5.2.2.2.2 Depends on the degree of curl: really need to be tacky for the photograph to relax enough to flatten (Daffner).

5.2.3 Surface Texture of photograph

5.2.3.1 Different surface

5.2.3.2 Ferrotyped surface

5.2.4 Blotter texture Machine made blotters have inherent air (luft) and screen sides that are discernable in raking light. 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, but smoothness in blotters can also be achieved during manufacture using a heated roller towards the end of production. (Watkins)

5.2.4.1 Blotter content (ie. dyes, and optical brighteners) (McElhone) Contemporary photographic grade blotters are comprised of 100% cotton linter.

5.2.4.2 Thick blotter, absorbency is very good (Daffner).

5.2.5 Interleaving materials

5.2.5.1 Spun poly(ester) webbing support types include, alphabetically by brand name: Bondina, Cerex, Hollytex (also spelled "Holytex," in previous literature), Reemay

5.2.5.1.1 Bondina is a non-woven 100% polyester 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 (http://www.cerex.com) (McElhone). Hollytex is manufactured with varying "openness" and comes in a lightweight and heavyweight, smooth, continuous sheet (Daffner, Watkins).

5.2.5.1.2 Poly(ester) webbing textures vary from very smooth, continuous to more open webbing. Choose carefully based on the many varieties available. It is advisable to avoid any polyester web 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). Using a fresh, pristine sheet of webbing will reduce potential embossing occurring during the flattening procedure.

5.2.5.2 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).

5.2.6 Pressure to apply

5.2.7 Position of photograph

5.2.7.1 Face up or down?

5.2.7.2 Does the distance to the edge of a pressing stack matter?

5.2.8 Ambient Humidity

5.2.9 Blotter Changes The frequency of blotter changes initially and over the course of the drying period will depend on the treatment itself as well as the philosophy of the individual conservator. If a photograph has been immersed, conservators will sometimes blot it on both sides, then place it between polyester web, blotters, and glass or acrylic sheets. Sometimes the blotters will then be changed frequently at the start, with ever increasing time increments between changes. 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! The inclusion of corrugated boards in drying stacks (between the blotters-polyester web-photograph-polyester web-blotters groupings) is sometimes seen as a way that air can circulate through the stack, reducing the need for blotter changes. See below for more detailed descriptions of these as well as alternative treatment methods such as the use of desiccated blotters (Kennedy).

5.3. Treatment Variations

5.3.1 Copy Press (Book Press)

5.3.1.1 Basic configuration (illustration)

5.3.1.2 Variations

5.3.1.3 Pros

5.3.1.4 Cons

5.3.1.5 Other tips

5.3.2 Blotter Stack for Pressing and Drying

5.3.2.1 Basic configuration (illustration)

5.3.2.2 Variation

5.3.2.3 Pros

5.3.2.4 Cons

5.3.2.5 Other tips: use dessicated blotters to speed up the drying time. Dessicate 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).

5.3.3 Blotter Stack with Inserted Corrugated Boards

5.3.3.1 Basic configuration: prepare a stack of sets of blotters and corrugated boards. Corrugated boards serve as air channel to dissipate moisture in the photographs. Don’t need too many blotters in between, defeats the purpose of having air channel. Can have more than one blotter in each set. Do not need a lot of pressure, but even pressure is very important (Edmondson). The configuration is illustrated below: [The diagram in original text goes here. Stay tuned or create a new one.]

5.3.3.2 Pros: The use of corrugated board that allow more air circulation through the cores, helps to speed up the drying time.

5.3.3.3 Cons: The use of corrugated board limits the amount of pressure possible to exert as the board can impart a corrugated impression into the photographs. The minimal pressure could be beneficial for gelatin emulsions (Brown).

