The "secret recipe" hidden in the sound of firecrackers: "75% saltpeter" rewrites the history of the invention of gunpowder

As the Spring Festival approaches, I am reminded of Wang Anshi's famous poem "New Year's Day": The year is over with the sound of firecrackers, and the spring breeze brings warmth to Tusu. On this day, every household will replace the old charms with new ones. In fact, the sound of firecrackers also hides the chemical "secret recipe" of the wisdom of the ancient Chinese working people.

Setting off firecrackers during the Spring Festival (copyright image from the gallery, reprinting may lead to copyright disputes)

01A brief history of the evolution of firecrackers

In ancient my country, people worked all year round, and there was no Western church. Therefore, there were many folk festivals during the Tang and Song dynasties, which were the main time for working people to rest and relax. Western culture is based on gods. On Sundays, people refuse to work and emphasize that gods and humans should rest together; Eastern culture is based on people. The purpose of holidays is to give people a rest, not gods. The invention of firecrackers was probably a tool used by people to make trouble for gods. In addition to setting off firecrackers during the Spring Festival to bid farewell to the old and usher in the new, people also set off firecrackers during major festivals and celebrations, such as the Lantern Festival, Dragon Boat Festival, Mid-Autumn Festival, weddings, house building, and business openings, to show celebration and auspiciousness.

The "Jingchu Sui Shi Ji" of the Northern and Southern Dynasties recorded: On the first day of the first lunar month, people woke up at the crowing of cocks and set off firecrackers in front of the courtyard to avoid the evil spirits of Shansao. According to the "Shenyi Jing" of the Han Dynasty, in ancient times, people would camp in the mountains and light bonfires at night, one to cook and keep warm, and the other to prevent attacks by wild beasts. However, there was an animal in the mountains that was not afraid of people or fire, and often stole food when people were not prepared. In order to deal with this animal, people thought of setting off firecrackers in the fire and using the crackling sound of bamboo to make it flee far away. The animal mentioned here is called "Shansao". The ancients said that it can make people feel cold or hot, and it is a ghost that makes people suffer from cold and heat diseases. Scare away Shansao, that is, expel the plague, and you will get good luck and peace.

Description of Shansao in Shen Yi Jing (Image source: screenshot from bilibili video)

At the beginning of the Tang Dynasty, plague broke out. Li Ting, a native of Liling County, Hunan, put saltpeter in bamboo tubes and ignited them to make louder noises and thicker smoke. As a result, the miasma in the mountains was dispelled and the epidemic was stopped. This was the earliest prototype of saltpeter firecrackers . Later, gunpowder appeared, and people filled bamboo tubes with saltpeter, sulfur and charcoal to burn, creating "explosive firecrackers" . In the Song Dynasty, people began to use paper tubes and hemp stems to wrap gunpowder and weave them into strings to make " woven cannons " ( i.e. firecrackers ).

Regarding the evolution of firecrackers, the Qing Dynasty's "Popular Collection·Paiyou" records: "In ancient times, firecrackers were all made of real bamboo and set on fire, so Tang poets also called them "explosive poles". Later generations made them out of rolled paper and called them "firecrackers". The sound of firecrackers is a sign of bidding farewell to the old and welcoming the new, and an expression of festive mood. The sound of firecrackers during the Spring Festival also sets off the festive atmosphere of the New Year.

02 Gunpowder: The "Soul" of Firecracker Making

Since Joseph Needham proposed the Four Great Inventions of China, gunpowder has become famous at home and abroad as one of the Four Great Inventions. The main ingredients of firecrackers are black powder , which contains sulfur, charcoal powder, potassium nitrate , and some also contain potassium chlorate , and are wrapped with strong paper on the outside. The effect of firecrackers depends mainly on the proportion of various ingredients and the strength of the outer wrapping material . From the perspective of fire science, gunpowder is a fuel with its own oxidant . It can directly react from solid fuel to generate gas products without oxygen, avoiding the process that ordinary fuel must first release gaseous fuel and then ignite and burn. Therefore, it is relatively fast, and the volume expands about a thousand times in an instant. However, if the wrapping material is not strong, it will prolong the reaction time and reduce the sound of the explosion .

