Gold leaf from the north and banana fiber cloth from the south: Cutting-edge science elucidates artisan handiwork to help preserve traditional crafts

Gold leaf is renowned as a craft of Kanazawa and Kaga and has been used in artworks and historical structures such as castle towers. Bashofu (banana fiber cloth) is a breezy fabric passed down from the Ryukyu Kingdom in Okinawa through the ravages of war. Research findings have revealed how cutting-edge science can explain the processes carried out in these traditional crafts through the handiwork of skilled artisans. As artisans continue to age, this demonstrates the potential for science to help pass down precious traditional crafts to the next generation.

Gold leaf is one-hundredth the thickness of aluminum foil

Gold leaf is a craft in which Kanazawa takes pride. It is said to have been made since the time of Toyotomi Hideyoshi. Gold is beaten and stretched on paper soaked in various solutions, processed to one-hundredth the thickness of aluminum foil. Used on folding screens in artworks and in building interiors, it is a masterpiece symbolizing magnificent and splendid culture.

Regarding its thinness, Professor Emeritus Kazuo Kitagawa of Kanazawa University had previously announced that "when gold leaf is viewed under an electron microscope, gold crystals are aligned in the same direction. This special structure is what makes the gold into foil." However, the detailed mechanism of why it can be made so thin, and shine without losing its luster, was not understood.

When a Chinese graduate student studying under Professor Yoshifumi Oshima (Nanostructured Materials Physics) of the Nanomaterials and Devices research area at Japan Advanced Institute of Science and Technology (JAIST) began saying they wanted to "solve the mystery," Professor Oshima decided to investigate why the cubic crystals "align" in the same direction.

Oshima's specialty is operating transmission electron microscopes that can visualize electron arrangements. The finished gold leaf product (100 nanometers thick; nano means one-billionth) had sugar cube-shaped crystals neatly aligned with their faces upward, just as Kitagawa had described. However, when he checked the gold leaf at each production stage, the crystal orientations were random up to the intermediate production stage, known as "kinzumi" (1 micrometer thick; micro means one-millionth).

Questioning conventional wisdom in metal processing: Discovery of 90-degree "slip"

When metal is beaten, it becomes thinner while also extending sideways. Such deformation can be explained by numerous "slips" occurring within the crystals. Faults seen in geological strata are also the result of this slip occurring, and the same thing happens in metals.

However, conventional wisdom holds that this slip only occurs on certain planes, determined according to the crystal structure, and this decides the inclination of the crystal orientation. In the case of gold, the crystals theoretically align at 45 degrees, creating surface irregularities on the gold. During the foil-beating process, the thinner regions of these surface irregularities tear and become powder, making it impossible to obtain a thin film of 100 nanometers.

To make gold leaf, the surface needs to be flattened as if the sugar cubes are lined up without alteration. For this, a "structure of sugar cubes lined up tilted at 45 degrees" needs to "tilt" into a "structure of sugar cubes lined up without tilting." However, this mechanism could not be explained by "conventional wisdom."

Oshima hypothesized that for the crystal orientation to tilt from a structure of sugar cubes lined up at 45 degrees to a structure of sugar cubes lined up without tilting, there must be a "new slip" that induces this tilt. He attempted to search for traces of it using an ultra-high voltage transmission electron microscope. This is because if a slip had occurred, traces of that slip should remain.

He then found that traces of multiple slips existed at 90-degree angles to each other. Considering the angle, these traces could not be explained by conventional slip, suggesting that a new type of slip called "non-octahedral slip" was occurring.

Low-temperature processing prevents recrystallization, making surface flat

However, there was another issue to solve. According to conventional metal processing theory, when many crystal slips occur in metal, the entire gold leaf becomes unstable. To resolve this instability, crystal reconstruction occurs irregularly and sporadically throughout the wide gold leaf. This is called recrystallization.

This recrystallization causes crystal orientations to become random at each location, conversely making it difficult to flatten the gold leaf surface. As a result of Oshima's verification of this point, he determined that by beating the foil at low temperatures below 100 degrees, recrystallization does not occur within the gold leaf, and the structure of sugar cubes lined up at a 45-degree tilt can be maintained.

To summarize the experimental results conducted by Oshima: through foil beating, numerous "conventional-theory" slips occur, and the entire gold leaf becomes a structure of sugar cubes lined up, tilted at 45 degrees. After this, the conventional-theory slips become entangled and can no longer move, but recrystallization is prevented due to the low temperature. As a result, an unexpected slip called non-octahedral slip occurs, and this succeeds in tilting the crystal orientation so that the entire gold leaf becomes a structure of sugar cubes lined up without tilting. As a result, the surface becomes flat, and the entire gold leaf can be made uniformly thin.

Completely refuting objections: Gold researchers don't quarrel

When this theory was explained to researchers specializing in metals, they objected, saying "Metals don't slip in perpendicular directions. Non-octahedral slip systems are theoretically impossible." However, when Oshima showed them microscope images, they said "Oh... it is happening" and "readily accepted it," he laughed. Being able to completely refute objections—not "the rich don't quarrel," but "gold researchers don't quarrel," perhaps.

Based on this research, Oshima wants to investigate what slip systems occur not only in gold but also in thin films of platinum, silver, and copper with different processing temperatures. He said, "What we learned this time is that if processing heat can be suppressed, gold develops a unique slip system. Now that we understand the mechanism, we might be able to mechanize gold leaf production by applying this. I want to consider whether we can use machines to solve the shortage of people able to carry out artisan handwork." A brilliant future for gold leaf has come into view.

