When a catastrophic flood inundated the archives in Florence in 1966, European conservators faced an enormous challenge. Traditional Western materials were failing when in contact with extremely waterlogged incunabula and documents. The breakthrough came with the adoption of a material from the other side of the world. – traditional Japanese paper. Thanks to its unprecedented wet strength, it has made it possible to save hundreds of thousands of invaluable cards.

Today, Japanese paper is an absolute cornerstone in workshops around the world. This is not a matter of historical sentiment, but of hard physicochemical parameters. Properly selected Japanese tissue paper can mend a torn page without obscuring the text, and thicker sheets can reconstruct a missing part of a cover. In this article, you will learn what causes the phenomenon of Japanese paper, how to choose the right weight, and how to correctly apply this material when repairing books and documents.

What makes Japanese paper second to none?

The secret lies in the raw materials and a specific sheet-forming process. Unlike Western papers based on short wood pulp, Japanese sheets are made from the long bast fibres of particular plant species. These are what give the material its unprecedented resistance to tearing.

In professional conservation, three main types of fibres are used, each with a different application:

The fibre length (up to 15 mm compared to 1–3 mm in western paper) ensures that even the thinnest, mist-like tissue paper retains its integrity when moistened with aqueous glue solutions.

How to choose the grammage of Japanese paper based on the type of damage

The golden rule of bookbinding and conservation states: The repair material introduced must be slightly weaker than the original object. In the event of unforeseen stresses (e.g. during a sudden page turn), the repair patch should tear, protecting the historical document. This is why the selection of the appropriate paper weight is so important.

Japanese papers and tissues is usually divided into three weight categories:

• Ultra-thin papers (1.6 g/m² – 5 g/m²): Real masterpieces of stationery (e.g. type TengujoIts overriding feature is complete transparency. They are used to repair cracks that go directly through printed text or illustrations. They strengthen the structure without reducing the readability of the print.
• Thin and medium papers (6 g/m² – 20 g/m²): The happy medium between transparency and strength. Ideal for standard repairs of edge tears, stabilising weakened card edges, and for adhering pages that have fallen out of the book block.
• Thick paper (from 21 g/m² to over 100 g/m²): Papers with a compact mass, completely opaque. They are used for backing weakened maps or posters, reconstructing missing corners, and creating strong reinforcements for the spines of a book block.

Why do we avoid standard tapes?

Using the best Japanese paper is pointless if you combine it with the wrong adhesive. Ordinary office adhesive tapes are the greatest enemy of paper. Their synthetic glues penetrate the cellulose structure over time, turn drastically yellow, and become impossible to remove without the use of toxic organic solvents.

Instead of these, professionals use a starch paste (most commonly from wheat starch). This is a chemically inert, aqueous binder that has a strong affinity for cellulose and does not degrade in a harmful way. To remove a patch after years, it is enough to gently moisten it. To prepare such a binder, you will need Specialist adhesives Top quality (pure starch, methylcellulose), free from biological contamination.

If the situation requires a faster intervention in a library or archive, an alternative is certified conservation tape, such as Neschen Filmoplast. Built on a foundation of acid-free paper and safe acrylic adhesives, they offer a compromise between speed of work and collection preservation.

Water brush technique – seamless repair step-by-step

The key to an invisible and durable repair using Japanese paper is the correct preparation of the patch. Never cut Japanese paper with scissors or a scalpel. Sharp tools crush and unnaturally sever long fibres, creating a hard edge which will show on the document after gluing and cause dangerous stress.

A highly precise „water brush” method is used here, which results in so-called feathered edges. These fibres blend seamlessly with the document being repaired.

Here's how to conduct this process:

1. Diagnosis Assess the damage. Plan the patch size with an allowance of approximately 3 mm on each side of the tear.
2. Waterline determination Place the sheet onto a smooth backing. Using a retouching brush dampened with distilled water, paint the shape of the planned patch.
3. Fibre splitting: Gently but firmly, tear the moistened material apart with your fingers along the wet line. You will see long, frayed fibres being released.
4. Binder application: Place the patch onto the polyester interlining (smooth side down). Using a brush, apply the starch paste with movements from the centre outwards, ensuring the edges are well saturated with adhesive. For applying glues and for precision work, a suitable Workshop equipment, including dedicated brushes and bookbinding bone folders.
5. Assembly and drying: Using tweezers, transfer the patch to the tear. Cover the mended area with separating material and blotting paper. Place the whole thing under a weight (press or counterweights) for at least several hours. Slow drying under pressure prevents warping of the card.

What to watch out for when working with Japanese paper?

Even the best materials require conscious use. Several myths have grown around East Asian papers, which can pose a threat to historical artefacts.

• With absolute absence of acidity: In the hobbyist environment, there's a belief that all Asian paper is naturally acid-free and safe. This is a mistake. Many cheap, mass-produced papers contain admixtures of wood pulp and residues from aggressive bleaching agents. Professional Japanese paper for restoration must meet strict archival standards (e.g. ISO 9706) or pass a rigorous Photographic Activity Test (PAT), which guarantees no chemical reactions with the object.
• Halo effect The structure of traditional Japanese paper is extremely open and highly absorbent of water. If you apply a wet, glued patch to a very dirty or acidified old print, the water can dissolve dormant impurities within it. The moisture will deposit dirt on the edges of the patch, creating a dark, extremely difficult-to-remove halo. Therefore, before carrying out structural wet repairs, it is often recommended to gently clean the document or carry out spot washing.