THE ROLE OF GELATIN IN PAPER PERMANENCE
TIMOTHY BARRETT, & CYNTHEA MOSIER
Modern papermakers and conservation researchers have obtained useful data from the analysis of historical paper specimens. Most notably, William Barrow offered important new standards for permanent and durable machinemade papers based on his evidence that longer-lasting historical papers could be characterized by their alkaline pH, alkaline carbonate reserve, high-purity cellulose fiber, and freedom from highly acidic sizing materials (Barrow 1974).
Gelatin was commonly used as a sizing material between 1400 and 1800 to prevent the bleeding of ink. Added advantages were its ability to improve the strength, abrasion, and soiling resistance of the paper. Since gelatin was apparently common in paper of this period, we feel its role in paper permanence deserves research attention. It has been proposed that gelatin may act as an acid acceptor (Wilson and Parks 1983), meaning that component amino acids in gelatin act as a buffer (Courts 1980) and may make gelatin a significant buffer in paper (Erickson 1994; Swartz 1994). These proposed effects might give gelatin a role in protecting paper from acidic environmental pollutants and acidic degradation products. Other possible roles for gelatin include limiting oxygen or pollutant gas access to the cellulose or minimizing the negative effects of humidity cycling via a relative humidity buffering effect.
If gelatin is a factor in paper permanence, adjustments may need to be made in our current standards for permanent and durable papers. More important for conservators, we may need to pay closer attention to how much gelatin is being removed during conservation treatments and to consider whether or not it should be replaced.
Gelatin is a protein derived from the larger protein molecule collagen, which is a major constituent of hard and soft connective tissues in animals. When a tissue such as hide or bone is subjected to an alkali or acid pretreatment followed by extraction in heated water, chemical bonds in collagen are broken somewhat randomly, resulting in the shorter chains of amino acids we call gelatin. (We should point out that this is the present-day method of manufacture and that historically, gelatin may have been manufactured by extraction in heated water alone.) The length of these amino acid chains, which affects many of the properties of gelatin, can vary greatly within one batch, but in general a milder treatment will break fewer bonds and thus yield a larger number of longer molecules than will a more severe treatment.
Despite the variation in gelatin molecule lengths, the amino acid composition of all gelatins is distinguished by its relatively high content of 4-hydroxyproline, an amino acid found in large amounts in no other animal protein. Approximately one out of every 11 amino acids in gelatin is hydroxyproline. The percent hydroxyproline by weight in mammalian gelatins varies slightly depending on the source of the tissue, the method of manufacture, and the purity of the gelatin (Leach and Eastoe 1977); but as an example, a hide glue, the moderately pure gelatin similar to that likely used for paper sizing, contains about 12.6% hydroxyproline by weight (TAPPI 1991a).
There is an accurate method for determining the protein content of paper, but it is not specific for gelatin (TAPPI 1991b). If other proteins or nitrogenous compounds are not known to be absent, gelatin content is more accurately determined by measuring the hydroxyproline in the sample and using that value to calculate the amount of gelatin present. TAPPI Official Method T-504 (TAPPI 1991a), revised 1989, is a spectrophotometric procedure for the determination of glue or gelatin in paper by determining hydroxyproline. TAPPI states that this determination is not affected by the presence of glycerin, alum, or rosin. We used a modified microscale version of this procedure in our study, as described in section 3.