Volume 17, Number 4
Sep 1993

The Role of Alum in Historical Papermaking

(This article is based on a talk presented at the IPC Conference 1992 in Manchester, England. A paper published in the conference proceedings contains additional information not included here.)

As a sizing ingredient, alum has been one of the most important materials in the history of papermaking. Since the late 19th century, it has also been mentioned as a primary cause of paper degradation, but little detailed information on the manufacture and use of alum has been available in the readily accessible literature. The two major alum varieties employed in papermaking have not always been distinguished for their different properties. Aluminum potassium sulfate was used throughout the history of papermaking until the 19th century. It was then replaced by the newly developed aluminum sulfate, a cheaper and more concentrated source of aluminum compounds. Although both aluminum potassium sulfate and aluminum sulfate tended to introduce different impurities into paper, the negative effect of aluminum sulfate on paper degradation overall is more significant. Certainly, an understanding of the characteristics of the two compounds can provide certain insights into the aging properties of paper containing either alum variety.

A. ALUMINUM POTASSIUM SULFATE

Aluminum potassium sulfate had been imported into Europe since antiquity for application in various trades such as fabric dyeing, and it was the first alum used in papermaking. It could be obtained from minerals such as alunite which occurred in sulfur-containing volcanic sediments. Mining sites were sometimes located in volcanic crater bottoms where the stones were extracted with naturally heated water, alum crystals forming in the evaporating solution.

A.1 Alum Production

In the mid-15th century, the first European alum mines were exploited at Tolfa, a volcanic area north of Rome in central Italy. This particular site is of interest to us as the source of what appears to have been one of the best varieties of alum in papermaking: the so-called Roman alum.¹ Known for its high quality until the 19th century, Roman alum was recognizable by its distinct reddish appearance, which was a result of dusting the alum crystals with a pigment--probably iron oxide.² The pigmentation served as a trademark of Roman alum. Before use, the alum could be rinsed under cold water to remove the pigment without dissolving significant amounts of the alum itself.

Slate and shale were other minerals which yielded alum when subjected to several production steps which can be summarized as follows: the aluminous rocks were piled up, roasted, and subsequently extracted in water; potassium hydroxide was added to the resulting solution; the crude alum crystals which formed in the evaporating solution were rinsed and redissolved in boiling water to purify the alum; the solution was transferred to large wooden casks where the alum crystals formed on the inside walls; and finally the casks were dismantled and the crystals removed.³

Alum could be contaminated with byproducts of its manufacture, iron oxides and iron sulfates. Iron compounds significantly impaired the performance of alum as a mordant of textile dyes and were more likely to discolor gelatin-alum sizes, as is indicated by the concern of 18th-century papermakers for good quality alums. The repeated recrystallization of the alum effectively freed it from iron contaminants.

A.2 Historical Gelatin-Alum Sizing

In Europe, the use of alum for hardening gelatin sizes is recorded during the 16th century. In 1579, Samuel Zimmermann in Germany wrote in a treatise on so-called secret arts, which included papermaking: "The printing and other paper. . . is drawn through alum-water and dried again."⁴ This quotation apparently refers to the separate application of gelatin and alum, a practice which was continued until the 18th century.⁵ In the process, the paper was first gelatin- sized, then steeped in a vessel containing the alum solution, and finally pressed for removal of excess size.

Gelatin solutions already containing the alum were more economical to use. They offered the advantage of preventing the rapid spoilage of the size during storage since the alum crystals were added to the freshly cooked gelatin sizing solution. Despite these advantages of the latter method, both sizing processes appear to have been used contemporaneously.

Like other 18th century writers, Lalande provides more detailed information on alum than is available from earlier sources. Roman alum, he states, was preferred in papermaking over what was designated only as "rock alum."⁶ Rock alum was a general term for an inferior quality alum which could affect the paper brightness. Until the 19th century, paper mills prepared their own gelatin size solutions by cooking raw animal parts. The reported weight of alum added to such a solution was typically based on the weight of the animal parts rather than the gelatin concentration of the solution. This information only allows an estimate of the ratio of alum and gelatin in the size. Percentages of 5% to 40% have been mentioned in the literature. Barrett used Lalande's text to calculate that the size described in his book could have had an alum content of 27%, based on the weight of the dry gelatin.⁷ It is interesting to note that aluminum ammonium sulfate was a not uncommon substitute for regular alum in paper sizing until the 1800s.⁸

A.3 Miscellaneous Historical Uses of Alum

Watercolorists brushed their gelatin-alum sized papers with an alum solution to ensure or improve their uniform moisture resistance. Alum was also believed to give additional luster to watercolors, as is noted in a 17th century manual.⁹ More significant, however, is the fact that some pigments used in watercolors are altered through the increased acidity of the alum-treated paper surface. In 1901, Church described how an artist can test the acidity of watercolor papers by applying ultramarine, chrome yellow and carmine washes. The subsequent visible alteration of pigment color indicated the presence of excess alum.¹⁰

In traditional bookbinding, alum was also widely used. For example, decorative paste papers, which were manufactured from the 16th century onward, were sometimes brushed with an alum solution after application of the pigment-paste mixture. This treatment increased the toughness of the paper surface.

