BPG Drying and Flattening

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Drying and flattening in paper conservation are 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 wetting or Humidification.

This page only covers drying and flattening of paper. The drying and flattening of Parchment is covered in a separate wiki page devoted to that material.

Wiki Compilers: Susie Cobbledick
Wiki Contributors: Susie Cobbledick, Hildegard Homburger, Katherine Kelly, Stacey Mei Kelly, Diane Knauf, Lydia Lyu, Bill Minter, Ewa Paul, Brook Prestowitz, Theresa J. Smith, Denise Stockman, Hsin-Chen Tsai, your name could be here
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 Page.

Copyright 2024. The AIC Wiki is a publication of the American Institute for Conservation (AIC). It is published as a convenience for the members of AIC. Publication does not endorse nor recommend any treatments, methods, or techniques described herein. Information on researching with and citing the wiki can be found on the Reference and Bibliography Protocols page.

Cite this page:

American Institute for Conservation (AIC). "BPG Drying and Flattening." AIC Wiki. April 25, 2024. https://www.conservation-wiki.com/wiki/BPG_Drying_and_Flattening.

Definitions

• Drying—To bring wet or humid paper into equilibrium with ambient relative humidity (RH). When paper has reached equilibrium with the environment and is considered dry, the paper still retains water. At RH of 20% to 70%, water content will be about 4% to 8% (Banik and Brückle 2018, 91). This is a process that will occur naturally, but in a conservation context, it is important that this process be controlled in order to achieve specific treatment goals.

• 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 water to paper. Methods of wetting can include misting, floating, blotting and immersion.

Forms of Planar Distortion Caused by Water

The following forms of distortion can emerge during the drying process after paper has been wetted or humidified. A properly chosen drying technique can minimize them.

• Curling—A rolling distortion that affects the entire paper sheet. It is caused by the uneven expansion or contraction of the two sides of the sheet due to the physical differences 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 paper sheet that has been moistened and subsequently air-dried without restraint. Sugarman and Vitale (1992) define cockles 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 2018, 450) 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.

When to Dry and Flatten

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
• Mending
• Local application of gels or other moisture
• Removal of adhesive residues

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

• It has been improperly stored i.e. paper tightly 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 Not to Flatten

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.

Paper is not a flat object. It is 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:

• Surface texture
• Plate marks
• Impressions
• Stretching in the center of leaves in books made during the hand press era
• Natural waviness of loft-dried sheets

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

• Possible damage to media
• Possible damage to the substrate

Factors to Consider

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. Processes should be chosen deliberately and with care keeping in mind the following factors:

Materials of Manufacture

Fibers--The characteristics of native fibers can be lost during pulp processing, but they may be a factor in how a sheet responds to water. The presence of hemicellulose, for example, can make paper more absorbent, which means that paper made from pure forms of cellulose, such as cotton or chemical pulps, will hold less water at the same relative humidity (Uesaka 2002). Fiber length can also affect dimensional stability as longer fibers can minimize wet expansion (Lindner 2018). Of the traditional European paper plants, linen has the longest fiber, followed by cotton, cotton linter, Manila hemp, coniferous wood and finally deciduous wood (Grant 1962). Of the traditional Japanese fibers, mitsumata and gampi fibers are shorter than kozo fibers (Nielsen 1997).

Noncellulosic components--Lignin, traditional rosin/soap size, and mineral fillers can interfere with fiber bonding and retard shrinkage (Banik and Bruckle 2018, 190 and 193). Less restraint may be needed when drying to maintain close to original dimensions.

Chemical and mechanical pulps--Whether a paper is made of a chemical or mechanical pulp has no consistent effect on the degree of wet expansion, but mechanical pulp paper does tend to shrink less as it has less ability to retain water (Uesaka 2002).

Recycled pulps--Papers made from recycled fibers expand and shrink less after exposure to moisture than papers made from virgin fibers (Caulfield 1988).

