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Archival Papers
In the ancient Greek seacoast town of
Abdera, 25 centuries ago, philosophers Leucippus and Democritus were teaching
that all matter is composed of empty space and indivisible bits of matter
called atoms. What little is left of Abdera is now only ruins. None of the
written works of Leucippus have been preserved over the millennia, and only a
few tantalizing fragments of the writings of Democritus still exist today.
However, their insight that all matter consists of atoms is, today, the foundation
upon which modern chemistry is based. To understand the chemical structure and
acid deterioration of paper, we need to know how atoms combine into molecules
which make up paper fibers.
The Atom
Today, physicists using massive particle
accelerators are continually breaking atoms into even smaller, more fleeting,
subatomic particles such as hadrons (pions, protons and neutrons), leptons
(muons, electrons and neutrinos) and quarks. Although fascinating, the subject
of subatomic particles would, at this point, unnecessarily complicate our
efforts to understand the atom and its role in the deterioration of paper
fibers.
The primary subatomic particles of atoms
are protons, neutrons and electrons. Atoms have a dense nucleus consisting of
positively charged particles called protons and neutral particles called
neutrons. A negatively charged cloud of particles called electrons orbits the
nucleus of the atom.
Protons and neutrons have almost 2,000
times the mass of an electron and, therefore, comprise nearly all the total
mass of an atom. This means, for example, that perhaps only one ounce of our
body weight is composed of electrons, the rest consisting of protons and
neutrons; yet we will see that these electrons form the bonds which unite the
atoms of elements such as carbon, hydrogen and oxygen into the molecules
(cellulose) which comprise paper fibers.
The reason atoms of elements differ from
each other is that they contain different numbers of protons, neutrons and
electrons. For example, an atom of hydrogen contains one proton and one
electron (hydrogen is the only element which has no neutrons). An atom of
carbon contains six protons, six neutrons and six electrons, and an atom of
oxygen contains eight protons, eight neutrons and eight electrons, while
uranium contains 92 protons, 146 neutrons and 92 electrons.
Nearly all known elements exist in
several isotopic forms. These isotopes are simply atoms of the same elements
which have the same number of protons, but differing numbers of neutrons. For
instance, the element calcium exists naturally in six isotopic forms. The
calcium isotopes thus contain 20 protons plus 20, 22, 23, 24, 26 or 28
neutrons, respectively. Atoms can also exist as electrically charged species
(called ions) meaning they have lost one or more electrons (positively charged
species) or gained one or more electrons (negatively charged species).
The electron cloud around the nucleus of
an atom is composed of a number of different energy levels or orbits.
If energy is absorbed by an atom, one or more electrons may jump from a lower
energy level to a higher one. When electrons return to a lower orbit
or energy level, they emit radiant energy in the form of visible light,
ultraviolet light, radio waves, x-rays or other wave lengths of the
electromagnetic spectrum. The number of electrons present in each of an atom’s
energy levels or orbits enables a chemist to make predictions about its chemical
properties. Atoms unite with other atoms to form molecules by sharing electrons
between the orbits of each atom. These chemical reactions occur primarily
between electrons in the outer energy levels or orbits of one atom with
another. When the electron clouds of separate atoms overlap and electrons are
redistributed among the outer orbits of these atoms, we find that
either one atom will lose one or more of its electrons to the other atom (ionic
bond), or each atom will share one or more electrons with the other atoms
(covalent bond). Since gaining or losing a negatively charged electron will
cause one atom to have either a less positive (negative) or more positive total
electric charge than the other atom, an electrostatic attraction is formed
between the atoms. This attraction is the chemical bond.
Covalent Bonds
The three atoms, carbon, hydrogen and
oxygen, when forming a cellulose chain (molecule), are held together by two
types of chemical bonds. The covalent bond, which is the primary holding force
between the glucose molecules making up a cellulose chain, and the weaker
hydrogen bond which plays an important role in forming cellulose chains into
adjacent sheets.
Covalent bonds occur when atoms share
one or more pairs of electrons between their outer energy levels or orbits.
This rearrangement of outer energy levels is such that the electrons are not
lost to an atom, but are shared between the orbits of the various atoms
comprising the molecule. This sharing of electrons between orbits, bond the
atoms covalently together into molecules. Atoms, such as carbon and
oxygen achieve stability by having eight electrons in their outer orbits.
However, an atom never has more than eight electrons in its outermost orbit.
For example, potassium has only eight electrons in its M orbit and one electron
in its N orbit, even though the M orbit of an atom is capable of
holding up to 18 electrons. Chemical reactions occur between atoms as they seek
to achieve a stable outer orbit (energy level) of eight
electrons.
Hydrogen Bonds
Different atoms vary in their ability to
attract electrons. Thus when atoms are covalently bonded together into
molecules, the negatively charged electrons will spend a disproportionate
amount of time orbiting the nucleus of the atoms which most strongly attracts
them. This causes the molecule to exhibit electrostatic polarity because one
end of the molecule will have a slightly more positive or slightly more
negative charge than the other. These oppositely charged ends of the molecule
are separated in the same manner as the oppositely charged poles of a magnet.
In the case of hydrogen and oxygen atoms which are being held together by
hydrogen bonding in adjacent cellulose molecules, the electrons spend more time
orbiting the oxygen nucleus, which gives it a more negative charge and the
hydrogen nucleus a more positive charge.
When molecules containing an oxygen atom
bonded to a hydrogen atom (cellulose) approach each other, the electrostatic
polarity of this bond will hold these molecules together. The positively
charged hydrogen atom on one molecule will be attracted to the negatively
charged oxygen atom on the other molecule. This attraction is called the
hydrogen bond and is responsible for holding adjacent cellulose chains together
to form sheets.
One additional bonding mechanism we find
in paper fibers is Van der Waals force. Van der Waals force is actually a
collection of three forces. 1) a dipole force such as the attraction you find
between the negative and positive poles of two magnets; 2) an induction force
such as that which causes a magnet to affect a non-magnetized piece of iron; 3)
the intermolecular forces in non-polar materials. This is the very weak
attraction all molecules have for each other. These forces act only at very
short distances.
We know that atoms of hydrogen, oxygen
and carbon are formed by combing protons, neutrons and electrons (see
illustration 3). These atoms bond together chemically to form molecules of
glucose as shown in illustration 4.