For centuries, alum has been used as an ingredient in Japanese dosa or gelatin sizing for the preparation of paper for painting or printing.

B. ALUMINUM SULFATE

Aluminum sulfate, also called alum, became an industrial product in the 19th century. It was made by treating either bauxite or china clay with sulfuric acid. Unlike true alum, aluminum sulfate could not be conveniently purified through recrystallization because of its greater solubility in water. This is one of the reasons why it often contained varying proportions of silica, iron and free sulfuric acid. By the early 20th century, however, commercial aluminum sulfate varieties were relatively uniform in quality. They were ranked according to grading systems and could be purchased in solid pieces as so-called "patent alum." Well-known varieties included cake alum¹¹, porous alum¹² and Turkey Red Alum.¹³ Because of its greater concentration of alumina (Al₂O₃) and cheaper production procedures, aluminum sulfate saved mill expenses and therefore replaced aluminum potassium sulfate for most purposes in papermaking and particularly in rosin sizing towards the mid-19th century. This earned it the name "papermaker's alum." Some 19th century European papermakers produced alum in their own facilities to ensure a more consistent quality at a lower price.¹⁴ At the mill, alum was emptied into tanks for dissolution in water prior to use.

B.1 Rosin-Alum Sizing

In the early 19th century, papermakers learned the correct use of the newly-developed rosin sizing technology by trial and error methods. The first valid theory explaining the principles of rosin sizing was not put forward until 1879. To precipitate one part rosin onto the paper fibers, 1.5 parts alum were required in the pulp solution, but an excess of alum was frequently added to ensure sufficient sizing. (Through improvements in 20th-century paper manufacture, the proportion of rosin and alum added to the pulp has significantly declined.) Not all of the alum was retained by the paper and some mills recovered it from the white water drained from the wire of the paper machine. Papers which needed to be particularly strong could be sized twice--either with rosin and a subsequent gelatin size or with two gelatin size applications. In such cases, the alum concentration was sometimes increased in the second size bath.

B.2 Miscellaneous Historical Uses of Aluminum Sulfate in Paper Manufacture

In 19th-century paper mill operations, alum was introduced for miscellaneous purposes besides sizing. It became what a critic called "one of the most used (and most abused!) non-fibrous materials" in the papermaking industry.¹⁵

For water clarification, blocks of alum were placed at the bottom of water tanks feeding into the mill. There the slowly dissolving alum precipitated suspended dirt particles and alkaline earth carbonates. The presence of either was particularly undesirable in rosin sizing.¹⁶

In the chlorine bleaching of pulp, small quantities of alum were added to speed up the chemical reactions. The risk of damaging the cellulose fibers increased through this treatment.¹⁷

Alum helped to retain clay fillers, pigments, resins and fiber fines in the pulp by aggregation. They were otherwise easily drained away with the white water from the pulp on the paper machine.¹⁸

Alum was effective in pitch control. Pitch, composed of resinous unsaponifiable chemicals in certain types of pulp, accumulates in the headbox and on the wire of the paper machine and can interrupt its smooth operation. The frothing or foaming of dispersed sizing materials on the paper machine was also inhibited through the presence of alum.¹⁹

Aluminum ions also act as deflocculating agents in pulp slurries.²⁰ They react with the paper fibers and give them electrical charges of the same sign. The repulsion of their like charges keeps the fibers apart and in even suspension. On the other hand, alum helped to increase bonding between fibers during sheet formation on the paper machine. This increased the wet strength of the paper.

Figure 1. Presence of alum in historic book papers. The percent of books that contained alum is indicated for each century. Average pH of tested books is indicated in each bar. (Source: Barrow.Research Laboratory, 1974, Table 2)

Overall, the use of alum increased between the 16th and the 20th century. (Fig. 1) Simultaneously, the average pH of book papers shifted from 6.7 to 4.8.²¹ Barrett, in his X-ray fluorescence analysis of book papers from 1500 to 1800, found that papers in good condition contained less aluminum, potassium and sulfur than those in poor condition.²² Twentieth-century sizing trends can be further exemplified by the postwar US rosin consumption, which steeply increased during the late 1950s.²³ At the same time, up to 500,000 tons of alum were used each year worldwide by the paper industry, which amounted to 60% of the worldwide alum production. Only a small percentage of this alum was sold as being iron-free.²⁴ The use of rosin decreased significantly during the 1980s.

