PAPER DEACIDIFICATION: A PRELIMINARY REPORT1
RICHARD DANIEL SMITH
INTRODUCTION
ONE of
the critical problems facing librarians in their efforts to preserve recorded knowledge is the
deterioration of paper, the basic material of communication. A major cause of paper deterioration
over prolonged periods is identified as the acid-catalyzed hydrolysis of the cellulose in paper
fibers.2 It is estimated that 90 per cent of the books published from 1900 to 1949 will be unusable
for general library purposes within thirty-five years if protective measures to reduce the rate of
hydrolysis are not taken.3 The deacidification of paper is known to reduce the rate of this deterioration, and it is believed that a de-acidification treatment would extend the useful life of such
library materials by several times.4 Although deacidification processes are applied to preserve rare
and valuable paper records, there appears to be no practical process for librarians to use in the
deacidification of ordinary books.
The purpose of this paper is to develop an approach to a practical treatment for the wet
deacidification of paper in books stored in libraries. The
1
Paper presented at the annual meeting of the International Institute for Conservation of Historic
and Artistic Works—American Group—at the Art Institute of Chicago, June 6, 1966. It is a preliminary report of work being done on the author's Ph.D. dissertation at the Graduate Library
School, University of Chicago.
2
Paper deterioration originating from lack of durability may be contrasted to the deterioration
which derives from lack of permanence. A fragile Bible paper may be highly permanent but not
durable. Sandpaper is durable but is not required to be highly permanent. Durability is the degree to
which a paper retains its original qualities under continual usage. Permanence is that property
ascribed to a material which, under specified conditions, resists changes with the passage of time.
The permanence of paper refers to the retention of significant use properties, particularly folding
endurance and color, over prolonged periods. It is affected by temperature, humidity, light, and the
presence of chemical agents (see American Pulp and Paper Association, 1965, pp. 182, 335-36; and
the discussion of Clapp, 1963). For an exhaustive discussion of the mechanisms of cellulose
degradation, see Ott, Spurlin, and Grafflin (1954), pp. 99-191. For specific references to questions,
see Jarrell, Hankins, and Veitch (1936) ; Shearon (1957); and Barrow and Sproull (1959).
3
See R. W. Church (1959), pp. 14-16. If the books (then 50 to 100 years old) would fall into a less
frequently used category, however, they might withstand infrequent though not general library use
(see Fussier and Simon, 1961, particularly Appendix H, "Distribution of Books by the Frequency of
Their Use," pp. H1-H3).
4
Barrow (196Sa), pp. 6-7.
methodology for this search will consist of:
1. Summarizing the current wet processes for deacidifying paper;
2. Considering certain improvements in practice which appear desirable; and
3. Postulating, based on preliminary findings, how these improvements might be attained.
LIMITATIONS
This discussion will focus upon the degradation of the cellulose in paper fibers which is caused by
acid-catalyzed hydrolysis. It will not consider other causes of paper deterioration unless a very
direct relationship with acid hydrolysis is involved, nor will it concern itself with the sources of the
acidity. This investigation assumes that the primary cause of the lack of permanence exhibited by
book papers in libraries is the acid-catalyzed hydrolysis of cellulose when the pH of an aqueous
paper extract is below 5.5 or perhaps 6.5
Generally a paper with a nearly neutral or mildly alkaline pH (as measured by an aqueous extract) is
found to deteriorate more slowly during extended storage than a similar paper with a significantly
lower pH.6 The rate of paper deterioration usually increases as the pH decreases.7 Therefore, this
report considers that the permanence of an acidic paper is improved if a significant shift in the
hydrogen ion content (pH) results from a deacidifi-cation treatment.
The reader should remember that other causes of paper deterioration can be and frequently are more
significant than acid-catalyzed hydrolysis of cellulose. Several familiar examples come immediately
to mind:
1. The wear and tear caused by patron use;
2. The effect on paper of light, of heat, and of humidity variation; and
3. The results of bacteria, fungi, and insect and rodent attack.
In general, this presentation will consider that the cellulose in paper fibers, or the paper fibers
themselves, are in an idealized condition. This means that all of the paper fibers in the paper substrate are construed to contain a uniform quality of cellulose. Or more broadly stated, this report
will neglect the differences in the rate of paper deterioration caused by variation in fiber source,
manufacturing procedure, secondary (accessory) paper-making materials, and environmental
conditions of use and storage (except insofar as these involve acids).
ACID DEGRADATION OF PAPER FIBERS
"Cellulose, the primary constituent of paper fibers, is a linear polysaccha-ride, /?-!, 4-glucan of high
molecular weight. It is the main solid constituent of woody plants and occurs widely elsewhere in
the vegetable kingdom. Cellulose, from the point of view of the pulp and paper industry, approaches
the above ideal [paper fibers are not pure cellulose] but never quite reaches
5
Two methods arc used for preparing paper samples for pH measurement, hot or cold water
extraction. The results may be approximately the same on some papers. On others, the hot
extraction may give up to 1 or more pH units lower results than the cold extraction. The Si or
perhaps 6 pH value specified refers to results obtained by the hot extraction procedure. The
proposed investigation plans to report the pH value of both hot and cold methods because a
difference of opinion exists about which method gives the best pH value for estimating the
permanence of paper. The procedure for these tests is given by the Technical Association of the
Pulp and Paper Industry (T43S-S2; 1952).
