Talk:Drying and Flattening Paper

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This page is a draft version for an updated Drying and Flattening Paper page. When the main page is updated, it will revert to a talk or discussion page for the topic. As of July 2019 those suggestions, which includes those offered by the BPG Publications Committee, PCC Task Force in 2007-2009 were as follows: Update priority: Medium. There are headings and sections with no text explaining the concept. There were only two contributors. Describe drying flat paper on a Dacron web or Plexiglas sheet.

The updated page is below:


Original Compilers: T. J. Vitale, Doris Hamburg
For a full list of the original contributors to this page, see the section below on History of this Chapter.
Wiki Compilers: Susie Cobbledick, Carolyn Frisa, Kesha Talbert
Wiki Contributors: Theresa J. Smith, Bill Minter your name could be here

Copyright 2019. The Book and Paper Group Wiki is a publication of the Book and Paper Group of the American Institute for Conservation. It is published as a convenience for the members of the Book and Paper Group. Publication does not endorse nor recommend any treatments, methods, or techniques described herein. There is an ongoing project to update the BPG Wiki. We welcome contributions and feedback. If you would like to get involved in this effort, please contact the wiki team at [email protected].


Drying and flattening are common processes in paper conservation typically carried out simultaneously after washing or other aqueous treatments. Most drying techniques are also flattening techniques. Flattening, however, can constitute a treatment in its own right undertaken outside of the context of aqueous treatment, though it often involves the incorporation of moisture.

Definitions

  • Drying—To bring wet or humid paper into equilibrium with ambient relative humidity (RH). This is a process that will occur naturally without human intervention, but in a conservation context it is important that this process be controlled in order to achieve specific treatment goals. Dry paper still retains water. At RH of 20% to 70%, water content will be about 4% to 8% (Banik and Brückle 2011, 91).
  • Flattening—Removing or preventing the formation of planar distortions in paper with the goal of maintaining or recreating as closely as possible the paper’s original dimensionality. Unlike drying, this is not a naturally-occurring process and requires intervention, though it does not always require water.
  • Humidification—The exposure of paper to water vapor. This will plasticize the paper by breaking bonds within fibers, but will not result in the breakage of fiber to fiber bonds (Sugarman and Vitale 1992).
  • Wetting—The introduction of liquid to paper. Methods can include misting, floating, blotting and immersion.

Common forms of planar distortion caused by water

  • Curling—A rolling distortion that affects the entire sheet. It is caused by the physical differences between the two sides of the sheet built in during the manufacturing process. It can also be caused by moistening only one side of a sheet.
  • Cockling—Distortions that affect sections of a sheet that has been moistened and subsequently air-dried without restraint. Sugarman and Vitale (1992) define this section as being larger than 1 cm square and smaller than the whole sheet. It is caused by differences in evaporation rates across the sheet due to uneven distribution of fiber (Banik and Brückle 2011, 400) and/or an uneven distribution of water (Sugarman and Vitale 1992).
  • Roughness—An overall increase in surface texture and loss of smoothness after a sheet has been wetted. It is caused by fiber swelling, uneven distribution of fiber and/or the release of dried in strain. The use of humidification instead of wetting helps to conserve calendared and burnished surfaces, as does the use of restrain drying.

When are these processes used?

Anytime paper is wetted or humidified, it must be dried in a deliberately chosen, controlled way that will prevent the emergence of out-of-plane distortions.

Paper will need to be dried and flattened after:

  • Aqueous treatments such as washing and deacidification.
  • Humidification for relaxing paper and removing distortions.
  • Mending with starch paste.
  • Local application of gels or water vapor to clean or remove adhesive residues.

Paper may require flattening with or without the use of water when:

  • It has been improperly stored i.e. paper rolled in a tube or a book allowed to sag on a shelf
  • It has been improperly handled i.e. folded or crushed
  • A previous treatment was not well-executed i.e. paste and tissue mends not dried under weight.

When are these processes not used?

plate mark
normal undulation in hand-press era book paper

Paper must always be carefully dried after it has been wetted and humidified, but the extent to which it should subsequently be flattened is an important consideration. It is also important to decide what constitutes distortion in a paper leaf.