Method of Formation

Machine—Machine-made papers have several 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, 2) these machine direction fibers are dried under considerable restraint, and 3) these papers generally have dried-in strains as a result of the tensions placed on the sheet during manufacture. When exposed to water, these papers will expand little in the machine direction, but may expand considerably across the grain (Lindner 2018, 13). If dried without restraint, they will naturally cockle (Smith 1950), and if the paper is relatively new, there will likely be some irreversible, one-time shrinkage in the machine direction as tensions are released (Uesaka, Moss and Nanri 1992). 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

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.

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 when exposed to water and dried.

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.

Overall Strength or Weakness

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 polypropylene fleece.

Local Weakness and Damage

Local areas of damage and weakness (e.g., tears, cracks, mold damage, etc.) can be may worse in the course of treatment, especially if their presence is not known. Examine the paper closely, preferably on a light table, and identify damage and areas that are thinner than surrounding paper. Damaged sheets are best treated using drying processes that involve pressure being spread evenly over the entire sheet. Friction drying is a good option.

Subsequent Use

The level of acceptable changes to the paper that occur during aqueous treatments and subsequent drying and flattening needs to be determined and will inform treatment. For example, dimensional change may need to be minimized if the paper is to be bound back into the original book boards or the texture may be need to be retained as an important element in the aesthetic qualities of a print.

Texture and Surface Finish

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.

Treatment Processes

In the process of treatment, a sheet may be changed in ways that will affect how it responds to moistening and subsequent drying. If the paper has been lined, for example, the expansion and shrinkage of the lining paper need to be considered in addition to the characteristics of the paper being stabilized (Nielsen 1997). Repairs such as mending or filling can also locally change how paper responds to drying and flattening processes.

Media

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

See also: Media Problems.

Equipment

Tools to Apply Restraint

• 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

custom made drying rack at the Penn State University Libraries' conservation lab designed by Bill Minter

• Drying racks—These can be purchased from conservation, art or baking suppliers. Conservation drying racks have moveable 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 woven polyester fabric 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 sandwiched between polyester, non-woven, permeable supports throughout treatment as long as the papers are wet or humid. These polyester webs 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. 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 thickness. Distorts when wet.
• 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 thickness 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. To the extent that felt is thinner and less dense, it tends to shrink when wet.
• Tek-Wipe—A non-woven material made of 45% polyester and 55% cellulose. Washable and reusable. This material also contains optical brighteners, though their effects on treatment outcomes are currently unknown. Slight distortions when wet.
• Evolon®--A non-woven material made of 70% polyester and 30% polyamide microfiber. Also includes a small amount of titanium dioxide. Washable and reusable. Dimensionally stable when wet (Molina and Hughes 2016).
• Diatomaceous earth bath stones--These home products have been suggested as a cheaper and more sustainable option to replace cotton blotters in some drying procedures in paper conservation. DE stones are made of material similar to Fuller's Earth, they dry out instantly (holding up to 150% of their weight in water), prevent bacterial and mold growth, and are made without additives. (More details in this PDF written by Ewa Paul, 2022)

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 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.
• Leave wet or humid sheets sandwiched in polyester web during treatment.
• Drying should ideally be done slowly and evenly.
• Avoid the introduction of heat in the drying process, especially heat applied to only part of the paper sheet. Heat can accelerate drying and cause unwanted distortion and stress.
• Most drying processes should be started after the free water gloss is off of your object, and it has a matte surface (Sugarman and Vitale 1992). Free water can be removed by:
  • Evaporation
  • Blotting
  • Draining at an angle
• The amount of weight put on humid or wet paper, especially wet paper, does make a difference in outcomes. 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).
• 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.

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. Lining techniques may also involve drying paper under restraint, but those techniques are covered in a separate section.

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 2018, 455) 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 new heavily-beaten, machine-made papers, especially in the machine direction. (Uesaka, Moss and Nanri 1992).

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. Thin strips of paper (such as newsprint) can be laid along paper edges that are perpendicular to the grain. This moves the edge of evaporation away from the sheet and prevents edge cockling. 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.

Microclimate

Creating a microclimate is conducive to slow, even drying. Can be used on wet or humid paper.