6
See Byrne and Weiner (1964) ; and Kantrowitz, Spencer, and Simmons (1940).
7
W. J. Barrow Research Laboratory (1963), Fig.
it."8 Wood cellulose, for example, is that material remaining after most, or essentially all, of the
lignin and certain of the carbohydrates, other than cellulose, have been removed by the pulping and
bleaching operations.
A cellulose molecule (chain) consists of an unspecified number of anhydrous glucose units (degrees
of polymerization or D.P.) connected through an oxygen bridge, that is, a hemi-acetal
FIG. 2.—Acid-catalyzed
hydrolysis of cellulose
linkage. In natural cellulose, the maximum D.P. is approximately 10,000. The structure and
chemical structure of cellulose can be represented as in Figure I.9 C stands for carbon, H for
hydrogen, and O for oxygen. The looped arrow at each end indicates that the molecule continues
indefinitely but within the constraints expressed above.
Hydrolytic attack causes degradation of cellulose by random scission of the hemi-acetal links
leading to hydrolyzed cellulose (hydrocellulose). The reaction for the acid-catalyzed hydrolysis of
cellulose is expressed by Figure 2.10 (A simplified structural formula for cellulose is used for ease in
following the reaction.) The hydrogen ion catalyzes (speeds up) the rate of hydrolysis; the result of
hydrolysis is a chain scission or cleavage. The product of hydrolysis is hydrolyzed cellulose, a
material analogous to cellulose, but with shorter chain length.
The result of an extreme degree of hydrolysis on a very minute (submicro-scopic) portion of a paper
fiber is shown in Figure 3.11 Four cellulose chains are represented in a by the long horizontal lines.
Hydrogen bonds and van der Waals forces (weak intermo-
8
American Pulp and Paper Association (1965), p. 109.
James P. Casey (1960), 1, 6. An informative, colored cellulose molecular model is given as the
frontispiece in Ott ct at. (1954).
10
Ibid., p. 104.
11
Adapted from Mauersbcrgcr (1954), p. 84.
9
lecular bonds) are represented by the short vertical lines. As shown in b, both the glucosidic (hemiacetal) links along the chain and the hydrogen bonds and van der Waals forces between the cellulose chains rupture at points where hydrolysis occurs.
The physical properties of a paper change relatively little during the greater part of the cellulose
degradation process.12 Figure 4 indicates that a paper's strength properties remain essentially
constant until the average D.P. of the cellulose in a typical paper fiber falls to a critical range of
about 400-500 D.P. Below this range, the paper rapidly loses strength and becomes embrittled if the
degradation continues.
The cellulose of a paper fiber may be divided into two classes, accessible and non-accessible
cellulose. The accessible cellulose is experimentally defined by its availability to enter into
chemical reactions. Non-accessible cellulose is the remainder, and it is very much less available to
enter into chemical reactions. Occasionally, the terms "amorphous" and "crystalline" are used
instead of "accessible" and "non12
B. L. Browning suggested in an interview on April 22, 1966, that the critical range for a paper's
strength properties might be represented as in Fig. 4.
accessible." This terminology complicates the problem because the percentage of crystalline
cellulose does not always correspond to the percentage of non-accessible cellulose when both are
measured by appropriate methods. The fact that some cellulose in a paper fiber is non-accessible for
acid degradation is demonstrated by studies which show that the average D.P. rarely falls below
200.13 This remains true in cases of severe acid hydrolysis. Hence, one reason that some papers are
more resistant to acid-catalyzed hydrolysis than other papers is because the amount of nonaccessible cellulose varies from one type of fiber furnish to another.
LITERATURE SURVEY
The major purpose of this paper is to propose an approach to a practical treatment for paper
deacidification in libraries. Various commentators indicate that a workable solution has not been
fully developed.14 The most useful list of studies is the annotated bibliography by Byrne and Weiner
(1964). Although this largely duplicates and extends the 1940 Kantrowitz annotated bibliography,
the latter yields additional information because different abstractors prepared many of the annotations. Bibliographies on other aspects of paper technology are published by the Institute of Paper
Chemistry and the Technical Association of the Pulp and Paper Industry (TAPPI).13
Other guides to the literature are APCA Abstracts; Abstract Bulletin; Abstracts . . . on Archaeology
and the Fine Arts; Environmental Effects on Materials and Equipment, Section A; Preservation
from Deterioration Abstracts; and, of course, Biological Abstracts and Chemical Abstracts.16 Barrow (1955, 1965a, 19656) Belen'kaya (1964); J. A. Church (1961); Grove (1964); Hansen (1939);
Kathpalia (I960, 1961a, 19616, 1962); Pravilova (1964); Spinner (1962); and Ya-brova
(1964) are contributors of interesting review and summary articles. The books by Britt
(1964); Casey (1960); Greathouse and Wessel (1954); Libby (1962); Ott, Spurlin, and
Grafflin (1954); and Plenderleith (1962) are suggested for background on cellulose, conservation
and deterioration, and paper technology. The APPA Dictionary of Paper (1965) is recommended
for clarification of paper terminology. Richardson recently contributed an annotated bibliography
on air and water pollution.
Most of the contributions on paper durability and permanence in the literature of library science are
non-technical. These works deal with such questions as denning the problem, speculating as to the
culprit, and formulating plans to find a solution or reporting on the efforts made. The technical and
fundamental literature is largely contributed by practicing scientists. Their articles are usually found
in the industrial and professional journals of the paper and textile fields. Although much about
paper deterioration is imperfectly understood, the impression received from these information
sources is that existing knowledge is not fully applied to improve the permanence of paper in books
stored in libraries.