Perhaps it is best not to think of paper as being flat; rather think of it as an object with three dimensions. Hand-made papers especially have an intrinsic undulating character that should be respected. There are other kinds of out-of-plane evidence that should be respected and retained. These forms of dimensionality are not distortions in need of correction, and every attempt should be made to protect them in the course of wet or humid treatments. They include:

  • Texture whether smooth, rough or in between
  • Plate marks around intaglio prints
  • Type impressions
  • Stretching in the center of leaves in books
  • Natural waviness of loft-dried sheets

There are other situations that necessitate forbearance in the matter of flattening:

  • Possible damage to media on the paper substrate
  • Possible damage to the substrate itself

Factors to consider when choosing drying/flattening method

Paper that is wetted or humidified is changed in irreversible ways. The methods chosen to subsequently dry and flatten the paper will also alter the paper. Choose processes deliberately and with care keeping in mind some or all of the following factors:

Materials of manufacture

Noncellulosic components—The lignin present in mechanical wood pulp, rosin size and mineral fillers all interfere with fiber bonding and retard shrinkage (Banik and Bruckle 2011, 190 and 193). Less restraint may be needed when drying to maintain close to original dimensions.

Method of paper formation

Machine—Machine-made papers have 2 characteristics that affect the way they respond to water. 1) They have a definite grain direction. Most of their fibers are oriented parallel to the belt on which the paper was formed, and 2) they have dried-in strains as a result of the tensions placed on the sheet in the machine during manufacture. When exposed to water, these papers will expand more across than along the grain. If dried without restraint, they will naturally cockle (Smith 1950). If dried with restraint, some of the cross-grain expansion may be retained.

Hand—These papers have grain direction but it is less pronounced than that of machine-made papers. At the time of their manufacture, they are dried under significantly less tension than machine-made paper and have significantly less dried-in strain. They will expand almost evenly in both directions when exposed to water. These sheets can usually be dried without much restraint.

Beating—The more heavily fibers in a paper had been beaten, the more that paper will expand when exposed to water and subsequently shrink when dried without restraint (Munoz-Vinas 2009). Western, machine-made papers tend to be more heavily-beaten than hand-made papers and Asian papers.

Weight of the paper

The heavier the paper, the less it will cockle when exposed to water (Kajanto and Niskanen 1998). Heavy papers will need less restraint when drying.

Paper history

Conditions of storage—Papers that are subjected to repeated temperature and humidity fluctuations lose their ability to absorb water, probably as a result of an increase in hydrogen bonding and density (Knop 2007). These papers will likely expand and shrink less.

Age—As papers age they tend to lose dried-in strains and become less likely to shrink when exposed to water (Munoz-Vinas 2009).

Previous treatment—As with aging and storage in fluctuating environmental conditions, previous aqueous treatment can leave paper more dense, less absorbent and less likely to have retained dried-in strains.

How much water needs to be removed

Some drying and flattening techniques are obviously more appropriate for wet paper than for humid paper. Wet paper is more plastic and can change significantly in dimension and surface finish in the course of drying. In the case of wet paper, especially, decisions need to be made about desired outcomes and how to achieve them with appropriate processes.

Strength or weakness of paper

Thin and/or brittle paper can be easily damaged by some drying and flattening processes. Lateral restraint techniques are not appropriate for brittle papers, for example, while thin papers respond well to friction drying and the use of soft materials such as fleece.

Subsequent use: Book, framed print, hanging scroll, etc

Aqueous treatment and subsequent drying and flattening techniques can change paper in a number of ways. Decide which kinds of changes are acceptable and which are not depending on the paper’s subsequent use. Dimensional change may need to be minimized if the paper is to be bound back into original book boards, for example, or texture may be need to be retained as an important element in the aesthetic qualities of a print.

Texture and surface finish of the paper

All aqueous processes will cause paper surfaces to roughen, but the effect is especially strong in wet treatments and in paper containing significant amounts of lignin, such as mechanical pulp paper (Forseth and Helle 1997). Roughening can be minimized by using humidification or dry processes to flatten, but if the paper has to be wetted out, there are some drying and flattening techniques that minimize changes in surface texture, including wet restraint and the use of smooth materials such as mylar and Gore-tex against the paper surface.

Media on paper

The stability of media must be considered when choosing a drying and flattening method. Media softened by aqueous treatment should not be placed under weights, while brittle media may not respond well to processes that result in changes of paper dimension.

Equipment

Tools to apply restraint

The amount of weight put on humid or wet paper, especially wet paper, does make a difference in outcome. The greater the weight, the more likely wet expansion will be dried in; the lesser the weight, the more likely shrinkage will take place (Munoz-Vinas 2009). Weight can be applied by:

  • Hand presses—Many conservation facilities have presses of various kinds, such as standing and nipping presses. They can be used to dry paper under restraint while saving table space. The pressure applied by hand press platens can be significant and adjustable, but it is hard to know exactly how much pressure is being applied.
  • Glass or other stiff material such as wood or thick Plexiglas, with or without added weight on top--These arrangements take up more table space. The weight that can be applied is limited, but at least it can be known.
  • Pneumatic press—Expensive, but allows for a greater range of known weights to be applied.
  • Weights--These can be added on top of glass, wood or Plexiglas or used on their own in lateral restraint drying. Weights come in many forms, including lead shot in Nalgene bottles, old irons, bricks, etc. Weights that are not themselves made of clean, inert materials should be contained in clean, inert materials, such as Tyvek or polypropylene bottles. Look out for crumbs and rust. Iron objects may need to be cleaned and sealed before use.

