User:Scobbledick/Sandbox

Introduction

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 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 2 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?

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 absorbent 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.

General Guidelines

• 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

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 (Vitale 1992). Some sources even suggest that drying under pressure increases interfiber bonding (Banik and Brückle 2011, 405). Helps to retain the paper’s original texture. 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).

Equipment

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.
• Weights on glass or other stiff material such as wood or thick plexi-glass--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.

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.
• 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.

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 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.
• Polypropylene or cotton fleece—these materials are absorbent and very soft. They can be used in restraint drying of thin, fragile papers.
• Tek-Wipe—a non-woven material made of polyester and cellulose. Washable and reusable.

Drying and flattening methods

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.

Time?

Leave wet paper to air dry for about 48 hours.

Microclimate

Using a drying rack or stack of screens 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 paper to maintain something close its original dimensions, plate marks, type impressions, undulations, etc.
• Cons—Not a good drying method for newer, machine-made papers due to cockling and shrinkage. All papers lose strength when air-dried, while their textures change and become more pronounced.

Face-to-face screens

Wet or humid paper 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

Friction drying (Fletch and Walsh 1979; Keyes 1984)

Equipment

Pros

Useful for thin sheets

Cons

Imparts texture

Lateral-restraint

Equipment

Pros

Good for fragile surfaces and distorted materials

Useful for thin sheets

Cons

Changes in dimensions

Uneven drying

Not good for brittle paper

Light weight

Equipment

Variations

Thick felt

Taut screens

Pros

Good for hand-made papers

Cons

Results in distortions in machine-made sheets

Mylar/teflon (Nicholson 1988)

Equipment

Pros

Good for thin papers

Retains calendared surfaces

Cons

Thorough wetting not good for all media

Imparts smooth surface

Evaporation is all from one side

Karibari style

Equipment

Pros

Good for thin papers

Retains calendared surfaces

Cons

Evaporation is all from one side

Imparts smooth surface

Restrained

Nonaqueous flattening

Bibliography

This is the citation for Banik and ...