13
Ibid.
See "Break for Bibliophiles" (1966); Clapp (1966); and Williams (1966).
15
Institute of Paper Chemistry (1936 to present) ; and Technical Association of the Pulp and Paper
Industry (1929 to present).
16
For bibliographical citations to works and authors mentioned in this paragraph, see the appended
bibliography.
14
SPECIFICATIONS OF AN IDEAL WET DEACIDIFICATION PROCESS
It will be useful to define an ideal wet deacidification process for use (1) as a yardstick in analyzing
the present wet deacidification processes and (2) as a guide to desirable improvements in practice.
The following five specifications provide one route toward these goals.
1. The treatment should reduce the active acidity (pH) to a level where other causes of paper
deterioration define the limit of useful life.
2. The treatment should deposit a protective residue in the paper substrate to act as a buffering and
stabilizing agent.
3. The treatment should not be harmful to paper and other book components or to book users.
4. The treatment procedure should be simple and within the capabilities of non-professional people
working with restricted facilities.
5. The treatment procedure should be fast, cheap, and lend itself to hand or mechanized
application.
Specification 1 is aimed at insuring that unit costs are minimized. For example, it seems wasteful to
promote maximum resistance to hydrolysis in a book whose useful life is determined by something
else.
Specification 2 derives from the accepted practice of precipitating a benign buffering agent, for
example, calcium carbonate, in the deacidified paper. Other benefits might result by an extension of
this procedure. Czepiel (I960, pp. 289-90) found evidence that the brightness reversion of paper is
accelerated by the presence of cupric, ferrous, and ferric ions." Studies by Hudson, Grant, and
Hockey (1964) on the sulfur dioxide pickup (from air) by paper indicate that enough sulfur dioxide
may be adsorbed into the margins of the leaves to cause deterioration.18 Their work supports LangwelPs (1953, p. 214)
contention that the activity of metal ions in catalyzing sulfur dioxide to sulfur trioxide should be inhibited. Perhaps the activity of these ions would be reduced as a natural consequence of
deacidification. The available copper and iron, when in ionic form, might precipitate under alkaline
conditions as the respective hydroxides.19 Therefore, Specification 2 is interpreted as implying also
the desirability of inhibiting the activity of the cupric, ferrous, and ferric ions.
Specification 3 treats the problems of reversibility, harmful or dangerous ingredients and reaction
products, and physical or chemical damage to the
17
"Brightness. As commonly used in the paper industry, the reflectivity of a sheet of pulp, paper, or
paperboard for specified light measured under standardized conditions on a particular instrument
designed and calibrated for the purpose. Strictly speaking, brightness is not a colorimetric quality.
"Brightness Reversion. A term applied to pulps, especially bleached market pulps, and also to
'white' papers and paperboards to indicate loss of brightness" (American Pulp and Paper
Association [1965], pp. 83-86).
18
The terms "absorption," "adsorption," and "sorption," are used to describe certain
phenomena by which materials may enter, remain in, and leave paper. These terms are defined by
Van Nostrand's Scientific Encyclopedia (1958), pp. 5, 23, 1534.
19
Interview with Dr. Leon Stock (1966). Support for the theory that a small quantity of calcium
carbonate, which is buffering the development of acidity, will also inhibit the brightness reversion
of paper at the margin of a book's leaves may have been provided by Hansen. The margins of
leaves, from the 1576 imprint studied, darken progressively at the edges (in contact with the atmosphere) as the quantity of calcium carbonate in these leaves decreases. See Hansen (1937, Figs. 1-6,
pp. 14-16A). Hansen also implied that this small quantity of calcium carbonate (minimum in good
leaves was 2 per cent CaCOs by weight) might be the factor which gave the good leaves
exceptional resistance to accelerated light aging (Hansen, 1937, p. 10).
book or its components.20 These problems indicate that deacidification treatments should deposit
benign residues whose ingredients are known to promote permanence over extended time periods.
The paper should retain as much of its strength as possible during treatment so as to minimize the
possibility of damage from handling.
Specification 4 aims toward the development of simple rules to determine the suitability of a given
paper or book for treatment. The latitude for error in following the treatment specifications should
tolerate considerable abuse and still provide satisfactory results.
Specification 5 is included because any acceptable deacidification treatment must be both
economical and practical. Unit costs and production schedules which librarians would find
acceptable are not available at this time.
ANALYSIS OF CURRENT WET DEACIDIFICATION PRACTICES
The concept of introducing an alkaline earth stabilizing agent into paper to increase its resistance to
the deterioration caused by acid-catalyzed hydrolysis appears to have developed initially at the
Ontario Research Foundation in the early 1930's.21 Schierholtz (1936), who assigned his patent
rights to the Foundation, claimed a process "whereby it [paper] is rendered non-tarnishing to metals,
such as silver, bronze, etc., with which it is brought into contact and inert with respect to inks and
colors, etc., applied to the paper, and more durable with age." He states: "As a result of the
treatment the paper is left substantially unchanged excepting with respect to its durability with age
and tarnishing properties."22 Aqueous solutions of barium, or calcium, or strontium, etc.. as their
respective bi-carbonates or together with their carbonates (in suspension) were impregnated into
paper by immersion or spraying. The timing of this treatment followed the other steps of paper
manufacture. Schierholtz (1936) also claimed that paper could be impregnated with an alkaline
earth hydroxide and then treated with carbon dioxide, either in gaseous form or in solution, so as to
form the alkaline earth stabilizing compound, that is, carbonate.