5.3.4 Alternative Blotter Stack with added air flow—vacuum pulling the air through stack

5.3.4.1 Basic configuration (illustration)

5.3.4.2 Variation

5.3.4.3 Pros

5.3.4.4 Cons

5.3.4.5 Tips

5.3.5 Dry Mount Press Without Heat

5.3.5.1 Basic configuration (illustration)

5.3.5.2 Variation

5.3.5.3 Pros: The press has uniform pressure; the pressure can be adjusted through how tight the clamp is and kept consistent. If lighter pressure is desired, the press platen can function as a heavy, uniform weight if it is not clamped at all.

5.3.5.4 Cons: Rippling at the edges of the photograph may result from the insufficient drying required of this method (Albright).

5.3.5.5 Other tips:

5.3.6 Pressure With Heat

5.3.6.1 Basic configuration (illustration)

5.3.6.2 Variation 1: Moderate Heat and Short Time Use moderate heat (recommended as 170 to 200 F) and short time (approximately 45 seconds) (Hoffman).

Steps:

  1. A silicone release paper is necessary to face the gelatin emulsion. Any cockling or wrinkles in the silicone release paper will transfer to the photograph being flattened. Always use clean and smooth release paper. (Fischer)
  2. Use moderate heat (recommended as 170 – 200oF) and short time (approximately 45 seconds) (Hoffman).
  3. rotate the photograph a dry spot on the silicone release paper several times
  4. 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 complete cooled down.
  5. 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.
  6. Resin-coated photograph can not subject to heat treatment
  7. With differing opinion - 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 200F, and preferably not higher than 180F. Heat is often the only practical method of removing planar distortions from RC photographs. (Weaver)
  8. Pros: can be very effective to make a gelatin print flat
  9. Cons: can accelerate silver image degradation; long term effect of heat on photograph is unclear. It has been observed that silver gelatin B&W photographs around 1970 could experience image color shift in this condition (Messier). Photographs may need to be pressed twice with heat to avoid cockling of support and rippling of edges (Fischer).
  10. Should indicate that 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)
  11. Changes such as increased mirroring/silverng can occur with matte gelatin photographs using the above technique (Fischer).

5.3.6.3 Variation 2: Moderate Heat and Long Time

Moist photographs are not suitable for the method.

Steps:

  1. Lower heat: let the press heated no more than 170oF and turn off
  2. Put the photograph sandwiched with silicone release paper or smooth paper/blotter and left the package in the heated press until the press cool down to room temperature
  3. Remove the photograph from heat press and press longer in regular pressing stack.

5.3.7 Stretched Method

5.3.7.1 In combination of lining (see Chapter of MOUNTING for details)

5.3.7.2 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 strip and glass strips around the edges. (Kennedy)

5.3.7.2.1 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.

5.3.7.2.2 Cons: One has to be aware of ambient relative humidity. 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 an in extreme cases may tear!! (Kennedy)

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

5.3.8.1. Basic Configuration: Felt - blotter - polyester web - photograph - polyester web - blotter - felt (Casella).

5.3.8.2. Pros: Low pressure on the image layer (Casella).

5.3.8.3. Cons: Cushioning and low pressure may not result in complete flattening as with other methods (Casella).

5.4. Other Methods Specific for Certain Photographic Processes

5.4.1 Photograph of One Layer Structure

5.4.2 Albumen

5.4.3 Collodion POP/ Matte Collodion

5.4.4 Gelatin POP/ DOP

5.4.5 Coated Photograph

5.4.6 Mounted photographs (it is possible to do it, but mount needs to be thinner, not card board type)

5.4.7 Carbon/ Woodburytype

5.4.8 Waxed Paper Negative

5.4.9 Resin-coated 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 170F may be required, but be careful above 190F as damage to the RC support can be sustained. (Weaver)

5.4.10 Film-based negatives

6.5. Bibliography

  • 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.
  • 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. Find article at: http://albumen.stanford.edu/library/c20/hendriks1.html
  • 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 Orgznization 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.
  • 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-Goodheart 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”
  • 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.
  • Thomasset, Annie. 1993 (Spring). “Conservation of a Hand-Colored Albumen Photograph”. Jose Orraca Conservation Studio.
  • Vischi, Christophe and Greg Hill “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. Pp. 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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