This section presents our responses to the questions and goals stated at the beginning of the project. Our conclusions are based on the analysis of the 1,578 specimens tested and may or may not reflect results that might have been obtained from a much larger sample set. We consider the work valid, nevertheless, as it answers some questions and raises important new ones about the history of papermaking and paper stability.

Pre- versus Post-1500 Papermaking
Papers made before 1500 contained higher concentrations of gelatin and calcium than papers made in subsequent centuries (plots 1 and 2). The pre-1500 papers were also thicker (plots 7 and 8), and lighter in color based on delta L* values (plot 9) (with the exception of papers whitened with chlorine bleach after the early nineteenth century). 1 We believe this evidence helps explain the commonly observed superior stability and intriguing aesthetic properties of these early European papers. The enhanced characteristics of the pre-1500 papers were likely connected with the fact papermakers in the pre-printing era made their high quality papers primarily for writing. At the same time we believe they attempted to make their sheets more parchment-like in order to better compete with parchment and vellum, the well-established and readily available writing substrates of the period. Gelatin became far less necessary in paper after the arrival of printing.

Materials and Workmanship
We found that papers with the highest materials and workmanship (M&W) grades (indicating superior formation quality, freedom from knots and debris, etc.) were associated with lighter color delta L* values (plot 52).2 A similar but less pronounced trend was seen with gelatin and calcium (plots 46 and 47). Specimens with the lowest M&W grades were associated with higher iron concentrations (plot 50). There was no trend evident with K or S levels (plots 48 and 49).

Variables Impacting Paper Stability
All papers are best thought of as a system (both chemical and physical) impacted by a complex mix of internal and external variables. Looking at any one of these variables independently is bound to amount to an over-simplification of the reality. 100% rag raw material is no guarantee of a long lasting paper if the water used to make it is loaded with dissolved Fe. Research may eventually show that a large amount of alum in paper is not necessarily a problem if there are high levels of Ca and gelatin also present. And poor storage conditions can ruin any paper given enough time. With this caveat about paper as a system, we consider the following variables independently:

Although it was commonly assumed that papermakers utilized gelatin sizing throughout the 1300–1800 period, this is one of the first times data has been gathered on the actual gelatin concentration in many hundreds of historical specimens. The earlier perception turns out to be accurate: gelatin was routinely added to paper. In addition, we found an association between higher gelatin content and lighter delta L* and delta a* color (plots 17, 77 and 24), the lighter colored specimens in two subsets (plots 55 and 71), and the highest versus the lowest M&W grades (plot 46). Based on this evidence, gelatin appears to be a net asset in paper. This conclusion mirrors similar findings by Anne-Laurence Dupont following her extensive accelerated-aging studies of gelatin-sized papers.3 The manner in which gelatin makes this positive contribution is likely to be a combination of pH buffering,4 added strength,5 preferential hydrolysis of the protein molecules over those of the cellulose (thereby slowing the acid-catalyzed hydrolysis of cellulose caused by alum),6 resistance to penetration of pollutant or other gasses that can contribute to deleterious reactions, and/or moderation of RH within the cellulose via a thin-film sealing effect. The latter two mechanisms, to the best of our knowledge, have not yet been investigated or demonstrated.

Gelatin appears to have a positive influence on paper color, but it is not as strong as that of Ca. Fe had a negative impact on our color measures delta L* and delta a*. K and S, related to alum content, were also found in higher concentrations in the darkest specimens. Therefore if a paper is known to be gelatin sized and is found discolored, the cause may be other components in the paper rather than the gelatin itself. That said, poor storage conditions could indeed result in a discolored gelatin-sized paper that is relatively free of Ca, Fe, K, or S. This is because paper tends to discolor more quickly when subjected to the high RH and/or temperatures associated with poor storage (or accelerated-aging studies).

We found an association between higher Ca content and lighter delta L* and delta a* color (plots 18, 78, and 25 and SBDP subset impact rankings), the lighter rather than the darker specimens in two subsets measured on the delta L* scale (plots 56 and 72), and the highest vs the lowest M&W grades (plot 47). These trends were equal to and often stronger than those seen for gelatin.

