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Rewetting and Flattening of Historic Paper Supports
1 - Introduction
2 - Stages of Air Drying
Tim Vitale © 2011 v8
1
2
Figure 1: Seven stages of paper drying taken from S & V (JAIC, 1992)
2
Table 1: List of the stages of paper drying with visual appearance description
2
Figure 2: Cross-section of paper - depicting the seven stages of drying
3
3 - Hydrogen-Bonds between Fibers
4
Figure 3: Rance 1963 graphic - hydrogen bonding between fibers
4
Note 1: Terry Towels for Air Drying
5
4 - Formation of Fiber Pulp into Paper
5
Figure 4: Handmade papermaking process
6
Figure 5: Taken from Dard Hunter Fig 152 & 153,
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Figure 6: Fourdrinier End of a Westvaco’s circa 1980 papermaking machines
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Figure 7: Westvaco’s papermaking machine’s Press and Dryer End
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5 - Paper Texture is Created by Pulp Refining, Sheet Formation & Pressing 8
6 - Recreating Historic Paper Texture Using Flattening Treatments
9
Note 2: Monitoring moisture content: 1) IR thermometer or 2) contact paper moisture meter 9
7 - Using a Texture Ruler
10
Figure 8: Workshop participant using small texture rulers
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8 - Paper Texture Size Domains - Evaluating Paper Texture, Flatness
11
Table 2: Six paper texture size domains
11
Figure 9: Meter Scale - Full sheet of handmade paper
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Figure 10a & 10b: Decimeter and Centimeter scale on the same handmade paper
12
Figure 11a & 11b: Images depict the Millimeter Scale size domain
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Figure 11c & 11d: Images depict the Many-Microns Scale size domain
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9 - The Flattening Process
15
Figure 12: Version of the auto tire inner-tube press
17
Figure 13: Gaehde’s mechanical press
17
10 - Scientific Data: Relationship of Sheet Formation to Re-drying a Sheet 18
Figure 14: Plot showing strength, shrinkage and dryness (time) against percent solids
19
Figure 15: Rance’s shrinkage data
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Figure 16: Parker’s paper formation drawings
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Figure 17: Robertson’s tensile strength data
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Figure 18: S & V drying of three sized papers
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11- References
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1 - Introduction
Some print and drawing curators do not respect the work of paper conservators.
They claim that all reflattening is too obvious, acting as a telltale that an artwork has been treated.
Determining optimal blotter flattening procedures requires experimentation with your materials &
equipment. Weight on the blotter stack seems significant, but initial dampness and timely release of
flatting weights are the significant variables based on past studies: see S&V 1992. Achieving an
historic appearance by:
● avoiding surface texture loss,
● preventing platemark reduction and
● introducing a broad warp
is the goal of this write-up. Getting traditional flatting and drying process to work properly, when it is
usually taken as granted, is a difficult challenge. It is for me, anyway; getting the platemark to look
historic has always been a challenge.
Stretch drying and its variants are not included because the procedures do not parallel the
papermaking process. Those procedures are useful when an artwork’s surface can’t take weight, but
don’t apply to the majority of artworks treated. Details on the full range of drying and flattening
technologies can be found in the AIC, Book and Paper Group’s Paper Conservation Catalog’s chapter
on Drying and Flatting.
Research published in 1992 (Sugarman & Vitale, JAIC) shows how drying paper parallels the initial
sheet formation process (papermaking); see Sections 2, 5 & 10. Science drives our understanding of
the process that is why the discussions are heavy with analysis and probable proofs; see Sections 8 &
10. Knowledge of papermaking (Sections 4 - 6) and interfiber hydrogen bonding (Section 3) are
critical to understanding paper flattening (Sections 8 & 9) in blotter stacks.
The ability to evaluate small changes in a paper before and after an experiment is critical to improving
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one’s procedures. It was shown that hum
humans are better than technology for judging
ing flattening detail.
A trained eye, using a Texture Ruler (Section 7) with a systematic framework for judging paper texture
is a useful experimental tool.. Paper texture size domains have been defined in Section 8.
8 While this
may appear pedantic and overly analytical, having an intellectual structure to define properties and
terms has proved beneficial to discussions on this topic. Specific procedures for influencing outcomes
are offered for each texture domain
domain.
Most conservators have the skill needed to flatten works quite effectively. However problems arise
with flattening treatments, from time to time
time. The loss of control in such a basic procedure, flattening
capabilities, can be critical. One particular fau
fault is common. The
he artwork comes out
ou of the blotter
stack flat, but when it is checked some time later, a broad warp is has developed..
There may be periods when a particular blotter stack gets used frequently and becomes damp.
damp This
can result in the blotters
otters becoming damper tha
than the surrounding air. This condition can lead to an
imperceptibly slow warp after a sheet is removed from a slightly damp blotter stack. Warp is one type
of paper texture; Section 8 has details on controlling texture types.
2 - Stages of Air Drying
Figure 1 shows the seven stages of d
drying; from S & V (1992). It depicts drying from a saturated
state to air dry. The observations are a critical description of the paper drying process. Drying in a
blotter stack, cannot be observed,, measured (weighed) and described in a critical moment-bymoment
moment manner. The process is hidden.
Figure 1: Taken from Sugarman & Vitale JAIC (1992), the diagram shows the seven stage of paper drying.
The data for Figure 1 was observed by Jane Sugarman
Sugarman. Percent solids (y-axis) was calculated by
observing the air drying performance of six papers averaged together against weight of the paper
sample in a 4-place
place analytical pan balance. Appearance was observed against weight, which can be
expressed as percent solidss or as moisture content relative to the bone dry weight of the cellulose
fibers (and sizing)
g) making up the paper
paper. The drying time is plotted along the x-axis
axis (horizontal axis).
Moving from left to right along the xx-axis follows the drying process from saturated,, at about 40-42%
solids (or 60% water), to air dry at about 92-94% solids (6-8% water). The rising multicolored
diagonal line in the chart window shows th
the percent solids at a drying time relative to water-saturated
state beginning at time zero.
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The Drying Process
Stage 1 is defined as flooded; the sheet appears dark and glossy. As the water drains away, the
point where no free water (gloss) can be seen, the appearance of the paper becomes dark and matte.
Stage 2, is defined as the point where air begins to intrude back into the sheet, causing the
appearance of the sheet to become a lighter shade of gray. For the six papers observed this
averaged at 47% solids. When air begins to intrude back into the sheet and the tonal appearance
begins to lighten, because water goes away and air takes its place in the fibrous structure. The water
saturates the solid structure making it appear dark. After 47% solids, within Stage 3, water exists only
in the paper pores; as more water goes away the darkness gives way to light. The appearance of
lightness is caused by light scattering from the surface of fibers that were formerly saturated by water.
Stage 4 begins at about 67-68% solids. Significant water has gone away. Air has intruded into most
of the sheet. The point has been observed to be critical because the nominally flat conformation of
the sheet changes, to one of flat with a slight warp. The sheet has changed shape without any
external stimulus. Internally the sheet is drying and shrinking. As fibers loos water the shrink in width
and length. There is unseen movement inside the paper: fiber shrink while surrounded by water,
slipping past one another. Once the amount of water between fibers become small enough the
surface tension draws the surfaces of the fiber into vet intimate contact and a hydrogen bond is
formed between the –OH groups on the surface of the fibers; see Figure 3 (Rance, 1963).
1
2
3
4
5
6
Stages of Drying Seen in Cross-Section
Paper Flooded with Water
First Air Intrusion into Fiber Network
Water Only in Large Pores
Water Just in Small Pores between Fibers
Onset of Physical Distortion; Bonded Fibers Shrink
Water Evaporating from Fibers: No Water in Pores
Appearance
Glossy - Wet; Saturated; Darkest Gray
Glossy with Matte Regions
Dark Gray (Matte no Glossy Areas)
Light Gray (Matte)
Edges Lift up – Even Lighter Gray
Very Light Gray
Table 1: Description of the stages of paper drying, with the visual appearance at each stage
Stage 5 is a critical point in the drying process, the average was found to be at about 68%  3 solids
on the y-axis. Onset occurs after about 60 minutes of air drying. The point was called “Onset of
Physical Distortion” because a significant change in appearance occurs.
