Understanding Acidity in Japanese Paper: What “Acid-Free” Really Means
Japanese papers are renowned for their elegance and strength, long used in conservation, bookbinding, and fine art applications. Often described as “acid-free,” they are frequently trusted for archival work—but that label, while generally true, isn’t always the full story.
While many Japanese papers are close to neutral in pH, they can still lean slightly acidic or alkaline depending on factors such as fiber type, additives, and processing methods. Let's dive into the nuances of pH in Japanese paper and see why they remain among the most trusted materials in the preservation field.
More Than Just “Acid-Free”: Why Japanese Paper Is Trusted in Conservation
Before getting into the details of pH, it’s important to recognize why Japanese papers are so widely used in archival and conservation contexts:
1. Long Fibers, Incredible Strength
Japanese papers are made from bast fibers like kozo (mulberry), gampi, and mitsumata, which are far longer than the fibers found in Western wood-pulp papers. These long fibers give the paper exceptional strength, even when incredibly thin.
This makes Japanese papers ideal for delicate tasks like lining fragile documents, repairing tears, or mending parchment—offering mechanical support without bulk.
2. Low Lignin, Naturally Stable
Traditional Japanese fibers are naturally low in lignin, the component in wood pulp that leads to acidity and embrittlement over time. As a result, these papers often maintain their integrity for centuries—even without modern chemical treatments.
3. Thin Yet Strong and Transparent
Japanese papers can be made in extremely lightweight weights—some as light as 4–5 gsm—while still offering strength and durability. This allows conservators to make virtually invisible repairs or overlays without compromising the aesthetics or legibility of the object.
4. Flexible, Absorbent, and Reversible
Because they’re handmade or lightly processed, many Japanese papers are also highly absorbent and flexible, making them receptive to adhesives, dyes, and water-based treatments. When used with reversible adhesives like wheat starch paste, these papers also support conservation ethics of reversibility and non-invasiveness.
The Meaning of “Acid-Free” and “pH Neutral”
Now, let’s turn to the chemistry.
Acid-free usually refers to papers with a pH above 6.5 or 7.0, but standards vary.
pH neutral refers to a pH very close to 7.0—neither acidic nor alkaline.
In casual use, “acid-free” has come to suggest a paper is "archival," but that’s not always the case. In practice, these terms are often used loosely. A paper might be labeled “acid-free” even if it leans mildly acidic or alkaline due to its materials or processing methods.
Why Some Japanese Papers Lean Acidic or Alkaline
1. Fiber Additions and Blends
Many conservation-grade Japanese papers are made exclusively from bast fibers like kozo or gampi. However, some modern or machine-made papers may include wood pulp, rayon, or synthetic materials to reduce cost or adjust performance.
Wood pulp introduces lignin, making the paper more acidic.
Rayon and synthetic inclusions can also shift pH, depending on their composition and processing.
While these papers may still offer excellent workability, they might not be the best choice for long-term conservation work.
2. Bleaching and Brightening
To achieve bright white or consistent color, some papers undergo bleaching or lightening treatments, which can introduce residual acidity. To counteract this, many mills add a buffering agent like calcium carbonate, which can result in a paper with a slightly alkaline pH.
So, a paper may technically be “acid-free” due to buffering—even if acidic steps were taken earlier in production.
3. Surface Sizing and Treatments
Sizing agents—used to modify absorbency—can also influence pH. Some sizing materials are neutral, but others (like alum or certain synthetic resins) can lean acidic or alkaline. Traditional sizings like funori or nori tend to be more benign, but interactions with adhesives, mounting methods, and environment can shift paper pH over time.
4. Environmental Exposure
Even pH-neutral paper can become acidic depending on how and where it’s stored. Atmospheric pollutants, acidic backing boards, or off-gassing from materials nearby can acidify paper. This is particularly relevant for conservators using Japanese papers in conjunction with other, older substrates.
Is Slight Acidity Always a Problem?
Not necessarily.
In conservation, materials are chosen not just for their chemistry, but for their mechanical behavior, workability, reversibility, and compatibility with the object being treated.
A Japanese paper that’s slightly acidic—but thin, strong, and free of lignin—may still be far superior to a buffered Western paper that is more chemically stable but mechanically inferior. The key is awareness and informed decision-making.
Best Practices When Selecting Japanese Papers
Check the specs: Look at fiber composition, weight, and any mention of pH or buffering. TALAS product pages often list these details.
Use pH testing pens or cold extraction methods if needed for assurance.
Choose 100% bast fiber papers (e.g., 100% kozo, gampi) for highest stability.
Be cautious with machine-made or pulp-included papers if using them in high-value conservation settings.
Match the paper’s performance to the task at hand—sometimes strength and behavior matter more than pH alone.
Conclusion
Japanese paper remains one of the most trusted materials in conservation for a reason: its unique structure, strength, and versatility are hard to match. While acidity is a factor worth paying attention to—especially when long-term stability is key—it’s just one part of a broader picture.
By understanding the complexity behind terms like “acid-free” and “archival,” you can make smarter, more informed choices in your studio or lab.