Cellulose
Cellulose, from which our paper is
fabricated, is built from glucose molecules bonded covalently together into
long chains. Each alternating glucose ring of the cellulose molecule is flipped
over and the water molecule (H2O) has been split out leaving an oxygen molecule
between each ring. This chain or ribbon (the cellulose molecule) will continue
for 3,000 to 5,000 glucose units (see illustration 5).
These long ribbon-like chains
(molecules) are built up into sheets which are held together by
the side-to-side hydrogen bonding between the chains (see illustration 6).
The sheets of cellulose (shown in
illustration 6) are held in staggered layers, one on top of another by Van der
Waals force. The geometry of the short, carbon-hydrogen bonds minimizes the
distance between layers and, therefore, Van der Waals forces (which are proportional
to the inverse of the sixth power of the intermolecular distances) are
maximized (see illustration 7).
These small units of cellulose formed
through side-by-side hydrogen bonding and layered by Van der Waals forces are called
microfibrils. These microfibrils will crystallize (organize into units) into
bundles by the same side-by-side hydrogen bonding and layer-to-layer Van der
Waals interaction that formed the microfibrils. These bundles are then
crystallized into fibers by the same side-to-side hydrogen bonding and
layer-to-layer Van der Waals forces. The microfibrils have nearly perfect
bonding, both side-by-side and layer-to-layer, but each successive stage of
formation has a progressively less perfect degree of bonding because any
imperfection in the first stages of crystallization will be progressively
magnified during progression to the final fiber formation. These fibers are
mixed with water and often other chemicals, beaten into a slurry and spread
onto a forming screen. They are then pressed together and dried to produce a
finished sheet of paper.
Acid Deterioration
The paper just described and depicted in
illustration 8 is a completely pristine sheet made entirely from long chain, alpha
cellulose fibers with no additives or impurities of any kind. Alpha cellulose
is the pure, long chain cellulose depicted in illustration 5. Unfortunately,
most paper available today contains a variety of additives, impurities and
other less stable plant products which cause acid deterioration of paper. Other
culprits which also have a deleterious effect on paper are environmental and
atmospheric acids and pollutants. As you may have surmised when reading about
the construction of the paper fiber, the destruction follows essentially the
same route but in the reverse direction. Acids attack the bonds which hold
together the glucose rings, the cellulose chains, the microfibrils, the bundles
and the fibers.
What is an acid? A simplified, but
acceptably accurate description is that an acid is any substance which can
donate a proton. Earlier it was mentioned that the hydrogen atom is the only
element which has only one proton in the nucleus and one electron in orbit.
When hydrogen loses that negatively charged electron, it becomes
positively charged (an ion), consisting of only one proton.
This proton is strongly attracted to negatively charged electrons which overlap
and share outer energy levels or orbits with other atoms to form the
chemical (in this case, covalent) bonds which hold the long chain, cellulose
molecule together.
The oxygen atom (0), shown connecting
the two glucose units (rings) in illustration 9 has formed a covalent bond by
sharing the six electrons in its outer (L) orbit with one electron from each
carbon to form a stable outer orbit of eight electrons. The two
hydrogen atoms each share their single electron with the three electrons each
carbon atom has left. Combined, this provides another stable outer orbit of
eight electrons. Now an acid (a hydrogen ion - proton [H+]) is introduced (see
illustration 10).
The positively charged hydrogen ion +
(acid) is strongly attracted to a negatively charged electron. The hydrogen ion
combines with one of the electrons being shared between the outer energy levels
or orbits
of the carbon and oxygen atoms. The hydrogen atom now shares this electron with
the oxygen atom, breaking the bond between the two glucose units or rings of
the cellulose chain (see illustration 11). Now, instead of a single, long chain
there are two shorter, weaker chains. The right side of the ring is stable
because by sharing the electron from the hydrogen atom, the outer orbit of the
oxygen atom still contains eight electrons.
The left side of the chain, however, is
not stable. The hydrogen ion combined with one of the carbon atoms electrons
leaving the carbon atom with only five electrons. This loss of one negative
electron means the carbon atom now has a positive charge, so it is now a
carbonium ion. The positively charged carbonium ion now seeks to achieve the
same stability possessed by the right side of the ring shown in illustration
11. The presence of a water molecule will provide the opportunity for the
carbonium ion, and the left side of the ring, to become stable (see illustration
12).
The positively charged carbonium ion
accepts a negatively charged electron from the water molecule. This electron is
shared between the outer orbits of the carbon atom and the oxygen atom. The
left side of the ring is now also stable, having returned to the same number of
electrons (as shown in illustration 10). However, the electron now being shared
between the outer energy levels or “orbits” of the oxygen and carbon atom was
taken from the hydrogen atom. This leaves a free hydrogen nucleus (which is a
proton or acid) (see illustration 13).
The hydrogen ion (acid) that was
released, will break another covalent bond connecting the rings of a cellulose
chain, which will release yet another hydrogen ion. As the chain is broken into
successively shorter lengths, it becomes progressively weaker. When one half to
one percent of the bonds are broken the paper will be virtually useless. When
the cellulose chain is broken, it also weakens and often breaks the hydrogen
bonds which bind the ribbons, or chains, into sheets. The layers held by Van
der Waals forces suffer the same fate. The hydrogen bonds are relatively weak,
having a bond strength of 3 to 6, compared to the bond strength of 86 for the
carbon-oxygen bond shown in illustrations 9, 10, and 11. The hydrogen bonds
strength comes from the close proximity of the hydrogen atom to the oxygen
atom.
The geometry of the covalent bonds
connecting the rings in the cellulose chain is such that the hydrogen atoms are
forced into a certain plane close to the oxygen atoms. A long chain results in
a 
stronger, more rigid structure with
higher strength hydrogen bonds. As the chain is broken into shorter and shorter
lengths, this rigidity is lost. The hydrogen and oxygen atoms are no longer
forced into planes of close proximity and the bonds can progressively weaken
and break. Like the hydrogen bonds, Van der Waals forces are weak (with a bond
strength of 2 to 10) relative to the covalent bond holding the rings in the
cellulose chain together. Also, like the hydrogen bond, Van der Waals forces
are weakened and broken when the covalent bonds connecting the rings break
chemically (by acid). The strength of Van der Waals forces are also dependent
on the geometry of the short carbon-hydrogen bonds, which minimize the distance
and, therefore, maximize the strength between the layers. As the chain is
broken and rigidity is lost, the carbon-hydrogen bonds are no longer so
strongly forced into the geometric plane which keeps the layers at a minimum
distance from each other. A loss of strength is then suffered in the bonding
between the layers.