Rigid supports

Paper being air dried needs a flat support, preferably one that allows air to circulate above and below the sheet. For free water evaporation or no restraint drying, drying racks and screens offer support, air circulation and a microclimate that allows evaporation and drying to proceed slowly and evenly.

  • Drying racks—These can be purchased from conservation, art or baking suppliers. Conservation drying racks have moveable polyester screens that can slide out of a support structure. Art drying racks designed for use in print shops have metal wire shelves hinged at the back. They can’t be removed, but they can be flipped open. Bakery racks can be used, but they are designed to hold baking sheets, not screens. You will need to fabricate/purchase appropriate screens. Custom made drying racks are also an option.
drying rack made of stacked screens
  • Screens—aluminum or wooden frames stretched with polyester can be stacked to form an enclosed drying rack or used on their own to support fragile wet paper. Screens can be purchased from silkscreen suppliers.

Flexible supports

Reemay
Hollytex
Bondina
It is usually best to leave papers on polyester, non-woven, permeable supports throughout treatment. These polyester non-woven materials include:


  • Reemay—The option with the most surface texture. Provides a good grip on wet paper in the washing process.
  • Hollytex—Smoother than Reemay. Made using a calendaring process. Use if there are any concerns about texture transfer to soft media or paper
  • Bondina—Smoother than Hollytex with a more ordered and dense fiber arrangement. Another option if texture transfer is a concern.

Absorbent materials

Absorbent materials are often used in drying and flattening to accelerate the removal of water from paper. These materials include:

  • Blotter—Unsized, thick, lightly-beaten papers made of cotton or chemical pulp. The 100% cotton is preferred. Blotters become stained and warped with use and must eventually be replaced. Comes in different weights. Best used in combination with some cushioning material, such as foam or felt, since the stiff blotter sheet can’t conform well to a paper’s variable thicknesses.
  • Felt, wool and synthetic—Absorbent, durable, washable and reusable. Felt is available in a variety of densities and thicknesses. Softer, less dense felt has the ability to conform to the variable thicknesses found within every paper sheet, even machine-made sheets. Softer felts can also minimize damage to plate marks, type impressions and other dimensional evidence while still being part of a light restraint drying system. Bear in mind that wool contains sulfur and can adversely affect some pigments.
  • Tek-Wipe—a non-woven material made of polyester and cellulose. Washable and reusable.

Drying and flattening methods

Keep the following in mind before you start a drying and/or flattening process:

  • Every time a paper sheet is exposed to water, vapor or liquid, it changes permanently as bonds shift. The original size, density, strength and surface texture are all lost. Drying and flattening processes can be chosen to minimize unwanted changes but cannot prevent them.
  • The fewer times a sheet is wetted/ humidified and then dried, the better.
  • It is generally better to wet/humidify whole sheets than parts of sheets because the fibers in the interface between dry and wet/humid areas can be damaged.
  • Drying should ideally be done slowly and evenly.
  • Most drying processes should be started after the free water gloss is off of your object, and it has a matt surface (Sugarman and Vitale 1992). Free water can be removed by:
    • Evaporation
    • Blotting
    • Draining at an angle
  • The amount of time required for each process varies and is difficult to predict, but when in doubt, give paper a generous amount of time to dry. As a rule, the more restraint applied, the longer the drying/flattening will take. Machine-made papers will need more time than hand-made papers. Restraint drying machine-made papers can take weeks.

Unrestrained

Air drying paper in the absence of weights, lateral restraint or absorbent material. To minimize out-of-plane distortions, unrestrained drying should be done slowly and evenly.

Microclimate

Using a drying rack, a stack of taut screens or a cover over a taut screen can help create a microclimate conducive to slow, even drying.

  • Pros—A good drying method for older, hand-made papers that tend not to shrink, cockle or curl much when wetted or humidified. It allows these papers to maintain something close to their original dimensions, as well as plate marks, type impressions, undulations, etc.
  • Cons—Not a good drying method for newer, machine-made papers due to cockling and shrinkage. According to some sources, all papers lose strength when air-dried, while their textures change and become more pronounced (Vitale 1992, Sugarman and Vitale 1992).

Face-to-face taut screens

face to face screens

Wet or humid paper in a polyester sandwich can be placed on a screen, and then a second screen can be placed face down on top of the paper but with shims in place to keep the second screen from actually touching the paper. The paper is not restrained, but the second screen will prevent significant planar distortions from forming.