Air has already begun to intrude back into the paper for many minutes, starting in Stage 2. Many of
the possible hydrogen bonds between fibers have formed (or re-formed) beginning in Stage 2. The
appearance has shifted from light gray to very-light gray from Stage 4 to 5.
Stage 6 is not well defined. The water content of the sheet is now low enough so that fiber surfaces
are exposed to air on all sides. The sheet appears to become bright white, light is scattering from all
surfaces. To be fair, some small regions where groups-of-free-water-molecules will exist, but most
water-molecule-groups are gone. Excess water exists within the paper, but most of the water is
bound to the surface of the fibers, internal and external. It is bound by hydrogen bonds between the –
OH groups on both the water and cellulose surface; see Figure 3 (Rance, 1963).
At “Onset of Physical Distortion” (Stage 5) many hydrogen bonds have formed, individual fibers are
shrinking (Stage 6) because they are exposed to air and loosing water to evaporation. Continued
fiber shrinkage will result in sheet shape change – an edge will lift. Once lifted, that edge will always
be a problem because it has been set (locked) into place by the interfiber bonds (hydrogen bonds)
that have formed. If the edge that has lifted is pushed down, some other part of the paper’s edge will
lift. This shows a mechanical linkage within the sheet.
Stage 7 is defined when the appearance becomes even lighter with the loss of more bound water. At
88% solids, shrinkage is still occurring but the rate becomes slower. Note that the shape of the
curved line has become almost flat. This defines a shift in the rate of the “loss of water” process.
The internal water takes time to move to the surface of the fibers so that it can be evaporated away.
The process becomes a diffusion-driven process.
The stages depicted in Fig 1 have defined percent solids content and appearance descriptions but
most of the actual transitions are indistinct, except for Stage 5. Significant judgment is required to fit
the intellectualized process to individual papers during conservation treatment. The description is a
guide to the process, not a set of rules. There is a reason only Jane Sugarman made the
observations, precision and only one person’s variables.
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During the drying process the paper shrinks in size by about 2-6%. Shrinkage could not be studied in
detail because cockling and warping of the sheet affects its shape and thus size. Measurements
would have been imprecise. Weight was a very precise measurement. In a blotter stack, shrinkage
causes the development of strain because the sheet restrained by friction with the blotter surface and
the weight from above. The paper does not shrink uniformly over the entire sheet. See Figures 14
and 17 for the relationship between (a) shrinkage, (b) tensile strength and (c) percent solids.
Figure 2: Shows the stages of paper drying. Much effort was expended to capture actual images of paper drying in
cross-section using an Environmental Scanning Electron Microscope (ESEM), however, when the electron beam sat on
the site of interest it deformed (slightly melted) the fibers being observed, as they dried; the cooling medium, water, was
evaporating away. A drawing had to be used instead.
3 - Hydrogen-Bonds between Fibers Hydrogen-bonds between fibers are formed when fibers come
into very close physical contact, as the water content in the paper decreases http://en.wikipedia.org
/wiki/ Hydrogen_bond. The interfiber bonds are not chemical bonds; they are weak bonds that form
when hydrogen and oxygen molecules come into short-range contact with each other’s electron
clouds. They are called weak bonds because they are relatively easy to break.
Figure 3: Taken from Rance in Pulp and Paper Science (1963), it shows the hydrogen bonding process. Water helps
bonding until true fiber-to-fiber contact is achieved. As the space between fibers becomes very small, capillary
attraction helps to pull the fiber surface into good contact for optimal hydrogen bonding density.
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Water will break hydrogen bonds and drying (decrease in free water content) will reform hydrogen
bonds. Not the actual drying, but the process of removing water so that the bonds can reform
between fibers. Note that the surface of fibers are covered with the “Velcro” of hydrogen and oxygen,
as seen in Figure 3.
3.1 - Influence of Capillary Action of Hydrogen Bond Formation
Capillary action http://en.wikipedia.org/wiki/Capillary_action, creates pressure within the capillaries
formed between fibers, as seen in Figure 3 (Rance, 1963), between the fibers and fibrils. As water in
the capillaries (paper pores) evaporate, the internal force created on the capillary walls helps to pull
the surfaces close enough for the hydrogen bonds to form.
3.2 - Application of Hydrogen Bonding to Flatting Procedures
If a rewet paper fiber mat (paper) is held in the shape that is desired – flat – during the re-formation of
the hydrogen bonds, the shape of the paper created by the drying surfaces (blotters, etc.) will be
locked into place by the bonds. This is the key behind the paper drying process.
Gaining control requires holding the paper flat while the paper passes through a solid content of the
68%  3-5% to 94% solids (Stages 5 thru 7). Getting the sheet into the press while it is still in lateStage 4, insures that Stage 5 “Onset of Physical Distortion” occurs while being held flat within the
press.
If the sheet never gets wet enough to reach 68% solids (32% water), due to incomplete wetting, it
cannot be flattened to fullest extent possible. Incomplete rewetting is one way to control flattening,
but it assumes the sheet has no distortions, small or large scale that need modification.
Predicable outcome is enhanced by the use of an optimal drying protocol for your studio’s workflow;
see Sec 8, Paper Texture Size Domains. The “optimal” blotter release protocol in S & V (1992) is
(a) 5 - 10 minutes, the sheet will be damp, about half dry
(b) 30 - 45 - 60 minutes, the sheet will seem to be dry, but it will not be in equilibrium with room RH
(c) 4 - 18 hours, I generally go overnight.
On the other hand, different paper types and different paper issues require different drying
procedures. The basic drying stages are always valid, but the protocol may need to be adjusted to
overcome specific problems.
Flatting tracing paper is an example. Tracing paper needs internal tension during drying to pull it flat;
however, too much tension will cause ripping of this very expansive and thus very contractive paper
type. It swells markedly when rewet, up to 4% to 8%. Releasing drying strain and reconfiguring the
blotters in contact with the sheet are the reasons for opening the blotter stack; this step will need to be
modified to quicker and a greater number of blotter stack releases.
3.3 - Watch Paper Dry
Watching paper dry may seem a waste of time. However, it is instructive to observe the seven stages
of drying noted above (Figs 1 & 2 and Table 1). Noting the stages and even documented them for
later study is an excellent tool for increasing understanding. The S & V (1992) publication would be
impossible without this type of work. Most folks will need to control our multitasking urges, and
maintain focus during the open air drying process.
Using a terry cloth towel rather than a blotter, and, a source of gentle heat (a portable hairdryer
mounted about 4-5 feet away) will speed the air drying process. As usual, using too much outside
force (heat) will disrupt critical observations.
Note 1: Terry Cloth Towels for Air Drying
Air drying on white terry cloth towels encourages even drying over the entire sheet.
The procedure also saves blotters because the towels can be used over and over,
remaining flat because they are cloth. Changing the towel after about 20-30 minutes
will also speed air drying.
However, the slower the air drying process the more even the evaporation will be,
and thus, the less distorted the sheet will be at its completion. This is critical
because there will be less physical distortion to remove when it comes time for
flattening in a blotter stack.
The individual twisted cotton nibs in terry cloth are at approximately the same
distribution density as the largest pores in a sheet of paper. Observations suggest
that the nibs empty the largest pore by contact.
The dried sheet of paper is often found stuck to the surface of the towel, especially if
the paper has not been move during drying. The attachment can only be due to
hydrogen bonding between the cellulose of the paper and in the towel. The capillary
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action of water pulling the towel and paper surfaces together,, facilitates the weak
bonds between the two cellulose surfaces
surfaces.
The towel drying technology
echnology has been practiced at the Queen’s Conservation
Training Program for years.. It was introduced to me by Debra Fox in 1997.
4 - Formation of Fiber Pulp into Paper
Paper is formed from a slurry of paper fibers in water. The slurry can be well beaten into individual
fibers or less-beaten into clumps of fibers of various sizes and densities.