Kathpalia (1962) reviewed five wet and two dry methods of paper deacidification in 1962.23 He
reports that calcium hydroxide and calcium bicarbonate or magnesium bicarbonate in an aqueous
solution normally form the active ingredients of wet deacidification processes. Kathpalia also
provides useful details for the conservator as he apparently explains how the Barrow immersion
process is practiced at the National Archives of India.24 All of the wet deacidification processes
presented appear to be, chemically speaking, closely related. Hence, this paper proposes to limit its
discussion to the best-known wet deacidification processes.
Only one method of paper deacidification has proved itself through general acceptance and
remained in use over
20
Reversibility, in conservation terminology, is defined as the ability of the conservator to remove
any residue introduced into the paper by a preservation treatment. A conservator wants, in case of
need, the power to return a treated document to its condition just prior to treatment.
21
See Grant (1937), p. 194.
22
Schierholtz (1936), p. 1; and Ontario Research Foundation (1936), p. 1.
23
Kathpalia (1962), pp. 245^9. "' Ibid., pp. 245-47.
twenty-five years. Summarizing this wet deacidification process, Barrow states:
As developed in my laboratory, this process consists of placing the document to be treated between
sections of special bronze wire cloth to prevent injury, and passing it through two [water] solutions,
allowing it to remain for about twenty minutes in each. The first is a [water] solution of 0.15 per
cent calcium hydroxide which effectively neutralizes the acid and the second is a [water] solution of
approximately 0.20 per cent calcium bicarbonate which carbonates the excess hydroxide and
precipitates calcium carbonate into the fibers of the paper. After treatment, the [bronze wire] cloth
and document are air dried in a rack constructed for the purpose. The precipitated calcium carbonate
not only has a stabilizing effect upon the cellulose fiber, but also acts as a buffer against the
absorption of any acid at a later time.25
Barrow explains that his procedure will shift the pH of a document from the 4.0-5.0 range to the
8.4-8.8 pH range and that this treatment will theoretically extend the life expectancy of a document
by a factor of from 8 to 10 times.26 Such treated documents are not believed to retain any dangerous
or harmful chemicals. The calcium carbonate deposited in the paper is well known to "have existed
side by side with cellulose fibers for hundreds of years. In short, if the need for a deacidification
treatment is imperative, the Barrow procedure is the process to use.
Despite its success in reducing acidity, the Barrow process has significant shortcomings which
prohibit its general application to the paper in books. Most papers retain about 10 per cent of their
dry strength when wet by water.27 A wet and weak document requires very careful handling. Only
single sheets, not bound volumes, can be treated. The expansion and contraction of paper on wetting
and drying might break the book's spine and/or cause cockling of the leaves and warping of the
boards. The leaves might be bonded together on drying by the soluble components of the paper. The
problems of drying large numbers of books, wet by water, are not resolved. The alternative of
dismantling the book, deacidifying the leaves, and rebinding costs more than libraries can afford to
invest in the conservation of ordinary books.
Barrow developed spray deacidification as a possible solution to these problems. The active
ingredient, magnesium
carbonate, is prepared by bubbling carbon dioxide through an aqueous suspension of magnesium
carbonate for two hours. An addition of 10 per cent ethyl alcohol is made to the magnesium
bicarbonate solution to facilitate handling and to reduce the tendency of the wetted paper to cockle
on drying.28 Librarians apparently believe that this cheaper process (as contrasted to dismantling the
book, treating the leaves, and then rebinding) is also beyond their means. The literature indicates
that no other organization has taken up Barrow's recommendation for a pilot plant study.29
A picture of the present state of paper deacidification processes may be obtained by analyzing the
two Barrow treatments against the five ideal specifi25
Barrow (19656), pp. 5-6.
Barrow (1965a), pp. 6-7.
27
Libby (1962), 2, 123.
28
Barrow Research Laboratory (1963), pp. 22-26; and Barrow Research Laboratory (1964).
20
Barrow Research Laboratory (1963), p. 26. Williams (1966) does imply that spray deacidification
is practical, but this writer was unable to verify through the literature that the process has been used
outside of the Barrow Research Laboratory.
26
cations. The Barrow treatments pass or exceed Specification 1 and definitely deposit some quantity
of a benign residue to buffer the development of further acidity. No information was found as to
whether this residue inhibits the activity of the cupric, ferrous, and ferric ions. The possibility that a
magnesium or calcium carbonate residue might inhibit the brightness reversion does not seem to
have been evaluated.30 Consequently, some reservation must be retained as to whether the Barrow
treatments pass Specification 2. The Barrow process, that is, single-sheet immersion, would fail
Specification 3 unless the book were dismantled and rebound. Otherwise, both schemes could be
interpreted as passing Specifications 3 and 4. Both Barrow treatments fail Specification 5 because
the processes are slow and costly.
