Fireworks: Flame Tests in the Sky
By: Elise Hilf Levtne
After watching a vivid display of color and light during the Fourth of July fireworks show, one cannot help but wonder how such an awesome spectacle is produced. Moreover, one cannot ignore the chemistry involved in the art of pyrotechnics.
The history of fireworks is a “colorful” one. The earliest manuscript on pyrotechnics, Liber Ignium et Comburendos Hostes or the Book of Fire, contained 35 recipes that were collected from the mid-eighth century to the end of the thirteenth century. In the tenth century, the Chinese made fireworks from saltpeter (potassium nitrate), sulfur, and charcoal. This combination of ingredients was called “black powder” and is commonly known today as gunpowder, a mixture by weight of 75 parts potassium nitrate, 15 parts charcoal, and 10 parts sulfur. Using gunpowder, fireworks were manufactured and displayed under military control in Europe until the eighteenth century. After this time, the Italians began to use fireworks at festivals and celebrations.
In 1540, Vanuzzio Biringuccio wrote one of the earliest textbooks on fireworks. Unlike the pyrotechnics of today, early fireworks were effective because of the noise and flames they produced but they contained virtually no color. This changed in the 1630s when the French chemist Jean Appier proposed the addition of “verdigris,” a basic acetate of copper, to fireworks in order to produce a green flame. In 1792, another French chemist, Antoine Lavoisier, known as the father of modern chemistry, identified zinc as a substance that would produce a blue color for fireworks.
However, the brilliant colors that we associate with present-day fireworks were not developed until the nineteenth century. Claude-Fortune Ruggieri described the use of metal salts to produce colorful fireworks in his pamphlet on color formulas in 1845. Many of these salts are still used today. Later in the nineteenth century, the metals magnesium and aluminum were used to produce color enhancements for fireworks thanks to the brilliant light released when they burn.
During the combustion of gunpowder, both sulfur and carbon burn by using the oxygen contained in the saltpeter (KNO3). This produces ash, smoke, and large amounts of hot gas. In the smoke and ash resulting from this combustion, the most common products are potassium sulfide (ninety percent of the total mass), potassium carbonate, potassium sulfate, and gases such as carbon dioxide and carbon monoxide. Furthermore, although gunpowder burns rather slowly in the open, it burns much more rapidly in a confined space and thus contributes life and explosive qualities to fireworks.
All fireworks contain an oxidizer which is either a metal chlorate or a metal nitrate salt and fuel(s) that burn within a confined space, a combustion yields hot gases as products. The addition of metals or chemical compounds causes the six characteristic colors of fireworks. Aluminum and magnesium are used to produce white. Whereas sodium salts produce the color yellow, copper salt, in the presence of a volatile donor, are used to create blue. The addition of strontium nitrate or strontium carbonate produces red, and barium nitrate or barium chlorate is used to generate the color green.
These colors also result when the same salts are heated in a Bunsen burner flame, a technique used in flame tests. A flame test is the emission of visible light by excited atoms or radicals whey they return to a lower energy state or ground state. Thus, the heat resulting from the combustion of gunpowder serves to heat these salts so that the excitation of specific metal particles occurs and their unique emission spectra are observed by their attentive audience. The orange or amber colors in fireworks displays are created with iron metal or charcoal and other forms of carbon. In a particular type of pyrotechnic device called a salute, a titanium mixture is used to produce silver bursts in the sky.
Pyrotechnics has evolved over the last 1200 years and is an art which uses chemical substances to create color, noise, light, and flame tests on a spectacular scale.