Pulp beating can be seen in Fig. 4, Image 1. Prolonged beating can
an even involve defibrilization,
which thrashes the fibers apart. There is significant literature on this topic.. Basically, beating
b
influences paper lumpiness.
Making paper by hand involves dipping a rigidly supported open
open-porosity
porosity screen (mould with deckle
frame) into a vat of the slurry and pulling out a uniformly distribute aliquot of entangled fibers as a lowlow
solids mat. The rapidly
dly draining mat is held above the vat for excess water to drain from the screen.
When enough water has drained away, the deckle (frame) is removed from the mould and the screen
surface is turned up-side-down,
down, where in the formed fiber mat is couched o
onto
nto a papermaker’s felt.
The stack is called a post and is the first of two pressings in the high pressure press the paper will
receive. A stack of alternating felts and very wet sheets (fiber mat) is amassed in a “post.” When
enough sheets have accumulated,
ulated, the stack is placed into a (screw) press, at moderate pressure, to
further de-water
water and compact the cellulose fiber mats into paper. Figure 5 shows historic paper p
Figure 4: A series of images that shows traditional papermaking steps in an hi
historic
storic papermaking mill. Note the hydraulic
press in image 5; this image is from the 1950
1950-60s;
60s; the actual historic process uses a screw press with a very massive lintel,
see Fig 5.
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When handmade paper is removed from the first pressing, the felts are pulled from the stack (post)
of formed sheets. The felts are tossed aside for reuse in couching, while the individual sheets of
paper are stacked face-to-face for a second pressing; often called laying.
In the second pressing, the paper sheets are placed face-to-face and returned to the press for highpressure pressing, Fig 4, Image 4. There has been much speculation on the actual pressure created
in a wooden screw press; it is assumed that the range is from 5 to 50 psi. Hydraulic ram presses
(Figure 4 Image 5) are capable of 100-125 psi, over their 300 to 500 square inch platen surface.
When the stack is removed from the press, skilled workers separate the sheet from one another; they
do not meld into one another because there is no physical entanglement between sheets. The
surface of each sheet has, however, influenced the surface of adjacent sheets because they are
forced against each other at 10-100 psi; no physical entanglement can occur, but sheets are
hydrogen bonded to each other and have to be pulled apart.
Figure 5: Taken from Dard Hunter Fig 152 & 153, shows historic screw presses used to express excess water from
just-formed (handmade) sheets so that hydrogen bonds form, setting the sheet into a flat state. Note that very high
pressure is applied to the sheet as it shrinks; this forms the sheet under tension; flattening pressure is not released
until it is almost dry. The sheets are resting face-to-face in the block at the bottom of Fig. 152.
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Figure 16 (p. 21) depicts fiber entanglement during sheet formation. Entanglement is created as the
fibers fall out of waterr suspension for the first time when formed on the mould screen. Pressing
cannot re-entangle fibers, this can only be done when the sheet is formed.
Papermaking on a paper machine follows the same basic process; the fiber pulp slurry is laid onto a
moving wire screen (called the “Forming
Forming Fabric
Fabric” in Fig 6) from the headbox. The transport bed of the
continuous loop
op of the woven wire fabric is shaken side
side-to-side
side to encourage fiber entanglement and
distribution at right angles to the machine direction. Machine direction is the movement of the
Forming Fabric, which tends to orient the fibers in that direction.
Figure 6: Fourdrinier-end of one of Westvaco’s papermaking machines, it produced high strength unbleached Kraft
paper at very high web speed, circa 1980.
Water drains away from the de-watering
watering web as the forming fabric travels from left to right side of Fig
6. The fiber mat goes through its initial de
de-watering stage with only modest pressure.
pressure This is created
by suction boxes at the far left (Forming Board) right end (four narrow ribs in Fig 6) of the Fourdrinier
stage.
Figure 7: Westvaco’s Kraft Paper (circa 1980) Presses and Dryer End of their paper
papermaking machine.
At about 35-40% solids, the sheet is strong enough to be transferred to the next stage of dewatering
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(press side of paper machine); see Fig 7. Additional bulk water is expressed into a moving felt using
light pressure in the so called couching stage. In Fig 6, this occurs at the far right end, beyond the
Fourdrinier stage, which is labeled “press” in this drawing. The couching felt is depicted in gray
looped configuration above the low-pressure press stage. The low-pressure press/couch stage (far
right of Fig 6, left of Fig 7) is where the surface of the felt is applied to the web’s top side surface
texture. In the next stage (see Fig 7) the damp paper web is further compacted and surface texture
smoothed to both sides of the damp web. The speed of the upper and lower second stage rollers,
relative to web speed, imparts additional burnishing to the damp web increasing smoothness. Drying
against heated polished drums; stretching between drums while drying; surface sizing; and final
calendaring impart the finished surface texture (gloss) to machine-made paper.
5 - Paper Texture is Created by Pulp Refining, Sheet Formation and Pressing
The paper manufacturing process is directly responsible for the small, medium and large scale
surface texture. See the section below on paper texture size domains for extensive details on surface
texture.
The handmade and machine-made processes are similar; they can yield similar results. Handmade
papers, however, tends to have more variable textures with a coarser small-scale surface texture and
a somewhat involuntary large-scale warp. Edge cockles and large scale conformation distortions are
common. Handmade paper can be planished smooth using hot or cold press rollers.
Machine-made papers can have a moderate amount of small scale texture, but they tend to be
smoother as a group, and lie flatter, or, present a slight curl in the machine direction due to being
stored on a roll. Generally, machine-made papers do not have edge cockles.
In handmade paper, the paper has a surface texture created initially by
(a) beating - defibrilization of individual fibers or less work to produce clumps of various sizes
(b) evenness of formation - vatmans screen movements to entangle fibers and control thickness
(c) couching to a felt - surface can be rough or smooth, at low or modest pressure
(d) finished by face-to-face high-pressure pressing in a screw or hydraulic ram press
(e) surface sizing with starch to seal and harden surface (a third stage of pressing can be applied)
(f) sheet are air-dried after sizing imparting some native curl; slow drying encourages flatness
(g) calendaring may be used to smooth surface of dry sheet
On the papermaking machine, surface texture is generally much smoother because there are fewer
processing variables in the mechanized process. The cellulose fiber pulp slurry must be well refined
so that it can be pumped through pipes and distributed evenly in a headbox. The headbox deposits
the thin pulp slurry on the fast moving Forming Fabric screen (loop). This high degree of pulp refining
produces a smooth fiber furnish (slurry) that de-waters quickly. The process goal is a uniform smooth
surface for writing, copying, wrapping and repurposing into photographic or inkjet printing base. The
machine-made surface texture is defined by
(a) high degree of pulp refining so the slurry can be pumped through the system
(b) woven wire texture; this is the plain-weave screen that receives the “stuff” from the headbox
(c) dandy roll (not shown) middle of Fourdrinier stage, impresses texture & watermark in slushy web
(d) couching process imparts first “applied” felt surface texture to the top-side of the web
(e) first-stage press (low pressure side) applies felt surface to underside of web
(f) second-stage press rollers (high pressure side) smooth surface of both sides while web is damp
(g) dryer stages stretch the web while further smoothing the surface using polished heated drums
(h) surface sizing can be used to seal or coat surface, to further smooth the web
(i) calendaring rollers, after dryer stage, planish surface to add surface gloss to dry web
(j) over-dried web is rolled onto a core for transportation and storage.
There is a variant; between handmade and machine-made, it is often called mould-made. The
processes uses individual screen moulds on a machine feed belt, feed by a headbox operating a
much slower speeds. The sheet can have great texture because the pulp does not need to be as
heavily refined before entering the headbox. The sheets can have greater surface texture and
deckled edges, but they are machine-made.
6 - Recreating Historic Paper Texture Using Flattening Treatments
The heart of the flattening process is to restrain the “sheet conformation” (flatness) before significant
hydrogen bonds form between the fibers during drying. Secondary importance is to release the drying
strain in a timely way, without allowing the sheet to curl due to evaporation in the open air. The last
issues is to use just enough weight to hold the paper flat, while allowing for the quick manipulation of
weights, glass platen and blotters or felts during blotter changes, which release the drying strains.