POTENTIALLY USEFUL MATERIALS IN PAPER DEACIDIFICATION
Barrow uses precisely the chemicals known to be associated with the long-term permanence of
paper.31 The steps in aqueous deacidification processes which most increase cost are related to the
effect of water on paper. Various investigators theorize that a deacidification solution containing
organic solvents might provide a remedy for these water-caused difficulties.32
Their line of thinking suggests that a remedy for the deacidification dilemma might be found by
seeking the answers to three questions.33
1. Are other chemicals known which have been used to promote the permanence of cellu-losic
fibers?
2. Are there chemicals which might be introduced into paper and therein allowed to react and form
chemicals which are known to promote the permanence of paper?
3. If such chemicals exist, are there solvents available which do not have the undesirable properties
of water?
Conservators commonly use acetone, benzene, ethyl alcohol, white gasoline, hexane, methyl
alcohol, naphtha, pyri-dine, toluene, and trichlorethelene as cleaning solvents.34 The problems
which concern conservators who use organic solvents are related to inks or secondary paper-making
materials and not to the paper fiber itself.35
33
Additional questions which might suggest other approaches to paper deacidification or
stabilization are:
1. Are there chemicals and processes known which could be combined to deacidify and
stabilize paper effectively without the necessity of wetting the paper being treated? This
question asks if permanence might be improved when the de-acidifying agent was introduced as a
gas. Perhaps cyclohexylamine carbonate, suggested in Langwell's (1966) vapor-phase
deacidification process, would be useful.
2. Are there deacidification or stabilization treatments known or possible which might be shown
experimentally to promote the long-term permanence of paper? This query is related to Question 1
(text) but differs in that the requirement for long-term exposure of paper (Specification 3) to the
deacidifying and/or stabilizing agent is dropped. Some possibilities or springboards for
further development are use of (a) soaps or alkaline earth tanates as implied by Holden and
Maguire (1910) ; (6) aromatic and aliphatic amines as per Speel (1952), p. 55; (c) alkali metal
borates and phosphates as reported by Istrubtsina and Pravilova (1965) ; (d) polymers of
cyanamides or their methyol compounds, as suggested by Sergeeva (1962); (e) dicyanamides and
dicyanoguanadines, as invented by Studney, Pollard, and Landes (1949, 1951); (/) acid-binding salts
as invented by Lang-well (1965); and (g) alkali salts of organic poly-acids as, e.g., sodium
carboxymethylcellulose, by Raff, Herrick, and Adams (1966).
30
See n. 19. Spinner (1962) discusses brightness reversion, for example, yellowing of paper on
aging.
31
Jarrell el al. (1936), p. 12; Hansen (1939); and Barrow (196So), p. 7.
32
LangweII (19S7), pp. 37-41; Kathpalia (1962), p. 248; and Barrow Research Laboratory (1964),
pp. 16-17.
34
Interview with Paul N. Banks (1966).
35
Ibid.
Certain properties of the first three aliphatic alcohols are compared with those of water in Table 1.
Note that water has the greatest weakening effect on paper. The retained wet strength increases with
an increase in molecular weight, for example, from methyl to ethyl to n-propyl alcohol. The dimensional stability of paper follows an analogous pattern when wet by these liquids. Paper wet by water
exhibits the least stability, while paper wet with n-propyl alcohol has the greatest diTABLE 1 ALIPHATIC ALCOHOLS COMPARED WITH WATER
mensional stability.36 On drying, paper wet with water will cockle or warp to a greater degree than
will paper wet with alcohols. The lower boiling points of methyl and ethyl alcohol signify a possibility of faster and/or lower temperature drying. The conclusion, then, is that solvents with at least
some properties superior to those of water are commercially available.
Question 1 asked if other chemicals exist which are known to be associated with the permanence of
cellulosic fibers.37 The nineteenth-century dyers knew that certain dyes (e.g., aniline black and
sulfide colors) are capable of inducing tendering in cotton and linencloth on storage.38 This
particular tendering is identified as caused by the acid-catalyzed hydrolysis of cellulose, the acid
being introduced in the dyeing process or as a decomposition product from the dye itself.39 Indeed,
Holden and Maguire referred to a number of successful treatments on which they improved in their
1909 British Patent No. 3087. Some of these earlier treatments impregnate the cloth with an alkali
metal formate or acetate to reduce the acidity and to deposit a buffering residue.40 Such a salt, say
sodium acetate, might be dissolved to form a 1 per cent by
36
Interview, April 22, 1966, with Dr. B. L. Browning, Chairman, Organic Chemistry Section,
Institute of Paper Chemistry.
37
Although results reported in part subsequently have justified a patent application, this paper has
been prepared as a progress report and the mention of chemicals and procedures herein is not to be
construed as a recommendation for the treatment of deteriorating cellulosic materials. It is hoped
that a more comprehensive report, planned to extend this preliminary report and to incorporate the
findings of the proposed laboratory investigation, will (1) be available in mid-1967 and (2) include
recommendations'as to chemicals and procedures.
38
Sansone (1895), pp. 32-33.
39
Trotman (1964), pp. 54, 438.
40
Holden and Maguire (1910). The alkali metal formates and acetates may prove so strongly basic
that they will have limited applicability as paper-stabilizing agents over prolonged periods. The
alkaline earth formates and acetates, which are less basic, appear to offer more promise in paper
deacidifkation.
weight aqueous solution.41 The dyed and unstable cotton or linen cloth is passed through the
solution and dried without rinsing.
The excessive acidity is reduced according to Reaction 1 below. The sodium acetate reduces the
danger of future acid attack either as introduced (sodium acetate) or possibly, in time, as sodium
carbonate or hydroxide because of an intermediate reaction with carbon dioxide (from air) or water.