Calcium carbonate has long been considered advantageous in paper because of its ability to act as an alkaline reserve, minimizing the impact of any acidic components in the paper or entering it from external sources. Because of this capacity, its addition to modern archival machine-made papers was one of the key recommendations to come out of the Barrow study.7 It is therefore not a surprise that Ca appears in our data strongly associated with enhanced stability. It is important to emphasize that we do not know from this research exactly which compounds Ca is associated with. CaCO3 is the most likely prospect, but calcium sulfate from the water supply is another possibility. In a straight correlation test of our variables, K and S had a correlation of 0.613 while that for Ca and S was 0.385. We consider this one piece of evidence that the S in the paper is there primarily as a component in alum rather than calcium sulfate.

Assuming that all the Ca in the papers tested was associated with CaCO3, we calculated that roughly twice as much would be found in paper made before 1500 than after (plot 12).

K, S, and Fe
We found an association between higher levels of K and S and papers closer to black on the delta L* scale (plots 19 and 20) however the trends are not strong. The negative impact on color is stronger with Fe (plots 21 and 28) but still not as pronounced as the positive influence of gelatin and especially Ca on color (plots 17 and 24, and plots 18 and 25, respectively).

Interestingly, in an exploration of rosin and alum sized papers, Raymond Janes found discoloration was related to Fe contamination of alum, rather than to excess alum addition per se.8 However in the present work, based on correlation values, Fe and K or Fe and S do not seem to be strongly correlated.

Nondestructive Instrumentation for Use in Conservation and Paper Studies
UV-Vis-NIR and XRF instrumentation and techniques are promising methods for the analysis of artifacts on paper during treatment. While the degree of precision attained in this work needs improvement for single-item analysis, the current levels of precision are adequate when large numbers of specimens are tested and general trends sought. Determination of these elemental concentrations in artifacts on paper can help the conservator make better informed decisions about the efficacy of various aqueous-intervention treatments or storage-protocol options. XRF and UV-Vis-NIR surveys, for instance, could help pinpoint why some books or drawings in a collection are browned and weak while others are not. If the data show that the discolored papers are much higher in concentrations of elements associated with alum (K, Al, and S or Al and S post-1800), or Fe, and low in gelatin and Ca content, that information could help a conservation or collections-care team decide whether or not aqueous neutralization and deposition of an alkaline reserve, or treatment with a chelating agent, is warranted. Alternatively, the same data, depending on the levels found, might help the team recommend, as a cheaper alternative, new alkaline-buffered housing for the artifacts with problems.

To establish helpful parameters for evaluating the results of such a survey, we selected the two hundred most stable specimens from the full data set, based on our chosen stability measurements of delta L* and delta a*, and compared them to the two hundred least stable papers (see table A below).9 In order to eliminate the effect of the introduction of chlorine bleach and other nineteenth-century innovations, specimens from 1800 or later were not considered. The preselection sample set consisted of 1,423 specimens. Note that delta L* data include values 0 and lower, and delta a* data include values 0 and higher. The values reported are for the two hundred highest or lowest values for each stability measure. In other words, the two hundred specimens are a different and unique group for each measure. Observed data (error not included) from those two hundred most or least stable are then reported by individual variables in tables B and C (below).

We note that the two hundred most stable specimens in table A are also twice as thick as the two hundred least stable (thickness 1 = thickness of the single leaf analyzed in millimeters). Given the same raw material and degree of beating, a thicker sheet would be stronger than a thinner one. But it is also the case that a thick, soft sheet can be considerably weaker than a thinner paper made from strong, well-beaten fiber. The most stable specimens also have a noticeably higher concentration of gelatin (median 8.64% versus 1.9%) and calcium (9,509 ppm versus 1,957 ppm) compared with the least stable. M&W grades are at least one point higher in the most stable specimens versus the least stable. K, S, and Fe values are, in general, not significantly different. In summary, gelatin and calcium can be considered the primary indicators of stability when stability is measured by paper color (delta L* and delta a*).