  • Pros—Also good for keeping the out-of-plane evidence in older, hand-made sheets. Minimizes any distortions that may arise. Makes it more workable to air dry machine-made papers and subject them to subsequent flattening since there are fewer significant distortions.
  • Cons—Unless subsequently air-dried, the same problems as the above.

Subsequent treatment

After paper has been air dried, it can be subsequently flattened in 2 ways. 1) It can be humidified and then dried under weight, or 2) it can be placed under weight without humidification. The latter method will take significantly longer than the former, but dry paper can be flattened under weight if given enough time (think of weeks or even months).

  • Pros—According to Sugarman and Vitale (1992), the air dry/humidify/dry under weight method is as good as early wet restraint in maintaining the sheet’s original texture. Allowing papers to air dry before flattening may also allow them to maintain something close to their original dimensions as the sheets are humid, not wet, when dried under weight.
  • Cons—Unlike the wet restraint method, the air dry/humidify/dry under weight method does not help maintain the sheet’s strength (Vitale 1992).

Semirestrained

Light weight

  • Equipment
    • Soft felts, wool or synthetic
    • Flat, dimensionally stable surface or stiff but absorbent material such as honeycomb board

Light weight can be used with humid or wet paper. The paper in its polyester sandwich is placed on a flat, dimensionally stable surface, such as a clean table top, silicone mat or taut screen, which has the advantage of allowing evaporation from the bottom of the sandwich. Absorbent surfaces (honeycomb board with or without blotter or mat board) can also be used as long as they remain dimensionally stable. A thick but soft felt is laid over the paper. No other weight is needed. Several layers of thinner felt can also be used. Leave the stack over night and remove the felt in the morning. The felt should not need changing as it can evaporate out much of the water it is wicking. It is important that the felt be soft so that it can keep contact with the paper if/when it goes slightly out of plane.

  • Pros--Prevents significant planar distortions while preventing over flattening. Papers retain plate marks, type impressions, etc. Another good option for hand-made papers in addition to air-drying if a light restraint seems safer. Could also be an option in some cases of fragile media.
  • Cons--Has some of the same drawbacks as air drying. Not a good option for newer, machine-made papers, which will be left with planar distortions and probable shrinkage. Surface texture will roughen. According to Vitale, paper dried under light weight, like air-dried paper, will lose strength (Vitale 1992).
  • Variation--An additional light weight such as a taut screen turned over on its face can also be added to the stack.

Karibari style

  • Equipment
    • Karibari board
    • Asian style paper
    • Smoothing brush such as nadebake

A karibari board is a tool used in traditional Japanese paper mounting. It consists of a wooden lattice covered by several layers of paper forming a surface that both breathes and shifts slightly. It is ideally suited for the treatment of Asian papers, especially for lining (Kato and Kimishima 2017). If it is to be used for drying and flattening without lining, then a false liner is used. The paper to be treated needs to be wet rather than humid. It is laid face down on a protective surface, such as a non-woven polyester web. If it is not already wet, it may be humidified and then misted with water. A larger, medium weight sheet of moist Asian style paper is laid on the back of the paper to be treated and the two sheets are brushed or pressed into close contact, the water holding them together. A brush, such as a nadebake, can be used but so can a bone folder, though in that case place a polyester web over the papers before applying the folder. Make sure that the false liner extends about an inch beyond all four edges of the paper being treated, and it is usually best that the grain directions of the two sheets match. After the false liner is in place, turn the two sheets over and apply paste to the edges of the false liner. Take the two sheets to the karibari board and use a brush or bone folder to adhere the package to the board surface, the face of the paper being treated against the board. Place the board upright against a support (ie wall or table) to save space and allow for air flow across the back. The papers will dry quite quickly (in less than a day), but in Japan, papers could be left on the board for extended periods of time in order to pass through seasonal cycles of high and low humidity.

  • Pros--The traditional karibari board allows for evaporation from both sides and gives slightly as the papers dry, so it is a rather gentle form of restraint. Well-suited to Asian style papers. Ideally suited to Japan's climate.
  • Cons--Fast drying could damage weak papers, especially short-fibered acidic Western style papers. Does not provide enough restraint to prevent out of plain distortions in Western machine-made papers.
  • Variation--The process of drying can be slowed down by adding layers of moist paper over the false liner. Other kinds of boards, such as wood, glass or Plexiglas can also be used though they are not as gentle as the traditional karibari.

Restrained

To restrain or not to restrain

The extent to which paper is restrained when drying can affect planarity, texture, strength and dimension. As a general rule, consider drying machine-made paper with more restraint than hand-made paper. Restraint can be created by either placing weight on a sheet or securing its edges.

