Oliver Whang

In 2017, Soyeon Choi, the head paper conservator at the Yale Center for British Art, received three folios of a letter written in 1753 by Eliza Pinckney, a prominent American agriculturalist. The scrawling lines on the cracked paper were elegant, but on close inspection they almost seemed to dance. And when Ms. Choi looked even more closely, through a binocular headband magnifier, she saw tiny rips around certain letters, and jagged holes around others.

Paper deteriorates for many reasons: fungi, moisture, heat, light, atmospheric pollutants. In Pinckney’s letter, as with many Western writings before the 20th century, the ink itself was eating through the paper, in a process called iron gall ink corrosion.

Ms. Choi, who has worked as a paper conservator for more than 20 years, and a number of other paper conservators I spoke to around the country, said their duty is to stave off these kinds of deterioration — to preserve the character and content of historical documents far into the future, long after their authors and conservators are gone.

Staring at those tiny splits and holes in Pinckney’s letter, Ms. Choi decided to proceed in the least obtrusive, most delicate manner possible: with the thinnest paper in the world.

The paper, called tengujo, is made by a company called Hidaka Washi, at a factory in the Kochi prefecture of Japan. The ingredients are simple, and the process is “not a very secret, special kind of thing,” Hiroyoshi Chinzei, the owner and operator of Hidaka Washi, told me recently over the phone. But the product that emerges is almost magical. The fibers knit together into gossamer lattice that is as much empty space as substance; what starts as a wet, white sheet becomes, on drying, almost completely transparent.

Hidaka Washi has specialized in the paper’s production since Mr. Chinzei’s great-grandfather first opened the company in 1949.

Inside the factory, the air is steamy from warm baths of alkaline water in which bushels of kozo — the stems of mulberry trees — are soaked. Workers remove dirt from the kozo and pound it into strands of pulpy fiber, which they lay in a tub containing water and neri, a thick viscous liquid that is derived from the tororo-aoi plant, also known as sunset hibiscus. Reacting with the neri, the kozo fibers gain a sticky, gummy quality, which allows them to be broken down even further and pulled apart into long white ropes, which are removed from the tub and spread out evenly over a screen. The ropes are massaged together and flattened to the width of a couple of spiderweb-like fibers. As the liquid dries away, these fibers are left woven together, clinging to each other in a delicate sheet of paper.

Tengujo can help reinforce and repair damages from many sources. Ms. Choi calls it “the bread and butter in paper conservation” and “probably the most gentle way of reinforcing anything.” Sometimes tengujo is used for spot-treatment, other times to completely line a manuscript. Its long fibers provide structure and support while remaining almost completely unnoticeable.

The paper was traditionally crafted by hand for more than 1,000 years in the Kochi and Gifu prefectures, and was used for a number of practical and ceremonial purposes — for writing, printing and artwork. But in the 20th century, paper manufacturers started using machines, that could spread the kozo fibers more evenly than before. This allowed tengujo to be made thinner and thinner while still retaining its characteristic flexibility and strength. In Japan, and eventually around the world, it became the go-to material for sliding-door covers, kimono wrappings and tissue paper.

Tengujo can be made so thin that, at a certain point, it is too insubstantial for even the most gentle, decorative uses. At the width of a couple of kozo fibers, the paper becomes as thin as the wings of a mayfly. Only one use remains then: paper conservation.

Trying to aggressively mend a document is risky because long-term chemical and physical effects are highly variable and relatively unknown. “The more and more I am in this field, I feel that I should do less and less,” Ms. Choi said.

So, as far as reinforcement material goes, the thinner the better.

About six years ago, Mr. Chinzei was asked by the National Archives of Japan to develop a tengujo that weighs 1.6 grams per square meter, thinner and lighter than any other paper in the world at the time. It took him two years of trial and error, minutely varying the pressure of the pounding machine, the speed of the mixing, the density of the fiber and neri in the water. “My workers and I were amazed we could make it,” Mr. Chinzei said.

The width of this thinnest tengujo is the same as the diameter of a single kozo fiber: 0.02 millimeters. Thinner than human skin. No other company has been able to replicate it.

It is now sold around the world for paper conservation. Chinzei has sent rolls to the Library of Congress, the Louvre, the British Museum and the Yale Center for British Art.

Slicing a 3-millimeter strip of Hidaka Washi tengujo with an ethanol-activated adhesive brushed onto one side, Ms. Choi gently covered an imperfection in Pinckney’s yellowing page. With a little push, the papers melted into each other.

From a normal reading distance it looked as if nothing had been done, but under close examination you could see tiny strands of kozo gripping onto the ink, reinforcing the letter in the smallest way. Something that might, over many years, make a big difference.

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