Piling on a lot of 25lb weights (bags of shot) is problematic because they are difficult to shift and thus
do not allow for efficient movement during blotter changes.
Once the sheet is rewet, the final shape that one desires for the sheet (flat) needs to be set before the
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“Onset of physical distortion” occurs. Defining that “wetness” point can be approached in many ways.
The most efficient method of judging wetness is an experienced worker.
Two technological methods are useful, however. The most accurate technology is to measure the
moisture content of the paper after spraying using a contact moisture meter. The pin type devices
used for measuring wood moisture content are difficult to use effectively on paper.
Contact moisture meters can be found on eBay and through online sellers such as Grainer and
Professionalequipment.com.
The method used by the author, and many others, judges the relative tone of the paper when it is
rewet. Experience and judgment are required; however, it is efficient when preparing to flatten a
sheet.
The appearance of the paper at the “Onset” is light gray. The degree of grayness is not precise, but
the drape and floppiness of the damp sheet is an ancillary indicator. A specific average solids content
was calculated for the “Onset” point from six historic papers in V & S (1992), it was found to be 68% 
3% solids, or 32% water.
Some workers find it safer to spray the sheet with water to a saturated appearance of “gray,” which is
about 55% to 65% solids (45% to 35% water) thereby allowing for evaporation during the manipulation
of the sheet into the blotter stack.
Once the fibers pass the “Onset” point of 68% solids, where they begin to bond to one another, the
less that can be done to manipulate the pliable damp fiber-mat using the tools of
(a) blotter smoothness or roughness (sides are often different)
(b) medium or soft compacting felts (white felts tend to be uniform; gray felts are more variable)
(c) platemark templates (used to mimic the plate in the flatting process)
(d) weight (100 to 400 pound of lead shot in sealed plastic containers, 25-lb each)
Paper can never be the un-formed from the physically entangled mat created when the sheet was first
formed; see Fig. 4. Paper still has significant strength when wet because of the physical
entanglement of fiber within the sheet.
When the sheet appears “gray,” the wet fibers are free to be shoved past one another, within the limits
of physical entanglement. One cannot push a string. The wetter the paper, prior to local treatment,
the more effective these small scale manipulations can be. A crease can never be flattened; it can
only be coaxed back into plane. Mechanically creases fibers cannot have their surfaces smoothed.
The damaged fibers can be coaxed somewhat into the nominal plane of the sheet.
Many factors can work against predicable flatten behavior
(a) coatings causes one side of the paper to shrink or swell more than the other, the coating could
be a Japanese paper lining
(b) paper has been stretched out-of-shape by being stored (i) formation fault, (ii) misshapen over
time, (iii) mishandling or (iv) hard creasing
(c) flattening was outside of optimal conditions leading to a broad gentle warp or curl that develops
after the sheet is removed from the blotter stack.
Flattening problems can often be resolved by simply rewetting and reflattening the sheet in a
sympathetic manner. Introducing liquid water (spray) is generally the only method of releasing driedin problems (cockles, over-flattening curls warps, etc.) and other flattening issues.
However, if the drying fault keeps reoccurring or occurs intermittently, it is likely that a damp blotter
stack may be the culprit. Another stack may be required or a more staggered workflow may be
beneficial. Normal thin blotters will warp if dried from damp in the open air; thus a blotter stack must
dry under light weight. Thick blotters appear to be more forgiving, but if they are very damp (cool to
touch) they will also warp if not weighted during drying to equilibrium with room humidity conditions.
Thicker blotters (40 point) in blotter stacks are more efficient at eliminating moisture transferred from
paper to blotter during drying, because they offer more edge to the open air along the open sides of
the stack. In addition they are much larger, 42 x 60 and opposed to 24 x 40 for the 10 point type
blotters. It may seem efficient to use less water in the humidification or rewetting process, or to allow
some air drying before flattening in the blotter stack, but insufficient rewetting will be argued against,
below.
Note 2: Monitor moisture content using infrared thermometer or contact paper
moisture meter
One way to judge continued evaporation after removal from the blotter stack is to us a
non-contact infrared (IR) thermometer. IR thermometers can be found for $65-130 from
websites such as http://www.professionalequipment.com, http://www.extech.com and
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http://www.granger.com or your local big box hardware store
store. IR thermometers and
contact moisture meters sets can be found at Extech (MO290: Pinless Moisture
Moistu Meter &
IR Thermometer) http://www.extech.com/instruments/product.asp?catid=11&prodid=518
://www.extech.com/instruments/product.asp?catid=11&prodid=518,
://www.extech.com/instruments/product.asp?catid=11&prodid=518
they run about $270 at nationwide resellers such as WW Grainger’s,, or online.
online
Evaporation of liquid to vapor consumes energy
energy, or heat.. The use of the energy for
evaporation within a sheet for causes the sheet to cool. If a dry paper is at room
temperature, an adjacent sheet with water evaporating from its surface will be cooler.
cooler The
actual degree
e of coolness is limited by the dew point of the air.. When the
th dew point
temperature is reached,, evaporation will stall due to formation of water on the sheet as it
condenses out from the air at dew point
point. Evaporation will not proceed until the sheet
stops cooling due to evaporation. Note that a source of energy (heat) is needed for
continued evaporation to occur when dew point is reached
reached. A non-contact
contact thermometer
will show a slightly damp paper to be cooler than the rest of the paper in the room,
room
assuming that the dry paper is in equilibrium with the moisture content of air in the room.
7 - Texture Ruler
Effective experimentation is facilitated by building a T
Texture Ruler. The ruler is used to judge the
degree of flatness of samples before and after treatment. It is an excellent tool for judging small
changes in paper texture due to procedura
procedural or materials changes. Ability to measure changes after
treatment eliminates much of the guesswork. Jon Arney has conclusively shown that human vision is
the most sensitive technique for accessing paper texture.
A ruler is created by selecting 5 or mor
more samples of papers that represents the range
ange of paper texture
you common in your practice.. In the 1990
1990, Smithsonian CAL/MCI, Drying & Flattening Seminar, five
5” x 7” samples were use due to space limitations
limitations. If space is available, use 10’’ x 12”
1 samples, and a
6 -12 foot ruler. The texture ruler can be made of cardboar
cardboard, and folded-up
up when not being used.
used
Divide the ruler into 100 units, the length will depend on the size of the samples
samples.. One
O sample should
center on one value, but span about six, or so, units. The samples should be more-or-less
more
evenly
distributed along the ruler’s scale.
Lighting is critical. The
he light should be uniform and from the same direction. A single fluorescent tube
4 feet long would work well; several 2’ to 4’ bulbs in inexpensive fixtures,, end to end,
end will be superior
to moving the light along the scale. The light should be oriented at about 15° to 25°
25 off of the viewing
plane, and about 1 to 3 feet away frrom the ruler, behind the ruler;; in front of your viewing location.
locati
The track lighting shown in Fig 8 was not satisfactory. Blister-packed, 2-foot-long T5
T fluorescent tubeand-fixture
fixture sets can be found in hardware stores for under $10
$10,, they will be ideal for the task.
task
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Figure 8: This image was made at the SI-CAL
CAL (now called MCI) showing participant using small texture rulers with 5” x
7” samples. As many
any as 8 rulers were in use at one time, used to measure the effects drying procedures on samples.
While a known (control) is desirable, fixed texture exemplars define 3 to 5 points along the ruler, so that treated
samples can be measure against those exe
exemplars, before and after treatment.
8 - Paper Texture Size Domains - Evaluating Paper Texture, Flatness
Research has shown that paper flatness and paper texture exist across a range of size scales. That
is, when evaluating surface texture and flatness
flatness, many components make up the overall appearance.
Those components can be organized
ized by the relative size of the physical phenomenon being
observed. Jon Arney first defined this surface texture organizational concept [mid--1980s], where each
size domain is ten times larger than
n the previous level
level. In scientific studies, the concept is
i
summarized as “an order of magnitude larger
larger.” This concept evolved from the powers-of-ten
pow
used in
-1
scientific notation, where: 10 = 1; 101 = 10; 102 = 100; 103 = 1000; etc.