This combination of interrelated forces
and chemical reactions is the primary cause of the massive amount of
deteriorating paper artifacts found in libraries and archives throughout the
world today.
Hopefully, you now can understand not
only the devastating effect acid has on paper, but the mechanism via which this
deterioration occurs.
What Causes Acids to Be
Present in Paper?
Impurities such as lignin, hemicellulose
and hydrolyzed cellulose oxidize and produce substantial quantities of acidic
degradation products. Alum-rosin sizing [Al2(SO4)3. 18H2O] added during the
paper making process is a prime acid producer. Various deteriorative
by-products, such as acetic acid, are produced as paper and film age naturally.
These by-products of deterioration then catalyze (cause) further degradation
reactions. This deterioration-from-within is responsible for the fact that
pages adjacent within a book will deteriorate more quickly than if they were
removed and stored individually. Acidic gases and pollutants from the
atmosphere such as oxides of nitrogen and sulfur dioxide, form sulfuric and
nitric acid. Other culprits are ozone, various peroxides, peroxyacl nitrates
and cupric and ferric ions which promote carbohydrate acid through the
oxidation of carbonyl and hydroxyl groups. There are also many indoor sources
of deleterious pollutants and chemicals. For example, deteriorative agents such
as formaldehyde, peroxides, formic acid, and acetic acid can be emitted by
wood, plywood, particle board and chipboard. Protein-based glues and wool can
yield sulfides. Fumes from an underground parking area can cause elevated
interior levels of oxides of nitrogen, and sunlight entering a building can be
responsible for increased photolytic reaction rates, resulting in
concentrations of oxidative and acidic molecules such as ozone, peroxides,
nitric acid and other nitrogen-containing molecules which are present at higher
levels inside than outdoors. Acids also migrate from adjacent acidic materials,
which is why we can not line an acid box with acid free paper and
expect it to remain acid-free.
What Can We Do to Protect
Paper From Acids?
Insist that
the paper you use be made from high quality fibers (preferably,
alpha-cellulose) without alum-rosin (acid) sizing and with a maximum 30 parts
per million of iron and .7 parts per million copper. Specify and use papers
that are free from lignin. Lignin is a very large complex organic molecule
which binds the cellulose together in a tree. While a papermaker can increase
his paper yield per tree to 95% by using the lignin (as opposed to 35% maximum
for pure cellulose), the lignin will greatly hasten a papers demise by breaking
down in myriad different ways to yield many different acids and peroxides
(which can also damage photographic materials). A commonly used qualitative
test for lignin is the Phloroglucinol (1,3,5-Benzenetriol) test. This test was
designed to indicate the presence of lignin in quantities of 6 percent and
higher. Since even small amounts of lignin can cause significant problems, you
should not rely on this test. A quick visual clue to the presence of lignin is
the color of a paper or board. The brown kraft color of standard (and some acid
free) shipping and packing containers comes from the lignin in the
paper. This same lignin produced color is often seen in the center portions of acid
free solid and corrugated boards, so you should exercise caution (or
preferably switch to lignin free materials) if you are using products made from
these types of boards. Apparently, some time ago, some people were taught that
lignin was present only in groundwood (mechanical wood) pulps. This, of course,
is not true. While mechanical wood pulps do contain lignin, unbleached (brown)
kraft pulps such as those produced in vast quantities in the U. S. for
corrugated shipping containers and kraft wrapping papers also contain
essentially their full original complement of lignin. As mentioned, mechanical
wood pulp products such as those commonly found in newspapers, pizza and shoe
boxes, and low quality mat/mounting board also contain lignin. Some papers are
available which are partially or semi bleached. These papers and
boards are a lighter brown color than their unbleached counterparts. However,
they still contain lignin. Our Lig-free Type I paper and boards are fully
bleached alpha cellulose which we have dyed a pleasing light tan color with
special fade and bleed proof dyes to mask any soiling which may occur with
extended use. These Lig-free papers do not contain lignin.
Alkaline Buffers
A generally accepted level of alkaline
buffer added to paper intended for archival use is three to five percent. There
are exceptions to the inclusion of alkaline additives, particularly with regard
to papers meant for the preservation of certain protein based textiles and
photographic materials, but we will address this issue later. Typically the
alkaline buffer used in paper is calcium carbonate (CaCO3). Remember, an acid is
any substance which can donate a proton, and we have seen the havoc a proton
can inflict upon the bonds holding our paper together. A base, such as calcium
carbonate, is any substance that accepts protons. The negatively charged
(basic) hydroxal unit (OH) combines with the positively charged (acidic)
hydrogen ion (H) to form water. Assuming enough alkaline buffer (calcium
carbonate) is available, the potential exists for the acid to be neutralized
before it can damage the paper. This is why it is important to have alkaline
buffering in all paper products used in conservation except, as mentioned
earlier, those intended for use with certain protein-based textiles and
photographic materials where excess alkalinity could potentially cause
problems.
Molecular Traps
As important as alkaline buffering is,
conservation scientists now realize it does have important limitations.
Alkaline buffering does not deliver the degree of protection we once assumed it
provide to our collections. If an acid migrates to, or arises from within (in
the form of a by-product of deterioration), or forms from a pollutant coming
into an alkaline buffered paper, and, if this acid is in contact with a
particle of alkaline buffer, the acid will be neutralized. However, if the
deteriorative molecule is an oxidative species such as a peroxide, or an acid
precursor like an oxide of nitrogen or sulfur dioxide, or a pre-acidic
by-product of deterioration, it will not react with the alkaline buffer.
Instead it continues through the paper housing (i.e, box, envelope, folder,
mount board, etc.) and damages the artifact you are attempting to protect. If a
suitable molecular trap is contained within the alkaline buffered paper, it can
capture and remove those harmful molecules which passed by the alkaline
buffering. Our General Purpose MicroChamber Paper and MicroChamber boxboards
(except the MicroChamber/Silversafe boards) contain both activated carbon and
our SPZ zeolite. The black side of the General Purpose MicroChamber paper (also
used in the MicroChamber boxboards) contains alkaline buffers and an especially
effective activated carbon. The white side of the paper contains alkaline
buffers and our modified proprietary hydrophobic, acid-resistant SPZ zeolite
which we developed and engineered after extensive research to perform the
specific functions necessary to protect your collections. Our SPZ zeolite
removes acids, aldehydes, ammonia, pollutants such as SO2 and NOx, and despite
the incorrect information seen in a competitors catalog, it does remove
oxidative gases, even in very low concentrations (see doorway and bus photos
subjected to ANSI standard IT 9.15-1992 oxidative gas tests in MicroChamber
test section on our website). This SPZ zeolite was engineered to remove all of
the known deteriorative molecules that threaten our collections, even those
with very low polarization levels.