Equipment

• Drying rack or taut screens
• A cover for taut screens, such as Plexiglass

Place wet or humid paper protected in a polyester web on a drying rack, in a stack of taut screens or on a taut screen under a cover. These arrangements all create semi-enclosed microclimates conducive to slow, even drying that minimizes distortions.

Variations

• A microclimate can also be made by placing the paper on blotter, in a tray. The tray can then be covered with a piece of Plexiglass, with a slight opening to allow for air exchange.

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, 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

The Face-to-face taut screen method can be used on wet or humid paper.

Equipment

• Polyester web
• Taut screens

face to face screen schematic

The paper is placed in a polyester web sandwich, 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—

• While it may minimize cockling in machine-made papers, some cockling is still present.
• As with other air-dry methods, strength may be lost and texture may coarsen.

Subsequent Flattening

After paper has been air dried, it can be humidified and flattened or dry flattened via the semi-restrained, restrained, or dry methods below.

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. This is an effective method to use with machine-made book paper that needs to be bound back into original covers more or less unchanged in dimension.

Cons—

• Unlike the wet restraint method, the air dry/humidify/dry under weight method does not help maintain the sheet’s strength (Vitale 1992).

Semi-restrained

Light Weight

Light weight can be used with humid or wet paper.

Equipment

• Soft felts, wool or synthetic
• Flat, dimensionally stable surface such as a clean table top, silicone mat, taut screen or a stiff but absorbent material such as honeycomb board with or without blotter or mat board

The paper in its polyester web sandwich is placed on a flat, dimensionally stable surface. A taut screen has the advantage of allowing evaporation from the bottom of the sandwich. Absorbent surfaces 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 overnight 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 in contact with the paper if/when it goes slightly out of plane.

Variations

• An additional light weight such as a taut screen turned over on its face can also be added to the stack.

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).

Karibari Method

The karibari can be used on wet or humidified paper (Webber 2017). If humidified, the paper must be fully relaxed.

Equipment

• Karibari board
• Asian style paper
• Smoothing brush such as nadebake
• Starch paste

karibari schematic

The method described here is used for drying and flattening without lining, though it does use a false liner. The paper to be treated needs to be wet rather than humid. It is laid face down on a protective surface, such as a 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 (e.g., 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.

Variations

• The process of drying can be slowed down by adding layers of moist paper over the false liner.
• Substitutes for a traditional karibari board include a painting canvas on a stretcher that is covered with machine-made Japanese tissue or wove paper (Webber 2017).
• In China a similar method is used by drying on walls (Kato and Kimishima 2017).
• Other kinds of boards, such as wood, glass or Plexiglas can also be used though they are not as gentle as the traditional karibari.

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 Western style papers.

Restrained

Standard Method

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 matte 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).

Equipment

• Flat, dimensionally stable surface, such as a table top, heavy glass, plexiglass, or a wooden platen
• Absorbent material, such as blotter, felt, capillary matting, or Tek-wipe
• Heavy solid surface such as glass, Plexiglas, wooden platen
• Weights or press, such as a nipping press or standing press

standard schematic

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. If blotters are used, it may be helpful to back at least the top blotter with a softer material such as felt, capillary matting, Tek-wipes or foam in order to conform to the paper's natural variations in thickness. The sandwich is then placed in a press or under 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 (e.g., 200 g/cm²) Munoz-Vinas 2009 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 or longer. It is common 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 2018).
• Prevents out-of-plane distortions, especially in machine-made papers with dried-in stresses.
• Because weight is evenly-distributed over the sheet, localized stresses are less likely to 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 impressions.
• Not a good or necessary choice for loft-dried, hand-made papers that need to retain their natural undulations.
• Can result in dried-in wet expansion, especially in newer, heavily-beaten, chemically-pulped, machine-made papers.

Forced Air

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.

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, or wooden platen
• Weights or press

forced air schematic

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 the 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 two, with or without the fan. Machine-made papers may need several days or longer. 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.

Variations

• 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.

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 that need to maintain their natural waviness.

Friction Drying

This method can be used to dry and flatten both wet and humid papers.