The other
alkali metal, or alkaline earth, formates and acetates react similarly. Nevertheless, sodium acetate is
apparently the only one recommended in current textile practice.42
Barrow reports that magnesium acetate will deacidify paper, and confirms that magnesium acetate
will improve the permanence of paper.43 Magnesium acetate is recommended as an acid inhibitor in
cellulose acetate laminating films.44 In his 1965 British Patent Specification No. 1,007,981,
Langwell cites magnesium acetate as one of the chemicals which has utility in the preservation of
documents.45 Magnesium acetate is soluble in methyl and ethyl alcohol.
Question 2 asked what chemicals might be reacted in paper to form chemicals which are known to
promote the permanence of paper.46 A compound called an alkoxide is formed if the hydrogen of
the hydroxyl (OH) group in an aliphatic alcohol is replaced by a metal. The alkali and alkaline earth
metals form basic alkoxides which are usually soluble in their parent alcohol and sometimes in
other organic solvents. For example, magnesium methyl-ate, or more correctly, magnesium
methoxide is formed according to Reaction 2 and is commercially available as a 5 per cent by
weight solution in methyl alcohol.47
Magnesium methoxide, upon introduction into paper, would neutralize acidity according to
Reaction 3; or, in
41
Merrill, Macormac, and Mauersberger (1949), pp. 619-20.
Trotman (1964).
43
Barrow Research Laboratory (1964), p. IS, Fig. 1.
44
Wilson and Forshce (1959), p. 153.
45
Langwell (1965).
46
See n. 37.
47
Anderson and Thomas (1963), p. 832.
42
the event water (sorbed by the paper) was encountered before hydrogen ions, react according to
Reaction 4; forming magnesium hydroxide, which subsequently reacts to produce neutrality as per
Reaction 5; or precipitates into the paper as a mildly alkaline buffer and inhibitor.48
The use of the lower aliphatic alcohols and their alkoxides may offer significant advantages in the
deacidifica-tion of paper because: (1) the harmful effects of water on paper may be evad-
ed; (2) the organic solvent (in the sample case, methyl alcohol) would volatilize out of the paper;
and (3) the assertion that magnesium hydroxide would promote the long-term permanence of paper
is defensible.49
PRELIMINARY RESULTS
This investigator found, through experiments to justify this study, that single sheets and books
could be deacidified by the type of immersion treatments previously discussed, although the results
must still be regarded as tentative, awaiting more refined experimentation. For one thing, the
method used to estimate pH was primitive and probably inaccurate. A piece of Hydrion "A" or "B"
short-range indicator paper was laid upon the paper to be tested and a drop or two of distilled water
added at one end of this pH color-sensitive paper. A comparison was made with the standard color
range provided by the manufacturer at the peak of color intensity. An estimate of the pH of the
paper being tested was thereby obtained. Even though Barrow reported that a somewhat similar
method using pH indicator paper gave accurate results, it is believed best to limit the results of these
tests to implying that a pH shift did occur as a result of
48
The sorbed water content in the paper of books in libraries would vary with the relative humidity.
Normally, the water content ranges from 5 to 7 per cent by weight. With greater variation of relative
humidity, the water content might range from 4 to 8 per cent (personal correspondence with Dr. B.
L. Browning, June 29, 1966).
49
One defense might use ionization equilibria to compute the pH of magnesium hydroxide, magnesium carbonate, and calcium carbonate in saturated aqueous solutions containing an excess of
solid phase. Such values would not represent the true pH in a paper substrate which had been impregnated with and retained a residue of these chemicals. However, the computed pH values should,
at least, indicate the relative relationship. The computation indicates that the pH of the calcium
carbonate solution would approach 9.7, that of magnesium hydroxide 10.6, and that of magnesium
carbonate 10.9. The relative pH bounds of a paper impregnated with an excess of magnesium
methoxide would have the pH of magnesium hydroxide (10.6) as a minimum and that of magnesium carbonate (10.9) as a maximum. It seems possible that in time the magnesium hydroxide
might tend to convert to the carbonate. This argument rests on evidence presented by various
authorities (e.g., Barrow Research Laboratory, 1964) that magnesium carbonate promotes the
permanence of paper.
the treatment.50 That is, the pH values which are reported below should not be taken to indicate
literally the pH value which would be obtained if a water extract were made.
The leaves of a 1964 Dutch imprint were cut apart and half of each leaf immersed for periods of
approximately 2| seconds to eighty minutes in a solution containing 6 per cent magnesium acetate, 2
per cent diethylene glycol, 5 per cent water, and 87 per cent ethyl alcohol.51 The pH shift was from
about 5 on the untreated sheets to about 9 or 10 on the treated sheets. The time of immersion did not
appear significant. (The diethylene glycol was included to act as a plasticizer. No definite conclusion was reached as to whether it reduced paper brittleness.)
