(General) High Acyl Gellan Gum In Parchment Conservation by Cathie Magee


In recent years, gellan gum and other gels have become welcome additions to the conservator’s toolkit. Aqueous treatments to remove adhesive from parchment can cause cockling, shrinkage, tidelines, brittleness, transparency, and gelatinization. Having experienced the challenges associated with treating parchment documents, I looked forward to the opportunity to learn more about the application of gels in this type of treatment. It was helpful that talks about gels were grouped together in the General Session, reducing the amount of sprinting between the convention center's "Earth" and "Sky" concourses to hear talks in competing concurrent sessions.

The presenter began by explaining the different types of gellan gums. Gellan gum is available in two general varieties: high acyl and low acyl. After being prepared for use, the high acyl (HAGG) variety appears to be a nearly opaque, translucent white, soft, elastic gel, in contrast with the more transparent and rather rigid low acyl gellan gum familiar to many conservators. The advantage of high acyl gellan gum is its compatibility with organic solvents. With low acyl gels, the solvents can be added after gelation, but the solvents in a high acyl gel can be added to the colloid prior to gelation, making it easier to determine the exact solvent ratio. Using a solvent-modified gel, a conservator can apply aqueous gel treatments with a great degree of control over the amount of moisture in contact with the parchment.

Next, the presenter provided tips for gel preparation. Yes! You can cook it in the microwave, with vigorous stirring, of course. A hot plate with a magnetic stir bar is the preferred method (a magnet’s arms never get tired). The hydration temperature is 70-70 degrees C with gelation at 70 degrees C. Calcium acetate enhances the structural integrity of the gel (0.4 g/L calcium acetate). The prepared gel can be stored in the refrigerator prior to use.

The first case study was a book cover whose paste-downs were comprised of parchment manuscript waste. The conservator used HAGG prepared with a 1:1 ethanol/ water (magnesium hydroxide) solution.  Mechanically damaged areas were reinforced with remoistenable facing tissue (RK-0 coated with a 1:1 mixture of wheat starch paste and 3% methyl cellulose).

The second case study was a spine adhesive on an eleventh-century German Gospel. The tenacious adhesive resisted removal. Adding a weight to push the gel into closer contact proved ineffective in removing the adhesive. When the conservator added sodium chloride to increase conductivity, the efficacy of the gel improved (0.5 g NaCl). Unfortunately, the gel couldn't reach adhesive trapped between the quires of the volume. 

The third case study was a set of cleaning tests. The presenter tested 1% HAGG in deionized water with and without calcium acetate, sodium chloride, and ethanol. She also modified the gels with a chelating agent (citric acid monohydrate) and a strong base (saturated calcium hydroxide). The same solutions were also used for a tideline reduction test. Future plans include artificial aging of the test samples.

During the question and answer period, the presenter explained that she had not tested the gels on sewing supports. She clarified that all solutions were used at 1% HAGG, because it was difficult to mix the gel at higher concentrations.

One serious problem with gels is the tendency to leave behind residues, The presenter was asked about the use of interleaving or barrier papers; she had found that they diminished the benefits of the HAGG by reducing contact with the surface of the parchment. In her opinion, the gel residue was better for the object than the adhesive residue; this was a trade-off that she was willing to accept, even if hers was a somewhat controversial position on the issue.