C. ASPECTS OF THE ROLE OF ALUM IN PAPER SIZING

Aluminum ions show a characteristic strong affinity for cellulose and a complex behavior under various papermaking conditions. Some of the relevant research of the past years has been excerpted as follows:

Figure 2. Example for rosin-alum bonding to cellulose. (Source: Gess, 1989, p. 78)

The basic mechanism of rosin sizing involves, among a variety of other possible reactions, the formation of cationic aluminum salts from the rosin soap and the aluminum ions. As Gess observes, the aluminum salt "can then react with the cellulose to provide the bridged complex" between the cellulose and the rosin.²⁵ (Fig. 2)

Figure 3. Structure proposed by Gess for the aluminum/beta keto acid reaction product. (Source: J.M. Gess and R.C. Lund, "The Strong Bond/Weak Bond Theory of Sizing," Tappi J. Jan. 1991, p. 111 ff.)

In the sizing process with alkyl ketene dimers, the aluminum in the beta-keto acid complex serves a similar purpose by linking the dimer to the cellulose, as recently proposed by Gess. (Fig. 3) Only five pounds of alum are usually added per ton of paper produced.²⁶

In gelatin-alum sizing, the formation of crosslinkages between hydrated forms of aluminum ions and gelatin molecules has been suggested.²⁷

Figure 4. Retention of rosin and alum in paper as a function of pH conditions during sheet formation. (This graph shows two of the curves in Fig. 1.1, p. 28, from E. Strazdins, "Chemistry and Application of Rosin Size," in W.F. Reynolds, ed., The Sizing of Paper, 2nd ed., Tappi Press, 1989.)

Figure 5. Relationship between extractable acidity and sheet making pH, for pulp containing 30% alum. (Source: W.F. Reynolds and W.F. Linke, "The Effect of Alum and pH on Sheet Acidity." Tappi, Vol. 46, 7, 1963, p. 410 ff.

The alum and rosin retention in the paper is determined, among other aspects, by the pH of the pulp solution. There is evidence that the alum retention increases with increasing pH in the 4 to 6 range, whereas the rosin retention remains constant over that range.²⁸ (Fig. 4) Varying pH conditions also determine the extractability of acidity from paper as demonstrated for softwood pulp impregnated with aluminum sulfate. The amount of acidity, measured as sulfur trioxide, which could be extracted from the sheets was greatest when the sheets were formed at pH levels between 4 and 6.²⁹ (Fig. 5)

D. CONSERVATION RESEARCH

The effects of conservation treatments on alum-containing papers have been investigated by a number of individuals. Some of the findings can be briefly summarized as follows:

In the 1970s Daniels found that the bleach chloramine-T was retained by water-rinsed test samples impregnated with alum, whereas alum did not affect the removal of other oxidizing bleaches.³⁰

Wilson noted that, among selected deacidification agents, only magnesium carbonate was able to reduce the concentration of aluminum compounds in alum-impregnated papers, and its effect was slight.³¹

Prolonged aqueous conservation treatments may cause the partial removal of alum-containing sizes, as has been noted in a recent study on the light bleaching of paper carried out at the Conservation Analytical Laboratory, Smithsonian Institution, Washington.³²

E. EXPERIMENTAL OBSERVATIONS ON GELATIN-ALUM SIZING

In order to investigate the natural aging characteristics of gelatin-alum sized papers, the author carried out several experiments. Test papers were sized with 3% gelatin solutions containing varying proportions of alum. In some cases alkaline earth carbonates were added to simulate hard water papermaking systems. The papers were artificially aged at 90°C and 50% RH and tested for a change of physical properties. The resulting color change, ÆE, and tensile energy absorption, TEA, of the aged papers can be summarized as follows:

Figure 6. Relationship of alum content in gelatin size and color increase in sized and aged cotton linter papers. (Original data)

The increase in discoloration in units of ÆE (measured with a Minolta Chroma Meter) of sized and aged cotton linter papers was directly proportional to the increase in aluminum sulfate concentration, which ranged from 0 to 90% based on the gelatin content. The alum concentrations above 50% do not reflect papermaking practices but were included to show possible paper aging trends resulting from the alum increase. (Fig. 6)

The presence of iron compounds with the alum in the paper appeared to play a significant simultaneous role in inducing the color changes. Also, alum concentrations higher than 40% made the paper very water absorbent.