Efficacy of Molecular
Traps in MicroChamber Papers
Acids:
It is
interesting to note that our molecular traps are significantly more effective
than an alkaline buffer at removing acids, and unlike buffered-only papers,
they will remove by-products of deterioration such as aldehydes which form
acetic acid. This is important because acetic acid is the primary by-product of
deterioration produced both by paper and by photographic materials. One
of the most dangerous pollutants to paper is acetic acid… As the effects of
acetic acid build up in a paper artifact, it accelerates degradation…My goal
was to identify materials that would be most effective at absorbing and retaining
acetic acid, and that would be suitable for use in preserving artifacts. I
looked at about 18 different materials, including activated carbon, clays,
calcium carbonate, and several zeolites… The activated carbon and one of the
zeolites-called SPZ, (the zeolite Conservation Resources developed for use in
Artcare board, conservation boards, papers and materials) performed
significantly better than the other physical adsorbents… based on its
adsorption and retention of acetic acid-which can be assumed to inhibit
cellulose deterioration-the SPZ zeolite, incorporated in Artcare (and)
MicroChamber technology (products), is a very viable material for preventative
conservation applications. 1
The results from
our tests using gas chromatography show that if we have equivalent papers-for
example a 65 g/m2
interleaving paper, or a 130 g/m2, .006" thick envelope paper, or a
standard 250 g/m2 archival
file folder paper in both MicroChamber paper and buffered paper, the
MicroChamber papers have 170 times the acid-removal capacity of the buffered
papers. In other words, the buffered paper would have to be replaced 170 times
before you would need to replace the MicroChamber paper.
1.
From an interview with James Druzik, Senior Scientist, the Getty Conservation
Institute, printed in the October 2003 Decor magazine.
By-products of
deterioration:
MicroChamber papers are very effective at
removing pre-acidic by-products of deterioration, such as aldehydes. These
pre-acidic deteriorative by-products pass unaffected through traditional
buffered paper because the deteriorative by-products do not react with the
alkaline reserve in buffered papers. If we assume all of the acetaldehyde (a
precursor to acetic acid) removed as deteriorative by-products by the
MicroChamber paper will become acetic acid, we find the MicroChamber paper can
remove what would become 231 times as much acid as would form if only the
buffered paper were present.
Pollutants:
MicroChamber
products do provide protection against common oxidative and acid gaseous
pollutants such as ozone (O3), oxides of nitrogen (NOx, NO, NO2), sulfur
dioxide (SO2), as well as H2S, CS2, ammonia, formaldehyde, peroxides and a
great many other such molecules which can harm collections. The traditional
alkaline buffers in conservation papers do not react with or remove these
deleterious molecules. Furthermore, such molecules can pass unaffected through
even the thickest buffered boards 2, where they can contact and damage
collections housed within these buffered boards and papers. If, for example, we
look at New York City and at Los Angeles, the EPA (The U.S. Environmental
Protection Agency) gives us the maximum hourly rate of a variety of pollutants
measured in these two cities for one year. Using these maximum concentrations,
we can calculate the maximum amount of a pollutant such as SO2 in one liter of
air. Exposing a 24 x 36 MicroChamber folder to a fresh liter (slightly more
volume than a quart container) of polluted air every hour, we find, at the
maximum hourly concentration level of pollutants measured in New York and Los
Angeles, the MicroChamber folder has the capacity to remove the SO2 in NY city
for 8219 years, and in LA for 26,224 years. Obviously if the air exchange is
increased this figure will be lower. For example, if the air flow rate into the
folder was increased to 10 liters per hour, the figures would drop to 1233
years for NY City and 3933 years in Los Angeles, CA. Of course the MicroChamber
product will also pick up other harmful molecules, in addition to the SO2.
Therefore to the extent these other molecules are present and removed, the
maximum quantity of SO2 which can be removed will be lowered-but these figures
do at least provide a point against which you can form a comparison between the
effectiveness (zero) of buffered products and of MicroChamber products.
The preservation advantage offered by
our new MicroChamber boards and papers, which contain both specialized
proprietary molecular traps and alkaline buffers, is quite striking. While
traditional alkaline buffered conservation papers and boards do provide an
advantage over acidic commercial products, this improvement does not begin to
approach the phenomenal gain in protection offered by MicroChamber products
over traditional alkaline buffered products. Alkaline buffered paper is a
technology of the 1960s. MicroChamber materials give you the advantage of
technology from the 1990s. MicroChamber products offer new opportunities in
preventative conservation, increased life and thus reduced preservation costs
for all collections. See the MicroChamber product verses traditional
buffered-only test results on our website.
2. Guttman, C. M. and
Jewett, K. C. 1993 Protection of Archival
Materials from Pollutants: Diffusion of Sulfur Dioxide through Boxboard, Journal of the American Institute for
Conservation 32:81-91. Also, see MicroChamber test section on our website.
pH
The pH scale is a yardstick used to measure
the number of hydrogen ions (H+ [acid]) in solutions. The pH scale runs from 0
to 14. The lower numbers refer to acid solutions, while the higher numbers
refer to alkaline or basic solutions. At pH 7 (neutral)
the concentration of hydrogen ions equals the concentration of hydroxide ions.