  • Restraint
    • Pros—Prevents planar distortions and the loss of strength that results if the sheet is dried without restraint. Evidence for the loss of strength in sheets dried without restraint can be found in Vitale (1992) as well as MacKay and Smith (1994). In contrast, Smith (1997) found little change in strength when sheets were dried under light restraint. Some sources suggest that drying under pressure increases interfiber bonding (Banik and Brückle 2011, 405) and helps to retain the paper’s original texture (Sugarman and Vitale 1992). Significant pressure can minimize changes to even calendared or burnished surfaces and prevent vertical expansion.
    • Cons—Can remove important evidence of paper manufacture and history by obscuring plate marks, type impressions, paper dimensionality, etc. Can also result in dried-in wet expansion, leaving the sheet with larger dimensions than it had before treatment.
  • Little/no restraint
    • Pros—Preserves the natural undulations of hand-made sheets and other out-of-plane evidence. Older and hand-made papers dry to something close to their original dimensions, though often with small amounts of shrinkage.
    • Cons—Some loss of paper strength, surface roughening and planar distortions. Significant shrinkage in in new heavily-beaten, machine-made papers, especially in the machine direction. (Uesaka, Moss and Nanri 1992).


"Standard" method

  • Equipment
    • Flat, dimensionally stable surface
    • Absorbent material, such as blotter, felt or Tek-wipe
    • Heavy glass, Plexiglas, wooden platen or press
    • Weights if not using press

This commonly-used method is described by Vitale (1992), Sugarman and Vitale (1992) and Watkins (2002). It can be used for both wet and humidified papers. If the paper has been wetted, then free water should first be blotted, drained or allowed to evaporate off the paper before it is restrained. Once free water is gone, the paper should have a mat but still wet appearance. If the paper is allowed to dry past this point, distortion and bonding will have already started to take place, resulting in the more pronounced loss of original surface texture (Sugarman and Vitale 1992).

The humid or wet paper is placed between absorbent layers in the following sandwich described from bottom to top: flat surface/absorbent material/ polyester web/paper/polyester web/absorbent material/platen/weights. The flat surface can be a table top, or the same material used on the top of the stack as a platen, such as thick glass, Plexiglas or wood. If blotters are used, then they should be backed with a softer material such as felt, capillary matting, Tek-wipes or foam in order to conform to the paper's varying thickness. The top blotter, at least, should have one of these soft materials behind it while the blotter is in contact with the polyester web. A press, such as a nipping press or standing press, can be used instead of a platen and weights.

The absorbent material should be changed out with dry material about 4 times at increasingly long intervals, such as ten minutes, thirty minutes, two hours, etc. The amount of weight placed on the stack should vary with the absorbent material used and the nature of the paper. Heavy weight (ie 200 g/cm2) combined with blotters will result in flatter paper and dried-in wet expansion. This combination may not be appropriate for hand-made paper or paper with plate marks, type impressions or other dimensional evidence. These papers may best be dried under lighter weight using softer absorbent materials, such as felts. The amount of time that papers need to spend in the stack can vary, but when in doubt be generous with time. Hand-made papers may be stable after drying overnight. Machine-made papers may need several days. It is often best to remove machine-made papers from the drying stack after several days, take them out of their polyester web supports and then place them in a press for several weeks.

  • Pros--Well-suited to machine-made papers as it mimics some the pressures they were subject to during manufacture. Some research indicates that drying under weight prevents the loss of paper strength and may increase interfiber bonding (Vitale 1992, Banik and Bruckle 2011). Prevents out-of-plane distortions, especially in machine-made papers with dried-in stresses. Because weight is evenly-distributed over the sheet, localized stresses can't result in ripping, as may happen in lateral restraint, making this method appropriate for more brittle papers.
  • Cons--Not good for paper holding fragile media. Heavy weight may result in the loss of a paper's dimensional evidence, such as plate marks and type impressions. Not a good or necessary choice for loft-dried, hand-made papers possessing natural undulations. Can result in dried-in wet expansion, especially in newer, heavily-beaten machine-made papers.

Forced air

  • Equipment
    • fan
    • corrugated boards, preferably 2 or 3 ply, preferably archival
    • Flat, dimensionally stable surface
    • Absorbent material, such as blotter, felt or Tek-wipe
    • Heavy glass, Plexiglas, wooden platen or press
    • Weights if not using press

This drying process mimics the methods used in hand paper mills. It has been described by Minter (2002) and is especially useful when processing many sheets at once.