Paper Texture Size Domains
Imperial
Meters
Microns
Size Name
Scale Name
39”
1
1,000,000
Meter
Paper Warp & Curl
3.94”
0.1
100,000
Decimeter
Cockling
3/8”
0.01
10,000
Centimeter
Coarse Paper Texture
1/25”
0.001
1000
Millimeter
Fine Paper Texture
1/400”
0.0,001
100
Many Microns
Fiber Conformation
1/400”
0.0,001
100
Many Microns
Fiber Conformation
1/4000”
0.00,001
10
Micron
Surface Roughness
1/40,000”
0.000,001
1
Single Microns
Fine Surface
urface Detail
Table 2: Lists the six major size domains, a seventh is listed but only for the sake on completeness, its texture
is far too small to be seen by humans except as the presence or absence of gloss.
Figure 9: Shows a full sheet of handmade
de paper 12
12-1/4”x 19-1/4”,, this depicts the texture seen in the Meter size domain.
domain
This sheet was never re-dried;
dried; it shows an original conformation. The image was taken using raking light to emphasize
texture.
Meter Scale – Paper Warp & Curl
The large-scale size domain is on the order of multiple decimeters square, up to a meter square (40”
on a side). This scale is used to evaluate the drape, curl or warp of the sheet.
Generally, this size domain is influenced by (a) the dried
dried-in strain’s release, (b) the timing of the
releases and (c) the moisture content when removed from the drying stack.
If your drying protocol was developed during drying experimentation, a warp or curl should not be
present or develop over time after being pulled from the bl
blotter
otter stack. If warp or curl does show up,
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look for the source
ce in the paper itself or to an applied coating
coating. Suspect dampness
ampness in blotter stack.
The S & V (1992) “optimal”
ptimal” blotter release protocol to control this domain is
(1) 5 - 10 minutes, the sheet will be damp, about half dry
(2) 30 - 60 minutes, the sheet will seem to be all but dry, it will not be in equilibrium with room RH
(3) 4 - 18 hours, I generally go overnight.
Figure 10a & 10b: The paper sample on the left (Fig 10a) shows the decimeter size domain, it is roughly 1 decimeter
centimeter on a side (4” x 4”); the black line at the top is one decimeter (3.94”). The image on the right (Fig
(
10b) shows the
centimeter size domain, about ½” x ½”; the line near the top is 1 centimeter (½”). Note that both images are from the same
handmade paper shown in Fig 8, the upper left corner.
Decimeter Scale - Cockling
The next larger scale covers the relationship between groups consisting of many thousands of fibers,
to each other; an area of a square
e dec
decimeter (3.94" square). Cockling texture
exture is influenced most by
the higher than normal initial
nitial wetting
wetting, accompanied by a slight stretching while being held flat using a
prolonged first drying stage at light to moderate weight (drying pressure)
pressure). The goal
al is to get fibers in
the edge squish together, past one another in a state of maximum un
un-bonding,, while stretching the
inner part of the sheet by prolong the first blotter release
release. Optimal placement of weight will be helpful.
Degree of cockling is influenced
uenced by (1) formation, (2) moisture content of the paper (or blotters in
contact with the artifact in drying stack) upon release from th
the stack of blotters or (3
3) to the failure to
release the strain of drying byy not following the optimal drying protocol for your equipment.
Drying protocol reported has significant influence on this size domain. Releasing the dried-in strains
late during drying facilitates the management of this size domain.
Wetting a sheet with liquid water, using a spray gun, not a ha
hand-sprayer
sprayer (develop a lot of stray “big
drops,” making a mess) until the color of the white paper is about medium gray.
A ‘prolonged first stage” protocol might be
(1) 15 - 30 minutes, the sheet will be damp, about half dry
(2) 60 - 90 minutes, the sheett will seem to be all but dry, it will not be in equilibrium with room RH
(3) 4 - 18 hours, or overnight.
If the sheet is left in the open air during the firs
first blotter change for more than 30 seconds, the sheet
will warp. The wetting and flattening pro
process may as well be started over.
Obviously the
e first blotter change is the most critical and should be prepared for well in advance. The
second blotter change, if far less critical. However, if the sheet was left out it would irreversible warp
due to the slight imbalance of moisture content between it and the air of the room. If the blotter stack
is damp from heavy use, second and third stages will need to be far longer. Extend for as long as it
takes for the edges of the blotter stack to come into equilibrium with the surrounding air.
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Figure 11a th& 11b: The images depict the Millimeter Scale size domain.. The image on the left, Fig 11a, is a 4x4 millimeter
corner a 17 century antique laid handmade paper. The image on the right, Fig 11b, is ffrom the same handmade paper
shown in Figs 9 & 10.
Figure 11c & 11d: The images depict the Many
Many-Microns Scale size domain. On the left, Fig 11b, is a photomicrograph from
the surface of a modern rag paper at captured at 22X through a light microsco
microscope with a 100-micron
micron scale bar. Figure 11c
(right) is an scanning electron beam image ((SEM image) of a different handmade
made paper that was less beaten, but shows a
more compacted surface texture. Both image
images show individual fibers that range in size fibers from 10 to 50 microns wide.
Millimeter Scale – Fine Paper Texture
This size domain is on the order of a millimeter square
square, 1000 x 1000 microns,, or about 1/16” square.
square
Fine Paper Texture
exture is the relationship between groups of fibers,, such as a wire marks
mark in a handmade
papers structure as seen in Fig 11a
11a. Figure 11b shows an unfortunately blurry image,
image made from a
800 pixels square corner of the 4800 ppi digital capture of the sheet; it is from the same 12¼ x 19¼
handmade paper seen in Figs 9 & 10. The size domain also includes the evidence of letterpress,
letterpress
intaglio and lithography printing technology.
Fine texture is most influenced by the details of paper flattening technology
(a) blotter and felt surface used in direct contact
(b) degree of initial wetness
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(c) extended-period first blotter change to set impressed fine surface texture from blotter surface
(d) weight on drying stack.
The optimal drying protocol reported in S & V (1992) has marginal influence on this size domain. This
size domain is difficult to influence in a positive manner during drying and flattening procedures.
Weight can have an influence especially if a bare blotter surface is used against the very wet surface,
and the first blotter change is postponed for an extra 5 minutes. The separation during the blotter
change could be critical, so good planning and contingency forethought will be issues.
Drying the paper causes it to shrink; if the sheet is held in place by the weights on the stack, a stress
will build up in the paper. Strain is the physical act of stretching, stress is the resulting mechanical
load imparted to the material; stress is the mechanical load due to the strain.
If a sheet is held under an unreleased strain, the sheet will emerge from the blotter stack with a curl,
often in the direction of greatest expansion (across the machine direction – fiber width direction). If
the blotter stack has damp blotters, the sheet will emerge from the blotter stack flat and then curl over
time, as the moisture content of the paper dry to be in equilibrium with the surrounding air.
Should this occur the degree of cockling that evolves has to do with amount, and unevenness, of
moisture content within the sheet when the evaporation begins in open air. A lot of small uneven
drying imbalances will cause tight uniform cockling on the scale of 1”- 2”- 4” scale. Broad uneven
dampness upon emerging from the blotter stack will result in the larger scale warp of the sheet.
The more uniform the evaporation, the smaller the degree of cockle or warp. The more modest the
combined series of causes, the higher the likelihood that the process will push the deformation, warp,
into the larger Meter Scale.
Release of strain, trapped by failure to make blotter changes, without the influence of residual
evaporation, generally yields warping on the Meter Scale.
Many-Microns Scale – Fiber Conformation
This size domain includes the relationship between individual fibers and the relationship of one fiber
with its nearest neighbor. The relative height of one fiber to its neighbors is seen a surface degree of
roughness. The difference is well depicted in Figs 11 c and 11d. The fibers must be pliable to
influence this size domain. A surface can be smoothed or roughened, by selecting surfaces wet fibers
are dried against.
Evaluation criteria includes smoothness, fluffiness (see Fig 11b) and issues of isolated fibers (or
clumps) sticking up due to roughing the surface. After extended water treatment, a paper can appear
more open or fluffy because the sizing has been removed, inadvertently, during stain reduction.