Any solution with a pH lower than 7 has more hydrogen ions than hydroxide ions
in solution. Any solution with a pH higher than 7 has fewer hydrogen ions than
hydroxide ions in solution. The pH scale is a logarithmic progression. This
means numbers on the pH scale are based on powers of ten. A pH of 2 therefore,
indicates ten times fewer hydrogen ions than a pH of 1, pH 3 has ten times
fewer hydrogen ions than pH 2 and one hundred times fewer hydrogen ions than pH
1. A pH of 4 has ten times fewer hydrogen ions than pH 3, one hundred times
fewer hydrogen ions than pH 2, and one thousand times fewer than pH 1. Since we
have seen how hydrogen ions break the bonds holding the cellulose chain together,
and since pH is the measurement of these acid ions, are we, therefore, able to
specify a paper with a pH of 7.0 or higher with the expectation that it will be
archival? The answer, unfortunately, is no! For one thing weak acids may not be
fully disassociated. Therefore, you do not always get an accurate picture of
acids present by measuring pH. Let us imagine someone offered you a brown kraft
paperboard. It is purportedly acid free and, in fact, the pH is
8.0. On further examination you discover that the paperboard contains no
alkaline buffering such as calcium carbonate. Now the paperboard is, in a
technical sense and at least initially, acid free. However, this paperboard
should not be used for archival preservation. There is no alkaline buffer present
to neutralize the acids from pollutants in the surrounding environment, and the
paper is full of lignin which will break down and form acids which will sever
the bonds holding the cellulose chain together. Adding alkaline buffering to a
paper which is full of lignin will not keep this paper acid free. Remember, if
one half to one percent of the cellulose bonds are broken, the paper will be
virtually useless. It also will be a source of acid which can migrate to and
damage adjacent materials. Therefore, we should never rely only on the term acid
free to specify a paper we intend to use for conservation purposes. It
is important to know the pH of a paper product but pH must be used in
conjunction with other specifications to be meaningful. We will look at the
specifications required to insure a paper is archival, shortly.
Degradation of Paper by
Light
Absorption of light will not directly
cleave a bond in the cellulose chain. However, certain additives, impurities,
dyes, and metals such as iron (Fe++++, Fe++) will absorb energy from light
which raises their electrons to a higher energy orbit. When this energy is
released to the cellulose, bond cleavage can result and this, in turn, can
allow oxidative degradation. Incidentally, this same mechanism is used by
plants to provide the energy to grow the cellulose we use to make paper. A
molecule of chlorophyll, for example, absorbs quanta of light energy from the
sun, raising its electrons to higher energy levels. When the electrons fall
back to lower energy levels, they release the same amount of energy they
absorbed. This energy is used by the plant cell to fuel the chemical reactions
which produce simple sugars.
Oxidation of Cellulose
This degradation is found when oxygen is
absorbed at certain sites on the cellulose molecule. With this oxygen
absorption we will find an increase in oxygen containing groups such as
carbonyls and carboxylic acids. As this greatly increased level of acid is
released, the cellulose will be hydrolyzed (meaning the covalent bond between
the rings in the cellulose chain will be broken, forming two shorter chains and
releasing a hydrogen ion) (see illustrations 10 through 13). Oxidation will
also cause color changes in paper, particularly if such impurities as lignin,
iron, alum sizing and hemicelluloses are present. Hemicelluloses are
polysaccharides (multiple sugar units) which have many branches, so they do not stack
or fit tightly into microfibrils like alpha cellulose (glucose units).
Enzyme Degradation and
Mechanical Damage
Enzymes are protein catalysts coded for
by that very newsworthy molecule, DNA, and assembled from amino acids in the
ribosomes of living cells. Enzymes such as endoglucanases and cellobiohydrolase
can cleave the bonds connecting the cellulose chain at any link. This subject
is really beyond the scope of our discussion, but if you are interested in
additional information, your local paper conservator will be able to answer any
questions you may have. Mechanical damage can result in splayed, split or
broken fibers which can weaken paper just as surely as bond cleavage between
the rings in the cellulose chain. Remember, the longer the chain, the stronger
and less mobile the structure. The hydrogen atoms are forced into a plane close
to the oxygen molecules and the hydrogen and oxygen atoms connecting the sheets
are able to form better hydrogen bonds. For essentially the same reasons, the
strength of Van der Waals force connecting the sheets into layers is maximized.
This is the reason you should request a paper with high strength and durability
even when it seems unimportant for your particular requirement. Say, for
example, you are trying to choose a thick, stiff paperboard for use in making
cloth wrapped book boxes. Perceived stiffness is basically a function of
thickness, but a thick board constructed from multiple plys of paper with high
physical strength characteristics, such as high folding endurance and tear
resistance, will be manufactured from good, long fibers. This board will be
much more resistant to damage over an extended period of time, and not be as
likely to harbor acids from bonds broken by hydrolysis (see illustrations 10
through 13) that can migrate to historical paper artifacts stored within it.
MicroChamber Technology
and the History of Archival Boards in the U.S.A.
The first attempt at producing an
archival board for preservation housings in North America was made in the
1960s. This first boxboard, gray with a red pulpy center and a pH of 6.5 was considered
to be at the leading edge of archival storage technology. Information that
acids were the root cause of paper deterioration was beginning to be widely
disseminated among those concerned with preserving documents, books and works
of art on paper. By todays standards, this mildly
acidic board would be unacceptable for use as a preservation housing; however,
at the time, its production was quite an achievement. The board mills of this
era all utilized acid paper making systems, and even this mildly elevated pH
level caused severe problems for the mill which produced it.
This was the period when Frazer Poole
was beginning to lead the US Library of Congress preservation program into new
areas. The Library quickly established new standards which required a pH of 8.5
for preservation housing boards. The paper adapted for this purpose was an
unbleached (therefore brown) kraft. It retained its full complement of lignin,
and no alkaline buffer was added. Unfortunately it did not retain its alkaline
pH for long. The solution to this problem was thought to be the addition of
calcium carbonate as an alkaline buffer. However, as time passed, it became
apparent that the addition of alkaline reserve did not prevent the pH from
dropping into the acidic range in boards containing lignin.
Further progress was made in 1979 when
Conservation Resources introduced the first gray boards made with quite low
levels of lignin and alkaline buffering distributed evenly throughout the
entire board. In 1980 another advance was made Conservation Resources
introduced the first lignin-free and sulfur-free alkaline buffered board,
produced initially for the Library of Congress.
The goal, until very recently, was to
produce stable archival papers and boards which would not break down and
contribute to the deterioration of the collection housed within them. With the
removal of lignin and other substances which promoted further deterioration,
and with the inclusion of alkaline reserve throughout the board, we thought we
had achieved the ultimate in archival storage housings: a truly non-reactive
housing that met our passive preservation goals. However, as observational and
experimental knowledge increased, it became apparent we needed to find
additional methods of dealing with the shortcomings of contemporary archival
alkaline buffered preservation materials.