Equipment

• Asian-style paper
• Smoothing brush such as nadebake
• Flat, dimensionally stable surface, such as a clean table top or silicone mat
• Absorbent material, such as blotter, felt or Tek-wipe
• Heavy glass, Plexiglas, wooden platen or press
• Weights or press
• Support material, such as mylar or polyester web

friction schematic

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 (two inches around all sides). These will be the friction sheets. Wet out one friction sheet and place on a flat, dimensionally stable surface. Wet out the paper being treated and place face down in the middle of the friction sheet, using the smoothing brush 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 support material. The wet three-sheet sandwich can then be dried under pressure using the standard method or the forced air method.

Variations

• 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

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.

Equipment

• Smooth membrane like Gore-Tex, Sympatex or Hydra Air
• Smooth nonstick material such as Mylar, Teflon, 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 or press

Mylar schematic

Gore-Tex schematic

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/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 smooth side of the Gore-Tex should always face the paper. The sandwich can then be dried under pressure using the standard or 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

Fragile, thin, badly creased and transparent papers may need special handling when being dried and flattened. The inclusion of soft materials next to the sheet in the drying stack allows for the gentle and even distribution of pressure and prevents the damage and distortion that could result from the use of stiff materials like blotters or dense felts.

The hard/soft sandwich can take many forms using a variety of materials. Two approaches using two different soft layers are described below.

Polypropylene

Homburger and Korbel (1999) first developed this hard/soft sandwich to flatten architectural drawings. They used polypropylene fleece in their stack and got best results with humid rather than wet paper.

Equipment

• polypropylene fleece
• Flat, dimensionally stable and slightly absorbent material such as binders board or museum board
• Heavy glass, Plexiglas, or wooden platen
• Weights or press

hard/soft sandwich with polypropylene schematic

The humid paper is placed in the following sandwich described from bottom to top: board/polyester web/paper/polypropylene fleece/platen/weights. When the first or only layer of fleece is put on the paper, a brush, such as a nadebake, a hand, 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. The right amount of pressure is crucial and is generally more than you would use on a standard drying sandwich. 80 kg/m² is a general recommendation, increasing the weight with the number of polypropylene layers (Homburger 2020). Leave the sandwich to dry at least 2 days without opening. After opening, close and leave under weight for additional time depending on observed results. Repeat the entire process if necessary.

Evolon

Bone and Stenman (2021) used a hard/soft sandwich to treat a group of Curtis photogravures printed on gampi. They chose 58 gsm Evolon for their soft layer after testing a number of potential materials. They found that Evolon absorbed and wicked out moisture more quickly and evenly than Sympatex, Goretex or Tekwipe.

Equipment

• Evolon 58 gsm
• Flat, dimensionally stable and slightly absorbent material such as binders board or museum board
• Plexiglas
• Sympatex

hard/soft sandwich with Evolon schematic

Each photogravure was washed on an Evolon support. After removal from the bath, the object was blotted with Tekwipe, placed between dry sheets of Evolon and put into the following sandwich described from bottom to top: Plexiglass/board/Sympatex/Evolon/paper/Evolon/Sympatex/board/Plexiglas. The materials in the stack promote slow, even drying, with the Sympatex allowing moisture to move from the Evolon to the absorbent board. Board was chosen over blotter for its rigidity and smoothness.

Each print was left in the stack for a minimum of three days, with the top board being switched out after one day to promote drying. The print was then removed from the stack but left in its Evolon sandwich and stacked with other prints between dry boards for several weeks.

Variations

• This approach lends itself to variation since different soft and hard materials can be used in the drying stack. Soft materials may include polypropylene fleece, Tekwipe, Evolon and soft felts. Hard materials include cotton blotter, board, Plexiglass and hard felts.

Pros--

• Good for problematic papers liable to damage or distortion when air-dried or dried under blotters or hard, dense felts. These problematic papers include fragile, thin, badly creased and transparent papers.

Cons--

• Like other forms of drying under restraint, the use of this method will typically result in dried-in wet expansion.

Lateral Restraint

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.