Potassium nitrate (copyright image from the gallery, reprinting may cause copyright disputes)

Generally, the occurrence of fire requires heat transfer to the surface of solid fuel, release of gaseous fuel, ignition of gaseous fuel, and feedback to the fuel surface until a stable combustion reaction is established. The combustion rate depends on the rate at which oxygen penetrates into the gaseous fuel position. Faster penetration means faster reaction, such as a fire in a hyperbaric oxygen chamber, which is very dangerous because oxygen ensures accelerated combustion. The discovery of gunpowder puts the oxidant and fuel together, eliminating the speed restriction of the chemical reaction, so it can generate instantaneous pressure to propel ammunition (such as firearms) or explosions (such as firecrackers).

To make firecrackers achieve the desired effect, first of all, the fuel-oxidizer ratio must be appropriate, without any excess material. If there is an excess of oxidizer or fuel, the excess part will absorb heat without contributing, so the energy release is reduced. Secondly, it must be tightly wrapped to produce the expected explosive force. Without packaging (rules), firecrackers will only burn quickly and there will be no amazing explosive effect. Finally, keep it dry. Water is the most convenient flame retardant. Gunpowder will extinguish or not light at all when it comes into contact with water.

03Evidence mining in gunpowder formula

Dr. Joseph Needham concluded through his research on the entire history of gunpowder that the proportion of saltpeter in gunpowder increased gradually. It is very likely that the proportions of saltpeter, sulfur, and charcoal were equal at the beginning, or the proportion of saltpeter was less. Later, the power of gunpowder became stronger and the burning speed became faster and faster. Finally, the proportion of saltpeter gradually increased to 75% or 80%. This is the optimal amount in theory.

To prove this point, he and his collaborators persuaded the British Royal Armaments Research and Development Bureau to specially prepare some gunpowder with a nitrate content of between 20% and 90%. Then, they went to Fort Healthstead of Kent, where the research and development bureau was located, and ignited the gunpowder one by one to observe the explosion effect and verify the evolution trend in the ancient books.

Joseph Needham managed to sort out the different components of gunpowder recorded in historical documents and used charts for comparative observation. From the Song Dynasty's "Wu Jing Zong Yao", the Ming Dynasty's "Huo Long Jing" and earlier Chinese ancient books before the 14th and 15th centuries, it was found that the balance points of the components of saltpeter, sulfur, and charcoal were scattered all over the chart, indicating that various different formulas were still being tested at the time, and the later formulas also centered around 75% saltpeter ( oxidant ) .

From early Arab and European records, it was found that this balance point was always concentrated near the center of 75% saltpeter, which seemed to indicate that Arabs and Europeans knew how to prepare the best gunpowder from the beginning. This shows that gunpowder was introduced to the West from China. When knowledge of gunpowder was introduced to Europe, knowledge of the basic formula was also introduced to Europe . Westerners did not know what saltpeter was before the 13th century, but China's "Essentials of the True Yuan Mysterious Way" published in 850 recorded the use of saltpeter, sulfur and charcoal to make a gunpowder mixture. The "General Essentials of the Military Classics" in 1044 gave the earliest military gunpowder formula. Dr. Joseph Needham used rigorous experimental design to prove the original creation and development process of gunpowder by the ancient Chinese, which is worthy of learning and reference for future generations.

Bronze fire gun from the Yuan Dynasty in the National Museum of China (copyright image from the gallery, reprinting may cause copyright disputes)

As the saying goes, the year ends with the sound of firecrackers, and happiness is everywhere, but beware of fire; oxidizing fuels are dangerous, so don't forget safety when celebrating. As the Spring Festival approaches, fireworks and firecracker control policies have been introduced across the country. Everyone must strictly abide by the local control policies and spend a safe and happy Spring Festival . Finally, I wish you all good luck in the Year of the Dragon and all the best.

Author: Ma Tingguang, Associate Professor, School of Urban Construction and Safety Engineering, Shanghai University of Technology

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.