Preventative Conservation
It was
becoming evident that by-products of deterioration produced as paper, film and
other organic materials aged, played a prominent role in deterioration, as did
harmful oxidative and acidic molecules found in the environment surrounding
archival collections. People understood that pollutant molecules such as ozone,
sulfur dioxide, and oxides of nitrogen could damage their collections. These
pollutants also damage buildings, statues and even living ecosystems. However,
until recently, most people generally did not realize that indoor pollutant
levels could be quite high. {Indoor pollutants are present in much higher
concentration than those found outdoors, and can be significantly more harmful
to artifacts than typical open-air pollution....We were seeing damage from
pollutants occur even in a controlled museum environment.3} Indeed these compounds can even be produced indoors by a
variety of materials and furnishings, as well as by heating equipment and
various appliances. Deleterious pollutants and chemicals produced inside
include deteriorative agents such as formaldehyde, peroxides, formic acid, and
acetic acid, which can be emitted by wood, plywood, particle board and
chipboard. Protein-based glues and wool can yield sulfides. Fumes from an
underground parking area can cause elevated interior levels of oxides of
nitrogen, and sunlight entering a building can be responsible for increased
photolytic reaction rates, resulting in concentrations of oxidative and acidic
molecules such as ozone, peroxides, nitric acid and other nitrogen-containing
molecules which are present at higher levels inside than outdoors. Acids and
other harmful molecules also migrate from adjacent acidic materials. Because
the artifacts we save degrade over time and produce by-products of
deterioration, and because they are generally housed together in high density
storage areas, harmful compounds tend to accumulate in higher concentrations
within the storage area.
Another
common misconception used to be that the alkaline buffering in archival papers
and boards dealt effectively with these deleterious compounds. Conservation
scientists now realize it is important to understand that the protection
conferred by alkaline buffering does have limitations. If an acid migrates to,
or arises from within ( in the form of a by-product of deterioration), or forms
from a pollutant coming into an alkaline buffered paper, and if this acid is in
contact with a particle of alkaline buffer, the acid will be neutralized.
However highly reactive oxidative gases such as ozone and peroxides are not
acids, and pollutants such as sulfur dioxide and oxides of nitrogen do not
become sulfuric or nitric acid until they combine with oxygen and water to form
these acids. Dr. Charles Guttman and his team from the U.S. National Bureau of
Standards published important research (Protection of archival materials from
pollutants: diffusion of sulfur dioxide through boxboard, Journal of
the American Institute for Conservation 32: (1993) 81 - 92) showing how readily
pollutant molecules pass through alkaline buffered boards. Obviously severe
damage to a collection can occur when these harmful molecules pass through an
archival paper or board, unaffected by the alkaline buffer, and react with or
form acids on the artifact housed within the archival container.
3.
From an interview with James Druzik, Senior Scientist, the Getty Conservation
Institute, printed in the October 2003 Decor magazine.
MicroChamber
Archival Materials
We have
invented and produced a new generation of archival boards and papers which
address the shortcomings of traditional alkaline buffered products. By
including a mixture of specialized activated carbons and/or specially designed
and formulated SPZ zeolite with our alkaline buffers, we produce MicroChamber
paper and boards which overcome the limitations of conventional alkaline
buffered products. Activated carbon is inert porous graphite, and zeolites are
microporous structures such as crystalline aluminosilicates. They do not react
with the molecules they eliminate, but rather remove and neutralize them.
Molecules removed by our MicroChamber papers include acids such as ethanoic
(acetic) and methanoic (formic) acid, phenols, aldehydes, hydrogen peroxide,
ozone, sulfur dioxide, hydrogen sulfide, carbon disulfide, oxides of nitrogen,
ammonia and formaldehyde.
When we look
at the evolution of papers used for preservation purposes, it is clear they all
have a common theme, which is passivity. Inactive became a superlative when
applied to these traditional conservation papers and boards. The goal was
primarily to avoid harming a collection, a problem so many people had
experienced when using acidic papers and boxboard housings. While conventional
buffered papers and boards do display a degree of effectiveness with acids,
they are not as effective as they could, or should be. Moreover, they do not
address the issue of deteriorative compounds other than acids. MicroChamber
products do address these issues. MicroChamber materials actively work to
protect your collection, as opposed to the role of the traditional buffered
only paper, which is to passively avoid self deterioration.
MicroChamber
papers and boards have been used in aging tests with both new alkaline buffered
book pages, and with old, naturally aged acidic book pages. MicroChamber
products have been tested with photographic negatives and with photographic
prints (all MicroChamber and Artcare papers and boards produced have passed the
PAT test). MicroChamber materials have also been tested with newspapers, works
of art on paper, animation cells, and paper directly soaked with acid. They
have been used to line shelves and drawers, to act as scavengers in exhibit
cases, to wrap artifacts for shipping, and for myriad other non-traditional
uses. Of course they are also playing an important role as boxes, folders,
envelopes and other conventional housing forms.
During the
past few years, MicroChamber and Artcare products have become widely used
throughout the world. Often customers contact us with success stories involving
problems these products have solved for them. Some of these uses may be useful
to you, whether now or sometime in the future.
MicroChamber
papers and boards have eliminated smoke and other odor problems with many
different collections including textiles, and they have been extremely
effective when used with negatives suffering from vinegar syndrome, where
they remove the acetic acid, resolving odor problems, and allowing the
collections to be handled again, as well as significantly improving their odds
of long term survival. They are also used for cold storage of deteriorating
acetate sheet photographic film, where it is reported that according to IPI A-D
strips, it appears quite effective at absorbing acetic acid. Of course one
would expect these results because a MicroChamber paper has the capacity to
remove 170 times as much acetic acid as an equivalent buffered paper.
We have also
received a variety of reports from people delighted with its effectiveness when
used to remove smoke and fire related odors from prints, books, papers, African
masks, wood carvings, textiles, furniture, ivories, bronzes and various other
works of art. A collector of paperback novels called because he was so
delighted that MicroChamber paper had eliminated what was becoming an
increasingly strong order from the by-products of deterioration emitted from
his collection. We have had comparable comments from others with similar
collections of newspapers, comic books, films and various ephemera.
