‘Wabi’ Edo period Japanese fireworks sourced nitrates from human urine, silkworm poop

TOKYO — Millions of people are delighted by the sparkling spectacle of fireworks at festivals in Japan every summer, but did you know that Japanese fireworks once called “wabi” date back to the Edo period (1603-1867), and that human urine and silkworm feces were among the materials used in their production?

Ingredients for black gunpowder

“‘Wabi’ launched in the Edo period was a dark orange color. The light depended on the black gunpowder used,” said Takehiro Matsunaga, an invited researcher in explosives engineering at the National Institute of Advanced Industrial Science and Technology.

Even now that fireworks have become multicolored and varied, the black gunpowder is used as “ageyaku,” a type of propellant that shoots the fireworks balls into the sky.

Black gunpowder is thought to have been invented in China in or around the eighth century, and is the oldest form of gunpowder. The technology reached Japan along with matchlock guns imported from Portugal in the 16th century. It consists of a mixture of roughly 77% potassium nitrate (in mineral form known as niter), about 15% charcoal and some 8% sulfur. Wabi’s orange color is made by the burning charcoal.

However, because niter is soluble, it was not produced in humid Japan. For that reason, during the Edo period it was made by using things such as nitrogen-rich soil from under flooring and silkworm feces from silk culture. First, the nitrogen in the soil was bound together with oxygen due to the action of bacteria in the ground to form nitrate ions, and then reacted with potassium in wood ash to form potassium nitrate.

Because nitrogen is also found in the urea in human urine, in the Satsuma Domain in what is now Kagoshima Prefecture, urine was reportedly also used for explosive ingredients. Later, as substitutes for niter became available inexpensively through imports, these methods of production were abandoned.

Combustible reacts to oxygen integrated with gunpowder

Why is niter essential for gunpowder? Matsunaga said, “The foremost feature of gunpowder is that it burns without oxygen.” Usually, combustibles burn using oxygen in the atmosphere, but gunpowder has this already mixed in. In black gunpowder, oxygen is contained in the potassium nitrate, which combusts together with the charcoal. Because the oxygen is integrated with the powder, carbon dioxide and other gases are generated all at once, causing an explosion. Substances such as niter that supply oxygen are called oxidizing agents, and are an essential substance in the manufacture of explosives.

Aside from black gunpowder, modern fireworks mainly use things such as metal powders that sparkle when they burn and brightly colored flame reactions. Matsunaga advised, “Try watching fireworks with a scientific understanding of the history of our ancestors’ discoveries and the efforts of pyrotechnicians.”

(Japanese original by Yosuke Tsuyuki, Lifestyle, Science & Environment News Department)