Table A


Average delta L*

Median delta L*

Average delta a*

Median delta a*

thickness 1, mm

thickness 1, mm

200 most stable

-2.43 -2.63 0.70 0.77 0.21 0.21

200 least stable

-17.2 -15.8 6.20 5.84 0.12 0.12


Table B


Weight %

Median Ca ppm

Median K ppm

Median S ppm

Median Fe ppm

M&W grade

200 most stable delta L*

6.0 8866 383 616 331 4

200 least stable delta L*

2.5 2008 594 1060 457 2

200 most stable delta a*

6.2 8159 436 681 326 4

200 least stable delta a*

2.3 1938 579 1000 454 2


Table C


Weight %

Average Ca ppm

Average K ppm

Average S ppm

Average Fe ppm

M& W grade

200 most stable delta L*

6.0 9825 471 1591 341 4

200 least stable delta L*

2.8 2453 700 2140 473 2

200 most stable delta a*

6.1 8977 502 896 335 4

200 least stable delta a*

2.5 2226 698 1330 467 2


With the above figures, conservation specialists can obtain data with nondestructive instrumentation that indicate which artifacts are “very stable,” which are “very unstable or at risk,” and which fall in between.

Do the results of this research recommend resizing with gelatin in cases where a conservator has determined aqueous conservation treatment is necessary? In the opinion of principal investigator Barrett, yes, particularly if pretreatment nondestructive analyses indicate a measureable amount of gelatin present (more than 2%) and/or if the artifact will be subject to handling, as, for example, in the case of a rare book that is regularly called for by history of the book classes. Cellulose derivatives are often considered alternatives to gelatin in resizing, but the chemistry of the two materials is very different and the cellulose derivatives do not have the long history that gelatin has in papermaking and paper stability. It was not possible during this work to monitor the changes in gelatin concentration during aqueous conservation treatment; however, important related destructive experiments were accomplished by Terry Trosper Schaeffer and Rachel Freeman in 1995 and 2004, respectively.10 11 More important, recent research in NIR techniques holds promise for the development of accurate non-destructive evaluation of gelatin content during treatment in the near future.12 13

At risk of stating the obvious, any type of aqueous- or gaseous-deposition conservation treatment alters forever the value of the artifact for research such as this study. The item is no longer as originally made and naturally aged. All concerned need to balance this reality with the responsibility to preserve a given artifact for future generations. In many cases, artifacts are best left free of aqueous treatment and instead given the best possible housing and storage conditions.

This project benefitted greatly from conservator notations attached to the artifacts or their housings that stated clearly what, if any, intervention had taken place. On behalf of future material-artifact researchers, we encourage conservators to indicate when an item has been subject to intervention and in what way. That information should at least be attached in abbreviated form to the artifact or its housing, with more detailed information filed or recorded online, separate from the artifact.

Accelerated-Aging Studies and the Manufacture of Handmade and Machine-Made “Permanent” Papers
Conclusions reached during studies of naturally aged papers must be backed up by accelerated-aging studies of laboratory-prepared papers. As a result of this work, we recommend that future aging studies incorporate gelatin-sized controls. Anne-Laurence Dupont’s research in this regard is a major contribution to the field and an excellent complement to the present study. Advances in aging equipment and techniques should incorporate temperature and humidity cycling and exposure to pollutant gases to better evaluate the role of gelatin in paper permanence. Such research may eventually show that gelatin or a comparable substitute is required in the production of long-lasting archival papers.

What type of gelatin should be employed in conservation treatment, research, or in the manufacture of modern archival papers? This is an appropriate and important question without a good, single answer. Modern gelatin manufacturers produce high-quality but often much modified gelatins with a wide array of physical and chemical characteristics. One could argue that the most traditional and perhaps the most appropriate gelatin for conservation applications would be a size made in the lab from parchment clippings. Recipes for such sizing exist and are used by some conservators.14 At the UICB Research and Production Paper Facility we use an acid-extracted Gelita bone gelatin that contains 4,000 ppm Ca.15 A careful and comprehensive study of gelatin materials available for use in conservation has not been done and is much needed.