Follow the "Standard" method until the drying sandwich needs to be made. Place corrugated boards between each sheet: flat surface/corrugated board/absorbent material/ polyester web/paper/polyester web/absorbent material/corrugated board/absorbent material/ polyester web/paper/polyester web/absorbent material/corrugated board/repeat/platen/weights. The stack can be made as tall as needed. Make sure the flutes in the corrugated board all face the same direction. When the stack is finished, set a fan to blow through the flutes. The stack can be placed on a table under weights, or it can be placed in a large press, which is a great way to save space.

forced air set-up using a standing press

Run the fan all day or overnight. After turning off the fan, the papers can be checked for dryness. They may need to be left in the stack for another day or 2, with or without the fan. Machine-made papers may need several days. It is often best to remove machine-made papers from the drying stack after several days, take them out of their polyester web supports and then place them in a press for several weeks.

  • Pros--Alleviates the need to change out absorbent materials. Saves space and time by making it easy to process large numbers of sheets in tall drying stacks. As with the "Standard" method, this drying process works best for machine-made papers.
  • Cons--Can't be precise as to the amount of pressure each sheet is under. Sheets at the bottom of the stack are under slightly more pressure than those near the top. Not a good choice for hand-made papers.
  • Variation--A stack with corrugated boards can be used without the fan if the stack is not too high or the sheets inside not too moist.

Friction drying

  • Equipment
    • Asian-style paper
    • Smoothing brush such as nadebake
    • Flat, dimensionally stable surface
    • Absorbent material, such as blotter, felt or Tek-wipe
    • Heavy glass, Plexiglas, wooden platen or press
    • Weights if not using press

This method can be used to dry and flatten both wet and humid papers. Begin by choosing a medium weight, Asian-style paper that will expand and contract in the same way as the paper being treated. If you are treating another Asian-style paper, try to match the fiber ie gampi or kozo (Fletcher and Walsh 1979). Cut two pieces of this paper about four inches larger than the paper being treated. These will be the friction sheets. Wet out one friction sheet and place on a flat, dimensionally stable surface, such as a clean table top or silicone mat. Wet out the paper being treated and place face down in the middle of the friction sheet, using a brush such as a nadebake to make good contact. Wet out the other friction sheet and place on top of the paper being treated. Use the brush again to make good contact. It is generally a good idea to match grains in all the sheets. Wetting out the papers can be done with a sprayer in place (Keyes 1984) or by dipping the sheets in water and bringing them out on a piece of mylar or polyester web. The wet three-sheet sandwich can then be dried under pressure using the "standard" method or the forced air method.

For humid papers, the method needs to be slightly modified. Since no liquid water is present, smoothing the sheets together with a brush is not necessary, but extra pressure needs to be placed on the sandwich as it dries, making a press a better option than a platen with weights (Keyes 1984).

  • Pros--Can effectively flatten thin, fragile, damaged or creased papers. A good method for thin Asian-style papers.
  • Cons--The friction paper may impart texture to the sheet being treated, so this process may not be appropriate for calendared papers (Neufeld 2014).

Mylar/Teflon/Gore-Tex

  • Equipment
    • Mylar
    • Gore-Tex or substitutes like Sympatex or Hydra Air
    • Tefton or other smooth nonstick material such as silicone or Parafilm M
    • Flat, dimensionally stable surface
    • Absorbent material, such as blotter, felt or Tek-wipe
    • Heavy glass, Plexiglas, wooden platen or press
    • Weights if not using press

Aqueous treatment and to a lesser extent humidification tend to roughen paper surfaces, and while this effect can be minimized through early wet restraint drying (see "Standard" method), it can not be eliminated by these means. If a calendared surface is an important part of a paper's character, then special efforts need to be made to retain it in the course of drying, and this can be done by placing a smooth material against the paper in the drying stack in a variation of friction drying. This method works best with wet papers, but can also be used with humidified papers.

Mylar--This material can be used to bring the wet paper face down out of a water bath and can then be left in place throughout the drying process. Paper can also be wetted out with a sprayer while lying face down on the Mylar, using brush such as a nadebake to make good contact. Let the Mylar/paper drain at an angle until free water is gone and then place in a drying sandwich: flat surface/absorbent material/ Mylar/paper/polyester web/absorbent material/platen/weights (Nicholson 1988). The sandwich can then be dried under pressure using the "standard" method or the forced air method.

Teflon, silicon or Parafilm M--Use the same method as with Mylar. These non-stick materials are best used if there are any concerns about adhesive residues.

Gore-Tex and substitutes--Support the paper on a polyester web. It can be wetted out in place with a sprayer or pulled out of a water bath. Allow the free water to evaporate or drain off as described in the "Standard" method. Place in a drying sandwich: flat surface/absorbent material/ Gore-Tex/paper/polyester web/absorbent material/platen/weights. The sandwich can then be dried under pressure using the "standard" method or the forced air method. As a variation, Gore-Tex can be used on both sides of the paper. It can also be cut to size to fit inside plate marks (Dwan 1992). Gore-Tex and its substitutes have the advantage of allowing evaporation from both sides of the paper.