Additionally, it has been used for preservation in animation cell mats and to
remove the build-up of plasticizers being emitted by a collection of stuffed
toys. One gentleman even used it to eliminate the new car smell (likely due to
VOCs) he found a bit overwhelming. The point is the material is both very
effective and very versatile. Most, if not all of the molecules causing the
offending odors are also responsible for the deterioration we all seek to
prevent. The molecular traps in MicroChamber and Artcare papers and boards have
been engineered to remove deleterious molecules even when they are present in
extremely low concentrations. Clearly any collections will be better off if
harmful substances are removed as they become present, before levels are
allowed to increase to the point where we can smell them. The graphs and color
photographs of test results shown on our website in the MicroChamber test
section will help demonstrate the capacity and efficacy of these materials. You
can quickly see the preservation advantage offered by MicroChamber and Artcare
boards and papers is spectacular. While traditional alkaline buffered
conservation papers and boards do provide an advantage over acidic commercial
products, this improvement does not begin to approach the phenomenal gain in
protection offered by MicroChamber and Artcare products over traditional
alkaline buffered products. Alkaline buffered paper is a methodology of the
1960s. MicroChamber materials offer you technology from the 1990s. Browse
through the MicroChamber test section on our website and look at the test
results comparing the MicroChamber products to the traditional buffered-only
products. MicroChamber products provide new opportunities in preventative
conservation for all collections.
Specifications
for Archival Papers
We
will break these specifications into two parts. The first part will deal with
those requirements needed to insure the chemical purity of an archival paper or
paperboard. The second part will show how you can insure that high quality,
long chain cellulose has been used to make your archival paper by specifying
certain minimum physical strength parameters that must be met. These are, in fact,
the specifications we use for our Lig-free Type 1.
Specifications
for Lig-free Archival Papers
Part One
1. The
paper should be made from fully bleached, alpha cellulose pulp. It should be
free of lignin, ground wood, waxes, plasticizers, reducible sulfur, oxidizing
chemicals and potentially harmful non-cellulose products. It should be free of
particles of metal with a maximum 30 ppm Fe and .7ppm Cu. The board shall be
hard sized with chemically saturated organic compounds to a Cobb size test of not
more than 100 grams per square meter (TAPPI) T-441 (os-69). The surface of the
paper should be smooth and free from knots, shives and abrasive particles.
2. pH
range: The paper should have a pH of not less than 8.5 nor more than 10.2.
3.
Sizing: Alkaline sizing should be used in place of alum-rosin sizing.
4.
Alkaline reserve: The paper should contain a minimum of 3% calcium carbonate
(CaCO3), or other suitable alkaline buffer.
Part Two
(These
strength specifications are for .010" thick paper.)
5. Abrasion
test: The paper shall show maximum fiber loss of one-tenth of one percent after
100 cycles according to TAPPI 476.
6.
Smoothness test: The paper should show a minimum smoothness of 195 Sheffield
units following TAPPI UM-518 test.
7.
Folding endurance test: The paper should withstand a minimum of 1,000 double
folds in the weakest direction at 1kg. load after conditioning according to
TAPPI T 511.
8.
Internal tear resistance (Elmendorf): The paper shall have a minimum tear
resistance of 350 gr. per sheet after conditioning TAPPI T414.
9.
Stiffness test: The paper should have 2800 stiffness units in the machine
direction and 1400 stiffness units in the cross direction in accordance with
TAPPI 489.
10.
Bursting strength: The paper should have a bursting strength of 300 pounds per
square inch when tested in accordance with TAPPI T 807.
Part
Three
(This
part pertains to those papers which are colored [dyed] such as our
Lig-free,
Type I.)
11.
Color UNLESS otherwise specified the outer surface of the paper should be
natural tan dyed with light-fast and non-bleeding dye.
12.
Fading test: When the paper is exposed in a standard fadeometer TAPPI UM-461
for 30 hours, the difference in brightness TAPPI T 452, measured on the exposed
and unexposed portions of the sample shall be less than 5 points.
As we have mentioned, certain textiles
such as silk and wool and certain photographic materials may be at risk in an
alkaline environment. We, therefore, have developed a special non-buffered
paper for use with these materials. The specifications are:
Specifications
for Photographic/Textile Conservation Paper
Part One
1. The
paper shall be made from fully bleached alpha cellulose pulp. It shall be free
of lignin, groundwood, particles of metal, waxes, plasticizers, alkaline
buffers, coloring agents, reducible sulfur, oxidizing chemicals, additives and
potentially harmful non-cellulose products. The surface of the paper should be
smooth and free from knots, shives and abrasive particles.
2. pH
range: The paper shall be in the neutral range.
3.
Sizing: Alkaline sizing shall be used in place of alum-rosin sizing.
4.
Alkaline reserve: The paper shall not contain calcium carbonate (CaCO3) or
other alkaline buffers.
5.
Sulfur content: The sulfur content shall be less than .0008% reducible sulfur
as ASTMD 984-74 or TAPPI T 406, su 72.
6.
Tarnishing properties: The paper shall be non-tarnishing as per accelerated
tarnishing test ASTMD 2043-69 and TAPPI T 444, T 564. The paper must also pass
the silver tarnish test developed by T. J. Collings and F. J. Young, London,
England.
Part Two
This paper meets the same high fold
(minimum 1000 double folds in the weakest direction at 1 kg.) and other
strength requirements as Lig-free , Type 1 in 5 through 10, part two.
Part
Three
Not
applicable since no coloring should be added to this paper.
MicroChamber/Silversafe
and Lig-free Type II boards for the Preservation of Archeological Specimen,
Photographic Materials and Textiles
We have developed and currently
manufacture two archival boards which we are using to make containers for the
storage and preservation of photographic images and proteinaceous artifacts
such as ancient skills, parchments and leathers, textiles (silks and wools), anthropological
artifacts including insect collections, horn, bone, hides, teeth, feathers and
albumin and gelatin emulsions commonly used in photographic prints and
negatives. Before we describe our new boards, we would like to briefly explain
the structure and composition of proteins and the process by which they are
assembled in a living cell. Outlining these fundamentals of protein structure
and function will give people a better understanding of our reasons for
developing these boards.
A protein is
a long chain (polypeptide) of x-amino acids (2-amino carboxylic acids) which
fold into various three dimensional conformations, thereby controlling access
to the reactive chemical groups in the particular three dimensional patterns.
There are 20 different amino acids which can be placed end to end in any order
to comprise the links of this polypeptide chain. Unlike cellulose, which is
comprised of a long chain of identical glucose rings, the protein can exhibit
considerable diversity because of the vast numbers of sequences of amino acids
which are possible and the fact that each of these amino acids has a different
reactive group called a side chain, which is the primary determinant of the
property of a given protein. Also, protein molecules are often comprised of not
one, but several different polypeptide chains.
