a burning secret

The Japanese technique of burning wood surfaces creates a protective carbonized layer in bioarchitecture.

Credit: A. Di maria et al., 2025

Yakisugi is a Japanese architectural technique for charring the surface of wood. It has become quite popular in bioarchitecture because the carbonized layer protects the wood from water, fire, insects, and fungi, thereby prolonging the lifespan of the wood. Yakisugi techniques were first codified in written form in the 17th and 18th centuries. But it seems Italian Renaissance polymath Leonardo da Vinci wrote about the protective benefits of charring wood surfaces more than 100 years earlier, according to a paper published in Zenodo, an open repository for EU funded research.

Check the notes

As previously reported, Leonardo produced more than 13,000 pages in his notebooks (later gathered into codices), less than a third of which have survived. The notebooks contain all manner of inventions that foreshadow future technologies: flying machines, bicycles, cranes, missiles, machine guns, an “unsinkable” double-hulled ship, dredges for clearing harbors and canals, and floating footwear akin to snowshoes to enable a person to walk on water. Leonardo foresaw the possibility of constructing a telescope in his *Codex Atlanticus *(1490)—he wrote of “making glasses to see the moon enlarged” a century before the instrument’s invention.

In 2003, Alessandro Vezzosi, director of Italy’s Museo Ideale, came across some recipes for mysterious mixtures while flipping through Leonardo’s notes. Vezzosi experimented with the recipes, resulting in a mixture that would harden into a material eerily akin to Bakelite, a synthetic plastic widely used in the early 1900s. So Leonardo may well have invented the first manmade plastic.

The notebooks also contain Leonardo’s detailed notes on his extensive anatomical studies. Most notably, his drawings and descriptions of the human heart captured how heart valves can control blood flow 150 years before William Harvey worked out the basics of the human circulatory system. (In 2005, a British heart surgeon named Francis Wells pioneered a new procedure to repair damaged hearts based on Leonardo’s heart valve sketches and subsequently wrote the book The Heart of Leonardo.)

In 2023, Caltech researchers made another discovery: lurking in the margins of Leonardo’s Codex Arundel were several small sketches of triangles, their geometry seemingly determined by grains of sand poured out from a jar. The little triangles were his attempt to draw a link between gravity and acceleration—well before Isaac Newton came up with his laws of motion. By modern calculations, Leonardo’s model produced a value for the gravitational constant (G) to around 97 percent accuracy. And Leonardo did all this without a means of accurate timekeeping and without the benefit of calculus. The Caltech team was even able to re-create a modern version of the experiment.

“Burnt Japanese cedar”

Annalisa Di Maria, a Leonardo expert with the UNESCO Club of Florence, collaborated with molecular biologist and sculptor Andrea da Montefeltro and art historian Lucica Bianchi on this latest study, which concerns the Codex Madrid II. They had noticed one nearly imperceptible phrase in particular on folio 87r concerning wood preservation: “They will be better preserved if stripped of bark and burned on the surface than in any other way,” Leonardo wrote.

“This is not folklore,” the authors noted. “It is a technical intuition that precedes cultural codification.” Leonardo was interested in the structural properties of materials like wood, stone, and metal, as both an artist and an engineer, and would have noticed from firsthand experience that raw wood with its bark intact retained moisture and decayed more quickly. Furthermore, Leonardo’s observation coincides with what the authors describe as a “crucial moment for European material culture,” when “woodworking was receiving renewed attention in artistic workshops and civil engineering studies.”

Leonardo did not confine his woody observations to just that one line. The Codex includes discussions of how different species of wood conferred different useful properties: oak and chestnut for strength, ash and linden for flexibility, and alder and willow for underwater construction. Leonardo also noted that chestnut and beech were ideal as structural reinforcements, while maple and linden worked well for constructing musical instruments given their good acoustic properties. He even noted a natural method for seasoning logs: leaving them “above the roots” for better sap drainage.

The Codex Madrid II dates to 1503-1505, over a century before the earliest known written codifications of yakisugi, although it is probable that the method was used a bit before then. Per Di Maria et al., there is no evidence of any direct contact between Renaissance European culture and Japanese architectural practices, so this seems to be a case of “convergent invention.”

The benefits of this method of wood preservation have since been well documented by science, although the effectiveness is dependent on a variety of factors, including wood species and environmental conditions. The fire’s heat seals the pores of the wood so it absorbs less water—a natural means of waterproofing. The charred surface serves as natural insulation for fire resistance. And stripping the bark removes nutrients that attract insects and fungi, a natural form of biological protection.

Leonardo viewed wood as “not merely a construction material but a living organism—a system in balance with its environment,” Di Maria et al. concluded. “His interest is not limited to mechanical effectiveness but extends to the relationship between matter and environment, between natural processes and human intervention. This perspective positions the Florentine genius as a precursor to what we now call bioarchitectural practice: human intervention on materials must be calibrated to an understanding of their biological and physical properties.”

DOI: Zenodo, 2025. 10.5281/zenodo.17506250 (About DOIs).

Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban.