Paper Deacidification Methods

Most paper produced between the 1840s and the 1980s was manufactured using an acidic sizing agent — alum-rosin size — that causes the paper to become brittle as it ages. The acidity accelerates the hydrolysis of cellulose fibres, which is why newspapers from the mid-twentieth century disintegrate while much older handmade papers, which were sized with gelatin, often survive in readable condition.

Deacidification does not reverse existing damage. It neutralises residual acid and deposits an alkaline buffer — typically a calcium or magnesium compound — that slows future deterioration. The Canadian Conservation Institute notes that deacidification can meaningfully extend the functional lifespan of paper when combined with correct storage conditions.

Identifying Acid Degradation

Before treating any document, assess the degree of degradation. Treatment decisions should be based on what the paper can physically tolerate, not on what a particular method can theoretically achieve.

Visual indicators

• Yellowing or browning of the paper surface, beginning at the edges where oxygen exposure is highest
• Brittleness — pages that crack or break when flexed gently rather than returning to their original shape
• Tideline staining — wavy brown rings that indicate previous water exposure, which often accelerates acidic breakdown
• Foxing — small rust-coloured spots caused by a combination of fungal activity and metal impurities in the paper

pH testing

pH test strips calibrated for the 4–10 range can measure the surface pH of dry paper. Dampen a small area of an inconspicuous margin with distilled water and press the strip against it. A reading below 7.0 confirms acidity; a reading below 5.5 indicates significant degradation. pH pens — which change colour on contact with acidic paper — are faster but less precise.

Aqueous Deacidification

Aqueous methods involve immersing or washing paper in a solution of deacidifying agent in water. They are the most thorough approach and also the most demanding — the paper must be able to tolerate full wetting without losing structural integrity or causing media to run.

Calcium hydroxide solution

A dilute solution of calcium hydroxide in distilled water (approximately 2 grams per litre) is one of the longest-established aqueous treatments. The paper is floated in a tray of solution for several minutes, then lifted onto a polyester mesh screen and allowed to dry flat. As the water evaporates, calcium carbonate precipitates within the paper fibres, providing an alkaline buffer.

Magnesium bicarbonate solution

Magnesium bicarbonate solution is prepared by passing carbon dioxide through a suspension of magnesium hydroxide in water until the suspension clarifies. The resulting solution is less alkaline than calcium hydroxide and is gentler on fragile papers. It is the standard for aqueous treatment of highly degraded materials in many North American conservation studios.

Precautions

• Test all inks, dyes, and media for water-sensitivity before immersion. Iron gall inks — common in documents from the 17th to early 20th centuries — are generally stable, but some synthetic dyes will bleed.
• Use only distilled or deionised water. Tap water in most Canadian municipalities contains chloramine and dissolved minerals that can leave residue or react with degradation products in the paper.
• Support wet paper fully when lifting it from a tray. Wet paper loses tensile strength and tears easily under its own weight.

Non-Aqueous Deacidification

Non-aqueous methods use a deacidifying agent dissolved in an organic solvent rather than water. They are appropriate for water-sensitive materials — manuscripts with fugitive inks, maps with colour washes, photographs on paper supports.

Bookkeeper spray (magnesium oxide in HFE solvent)

Bookkeeper is a commercially available non-aqueous spray developed for archival use. It disperses magnesium oxide particles in a hydrofluoroether solvent that evaporates quickly and leaves no residue. It is available in Canada through conservation suppliers and through the Canadian bookseller network. The spray is applied in a fume-ventilated space; the solvent is not acutely toxic but should not be inhaled in quantity.

Wei T'o solution

Wei T'o, developed in the 1960s by a Canadian chemist, was among the first non-aqueous deacidification products widely adopted by libraries. It uses magnesium methoxy methyl carbonate dissolved in a freon or hydrocarbon carrier. The original freon-based formulation has been reformulated due to ozone depletion concerns; current versions use approved hydrocarbon carriers. It is sold for both spray and brush application.

Mass Deacidification

For collections involving hundreds or thousands of items, individual treatment is impractical. Library and Archives Canada has used mass deacidification processes — in which documents are treated in sealed chambers using vapour-phase or liquid-phase deacidifying agents — for portions of the federal collection. These processes are not accessible to individual collectors but are described in LAC technical documentation for institutional context.

After Treatment: Storage Conditions

Deacidification slows deterioration but does not eliminate it. Storage conditions after treatment determine how much benefit the treatment provides over time.

• Aim for a relative humidity between 30% and 50%. Humidity above 65% accelerates both chemical degradation and biological growth. Canadian winters commonly produce indoor humidity below 20% when heating runs continuously; a humidifier in the storage area is often necessary.
• Store documents in acid-free folders inside acid-free boxes. Archival-quality enclosures are available from suppliers including Gaylord Archival and University Products, both of which ship to Canada.
• Keep storage areas away from exterior walls, which experience the greatest temperature fluctuation, and away from water pipes.