Thoroughly rewetting, and then reforming the hydrogen bonds will help reverse a fluffy appearance.
Capillary action in the pores between fibers, during dry down, have orders of magnitude higher
internal pressure than can be applied with flattening weights (25 lb bas of shot) during flattening.
Water can be used to increase surface compaction. Resizing, with either gelatin or methylcellulose,
will gap-fill and help to seal the surface, but it won’t improve interfiber bonding.
Humidification and flattening have influence on this size domain. High wetness at the start of
flattening will influence the fine details more and help to compact the sheet. Experimentation with
blotter stack weight is worth analysis but the difference between (a) 50 pounds on 500 in2 (20” x 25”) of
blotters and (b) 200 pounds over the same area (500 in2) is 0.1 psi vs. 0.4 psi. About 500 pound of
weight would be needed to increase flattening pressure from 0.1 psi to 1.0 psi, an order of magnitude
difference. About 5000 pounds of weight would be needed on a 20”x 25” blotter stack (500 in2) to
simulate the lowest estimated pressure predicted for historic screw presses (10 psi).
Micron Scale – Surface Roughness/Glossiness
This size domain is the smallest in the list. Humans can’t resolve detail smaller than about 80
microns, 6-line-pairs-per-millimeters, or 300 ppi in the digital domain. We have no hope of seeing a
single micron in relation to another single micron.
Most paper fibers are about 10 to 50 microns wide; we don’t see them without magnification. This
size domain, therefore, includes the information involved in the evaluation of fine surface roughness
such as gloss or sheen.
The evaluation of the size domain includes how light is scattered, or not scatter, from the surface.
Gloss is most influenced by burnishing or application of a coating such as surface sizing.
A smooth surface is influenced by direct contact with a rough blotter; no polyester web release sheet
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would be used. The smooth and rough sides of blotters have slightly different effects.
When flattening to influence the “Micron” size domain: start with (1) higher than normal water content,
(2) avoid interleaving, (3) use greater blotter stack weight, and (4) use less frequent blotter changes.
This should facilitate mimicking the rough surface of the blotter and diminishing surface gloss.
The use of polyester paper interleaving will disallow the mild hydrogen bonding to develop between
cellulose of the sheet and the blotters; the surface should appear to be smoother than without
interleaving.
Felts and very high weight can be used to bring back certain types of original surface texture that
might be lost to surface treatments, such as local burnishing of creases. The hairs in a felt’s surface
are critical to restoring the handmade-paper-surface appearance; see Fig 11c.
9 - The Flattening Process
The flatting process must start with the sheet being rewet. Most of us have discovered that just
weight a curled edge will not flatten it back into place. The most effective method is to wet the sheet.
9.1 - Wetting
Humidification while in equilibrium with air at 100% RH softens the fibers, alone, as much as they can
be relaxed. The absence of liquid water prevents further softening of the sheet.
In general, unless there are humectants (water absorbing salts) or a high degree of sizing in the
paper, liquid water will not form within the paper pores due to the absorption of water vapor. That is,
the type of water that is allowed through the openings in Gortex, water vapor, single water molecules.
To achieve full softening of the sheet, liquid water must be introducing into the sheet. This can be
done by spraying with a “detailing” spray gun, a hand-pump sprayer, a pressurized metal body
sprayer (Dahlia Sprayer) or a hose attached to an ultrasonic humidifier.
Liquid water is needed to breach the fiber-to-fiber bonds (hydrogen bonds) as seen in Fig 2. Breaking
the bonds between fibers allows the sheet to fully relax, so that the most natural flattening can be
achieved.
Paper expands when it is wet. The more water that is used for wetting, the more pores within the
sheet will be filled and the higher number of fiber-to-fiber bonds that will be broken during wetting.
The greater the swelling the more the sheet expands.
Humidification will only soften and swell the fibers. On the other hand, immersion in water will
saturate the sheet immediately; filling all the open pores. In most cases however, the paper will still
float because some voids (closed pores) have yet to be filled with water; over time, these pores will
also fill and the paper will sink in a bath of water. Spraying the sheet with water will introduce liquid
water into the paper somewhere between humidification and immersion, depending of the amount of
water transferred into the paper.
The most accurate method for measuring moisture content is the weight of paper dripping wet vs
weight of the sheet completely dry. This is not a realistic method when treating artifacts. Judging the
degree-of-wetness based on the degree-of-grayness (saturating a porous material with water) works
reasonably well. There are drawbacks, one of which is remembering what the sheet looked like when
dripping wet or maybe the sheet has never been completely wet. What can be said, is that, as the
paper takes on water it turns gray.
Use of a moisture meter made for paper is the best method of accurately knowing the water content.
However, this may be more information than the experienced worker needs. Many workers use their
moisture meter actively when they first get them and the only occasionally once they get to know the
gray appearance of wet paper.
Curl develops when one side expands more than the other. The sheet curls on the side opposite to
the one expanded. When one side is sprayed, it will expand and curl (concave). In most cases, an
artifact should be sprayed evenly from both sides.
9.2 - Wetting Technology
Spraying the sheet with fine-mist, such as that from “detailing” spray guns (DeVilbis EGA gun), is the
method recommend by many workers; see the Paper Conservation Catalog entry on Drying and
Flattening.
Hand pump sprayers (Windex type) and Dahlia (type) sprayer have wide appeal because they can be
coaxed to work reasonably well; they are much less expensive and are more readily available.
Unfortunately, they do not output a consistently precise fan of fine water droplets as does air-powered
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spray equipment, such as “detail” paint sprayers. A compressor-powered spray gun is the only
repeatable and foolproof method of applying a very fine mist of water uniformly over the entire sheet.
This recommendation excludes the $150-250 paint sprayer kits that use house-paint spray guns
(purchased for under $35, alone). Theses paint sprayers are just too coarse for conservation
purposes. The DeVilbis EGA gun can be found on eBay for about $75-125; a compressor capable of
40 psi will be needed for the air supply. They cost about $100-175 at your local big-box building
materials supply stores.
Humidification using an ultrasonic humidifier will introduce small amounts of free water into paper.
The fog that is produced by ultrasonic humidification is made of very small water droplets, not
individual water molecules. Once the fog is in room air, if the air is warm enough, it will evaporate into
vapor. Water vapor consists of discrete (individual) water molecules; they have taken on the heat of
vaporization. A fog is made of small droplets of liquid water, the drops contain hundreds of
thousands of water molecules formed into a drop.
9.3 - Drying
Paper shrinks during drying.
As paper shrinks, the weight on the drying stack in the flattening process creates the “stress of
shrinkage.” This stress is held it in the paper by the weight on the blotter stack. The greater the
weight, the more the strain cannot release itself within the stack. Weak papers, even mended papers,
can tear if the strain of shrinkage is not released, by making a blotter change.
The first blotter change is the most critical, because that is where most of the shrinkage occurs.
Research has shown, S & V 1992, that releasing the strain of flattening, at about
(1) 5 - 10 minutes and
(2) 30 - 60 minutes into the flattening process
eliminates the broad curl that can occur in the Meter size domain. This is a flattening fault commonly
seen in papers that are “called” over-flattened. The final release from the blotter stack occurs
somewhere between
(3) 6 and 18 hours,
depending on how damp the paper was at the start and how often the blotter stack has been used. A
“much used” blotter stack will be damp. If the sheet is still slightly damp when pull from the blotter
stack, that is, it starts “flat” and then proceeds to slowly develop a warp it is curling from the
unrestrained drying of the final stage to equilibrium with room conditions. The final shape depends on
many factors discussed in the Meter and Decimeter scales discussed above.
The Process
Since over half of the sheet’s drying occurs in the first 5-15 minutes, the first release of the building
strain is released within that period (S & V 1992): 5-15 minutes. Almost 90-95% of total drying occurs
in about 45-90 minutes, so the second release is always within that period (S & V 1992): 45-90
minutes.