[1] For more on our delta L* and other color values, see UV-Vis-NIR Spectrometer, Color analysis under PROCEDURES, Instrumentation and Methods and Color under DISCUSSION, Nonchronological Plots.

[2] For more on our materials and workmanship (M&W) grades see PROCEDURES, Specimen Selection especially the discussion associated with figures 2 and 3.

[3] Anne-Laurence Dupont, “Gelatine Sizing of Paper and Its Impact on the Degradation of Cellulose During Aging: A Study Using Size-Exclusion Chromatography” (PhD dissertation, University of Amsterdam, 2003).

[4] John Baty and Timothy Barrett, “Gelatin Size as a pH and Moisture Content Buffer in Paper,” Journal of the American Institute for Conservation 46, no. 2 (2007): 105–21.

[5] Timothy Barrett, “Evaluating the Effect of Gelatin Sizing with Regard to the Permancence of Paper,” in Conference Papers Manchester, 1992, ed. S. Fairbrass (London: Institute of Paper Conservation, 1992), 228–33.

[6] Dupont, "Gelatine Sizing of Paper and Its Impact on the Degradation of Cellulose During Aging: A Study Using Size-Exclusion Chromatography."

[7] W. J. Barrow Research Laboratory, Physical and Chemical Properties of Book Papers, 1507–1949. Permanence/Durability of the Book 7 (Richmond: W.J. Barrow Research Laboratory, 1974).

[8] Raymond Janes, “A Comparison of the Color Stability of Certain Rosin Sizing Agents as Influenced by Light and Iron” (Bachelor of Science thesis, Department of Paper Science and Engineering, Western Michigan University, 1953) and personal communications with T. Barrett.

[9] For more on our delta L* and other color values, see UV-Vis-NIR Spectrometer, Color analysis under PROCEDURES, Instrumentation and Methods and Color under DISCUSSION, Non-Chronological Plots.

[10] Terry Trosper Schaeffer, “A Semiquantitative Assay, Based on the TAPPI Method, for Monitoring Changes in Gelatin Content of Paper Due to Treatments,” Journal of the American Institute for Conservation 34, no. 2 (1995): 95–105.

[11] Rachel Freeman, “Quantifying Gelatin Sizing Loss During Aqueous Conservation Treatments” (Senior Specialization Project, Graduate Program in Art Conservation, Buffalo State College, May, 2004), 1–9.

[12] M. Strlič, D. Lichtblau, J. Kolar, T. Trafela, L Cséfalvayová, M Anders, B de Bruin, et al., “SurveNIR project – a Dedicated Instrument for Collection Surveys.” Durability of Paper and Writing 2. Ljubljana, Slovenia, September 10, 2008. Accessed July 21, 2011.

[13] L. Cséfalvayová, M. Pelikan, I. Kralj Cigić, J. Kolar, and M. Strlič, “Use of Genetic Algorithms with Multivariate Regression for Determination of Gelatine in Historic Papers Based on FT-IR and NIR Spectral Data,” Talanta 82 (2010): 1784–90. doi:10.1016/j.talanta.2010.07.062

[14] P. Spitzmueller, “Selecting a Paper Re-sizing Agent and Its Concentration: A Look at Parchment Size and Photographic Gelatin,” in Manchester Conference Papers 1992, ed. S. Fairbrass (London: Institute of Paper Conservation, 1992), 214–18.

[15] Gelita photo bone gelatine (Type 8039, Lot 1), available from Gelita North America, PO Box 927, Sioux City, IA 51102 (tel. 888-443-5482).

Cite as: . “Conclusions.” Paper through Time: Nondestructive Analysis of 14th- through 19th-Century Papers. The University of Iowa. Last modified . .