What Is Book Deacidification? How to Slow Paper Deterioration
Paper deterioration is the single greatest long-term threat to book collections. The primary cause is acid — specifically, the acidic compounds present in most paper manufactured between approximately 1850 and 1990. These acids slowly break down the cellulose fibers that give paper its strength, causing the paper to yellow, become brittle, and eventually crumble. Deacidification is the process of neutralizing these acids to slow — though not reverse — the deterioration.
Why Paper Becomes Acidic
The Alum-Rosin Sizing Problem
Paper manufactured during the industrial era (roughly 1850–1990) was typically “sized” with alum-rosin — a chemical treatment that made the paper resistant to ink bleeding. Alum-rosin sizing produces sulfuric acid as it ages. This acid attacks the cellulose fibers of the paper, causing chain scission (breaking of the long cellulose polymer chains) that progressively weakens the paper.
Lignin and Wood Pulp
Before the mid-19th century, paper was made primarily from cotton and linen rags — fibers that are naturally acid-free and remarkably durable. (This is why medieval manuscripts on rag paper are often in better condition than 20th-century books on wood-pulp paper.)
The shift to wood-pulp paper in the mid-1800s introduced lignin — a natural polymer in wood that produces acids as it degrades. Paper with high lignin content (newsprint, mass-market paperback paper) yellows and becomes brittle fastest.
Environmental Acids
External acid sources also contribute:
- Air pollution (sulfur dioxide from industrial emissions)
- Acidic storage materials (cardboard boxes, wooden shelves)
- Adhesives used in binding and repair
- Previous owner’s handling (skin oils are slightly acidic)
How Deacidification Works
Deacidification introduces an alkaline (basic) compound into the paper to neutralize existing acids. The process typically deposits an “alkaline reserve” — excess alkaline material that continues to neutralize acids as they form in the future.
The Chemistry
The basic chemical principle: acids are neutralized by bases. Deacidification agents are mild alkaline solutions or compounds that:
- Penetrate the paper fibers
- React with existing acids to neutralize them
- Leave a residual alkaline buffer (typically calcium or magnesium carbonate)
The alkaline reserve is the key to long-term effectiveness — it continues working as new acids form from ongoing paper degradation or environmental exposure.
Deacidification Methods
Spray Deacidification
Bookkeeper spray: The most accessible deacidification method for individual collectors. Bookkeeper (manufactured by Preservation Technologies) is a spray-on solution containing magnesium oxide particles suspended in a non-aqueous carrier. You spray each page, the carrier evaporates, and the magnesium oxide remains in the paper as an alkaline reserve.
Application: Fan the pages and spray lightly. Allow the carrier solvent to evaporate (typically 15–30 minutes). The process does not wet the paper significantly, which reduces the risk of cockling (wrinkling).
Limitations: Spray application treats the surface of the paper but may not fully penetrate thick or dense pages. It is best for routine preventive treatment, not for paper that is already severely degraded.
Aqueous (Wet) Deacidification
Professional conservators use aqueous deacidification solutions — typically calcium hydroxide or magnesium bicarbonate dissolved in water. The book’s pages are immersed in the solution or the solution is applied by brush.
Advantages: Thorough penetration of the paper fibers. The alkaline reserve is deposited throughout the paper, not just on the surface.
Disadvantages: Wetting paper risks cockling, ink bleeding, and dimensional changes. Aqueous treatment is only appropriate for unbound pages or pages that can be safely wetted. This is a professional conservation treatment — not a DIY procedure.
Mass Deacidification
Libraries and archives use industrial-scale deacidification processes to treat entire collections:
Wei T’o process: Uses magnesium methoxide dissolved in a fluorocarbon carrier. Books are immersed in the solution, which penetrates the paper without wetting it. The carrier evaporates, leaving magnesium carbonate as an alkaline reserve.
Bookkeeper process (mass version): An industrial-scale version of the spray process, treating hundreds of books at a time.
DEZ (diethyl zinc) process: The Library of Congress developed this process, which uses diethyl zinc gas to neutralize acids. The process has been controversial due to the extreme flammability of diethyl zinc.
When to Deacidify
Preventive Treatment
Deacidification is most effective as a preventive measure — treating paper before it has deteriorated significantly. If the paper is still white, flexible, and strong, deacidification will help keep it that way.
Books at Risk
Prioritize deacidification for:
- Books printed on acidic wood-pulp paper (most books from 1850–1990)
- Books showing early signs of yellowing or brittleness
- Books stored in uncontrolled environments (attics, basements)
- Paperback originals and mass-market paperbacks (printed on the most acidic paper)
When Not to Deacidify
Already-alkaline paper. Books printed after approximately 1990 on acid-free paper (look for the “infinity symbol” ∞ on the copyright page, indicating acid-free stock) do not need deacidification.
Severely deteriorated paper. If the paper is already brown and brittle — if pages crack when flexed — deacidification will not reverse the damage. It may slow further deterioration, but the paper’s structural integrity is already compromised.
Books with sensitive inks or colorants. Some inks, watercolors, and colorants react with deacidification agents. Test on an inconspicuous area before treating.
Very valuable books. For high-value collectible books, consult a professional conservator before applying any treatment. Even well-intentioned treatment can reduce value if improperly applied.
Deacidification and Market Value
The Collector’s Dilemma
Deacidification is a conservation treatment — an alteration of the book’s original state. In the rare book market, any alteration — even a beneficial one — should be disclosed to potential buyers. Some collectors prefer untreated copies in their original condition; others prefer the knowledge that deterioration has been slowed.
Disclosure
If you deacidify a book and later sell it, disclose the treatment. A description might read: “Pages deacidified with Bookkeeper spray to slow acid deterioration.” This is professional and transparent.
Impact on Value
For most books, careful deacidification has a neutral or slightly positive effect on value — it demonstrates that the owner cared about preservation. For very expensive books ($5,000+), some collectors prefer no treatment at all, and the disclosure of deacidification may modestly affect perceived value.
Prevention vs. Treatment
The best approach to acid deterioration is prevention:
Store books in a controlled environment. Temperature (65–70°F), humidity (30–50% RH), and air quality all affect the rate of acid formation and paper degradation.
Use acid-free storage materials. Acid-free boxes, tissue paper, and shelving prevent external acid contamination.
Avoid exposure to light. UV light accelerates chemical reactions, including acid formation.
Handle with clean hands. Skin oils are slightly acidic and contribute to long-term paper degradation.
Deacidification is a valuable tool, but it is a supplement to — not a substitute for — proper environmental storage. A book stored in ideal conditions on acid-free shelves will fare better over the long term than a deacidified book stored in a humid basement.