THE ROLE OF GELATIN IN PAPER PERMANENCE
TIMOTHY BARRETT, & CYNTHEA MOSIER
2 RELEVANT LITERATURE
In a study of factors influencing the stability (as determined by retention of folding endurance, tensile breaking strength, tearing strength, alpha cellulose content, and copper number after aging for 72 hours at 100°C) of experimental papers made from two different sulfite pulps, researchers reported that “all the  papers of the series were relatively low in stability with the exception of tub sizing run no. 225, a waterleaf sheet sized with glue containing no alum, which retained 71% of the initial folding endurance and decreased but 1 percent in alpha cellulose content after the accelerated aging test” (Rasch et al. 1933, 22).
In a study using identical aging conditions, results showed that seven gelatin sized papers retained an average 56% of their original fold endurance compared with four papers containing no gelatin size, which retained an average 16% (Rasch 1931). In a different study investigating wood fibers as a papermaking material, the same researcher noted that gelatin surface-size appeared to help “retard chemical degradation” in paper, as evidenced by superior retention of fold endurance and alpha cellulose, again after 72 hours of aging at 100°C (Rasch 1929, 500). In an investigation of highly purified wood fibers as papermaking material, workers noted in their conclusions that “surface sizing with glue or starch improved the resistance to the aging test [retention of folding endurance, bursting strength, tearing strength, tensile breaking strength, percent elongation at rupture, alpha cellulose content, and copper number after 72 hours at 100°C], especially with glue sizing” (Rasch et al. 1931, 782).
When humidity has been combined with elevated temperature in aging studies, again interesting but inconclusive results have been noted. In a Japanese study, when handmade papers were sized with traditional combinations of glue and alum and aged at 80°C and 80% RH, the size had no distinguishable effect on deterioration rate constants (Inaba and Sugisita 1988). The glue did, however, improve the mechanical properties of the paper. (It should be noted that the alum added during this study was 50% the weight of the dry gelatin—much more than would be recommended when and if alum is required during the production of handmade conservation papers). After moist heat aging at 80°C, papers sized with gelatin or gelatin and alum (alum added at 5% based on the weight of the dry gelatin) showed a higher rate of deterioration but superior mechanical properties when compared with unsized aged base sheets (Barrett 1989). After exposure to automobile exhaust followed by heat aging, gelatin-sized papers in another experiment showed a similar higher rate of strength loss, but again, better after-aging mechanical properties compared with aged controls free of gelatin (Barrett 1992).
Barrett (1989) noted an inclination for gelatin-sized papers to discolor more compared to their counterpart base paper without gelatin after moist accelerated aging at 80°C. Rasch (1929) made a similar observation after aging rag papers with and without gelatin sizing for 72 hours at 100°C but, as cited above, the same report also suggests gelatin surface size appeared to help “retard chemical degradation” in paper.
To the best of our knowledge, prior to the work we describe here, only one study of gelatin content in historical paper specimens has been mentioned in the literature (Barrow 1972). In it the author reports that 10 writing papers from the 17th and 18th centuries submitted to the National Bureau of Standards in about 1940 for testing contained between 2.6% and 5.8% glue. The method of determination is not stated. In our earlier qualitative analyses of historic papers, we observed higher levels of gelatin in papers considered to be in good condition compared with a second group considered to be in poor condition (Barrett 1992). In a continuation of that study, we have now made quantiative measurements of hydroxyproline in a different selection of naturally aged papers.