Bashofu is the subtropical kimono, Beloved by both Ryukyu Royalty and commoners

Bashofu is one of Okinawa's representative crafts, said to have been woven in the 12th-13th centuries during the Ryukyu Kingdom. Though it nearly disappeared with the Battle of Okinawa, the late Living National Treasure Toshiko Taira revived it based on encounters at a spinning factory in Kurashiki, Okayama, where she was assigned for labor service. Beloved by both royalty and commoners for many years, it is still made mainly in the Kijoka district of Ogimi Village in northern Okinawa Prefecture, Taira's hometown.

Researcher Yoko Nomura at the Science and Technology Group at Okinawa Institute of Science and Technology (OIST) (Living Science and Biotechnology), originally studied clothing science. When she joined OIST ten years ago, she became interested in the historical bashofu, where she wondered, "What do people wear when it is so hot?"

Bashofu is divided into three types: soft fabric for kimono called kijaku, somewhat stiff fabric used for obi sashes, and hard cloth for interior use. Bashofu is completed through a total of 23 processes, including boiling material harvested from "itobasho," a type of banana plant, in alkaline solution to soften it, and removing impurities to extract fibers.

When Nomura investigated the manufacturing methods and products, she found that the three types of bashofu had almost the same processes. They were plain weave, a simple combination of weft and warp threads, with no major difference in the number of thread twists—about 2-3 times per centimeter. Thinking that the difference in the fibers themselves might determine the softness of the cloth, she began researching. In addition to a sense of crisis that "bashofu culture, where even people in their 70s are called young artisans, is declining," she also thought that "having scientific evidence, not just artisan intuition, will help preserve the uniquely wonderful culture of Ryukyu."

Analysis from a botanical perspective: Fiber's honeycomb structure disperses sweat

Joining this research was Researcher Koji Koizumi in the OIST Imaging Section (Botany). Koizumi focused on how artisans divide the harvest of the leaf sheaths, part of the leaves of itobasho, into four grades. From the outside, they are called waha, nahau, nahagu, and kiyagi; waha is used for tablecloths and bags, nahau for obi, and nahagu for kijaku (i.e., kimono), while kiyagi is too soft to be used as thread. Additionally, artisans further divide nahagu into weft thread called nuki and warp thread called hashi.

From a botanical perspective, 90% of what is used as fiber in any leaf sheath are the fiber bundles in the mesophyll tissue, with the other 10% being the fiber tissue portion within the vascular bundles. The researchers decided to investigate what structural differences exist among the four types of leaf sheaths. By analyzing the structure, they could attempt cultivation methods and breeding without relying only on artisan intuition.

First, they embedded thread received from artisans in Kijoka in resin, made sections, and observed the shape. Nahagu had a smaller area of fiber tissue compared with waha. In the surface structure, waha had impurities such as leftover mesophyll cells, but both the nuki and hashi of nahagu had clean structures without impurities and had a wrinkled appearance. These wrinkles are thought to be caused by microfibrilized (refined) cellulose fibers on the cell surface manifesting on the surface.

The fiber cross-section has a honeycomb structure, and because this structure contains air, it can disperse sweat. What makes it suitable for Okinawa's humid climate is not only the good feel when it is worn, but also that it can release sweat from the body surface.

Artisans select leaves with thin cell walls through touch intuition

Next, the researchers investigated the components in detail. Cell wall components are mainly composed of cellulose, hemicellulose, and lignin. Among these, lignin is a substance related to hardness, so they hypothesized that lignin content would differ by fiber part, but the results per unit area showed "no difference." In other words, they thought other elements determining softness must exist and reconsidered how to proceed with the research.

Finally, they tried measuring cell wall thickness and examining the "wall-to-lumen ratio," which shows the ratio of cell wall to the area of space. They compared fibers harvested from different fields in Kijoka. As a result, they found that cell wall thickness was 2.37 micrometers for waha, 0.97 micrometers for nahagu-nuki, and 1.42 micrometers for nahagu-hashi. It was found that nahagu types have thinner cell walls.

Moreover, the wall-to-lumen ratio was high for waha and significantly lower for nahagu. In other words, it was revealed that minute structural differences in cells determine fiber hardness, rather than components. Koizumi thought, "The reason itobasho fiber is successful as kimono fabric is because artisans visually examine the leaf sheaths and, through touch intuition, select and use those with thin cell walls."

Bashofu song lyrics: It won't become cloth if it's ripe?

According to Koizumi, because bashofu fibers become hard when banana fruit forms, the growers remove leaves and cut branches to make the itobasho grow thick, then make use of the soft fibers before the fruit forms. If this is the case, the lyrics "Minoreru basho ureteita" ("The fruiting basho, matured") from the famous Ryukyu folk song "Bashofu" with lyrics by Professor Emeritus Yasuichi Yoshikawa of Meio University would become "scientifically incorrect."

Was the ripened basho not for cloth but for appreciation? Since the lyrics continue with "Midoriba no shita washita shima Uchinaa" ("Under the green leaves, our island Okinawa"), it would make sense if the lyricist were feeling the beauty of the greenery, looking at itobasho leaves. Incidentally, according to Koizumi, in parts of Southeast Asia they also make cloth using fibers after fruit has formed, but since Yoshikawa passed away in July 2025, the true meaning of these lyrics can no longer be known.

Through this research, it became clear that thin cell walls result in supple and wearable bashofu fibers. Koizumi said, "By searching for other plants with thin fiber tissue cell walls, we might find alternative plants to replace itobasho. Also, these research results make it easier to establish cultivation methods such as soil and fertilizer conditions. If we advance cultivation method improvements and breeding within a range that doesn't deviate from basic traditional methods in the traditional craft, we can pass down bashofu, which is lacking in kijaku materials, to the next generation."