The
construction of a protein begins when an enzyme (which is also a protein)
called an RNA polymerase uncoils the DNA double helix in the nucleus of a cell,
exposing six bases or nucleotides at a time. Each three adjacent nucleotides
(called a codon) designates, or codes for, a specific amino acid.
Heterogeneous nuclear RNA (hnRNA) is formed using the DNA template to acquire a
complimentary sequence of nucleotides to those found in the gene (DNA) being
expressed.
Messenger RNA (mRNA) formed from the
hnRNA leaves the nucleus of the cell with the precise number and sequence of
nucleotides required to code for the specific protein called for by the DNA
(gene) in the nucleus of the cell. The mRNA travels through the cytoplasm of
the cell to a site comprised of four molecules of RNA and many different
proteins called a ribosome.
In the
ribosome transfer RNA, consisting of three nucleotides (a codon) and the amino acid
specified by the codon, attach to three complimentary nucleotides on the mRNA.
As the ribosome moves sleeve like down the length of mRNA, the transfer RNA
continues to combine to the next three complimentary nucleotides on the mRNA
bringing the next amino acid to the growing polypeptide chain. As the amino
acid carried by the tRNA is attached to the polypeptide chain, the tRNA is
released to the cytoplasm where it combines with another amino acid. This
process continues with the polypeptide chain of amino acids growing longer and
longer until the ribosome comes to a terminator sequence on the mRNA. The
completed polypeptide is detached into the cytoplasm where it folds into its
specific three dimensional conformation and becomes a protein. It is when it is
in this final three dimensional conformation that it exhibits those properties
we associate with this specific protein.
The side chains of the amino acids are
ionic and, therefore, form electrostatic bonds between each other which hold
the protein into its particular conformation. Changes in pH affect the ionic
forms in which the side chains of the amino acids exist and, therefore, changes
in pH also affect the formation of bonds by proteins. In order for an
electrostatic bond to exist between side chains, both positive and negative
changes must be present. Raising the concentration of H+ (increasing the
acidity) decreases the number of charged caboxylate ions and the carboxylate
group on the side chains lose their charge. If the concentration of H+ is
lowered (made alkaline) the H+ will leave the ammonium group which will lose
its charge. It follows then that these bonds are generally most stable near
neutrality. Also, for these amino acids with both carboxylate and ammonium
groups, there is a pH value at which the number of negatively charged
carboxylate groups will be exactly the same as the number of positively charged
ammonium groups. Various points along the side chain will be more negative or
more positive than others which allows the bonding to continue, but the total
charge will be zero. This is the isoelectric point of the protein and the pH at
which this isoelectric point occurs is where the protein is least reactive and,
therefore, most stable.
If the pH, or isoelectric point, of the
protein is altered, the side chains of the amino acids comprising the protein
will lose their ionic forms and the electrostatic bonds between them will be
broken. The protein can then unravel from the distinct form which gave it the
properties we associated with this specific protein and become a polypeptide
again. In addition, the peptide bonds between the amino acids are now exposed
to the possibility of clevage by hydrolysis. By comparison, if we break the
covalent bonds in a cellulose chain, we do not alter the properties we
associated with the paper. It looks the same and feels the same. It just gets
weaker.
This sensitivity and the potential
magnitude of damage, coupled with the persistence of several conservators and
conservation scientists, prompted us to look for an alternative to highly
alkaline buffered papers for the long term storage of proteinaceous artifacts.
Our objective was to invent an archival material from which we could make
containers that would in no way harm or interfere with the artifact stored
within it. A very pure, neutral, non-buffered paper which was itself at its
isoelectric point would provide precisely the neutral, non-reactive environment
we wanted. We added a thick layer of alkaline pH, alkaline buffered paper to
the neutral pH, non-buffered paper which forms the interior of our containers.
These boards, which we call Lig-free Type II, provide a securely neutral,
non-reactive, sulfur free interior while the outer plies of the board contain
alkaline buffering. This was the first, and until recently, the only boxboard
available which addressed the needs of collections felt to be sensitive to an
alkaline environment. We added a thick layer of alkaline pH, alkaline buffered
paper to the neutral pH, non-buffered paper which forms the interior of the
containers.
Lig-free Type II is currently available
in a corrugated board. The outer liner and inner corrugated medium are alkaline
pH, alkaline buffered paper while the inner liner is a neutral, non-buffered
paper. This board is strong, exceeding 250 lbs. pressure per square inch
bursting strength, and rigid. It is especially good for making large textile
boxes which can then be shipped and stored flat, for backing boards, and for
use as a support in conservation work. In Lig-free boards, all the buffered
papers are light tan in color while the non-buffered papers are white, so it is
easy to see which side is buffered and which is not buffered. All the papers
used in these boards, both buffered and non-buffered are pure long chain
cellulose and they are free of lignin, sulfur and other deleterious substances.
Complete specifications are detailed under the headings Lig-free Type II
(alkaline pH, buffered paper) and Photographic/Textile Conservation Paper
(neutral 
pH, non-buffered paper).
Now we also offer
MicroChamber/Silversafe products, the first boxboards and papers able to
address both the needs of alkaline sensitive collections, and the shortcomings
of alkaline-buffered-only archival storage products in dealing with pollutants
produced both indoors and outdoors, and by-products of deterioration. Standard
MicroChamber boxboards combine alkaline buffers, activated carbon and zeolite
molecular sieves, offering your collection the greatest protection available
from acids, by-products of deterioration and pollutants. Now
MicroChamber/Silversafe boxboards provide a combination of buffered Lig-free
board for strength, thickness, and support, coupled with tan MicroChamber paper
faced with a surface of neutral pH, unbuffered, soft white cotton Silversafe
paper. These products were developed so the advantages of MicroChamber
technology could be combined with neutral pH, unbuffered cotton Silversafe
paper, for use with protein-based textiles such as silks and wools, and with
certain photographic materials, and other artifacts which collection managers
feel may benefit from the neutrality of unbuffered cotton coupled with the
protective security offered by MicroChamber materials. MicroChamber/Silversafe
products are currently available in folders, folder paper, negative enclosure
paper and envelopes, as well as solid-fiber and corrugated boxboards, backing
boards and support boards.