Washi in a Western Case: Which Sheets Take Paste

A Japanese sheet goes into a western case when its fibre length and formation match the job: kozo for hinges and spine linings, gampi for thin endpapers, mitsumata where a soft, opaque sheet is wanted. None of the three takes paste like a western woodfree paper, and all three need a dry, restrained paste and a support underneath. The choice is made by weight and fibre, not by the name of the paper. This is the same logic that governs Japanese paper in a case: the sheet is placed, not soaked.

Which Japanese papers take paste without cockling?

Cockling is a function of how much water the sheet takes up and how fast it lets it go. Washi made from kozo, gampi or mitsumata is beaten long and drained slowly, so the fibres lie in a loose, open web. That web absorbs water quickly and swells across the grain. A western paper sized internally resists the first rush of water; a washi does not.

The practical answer is that all three take paste, but only if the paste is thick enough to sit on the surface and thin enough to be spread. We use wheat starch paste at a consistency like thin cream, cooked and cooled, and we pass it through a fine sieve. A paste that is too wet soaks straight through and the sheet cockles before the brush leaves it. A paste that is too dry grabs and tears the sheet.

Gampi is the least forgiving. Its fibres are short and dense, the sheet is thin, and it cockles at the first sign of excess water. Kozo is the most forgiving because the fibres are long and the sheet has more body. Mitsumata sits between them, softer and more opaque, and it cockles in a way that is harder to press out.

The trick that works on all three is to paste the western paper, not the washi. Lay the paste on the western sheet, let it lose its shine, then lay the washi down dry and press it with the palm. The washi takes up only what it needs.

How is a kozo sheet weighed and priced?

Kozo is sold by weight per unit area, and the trade uses grams per square metre, the same as western paper. A typical kozo for book work runs from 12 to 40 gsm. A 12 gsm sheet is nearly transparent and is used for repair and for thin endpapers. A 40 gsm sheet has enough body to serve as a spine lining or a hinge.

Pricing follows the same measure. A sheet sold as 20 gsm at a given size costs less than a 40 gsm sheet of the same size, but the comparison is not linear because the heavier sheet contains more fibre and more labour. Handmade kozo is priced by the sheet, not by the ream, and the price reflects the number of sheets the maker pulled in a day. Machine-made kozo is sold by the ream and is cheaper, but the fibres are shorter and the sheet is more uniform.

When you buy, ask for the weight in gsm and the fibre content. A sheet labelled kozo may be a blend with wood pulp, and a blend behaves differently under paste. A pure kozo sheet tears with a long, fibrous edge; a blend tears short and dusty. That tear is the quickest test at the bench.

Where does a washi lining go in a western case?

The spine lining is the first place. In a western case binding, the spine of the text block is lined after sewing and before casing in. A strip of kozo, 30 to 50 gsm, cut to the height of the text block and 40 to 60 mm wide, is pasted over the spine and rubbed down. It holds the sewing supports, reinforces the hinge, and stays flexible when the book opens.

The second place is the hinge itself. A narrow strip of kozo, 15 to 25 mm wide, is pasted along the inside of the joint between the text block and the case. This is the strip that takes the strain when the cover opens. A western paper hinge can crack; a kozo hinge stretches and recovers.

The third place is the endpaper. A thin gampi, 12 to 20 gsm, can be pasted as a single endpaper or as a guard. It is not strong enough to be the only endpaper in a heavy book, but it is used as a decorative or reinforcing layer over a western endpaper. Mitsumata is used where a softer, more opaque endpaper is wanted, often in a case where the board is visible through the sheet.

What does the fibre length do at the hinge?

Fibre length decides how the sheet behaves when it is pulled. Kozo fibres run from 5 to 15 mm, and they are long enough to bridge a joint and to distribute the strain. Gampi fibres are shorter, around 3 to 6 mm, and the sheet is denser; it is strong in tension but less able to stretch. Mitsumata fibres are short and fine, and the sheet is soft and opaque.

At the hinge, the sheet is asked to bend and straighten thousands of times. A long-fibre kozo sheet does this without tearing because the fibres slide and return. A short-fibre sheet fatigues and eventually cracks. This is why a kozo hinge outlasts a western paper hinge in a book that is opened often.

The same property explains why kozo is used for repair. A strip of kozo pasted over a torn hinge will hold because the fibres interlock with the paper underneath. A western tissue will hold for a while, then lift.

Which sheet for which job?

For a spine lining, use kozo at 30 to 50 gsm. It is strong, flexible and takes paste without cockling if the paste is applied to the spine, not the sheet. For a hinge, use kozo at 20 to 40 gsm, cut with the grain running along the hinge. For an endpaper, use gampi at 12 to 20 gsm, or mitsumata at 20 to 30 gsm where a softer sheet is wanted.

For a decorative endpaper in a case where the board is heavy, a western paper is still the better choice. Washi is not a substitute for a western endpaper; it is a different material with different behaviour. The mistake is to treat it as a drop-in replacement.

How do you paste a washi sheet without a fight?

Work on a dry surface. Paste the western paper, not the washi. Let the paste lose its shine. Lay the washi down and press it with the palm, working from the centre out. Do not brush the washi with paste. Do not lift the sheet to check it. If the sheet cockles, stop, let it dry under weight, and start again with a thicker paste.

A sheet that has cockled can sometimes be saved by damping it and pressing it between blotters, but the fibres have already moved. Prevention is the whole method. The same restraint applies to the whole operation: a washi sheet is placed, not worked.

What about weight and price in practice?

A 20 gsm kozo sheet at 500 by 750 mm weighs 7.5 g. A 40 gsm sheet of the same size weighs 15 g. The price per sheet follows the weight and the labour. For a book of 200 pages, a spine lining 50 mm wide and 200 mm high uses 0.01 square metres of 40 gsm kozo, which is 0.4 g of paper. The cost is small; the effect on the binding is not.

Buy a small quantity first. Paste a test strip on a scrap of board and let it dry. Open and close it twenty times. If the sheet holds, it will hold in the book. If it lifts, the paste was too wet or the sheet was too thin. The test takes ten minutes and saves a binding.

Sources

• The Metropolitan Museum of Art, "Washi: Japanese Paper" (Heilbrunn Timeline of Art History): https://www.metmuseum.org/toah/hd/washi/hd_washi.htm