The timing of final release from the stack is also critical because slightly damp paper will curl, cockle
and/or warp. The paper’s moisture content has to be in equilibrium with about 70-80% RH (or less),
for curl not to occur upon release from the drying stack, assuming the room is at 50% RH. The final
stage is 4 hours to overnight
9.4 - Flattening
Flattening technology manipulates the softness of wet fibers (hydrogen bonds within fibers); the
hydrogen bonding between fibers (interfiber bonding); the shrinkage of drying fibers; and the degree
of “shrinkage strain” within the sheet. Fiber shrinkage drives sheet curl, warp and distortion.
To control the flattening process Conservators use:
(a) softness and smoothness of flattening tools surface – felts and blotters
(b) separation between blotter cellulose and paper cellulose – polyester paper release sheet
(c) flattening pressure – weight on blotter stack
(d) surface sizing – used to gap fill and seal the surface
(e) release of the shrinkage strain during the drying (shrinkage) process
(f) weight on blotter stack.
9.5 - Flattening Materials
The use of two thin felts, with modest pressure, yields maximum fine and coarse texture with no
cockling. The use of thinner felts and greater pressure, yields a higher degree of flattening,
eliminating some fine and coarse texture. Using thicker felts requires greater pressure, but can yield
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a variety of desirable results.
Substituting a blotter for one of the felts produces even greater fine surface texture. Use of thin
polyester interleaving eliminate most of the (a) coarse and much of the (b) fine texture.
Using two blotters with coarse surface texture on both sides, with no interleaving, produces a sheet
with about 10-40% of the texture of the blotters used.
Using 10-point smooth blotters on both sides, produces a very smooth sheet in the Millimeter size
domain. Using 40-point blotters with coarse texture can produce quite a bit of new fine and coarse
surface texture in the Millimeter size domain. The greater the original wetness and the greater the
weight on the blotter stack, the more texture will be imparted. An overflattening (slight curl on the
Meter scale) appearance will result if the strain of drying is not released early when using thick
blotters.
Weight Issues
The use of weight is often misunderstood. The general rule of thumb is that more weigh results in
greater flatness. Very light weight, applied in an effective strain-release protocol can produce the best
results on sheets with no other drying issues.
If the pressure is too high during flattening, several size domains can be affected. Principally the
coarse and fine texture is obscured, however, if the “strain of drying” is not released because the
weight stack is too complex a broad warp will result. Evidence of printing can be lost, at even
moderate flattening pressure.
One standard blotter size is 24” x 40”, thus a common blotter stack surface area is 480 in2 (20 x 24).
The round number this size is 500 in2 for faster calculations. When four 25-lb bags of lead shot are
placed on that blotter stack, the average pressure per square inch is 0.2 psi (500/100 = 0.2 pound per
square inch) . With ten 25-lb-bags of shot, the pressure per square inch is only a half-pound
(250/500=0.5). It is almost impossible to use too much pressure on a blotter stack. Not releasing the
strain of drying is a much more grave flattening fault.
At the 1990 CAL/MCI Drying and Flattening
Seminar, ¾” x 4” planks using 7-ply plywood
framed upper and lower platens held apart by
⅜” threaded rod stock to connect the two
platens that use automobile tire inner-tube
presses [originally designed by Bob Futernick]
were capable of exerting about 20-25 psi before
the rubber inner-tube failed. Note the unused
2”x 4” double bolted frames in the lower right of
the image that were not used because they
could fail during flattening. For a 20 x 24 blotter
stack, that is 20 psi, the highest pressure press
that could be designed for the 1990 workshop.
The lowest pressure used was 0.1 psi (50
pounds of lead shot). In the background note
the other blotter stacks being experiment with
by enthusiastic teams seeking to come up with
a more favorable flattening outcome. The
results were evaluated using the texture ruler,
in preceding days as shown in Fig.8.
Figure 12: The automobile tire inner-tube press. The rubber
inner-tube was housed below the blotter stack as seen in the
The highest level of weight at the 1990 Seminar
open end; built by Messier for the 1990 Drying & Flattening
was well below the 50 to 150 psi pressure that
Seminar.
is suggested by folks who have posited
calculations on the size and force exerted in historic screw press.
No surface damage was observed when using 20 psi. Failure to release the “strain of drying” was
very obvious when using the inner-tube press. The major faults observed were (1) the loss of texture
from smooth blotters in contact with the artifact, or (2) the introduction of the impression of large
fibers/hairs on the surface of some of the felts used, for papers with no felt texture originally.
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Figure 13 shows the famous Christa
Gaehde mechanical press. Images
were obtained by Jane Sugarman when
she worked with Gaehde in the early
1990’s. Note the strain gage readout in
the mustard-colored box at the front of
the press platen. The pressure was
applied with a complex screw
mechanism. The maximum pressure
possible is unknown by the author. The
weights on the back wall of the image
on the right were counter weights to
facilitate moving the heavy top platen.
Overflattening
Overflattening is difficult to define.
Paper conservators and Curators know
it when they see it, however. Some
notable curators have also said they
can tell when a sheet has been
flattened. Generally, the property they
Figure 13: Christa Gaehde’s famous mechanical press shown open
and is use on the right. There was a motor that opened and applied the are observing has at least two markers:
pressure via a screw mechanism. Counter-weights on the wall behind
(a) a broad warp on the Decimeter (2 the press on the right image made handling the top platen easier.
10”) or Meter (15 -70”) scales and (b)
the loss of the plate or stone mark.
10 - Scientific Data on the Relationship of Pulp Sheet Formation to Re-drying a Wet Sheet
The following plot (Fig 14) brings together very different types of data from paper formation to
Figure 14: Plot showing strength, shrinkage and dryness (time) against percent solids; showing relationships. The xaxis shows percent solids from fibers floating in water at 10% solids, on the far left, where a fiber mat begins to form,
see Figure 8-2 (Parker). As water is extracted from the fiber mat it becomes a sheet at about 40% solids, the
remaining water is extracted until the formed sheet reaches equilibrium with the surrounding air, about 6-10% water
or 90 to 94% solids.
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rewetting and drying. It’s based on evaluations with one property (percent solids) common to the
work of Robertson (1954 & 1963) and Rance (1963 & 1964) and from the Paper Industry literature
along with and investigations by Sugarman and Vitale leading to their 1992 JAIC publication. The
point of the very complex plot is to show that there is a direct relationship in the process of drying
paper, whether it is just being formed or is re-dried from being rewet.
Note the color coded plot lines that are correlated to (i) author names, (ii) property and (iii) Y-axis
scale, plotted against the X-axis (percent solids) common to all plots. The data was repurposed from
plots in the original publications so that it could be use in Fig 14; also see Figs 15 thru 17.
Figure14 shows that when a sheet of paper is rewet it reached about 40% solids. As the rewet sheet
dries the percent solids increase (drying). On the x-axis, drying moves from left to right. At about
68% solids the “Onset of Physical Distortion of rewet Sheet” is shown with a dashed vertical green
line. Note that Rance’s shrinkage data plot (blue) shows a significant inflection point at about the
same percent solids.
Rance’s data (blue) shows that the initial point of increase in rate of shrinkage occurs at about 40%
solids. At this point, the fiber furnish begins to link together, transferring individual fiber shrinkage to
the sheet, via the formation of hydrogen bonds between fibers.
This is close to where Robertson’s tensile strength data (red plot) shows a dramatic increase (about
46% solids). Clearly, this is where large scale hydrogen bonding takes hold in the de-water mat, the
mat’s tensile strength increases as solids content increases.
.
Figures 15: Rance’s published data (TAPPI, 1964, Fig 3, below) on paper shrinkage which was re-plotted by the author to
reveal the change in property slope which defines a change physical state. The data is plotted in Fig 14 as the blue plot
line.
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Slope #1 in Fig 15 (20% to 40% solids) shows slow shrinkage of the sheet based on loss of water.
Figure 16 shows Parker’ss Figure 8.2 depicting the formati
formation
on of a sheet of paper from fiber slurry
(upper left) on the papermaking machine
machine. The upper left of Fig 16 helps to visualize the process
depicted by Rance’s slope #1 shrinkage data.
