Iron in Paper: Problems and Current Solutions – Part 2

chemicals used to prevent iron catalyzed damage in paper

Most of the conservation research regarding treatment options for Fe(II) catalyzed oxidative damage, sometimes displayed as discoloration and foxing in paper is focused on stabilizing iron gall inks in archival collections. As a conservator, my aim is to apply the findings of this research to address problems posed by iron contaminants that may be scattered throughout the paper sheet and develop an effective cleaning and stain reduction system whether the paper contains iron or not.

Treatment steps in such systems might include the following. Some modification may be necessary to protect sensitive media or paper surfaces, and in some cases treatment is not possible. (Keep in mind that Fe(II) ions are catalytic and water soluble; Fe(III) ions are insoluble.)

Water treatment: Uses solutions where the conductivity is tailored to optimize soil removal. This treatment removes soluble discoloration and some catalytic Fe(II) ions as Fe(II) is water soluble. (See Wolbers’ World: a Workshop Review in the September 2012 issue).

Reduction: Reducing agents or bleaches like sodium dithionite and sodium borohydride reduce overall discoloration and stains and convert some insoluble Fe(III) to water soluble Fe(II).

Chelation: Chelators are weak organic acids that can sequester metal ions and remove intractable soils that surfactants and water alone can’t budge. Citric acid and EDTA are two chelators sometimes used in paper conservation that reduce overall discoloration and stains and sequester some amount of Fe(II) and Fe(III).

Citric acid has less binding power than EDTA, which can be an advantage when treating items with vulnerable colours. The stronger binding powers of EDTA are useful, but can be hazardous to the media. Rinsing well after chelation is essential as any residual EDTA allows it to join catalytic Fe(II) and hydrogen peroxide in the same molecule a combination likely to trigger the dreaded Fenton Reaction: in a few weeks or months, the previously cleaned paper can become heavily spotted and discoloured.

Phytic acid may provide a better option than EDTA as phytic acid does not bind with hydrogen peroxide; any phytate-Fe residues that might be left in the paper after treatment are so efficiently bound that they remain chemically inactive. Because phytic acid is only slightly weaker than EDTA, media vulnerability is a consideration.

Oxidative bleaches: The above treatments may eliminate or minimize the need for oxidative bleaches. Gentle light bleaching may be sufficient and chemical bleaches can be applied in rigid gels for very targeted treatments.

Alkaline reserve: Raising the pH of the treated paper to the 8-8.5 range binds a proportion of any residual Fe(II) into insoluble hydroxides preventing further catalytic activity. Alkaline reserves also protect against future acid hydrolysis, which causes yellowing.

Antioxidants: More research is required, but antioxidants will likely be used in the future to prevent metal-catalyzed oxidative damage.

Resizing with gelatin: Creates a protective barrier between the paper and the outside environment to slow down the absorption of airborne pollutants like peroxides and buffer changes in Relative Humidity (RH).

Environmental controls: High humidity promotes chemical reactions, including iron catalyzed oxidation. RH-conditioned sealed framing and storage systems are protective. Moderate temperatures also reduce chemical reactions.

This summary of treatment options for paper conservation is intended for the general public. Chemical concentrations, modifications and pH ranges for their safe use have not been included.