The same solution was used to treat a variety of books selected from discards of the University of
Chicago Library. Each book was split down the spine and one section totally immersed (boards and
all) in the treatment solution for thirty minutes. No effort was made to insure that all parts of the
book would be wetted, that is, circulating the solution, splaying the leaves, or moving the book
about. The section was then removed and pressed, by hand, against a flat surface to squeeze out as
much of the solution as possible. The wet section was either (1) set on end with its leaves splayed or
(2) laid on its side, board down, with every second or third signature turned over and tucked into the
spine to facilitate airflow, and then allowed to air dry overnight. In each case, the untreated section
was retained to act as the control. It took several days for the pH of the treated sections to stabilize
due to the evolution of acetic acid. The location of the pH test was at the center of a leaf in the
middle of each section. The pH shift depended upon the original pH of the paper in the book being
treated; averages were from a pH of 4 before treatment to a pH of 6 after treatment and similarly
from 5 to 7 or 8. The spines and the boards were not tested but only checked to insure that they had
been wetted.
These results were encouraging in that the acidity of the paper undergoing treatment was reduced.
(It is my opinion that the treatment reached the innermost parts of the book.) The boards and leaves
of the treated sections showed essentially no cockling or warping on drying. The treated sections
were entirely usable. The ink and dyes from the type images of a few foreign imprints did bleed and
run. However, these foreign imprints remained entirely legible although they were aesthetically
unpleasant. The dyes from the speckle fore-edge and from the cloth bindings of a number of
domestic and foreign imprints were leached sufficiently to color the treatment solution. Although
this solution did darken the color tone of the leaves of these sections or those sections subsequently
treated, librarians who inspected the results judged the bluing in those cases to be inconsequential.
Preliminary experiments with a S per cent by weight solution of magnesium methoxide solution in
methyl alcohol were limited to treating single sheets. The procedure was simple; the test sheets
were dipped into the solution, moved about for a few seconds until wetted, and then removed and
hung up
50
Barrow Research Laboratory (1963), p. 25.
The 5 per cent water derives from the water of crystallization of magnesium acetate tetra-hydrate,
(CH3COO)2Mg • 4 H2O; the source of the 6 per cent magnesium acetate.
51
to dry. The pH shift was from about 4 to 10. Although more cockling occurred than with ethyl
alcohol, the amount of cockling was significantly less than when another sheet of this same paper
was wet with water and dried.
It appears, then, that the use of aliphatic alcohols as the solvent for and carrier of a basic formate or an acetate or alkoxide may offer a
practical de-acidification treatment for paper. It also seems likely that substantially greater
quantities of these stabilizing agents may be introduced into paper because of their greater solubility
in alcohol as contrasted to the solubility of conventional stabilizing agents (calcium hydroxide,
calcium bicarbonate, and magnesium bicarbonate) in water. It is my belief that some combination of
these materials will provide at least one route toward a major breakthrough in the wet
deacidification of single sheets. It is too early to predict the ultimate outcome of this investigation
with regard to treating books. There are too many unsolved problems in efficiently treating a
variable mass of leaves held immobile along one edge, as in a book.
The goal of the proposed study is to set the stage for a mechanized, pilot plant operation wherein
whole books are deacidified. Some probability of success is indicated by (1) the Latin American
practice of thoroughly wetting whole books with gasoline and (2) the non-aqueous deacidification
treatment developed by Baynes-Cope, British Museum Research Laboratory.52 If this study does
develop an efficient laboratory method of deacidifying whole books, there will remain formidable
problems in expanding these results to processing the quantity and variety of books in a research
library on a production-line basis.
PROPOSED LABORATORY INVESTIGATION
The laboratory investigation, in which the above treatments (and perhaps others) may be studied,
might be subdivided into three stages. The initial set of experiments could evaluate the effect of
different treatments upon several kinds of book papers. The book papers, themselves, might be
selected to act as constants with the treatments being evaluated as variables. The second series of
experiments could extend and support these initial results by evaluating effective treatments on
book papers selected to represent the varieties of paper found in books in libraries. In.this series, the
treatments would be
52
In an interview with Harold W. Tribolet (October 13, 1965), he asserted the feasibility of treating
books in organic solvents based upon the Latin American practice of soaking books in gasoline to
prevent insect and rodent attack. (Paper, when wet by hydrocarbon solvents like gasoline, does not
lose strength or expand.)
The normal British Museum practice is to use the Barrow immersion process as their routine
method of paper deacidification. A. D. Baynes-Cope, British Museum Research Laboratory, developed a non-aqueous deacidification process for documents which cannot be treated with water.
The criteria for selection of paper for this special treatment are (1) the nature of the ink, i.e., if the
ink is likely to be susceptible to water damage; and (2) the actual fragile nature of the paper, i.e., if
the paper would become dangerously weak if wetted by water.
The treatment solution is prepared by dissolving 1.86 gms. of barium hydroxide octahydrate,
Ba(OH)2 • 8 HSO, in 100 ml. of methyl alcohol to form a 1 per cent by weight-volume solution of
barium hydroxide, Ba(OH)2. This deacidification solution is applied (1) by an immersion treatment
or (2) by brushing or spraying when the paper is very fragile. The treated document is hung for air
drying so that any residue of barium hydroxide might react with carbon dioxide from the surrounding air to form barium carbonate. Sensible precautions should be taken in applying this
process because (1) barium and most of its compounds are toxic, (2} methyl alcohol fumes are both
explosive and toxic, and (3) many colored inks (e.g., ballpoint-pen inks) are soluble in methyl
alcohol (Werner, 1966).
considered as constants and the papers as variables. A final set of experiments might attempt to
expand efficient treatments to the deacidification of whole books.
The utility of the deacidification treatments could be evaluated using the standardized tests
established by the Technical Association of the Pulp and Paper Industry or by more rigorous
procedures designed by authorities on paper to improve the consistency of the TAPPI tests.