Accelerated aging (90°C, 50% RH) produced a color shift in cotton linter papers that depended on the composition of their size. Under the aging conditions employed, unsized paper remained relatively white. Gelatin-sized paper showed a tendency towards yellowish discoloration, whereas increasing alum concentrations in the gelatin size shifted the discoloration from yellowish towards grayish brown. Test papers impregnated only with alum also discolored to a distinct grayish brown tone.

Figure 7. Relationship of alum content in gelatin size and paper strength in sized and aged Whatman #4 filter papers. (Original data)

The value of the TEA measure of paper strength declined proportionally with an increase in alum content of gelatin-alum sized and aged Whatman filter papers. (Tensile energy absorption is a measure for the amount of energy necessary to stretch a sheet of paper to the point of rupture.) Interestingly, the test paper sized with only 1% alum on the weight of the gelatin did not suffer physical strength loss during aging. Papers impregnated with magnesium carbonate or calcium carbonate and then sized with a gelatin-alum solution showed better physical strength properties after aging than did similarly sized unimpregnated papers. (Fig. 7)

F. SUMMARY AND CONCLUSIONS

Pre-16th century accounts of the use of alum in papermaking are scarce, but alum was very likely known to papermakers by the 14th century. The presence of elemental aluminum and potassium detected in 15th-century papers indicates the application of alum. The early use of the substance is not surprising since it provided the most effective method of reducing the ink absorbency of gelatin sized writing papers. Papers sized only with gelatin remain readily moisture absorbent.

In sizing practice, alum was added to the gelatin size or, alternatively, was applied to paper separately after gelatin sizing. Both of these sizing methods are recorded in the 18th-century literature. Whether the aging properties of sized papers were affected by the method chosen remains to be investigated. The quality of the alum (as mainly determined by the extent of its contamination by iron) and of the gelatin probably was more important in influencing the aging properties of papers than was the method of application.

Aluminum ammonium sulfate was an occasional substitute for aluminum potassium sulfate in paper sizing. This should be borne in mind when elemental analysis of paper specimens shows the presence of aluminum, but not of potassium.

In sized and artificially aged test papers, alum decreased the paper strength and brightness. It was observed that, whereas the use of Whatman filter papers ensured the greater homogeneity of the sample population and therefore greater reliability of the test results, handmade cotton linter papers had aging properties more closely resembling those of naturally aged papers. The cotton linter papers, for example, discolored more readily upon artificial aging (90°C, 50% RH), probably because of impurities such as iron compounds in the pulp.

In the evaluation of historical papers, however, alum should not be regarded as the only cause of paper degradation. The effect of alum on paper can only be appreciated in context with the many interrelated aspects of paper composition, including the quality of the gelatin used; the pH of the sizing solution; the purity of the alum; and the presence of other pulp constituents. Although it will not be possible for a conservator to distinguish between these aspects for every paper examined, the history of alum application should be a reminder of the variability of paper as a historical medium.

Acknowledgements

The author expresses her gratitude to the faculty of the Art Conservation Department, State University College at Buffalo, especially to Cathleen Baker for suggestions and comments and to Dan Kushel and Christopher Tahk for technical advice and assistance throughout the project. She would also like to thank Timothy Barrett, Director of Paper Facilities (at the School of Art and Art History, University of Iowa and the UI Center for the Book), for carrying out X-ray fluorescence analysis and folding endurance testing. He, along with Pamela Spitzmueller, University Conservator, University of Iowa Libraries, generously shared their insights into papermaking practices and paper properties. The helpful historical literature suggestions provided by Tom Conroy in Berkeley, California, are also much appreciated. This paper was prepared during 1990-91 while the author was a Getty Senior Fellow at the Art Conservation Department, State University College at Buffalo.

REFERENCES

Barrett, Timothy. "Early European Papers/Contemporary Conservation Papers." The Paper Conservator, 1989, Vol. 13.

Barrow, W.J., Research Laboratory. Physical and Chemical Properties of Book Paper, 1507-1949. (Permanence/Durability of the Book--VII) Richmond, Virginia: Barrow Research Laboratory, 1974.

Beckmann, Johann A. "Anleitung zur Technologie." 4th edition from 1796, in: Karl Keim, Das Papier, 2nd edition. Otto Blersch Verlag, Stuttgart, 1956.

Chambers, E. Cyclopedia or an Universal Dictionary of Arts and Sciences. London, 1784.

Church, A. H. The Chemistry of Paints and Painting. Seeley and Co. Ltd., London, 1901.