  • Pros--Retains calendared surfaces on papers. Mylar/Gore-tex/Teflon/silicone/Parafilm act as supports for thin and fragile papers
  • Cons--If using Mylar, Teflon, silicone or Parafilm M, evaporation is only on one side. This could cause curling after drying.

Hard/soft sandwich

  • Equipment
    • polypropylene fleece
    • Flat, dimensionally stable surface or stiff but absorbent material such as honeycomb board
    • Heavy glass, Plexiglas, or wooden platen
    • Weights

Fragile, thin, badly creased and transparent papers may need special, gentle handling. For these sensitive papers, pressing under layers of soft polypropylene may be safer and more effective than blotters or dense felts. These papers need to dry under very uniform pressure, and only a soft material can provide it; blotter and dense felt both have surfaces with enough texture to prevent the pressure from being as uniform as that of layered polypropylene. Homburger and Korbel (1999) first developed this hard/soft sandwich to flatten architectural drawings.

The hard/soft sandwich is best used to dry and flatten humidified, not wet, paper. The humid paper is placed in the following sandwich described from bottom to top: flat surface/polyester web/paper/polypropylene fleece/platen/weights. The flat surface can be a clean table top or silicone mat. Absorbent surfaces (honeycomb board with or mat board) can also be used as long as they are smooth and remain dimensionally stable. When the first or only layer of fleece is put on the paper, a brush, such as a nadebake, or a bone folder should be used to smooth the fleece unto the paper. The more distorted the paper is, the more layers of polypropylene fleece will be needed, up to about four layers. Leave the sandwich to dry overnight and repeat if necessary.

  • Pros--Good for problematic papers liable to damage or distortion when air-dried or dried under blotters or dense felts. These problematic papers include fragile, thin, badly creased and transparent papers.
  • Cons--Doesn't work well with wet papers. Method provides no benefit for papers other than the sensitive ones described above.
  • Variation--soft felts can be used for a comparable effect.

Lateral-restraint

  • Equipment
    • Flat, dimensionally stable surface or stiff but absorbent material such as honeycomb board
    • non-woven polyester
    • strips of blotter
    • pieces of glass or other stiff material such as book board
    • weights.

This technique is especially useful for thin papers or papers that hold media that cannot be pressed. It is more effective for wet than humid paper. The paper on a non-woven polyester support is placed on a flat, dimensionally stable surface, such as a clean table top or silicone mat. Absorbent surfaces (honeycomb board with or without blotter or mat board) can also be used as long as they remain dimensionally stable. The edges of the paper(about .5-1 cm) are covered by strips of blotter, which are in turn covered by strips of book board or glass held down by weights. As the paper dries, it will become taut and flat.

  • Pros--Good for thin sheets and paper that holds fragile media.
  • Cons--Paper may be left with larger dimensions, and brittle paper may rip as uneven drying stress the sheet. Not good for paper whose edges are weak.
  • Variation--Place a cover over the paper as it dries to slow the process down and make drying more even.

Nonaqueous flattening

Sometimes water needs to be introduced in order to flatten paper. In less extreme cases, water speeds up the flattening process and usually makes it more effective. There are times, however, when paper needs to be flattened but the introduction of water is not an option such as when:

  • Paper holds iron gall ink.
  • Paper holds water-sensitive media.
  • The texture of the paper is considered an important feature.
  • The dimensions of the paper cannot change


In these cases, paper can be placed under weights or in a press and, given enough time, will eventually flatten out. This process can take weeks or months.

The sheets should be placed between stiff, protective, inert materials such as book board. Add a polyester web barrier around the sheet if there are any concerns about sticking. Be conscious of the pressure or weight applied and add soft materials such as felt or polypropylene fleece if there are dimensional features that need to be saved.