PROBLEMS IN DEACIDIFYING BOOKS
Certain problems in the wet deacidification of whole books, and sometimes single sheets, deserve
further discussion. Although cellulose is a relatively inert material, paper fibers are necessarily
surface-active. Such activity might adsorb some of the deacidification agent out of solution as the
treatment is penetrating into a closed book. Or as the book is drying, the deacidification agent might
be carried out to the book's surface and deposited near the point where the solvent evaporates. It is
evident that these actions must occur to some degree, and to the extent that additional protection is
required at the periphery of the book, these actions are desirable.
How can the handling problems of impregnating and drying books be mechanized? Will vacuum
processing techniques facilitate processing at ordinary temperatures? Or will it prove desirable to
dry books using dielectric heating whereby the book can be warmed uniformly? Would a centrifuging stage provide assistance by reducing the amount of solvent to be removed by drying?
What methods of solution treatment will be desirable so that recycling, as
necessary in a continuous process, can occur? Obviously, filtering will remove particulate matter,
but what of the colorants leached from the book by the organic solvent? Would these dissolved
materials be adsorbed by a surface-active filtrant or will another method, such as distillation, be
indicated?
If a given treatment is applicable to a certain kind of paper or book or condition of paper or book,
will the treatment be applicable for all kinds of papers and books?
Will formic and acetic acids completely volatilize out of the paper, and are these acids and their
parent compounds (for example, the alkali metal and alkaline earth formates and acetates) benign to
paper and book materials as some of the literature indicates?
What is likely to occur with different inks and their colorants and media? Fortunately, most book
printing inks utilize carbon-black pigments and the boiled linseed or tung-oil-type media. These
materials tolerate both alcohols and shifts in pH range. However, the small quantities of blue-black
dyestuffs incorporated by present-day practice into printing inks do bleed in alcohols. The result,
technically speaking, would be a grayer paper and a slightly browner type image. Actually, the
treated paper might appear whiter to the human eye because the dyestuff will tend to neutralize the
yellowish hue of the paper. The problem of colored illustrations is considerably more serious as a
very great variety of dyes and pigments are used. Some dyes and pigments are understood to be
unstable in alkaline conditions, and they or other ink components might be soluble in alcohol. Although verification is necessary, it is believed that most inks used in printing and illustrating books
will tolerate the
sort of treatment this discussion has proposed.53
The secondary paper-making materials, such as fillers, starch, coatings, and alum-resin size, should
not be detrimentally affected by this treatment. Likewise, cotton and linen cords or cloth and
boards, which are cellulosic in nature and comparable to paper, should not be affected. Animal
glues and starch pastes should be unaffected. However, the pyroxylin clothbound books may create
a problem. The pyroxylin, that is, cellulose nitrate, is soluble in alcohols.
It is probable that leather bookbindings should not be immersed into or wetted by the treatment
solution because the alkaline solution might detan the leather, and the alcoholic solvent might tend
to dry and harden the binding.54 Some leather colorants are soluble in alcohol and could stain the
book's leaves. It may be possible to evade this problem through a hand treatment for leather-bound
books. The leaves could be immersed while holding the boards up out of the solution. A special
rack would be necessary to support the book and hold the boards away from the leaves during
drying. A conditioning treatment might be required to protect the flexibility of the book's spine or
hinges.
SUMMARY .
This paper has:
1. Superficially sketched the mechanism and effect of the acid-catalyzed hydrolysis of cellulose on
paper fibers;
2. Briefly reviewed current wet deacidification practices as an aid in suggesting desirable
improvements in practice;
3. Introduced several new approaches to the problems of paper deacidification in books stored in
libraries;
4. Described a few of the preliminary experiments indicating probable success of the proposed
investigation;
5. Cited a number of problems on which more information is needed; and
6. Outlined a plan by which these ideas on paper deacidification could be investigated.
ACKNOWLEDGMENTS.—This
study was made possible through encouragement and guidance from
the Institute of Paper Chemistry, Ap-pleton, Wisconsin, and from the Department of Chemistry and
the Graduate Library School, University of Chicago. The author appreciates the assistance received
from Paul N. Banks, Conservator, Newberry Library, Chicago; Forrest F. Carhart, Jr., Director',
Library Technology Program, American Library Association, Chicago; and A. E. Werner, Keeper,
British Museum Research Laboratory, London. The author is particularly indebted to B. L.
Browning, Chairman, Organic Chemistry Section, Institute of Paper Chemistry, Appleton,
Wisconsin; Marvin C. Rogers, Director, Chicago Paper Testing Laboratory, and Executive Director,
Photoengraving Research Institute, Chicago; Leon Stock, Department of Chemistry, University of
Chicago; and Harold W. Tribolet, Manager, Extra Binderv, R. R. Donnelley and Sons Co., Chicago,
for Background on the nature of cellulose, paper, and books. Thanks are also due to Herman H.
Fussier, Director, University of Chicago Library; George W. Troyan, Assistant District Sales
Manager, Allied Chemical Corporation, General Chemical Division, Chicago; and James D. Innes,
Assistant to the Director of Marketing, Stauffer Chemical Company, New York City, for gifts of
books, magnesium acetate, and sodium and magnesium methylate (methoxide).
53
Rogers (April S, 1966).
Roddy (1966).
54
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