Slope #2 in Fig 15 (40%
40% to 75% solids
solids) shows a faster shrinkage process. The
he mechanically
intertwined fibers link the fiber mat together so that contraction of sheet size produces shrinkage of
the fiber mat. The rate is faster because the mechanical intertwining
intertwining;; this is depicted Fig 16 in the
drawing on the right.
Slope #3 in Fig 15 (75% to 94% solids) shows an abrupt change at about 75% solid,
solid the fibers are so
close, due to the loss of water, that the capillary action of water helps bridge the gap, that hydrogen
bonding between fibers lock fibers to one another impa
imparting
rting even greater contraction of size to the
sheet.
Figure 16: Graphic was taken from Parker, Fig 8.2
8.2; however the actual reference was lost. The drawings show the fibers
falling out of the slurry as the intertwined fiber mat forms, on the left. The drawing on the right shows the thickening fiber
fib
mat.
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Figure 17: Original graph (Fig 16) from Robertson’s (1954), with handwritten data called out. The data was repurposed
for use in the Fig 14 as the red plot line, which depicts the increase in strength of the forming fiber mat.
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Figure 18: Sugarman & Vitale (1992) data gathered while observing the drying of three sized papers. Data from all six
papers evaluated (three of which were unsized) was averaged for the “composite” line used for the green plot line in
Figure 14.
11- References
a) Primary reference
Sugarman, J. & Vitale, T.J. 1992. Observations on the Drying of Paper: Examination and Application of Drying
Methodologies to Treatment. JAIC. 31(2): 175-197. <ttp://aic.stanford.edu/jaic/articles/jaic31-02-003.html>
b) Direct references for S & V 1992
Arney, J. S. & Gammell, J. R. 1990. The interaction of light with objects of art: Aging and seeing. The Spectrum 3(3):9–13.
ASTM. 1982. D644–55. In 1983 Annual Book of ASTM Standards Section 15, Vol. 15.09. Philadelphia: American Society for
Testing Materials.
Christiansen, P. K. & Giertz, H. W. 1966. The cellulose/water relationship. In Consolidation of the Paper Web, ed.
F.Bolam. London: Technical Section, British Paper and Board Makers' Association. 1:59–89.
Rance, H. F.1954. Effect of water removal on sheet properties. TAPPI Journal 37(12):640–48.
Robertson, A. A. 1963. The Physical Properties of Wet Webs. Svensk Papperstidning66 (12):477–97.
Robertson, A. A. 1966. Measurement and Significance of the Water Retention Properties of Paper-Making Fibers. In
Consolidation of the paper web, ed. F. Bolam. London: Technical Section, British Paper and Board Makers' Association.
1:90–115.
Robertson, A. A.1970. Interactions of liquids of liquids with cellulose. TAPPI Journal53 (7):1331–39.
Rowland, S. P. 1979. Solid liquid interactions: Inter-and intracrystalline reactions in cellulose fibers. In Applied fiber science,
ed. F.Happey, London: Academic Press. 2:205–37.
Skelton, J. 1975. Interfiber forces during wetting and drying. Science190 (4209):15–20.
Sugarman, J. 1985. The effect of different drying techniques on the surface qualities of paper. Unpublished manuscript.
Winterthur Museum.
Vitale, T.J. 1992. Effects of Water on the Mechanical Properties of Paper and Their Relationship to the Treatment of Paper.
In Materials issues in Art & Archaeology Ill, Symposium Proceedings. eds. Vandiver, Druzak, Wheeler & Freestone.
Pittsburgh: Materials Research Society. 1992. 397-427.
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Vitale, T.J. 1992. Effects of Drying on the Mechanical Properties of Paper and Their Relationship to the Treatment of Paper.
In Materials issues in Art & Archaeology lII, Symposium Proceedings. eds. Vandiver, Druzak, Wheeler & Freestone.
Pittsburgh: Materials Research Society. 1992. 429-445.
Wink, W. A. 1961. The Effect of Relative Humidity and Temperature on Paper Properties. TAPPI Journal44 (6):171A–80A.
c) Indirect references used for Drying, Rewetting and Flattening study
Atalla, R. H. 1981. The Crystallinity of Cellulosic Fibers. Dependence on History and Influence on Properties. In
Preservation of paper and textiles of historic value II. Advances in Chemistry Series 193. Washington, D. C.: American
Chemical Society. 169–76.
Byrd, v. L. 1971. The Effects of Humidity on paper. Forest Products Department, U. S. Department of Agriculture, Chem.
26. 30–35.
Corte, H. K.1976. Perception of the Optical Properties of Paper. In The fundamental properties of paper related to its uses,
ed.F.Bolam. London: Technical Section. British Paper and Board Makers' Association. 2:709–11.
Corte, H. K.1982. The Porosity of Paper. In Handbook of Paper Science: The Structure and Physical Properties of Paper,
ed. H. F. Rance. Amsterdam: Elsevier Scientific Publishing Company2:1–70.
Daisley, P. A.1976. Quantification of Subjective Quality. In The Fundamental Properties of Paper Related to its Uses, ed.
F.Bolam, London: Technical Section, British Paper and Board Makers' Association. 2:709–11.
Gallay, W. 1976. Textural Properties of Paper: Measurements and Fundamental Relationships. In The Fundamental
Properties of Paper Related to its Uses, ed.F.Bolam. London: Technical Section, British Paper and Board Makers'
Association. 2:684–95.
Hemadi, S., Huszar, J. & Lengyel, P. 1977. Interactions between liquids and paper [Shortened version of the original
article, prepared by R. W. Hoyland, University of Manchester Institute of Science and Technology, Manchester.] In
Transactions of BPBIF symposium (Oxford): Fiber-water interactions in paper making. London: The British Paper and Board
Industry Federation. 693–707.
Page, D. H., & P. A. Tydeman. 1962. A new theory of the shrinkage, structure and properties of paper. In The formation and
structure of paper: Transactions of the symposium held at Oxford, September 1961, ed.F.Bolam. London: Technical Section,
British Paper and Board Makers' Association. 397– 421.
Page D. H., & P. A. Tydeman. 1966. Physical processes occurring during the drying phase. In consolidation of the paper
web, ed.F.Bolam. London: Technical Section, British Paper and Board Makers' Association. 1:371–96.
Robertson, A. A.1964. Some observations of the effects of drying papermaking fibers. Pulp and Paper Magazine of
Canada65:161–68.
Rowland, S. P.1977. Cellulose: Pores, internal surfaces, and the water interface. In Textile and paper technology, ed. J. C.
Arthur. Advances in Chemistry Series 49. Washington, D. C.: American Chemical Society. 20–45.
Scallon, A. M.1976. The accommodation of water within pulp fibers. In The fundamental properties of paper related to its
uses, ed.F.Bolam. London: Technical Section, British Paper and Board Makers' Association. 1:9–29.
Scallon, A. M., & J. Borch. 1976. Fundamental parameters affecting the opacity and brightness of uncoated paper. In The
fundamental properties of paper related to its uses, ed.F.Bolam. London: Technical Section, British Paper and Board
Makers' Association. 1:152–71.
Setterholm, V. C., & Chilson, W. A. 1965. Drying Restraint. TAPPI Journal48 (11):638–40.
Stamm, A. J.1950. Bound water and hydration. TAPPI Journal33 (9):435–39.
Stone, J. E., & Scallon, J. A. 1966. Influence on drying on the pore structures of the cell wall. In Consolidation of the paper
web, ed.F.Bolam. London: Technical Section, British Paper and Board Makers' Association. 1:145–73.
Tim Vitale
Paper & Photography Conservation
Digital Facsimiles & Digital Restoration
Migration of [still] Film to Digital
Preservation & Imaging Consulting
Vitale Art Conservation
2407 Telegraph Ave.
Suite 312
Oakland, CA 94612
510-594-8277
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Website: http://vitaleartconservation.com/
Resume: http://videopreservation.conservation-us.org/tjv/vitale_long_resume.pdf
Video Preservation: http://videopreservation.conservation-us.org/ (2007)
Albumen website: http://albumen.conservation-us.org/ (2001)