Equipment

• Flat, dimensionally stable surface or absorbent material such as honeycomb board with or without blotter or mat board
• non-woven polyester
• strips of blotter
• pieces of glass or other stiff material such as book board
• weights

lateral restraint schematic

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 can also be used as long as they remain dimensionally stable. The edges of the paper (about 0.5-1 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.

Variations

• Place a cover over the paper as it dries to slow the process down and make drying more even.

Pros--

• Good for thin sheets and paper that holds fragile media.

Cons--

• Paper may be left with larger dimensions.
• Brittle paper may rip as uneven drying stresses the sheet.
• Not good for paper whose edges are weak.

Suction Table

Suction table drying and flattening is a method that allows some control over dimensional changes in paper. It is a treatment option for papers with sensitive media that may offset onto interleaving materials and works that cannot be flattened inside a drying stack. It can be done with a basic suction table, or with a suction table equipped with a humidification dome. It is sometimes used to finish off treatments that were already being carried out on the suction table. This method can be used on wet or humidified supports.

While works can be dried completely on the suction table, suction table drying is also done in a two-step process, with initial drying done on the table and then transferring the work to dry between polyester web, blotters and/or felts under weight.

The suction table can also be used to treat parchment.

Equipment

• Suction table with or without dome
• Polyester web
• Blotter paper or other barrier material

Transfer paper to be dried and flattened to the suction table. A polyester web should be used as support. If the paper is already on the table after treatment, leave in place. A barrier material such as blotter or polyester web can be used between the table and the work. Curling can occur if the object is dried fully on the suction table. Place weights along the edges of the paper to prevent this. Polyester strips can also be placed over the edges to prevent them from drying out faster than the rest of the sheet. Start with low suction to ease out creases and undulations, and increase suction as the object flattens out. After the work has been flattened, and the media of the object appears to be dry to the touch, the suction can be turned off, and the object allowed to dry with the edges weighted (Weidner 1993). At this point, the object can also be transferred to a traditional blotter drying stack if desired.

If the paper has undergone severe expansion, splitting of the paper could occur. To prevent this, the weighted edges can be released and the suction turned off to allow the paper to contract briefly. Replace the weights along the edges. Once the object is flat, the suction can be turned off and the object allowed to dry in place (Weidner 1993).

Variations

• The rate of drying can be slowed down by working under a dome with an ultrasonic humidifier.
• The suction table can cause airborne pollutants and surrounding dust and dirt to embed itself into the paper fibers, a Gore-tex barrier has often been recommended as a filter. Gore-tex can also be used to slow down drying (Mowery 1991).

Pros--

• A treatment option for works with creases and cockling that require manipulation during flattening, and works that cannot be covered while wet (unfixed pastels, gouache, impasto, textured paper etc.) (Weidner 1984).

Cons--

• Not an ideal method for objects with sensitive media on both sides
• The use of the suction table can lead to changes in the texture and characteristics of the paper
• Not ideal for Asian papers
• Suction table drying dries works in an expanded state which can lead to curling, formation of creases and distortions, and sometimes tearing or splitting (Watkins 2002).
• Suction table flattening and drying can cause severe distortions on tracing papers compared to using a traditional blotter press (Reyden, Hofmann, and Baker 1993).
• Pastel pigments can sink into the support or condense or clump on the paper surface resulting in visible changes in the texture of the medium (Kosek 1990, Weidner and Zachary 1994, Daniels 1998).

Dry Flattening

Sometimes water needs to be introduced in order to flatten paper. Even when not necessary for flattening, water speeds up the 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. To avoid cracks in severely curled paper, and in fragile paper or media, work slowly to reshape the paper. Weight can be added slowly over several days or weeks or if using a press, the platen can be brought down as gradually as necessary ie. a quarter turn every day, every week, etc. 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.

Paper that has been stored rolled in a tube can be uncurled by securing it to a wall with paper-lined clips and allowed to slowly unroll (Watkins 2002). Since this process can take weeks a cover should be put over the paper to protect it from dust.

Pros--

• Avoids the problems associated with the introduction of moisture, such as changes in dimension, increased roughness and loss of surface finish.