Bibliography

Banik, Gerhard, and Irene Bruckle. Paper and Water: A Guide for Conservators. New York: Routledge, 2011.
Dwan, A. "Use of Goretex to Dry Smooth, Calendered, and Modern Papers." Book and Paper Group Annual 11 (1992): 22-23.
Fletcher, S, and J Walsh. "The Treatment of Three Prints by Whistler on Fine Japanese Tissue." Journal of the American Institute for Conservation 18 (1979): 118-126.
Forseth, T, and T Helle. "Effect of Moistening on Cross-Sectional Details of Calendered Paper Containing Mechanical Pulp." Journal of Pulp and Paper Science 23, no. 3 (March 1997): J95-102.
Homburger, Hildegard and Barbara Korbel. "Architectural Drawings on Transparent Paper: Modifications of Conservation Treatments." Book and Paper Group Annual 18 (1999):
Htun, Myat. "The Control of Mechanical Properties by Drying Restraints." In Paper: Structure and Properties, edited by J Anthony Bristow and Petter Kolseth, 311-326. New York: Marcel Dekker, 1986.
Kajanto, I, and K Niskanen. "Dimensional Stability." In Paper Physics: Papermaking Science and Technology Book 14, edited by K Niskanen, 223-259. Helsinki: Fapet Oy, Finnish Paper Engineers' Association and TAPPI.
Kato, Masato, and Takayuki Kimishima. "Karibari: The Japanese drying technique." In Adapt & Evolve 2015: East Asian Materials and Techniques in Western Conservation. Proceedings from the International Conference of the Icon Book & Paper Group, London 8–10 April 2015, 91-98. London: The Institute of Conservation, 2017.
Keyes, K. "The Use of Friction Mounting as an Aid to Pressing Works on Paper." Book and Paper Group Annual 3 (1984): 101-104.
Knop, A, G Banik, I Bruckle, and U Schade. "Paper and Board in Closed Boxes: Alteration of Water Sorption Capacity During Cyclic Temperature Fluctuations." Restaurator 28 (2007): 218-224.
Mackay, Christine, and Anthony Smith. "The Effect of Wetting Agents on the Tensile Strength of Paper." In Conservation of Historic and Artistic Works on Paper, edited by Helen D Burgess, 199-203. Ottawa: Canadian Conservation Institute, 1994.
Minter, B. "Water Damaged Books: Washing Intact and Air Drying--a Novel Approach." Book and Paper Group Annual 21 (2002): 105-109.
Munoz-Vinas, Salvador. "The Impact of Conservation Pressure-Flattening on the Dimensions of Machine-Made Paper." Restaurator 30 (2009): 181-198.
Neufeld, Laura. "Review of Flattening Techniques for Thin or Transparent Papers." Book and Paper Group Annual 33 (2014): 104-105.
Nicholson, C. "The Conservation of Three Whistler Prints on Japanese Paper." In The Conservation of Far Eastern Art, edited by John S Mills, Perry Smith and Kazuo Yamasaki, 39-43. London: International Institute for Conservation of Historic and Artistic Works, 1988
Smith, Anthony W. "Effects of Aqueous Treatments on the Mechanical Properties of Paper." In British Museum Occasional Paper Number 16: The Interface between Science and Conservation, edited by Susan Bradley, 59-65. London: The British Museum, 1997.
Smith, S F. "Dried-In Strains in Paper Sheets and Their Relation to Curling, Cockling and Other Phenomena." The Paper-Maker and British Paper Trade Journal, March 1950, 185-192.
Sugarman, Jane E, and Timothy J Vitale. "Observations on the Drying of Paper: Five Drying Methods and the Drying Process." Journal of the American Institute for Conservation 31, no. 2 (Summer 1992): 175-197.
Uesaka, T, C Moss and Y Nanri. "The Characterization of Hygroexpansivity of Paper." Journal of Pulp and Paper Science 18, no. 1 (January 1992): J11-16.
Vitale, Timothy. "Effects of Drying on the Mechanical Properties of Paper and Their Relationship to the Treatment of Paper." In Materials Issues in Art and Archaeology III, edited by Pamela B Vandiver, James R Druzik, George Segan Wheeler and Ian C Freestone, 429-445. Pittsburgh: Materials Research Society, 1992.
Watkins, Stephanie. "Practical Considerations for Humidifying and Flattening Paper." Book and Paper Group Annual 21 (2002): 61-76.

History of This Chapter


BPG Wiki
In 2009, the Foundation for Advancement in Conservation (FAIC) launched the AIC Wiki with funding assistance from the National Center for Preservation Technology and Training (NCPTT), a division of the National Parks Service. Along with catalogs from other specialty groups, the published Paper Conservation Catalog and the unpublished Book Conservation Catalog were transcribed into a Wiki environment. In 2016, the BPG Wiki Coordinators reformatted this chapter by removing the legacy numbered outline format and improving internal links. In 2018, Susie Cobbledick, Carolyn Frisa, and Kesha Talbert formed a compiler group to revise and update this chapter.

Paper Conservation Catalog (print edition 1984-1994)
Prior to the creation of the AIC Conservation Wiki, this chapter was created in 1984 as Chapter 28: Drying and Flattening of the 1st edition of the Paper Conservation Catalog, (print edition 1984-1994) by the following:

Compilers: T. J. Vitale, Doris Hamburg


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