Daniels, Vincent. "The Elimination of Bleaching Agents from Paper." The Paper Conservator, Journal of the IIC United Kingdom Group Paper Group, 1976, Vol. 1.

Davis, Charles T. The Manufacture of Paper. Philadelphia, 1886.

Gess, J. M. et al. "The Strong Bond/Weak Bond Theory of Sizing." Tappi Journal, January 1991.

Gess, J. M. "Rosin Sizing of Papermaking Fibers," Tappi Journal, July 1989.

Grant, J. A Laboratory Handbook of Pulp and Paper Manufacture, 2nd edition. Edward Arnold & Co., 1944.

Hofmann, Carl. Praktisches Handbuch der Papier Fabrikation. Verlag der Papierzeitung, Berlin, 1891. Vol. 2.

Jenner, Thomas. A Book of Drawing, Limning, Washing or Colouring of Maps and Prints. Printed by Simmons for Jenner, London, 1652.

Kragh, A. M. "The Effect of Aluminum Sulphate and other Polyvalent Metals on the Viscosity of Gelatin Solutions." Journal of the Science of Food and Agriculture, June 8, 1957.

Lalande, Joseph de. The Art of Papermaking (1761), transl. by R. Atkinson. County Clare, Ireland: Ashling Press, 1976.

Libby, C. Earl. Pulp and Paper Science and Technology, Vol. II. McGraw-Hill, New York, no date (probably 1960s).

The Manufacture of Pulp and Paper, Vol. III. (Joint Executive Committee on Vocational Education Representing the Pulp and Paper Industry of the United States and Canada) McGraw-Hill Book Co., New York, 1922.

Rance, H. F. The Raw Materials and Processing of Papermaking. Handbook of Paper Science, Vol. 1. Elsevier, New York, 1980.

Reynolds, W. F. et al. "The Effect of Alum and pH on Sheet Acidity." Tappi, Vol. 46, No. 7, 1963.

Schaefer, Terry T., et al. "Effect of Aging on an Aqueously Light Bleached, Mixed Pulp Paper." The Book and Paper Group Annual, Vol. 10. American Institute for Conservation, Washington, D.C., 1991.

Singer, Charles. The Earliest Chemical Industry: An Essay in the Historical Relations of Economics and Technology Illustrated from the Alum Trade. 1st edition. The Folio Society, London, 1948.

Tomlinson, Charles (ed.). Cyclopedia of Useful Arts, Mechanical and Chemical Manufactures, Mining, and Engineering. London, 1862.

Ure, Andrew. A Dictionary of Arts, Manufactures, and Mines, Vol. 1. Appleton & Co., New York, 1866.

Wilson, William K. et al., "The Effect of Magnesium Bicarbonate." Preservation of Textiles and Paper of Historic and Artistic Value II. J. C. Williams, ed. (Advances in Chemistry Series, 193) American Chemical Society, Washington, D.C., 1981.

Witham, G. S. Modern Pulp and Paper Making. Chemical Catalog Company, New York, 1920.

Zimmermann, Samuel. Von Gehaimnuß verborgner Künsten/ New Titularbuech Das ist/ wie man ainer Jeden Person/ sey was Wurdigkait sie wolle/ in zwen Thail gethailt/ sambt etlichen hinzugethanen fürtreffentlichen Gehaimnussen/ verborgenen Mechanischen Apochryphischen und gleichsam Übernatürlichen Künsten/ des Lesen und die Schreiberey betreffend/ Desgleichen vor niemals in Truck aussgangen. Ingolstadt, 1579.

Notes

1. Tomlinson, p. 40

2. Chambers, p. 9

3. Ure, p. 52 ff.;
Singer, p. 193 ff.

4. Zimmermann, p. 107

5. Beckmann, p. 58

6. Lalande, p. 46

7. Barrett, p. 27

8. Davis, p. 436

9. Jenner, p. 22

10. Church, p. 12

11. Grant, p. 129

12. Davis, p. 438

13. Singer, p. 289 ff.

14. Hofmann, p. 342

15. Grant, p. 127

16. Davis, p. 440

17. Manufacture of Pulp and Paper, 9, p. 26

18. Libby, p. 116

19. Witham, p. 250

20. Rance, p. 19

21. Barrow, p. 40

22. Barrett, p. 33

23. Unpublished manuscript, Hercules Co., 1991

24. Libby, p. 128

25. Gess, p. 78

26. Gess, p. 112

27. Kragh, p. 351 ff.

28. Reynolds et al., p. 410 ff.

29. Ibid.

30. Daniels, p. 9

31. Wilson et al., p. 104

32. Schaefer et al., p. 208