Cons--

• Generally less effective and takes longer than other methods.
• Flattening brittle or fragile paper in this way can cause cracks.
• Media may also flake off of the paper.

Special Cases

Books

Books are complex hybrid objects that often require special methods to treat their paper components. Whenever possible, avoid pulling the book and treat its components in situ.

• Distorted textblock paper--Leaves in books may be folded, crumpled or cockled due to water damage. To flatten these leaves, isolate them from the rest of the textblock by placing mylar sheets on either side of the damaged section. Insert a piece of slightly moist or humid blotter between the mylar and the damaged leaves, protecting the leaves from the blotter with a piece of polyester web. If the damaged section is thick enough, place a second piece of moist blotter on the other side of the section, again with a piece of polyester web protecting the leaves direct contact with the blotter. Leave in place for about twenty minutes. The volume can be closed or open during this humidification process. Remove the blotter and polyester web and close the book. The book may be weighted down under a board or placed in a press. Observe the book's natural paper density and apply more weight for a dense book and less or no weight for a book whose textblock contains a significant amount of air. As a general rule of thumb, books produced before the 18th century are less dense. Thin sheets of waste paper may be added to the humidified section to adsorb moisture. These should be removed or changed out as needed. Leave the book to dry closed overnight.

bifolium being placed in a folded polyester web

bifolium secured in polyester web

• Warped boards--Warped book boards can place stress on sewing structures and damage textblocks, so treating them to bring them back into plain may be necessary. When the boards are made of cellulose (paste, mill and pulp boards) they can be flattened through humidification. Open the book so the board lays flat on a surface, leaving the pastedown exposed. Support the rest of the book with a weight, polyurethane wedge, etc., holding it almost upright. Cut a piece of blotter a little smaller than the paste down, moisten it slightly, and then place it in the following sandwich: board/polyester web/moist blotter/mylar. If the board is part of a leather bound book, cut the blotter so that it is slightly smaller than the turn-ins. This will protect the leather under the pastedown from being adversely affected by moisture. Leave the humidifying sandwich in place for about twenty minutes and then remove the blotter and polyester web, leaving the mylar in place. Put the book in a press between two pieces of clean book board and leave it there for several weeks. It may be necessary to repeat the process. If the boards are detached from the textblock, follow the same procedure for flattening them and treat them before reattaching them.

• Drying and flattening bifolia--If the book has been pulled in order to treat the paper of the textblock, try to keep the bifolia folded throughout treatment. This will relieve stress on the weakened paper on the fold. Each bifolium can be placed in a folded sheet of polyester web with an additional polyester web insert placed between the leaves. The additional polyester web insert should be pulled snug into the paper fold, holding it against the polyester web fold. Sew threads through all three layers of web on either side of the bifolium and tie loosely. This sewing will hold the bifolium in place throughout treatment. Once the bifolium has been treated (e.g., washed, deacidified, etc.), it is left in the same polyester web for resizing and then drying. As a general rule, paper for books is best dried using a no or low restraint method. Since the paper needs to return to wrappers or covers, it is important that its original dimensions be retained as much as possible. In the case of hand made paper from the hand press era, there is also important dimensional evidence (type impressions, natural undulations, etc.) that may be lost if the paper is dried under pressure. Dry the bifolia overnight and then remove from the polyester web. Place them in a stack, rotating each bifolum 180 degrees in relation to the bifolium below it, so that each fold is cover by a fore edge and so forth. For hand made paper, place a light weight on top of the stack and leave overnight. For machine made paper (which will likely be much more cockled) place the stack in a press between two pieces of clean book board, apply pressure and leave in place for at least 48 hours but longer if possible. It is usually not necessary to rehumidify bifolia to help them dry flat, but it is an option if dry pressing for several weeks doesn't work.

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History of This Page

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 became the compiler for this page. With the help of the contributors listed above, a fully revised page was launched in November 2019.

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 1^st edition of the Paper Conservation Catalog, (print edition 1984-1994) by the following:

Compilers: T. J. Vitale, Doris Hamburg