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Fireworks: having a blast with color!Bradley J. Waffa |
When the flame of powder toucheth the soul of man, it burneth exceeding deep.
Roger Bacon, 1242
Why explore fireworks?
Have you ever heard of a "green man"? And I'm not referring to the Jolly Green Giant on the canned vegetables. Green men were the pyrotechnicists (not "pyrotechnicians") of the early16th century that covered their naked bodies in soot and leaves while they worked. This strange attire not only helped protect their skin from sparks and hot mortars, but it camouflaged them as they ran around in the dark, frantically lighting fuses during their firework performances! See, you do learn something new everyday! Fireworks have a long and fascinating history, all of which is related to light and color. Be it green men, or the historical evolution of colored flames, or the chemistry of light in a firework display, light and color are a constant theme in the study of fireworks, and therefore a perfect topic for exploration in NOND 102- The Science of Color.
A brief history
Fireworks exploded onto the scene in China, thousands of years before Christ. More accurately, they popped and fizzled onto the scene. The first "fireworks" as we now know them, were simply green bamboo shoots called pao chuk ("bursting bamboo") by the Chinese peoples who burned them. Bamboo, a fast-growing woody grass, traps pockets of air its sap and shoots. When heated, as in a fire, this air expands rapidly bursting the bamboo shoot open with a loud, violent crack. Although this early fireworks display was not much to look at, appealing mostly to the auditory rather than visual senses, it awed and humbled people in religious ceremonies for millennia. It demonstrated again, possibly for the first time since the first hunter-gatherer campfire, man's ability to conquer the elements- to control fire. Pao chuk was, perhaps, the cornerstone of today’s fireworks.
The next necessary precedent in the formation of modern fireworks was the accidental discovery of huo yao (literally "fire chemical"), or gunpowder as we know it. About 2000 years ago in 9thcentury China, alchemists were trying to perfect a recipe for a thick toffee that was thought, if created properly, to bring eternal life to whoever ate it. Ironically, those who were not blown up steeping it often died from eating it. The honey, sulfur, and saltpeter (the old term for potassium nitrate) brew, when ignited, produced a flaky black mixture that burnt up quickly over an open flame. It was not long before this black powder was added to pao chuk, and the first man-made firecracker was born.
Since the discovery of gunpowder, fireworks have evolved drastically in construction, application, and style. Bamboo stems, for example, were replaced by paper tubes with fuses. Where pao chuk was once used to scare away evil spirits, developing firecrackers were used to scare away the enemy in warfare (and eventually to blow him up). During the Renaissance, fireworks became a true "art" form, when sculptors, craftsmen, and pyrotechnicists worked together to create miniature castles and palaces adorned with fountains and wheels that would spray brilliant orange sparks, or spin so quickly that the viewer witnessed a spectacular "ring of fire" hovering above the castle during a nighttime display.
However, despite countless changes and advancements in the way fireworks were created and used, the color they produced remained orange and bright white for nearly 2000 years! It wasn't until the1830s when advancements in chemistry led to the discovery that color could be produced by burning a metallic salt and a chlorinated powder simultaneously. Chemists applied this discovery to "qualitative analysis," using flame tests to categorize new elements based on their unique emissions spectra. Pyrotechnicists, however, began utilizing these compounds in their fireworks. Other advancements in chemistry, like the discovery of electrolysis, allowed chemists to obtain magnesium and aluminum in their purest forms. These elements, applied to colored fireworks, allowed them to burn even brighter than before!
Today, fireworks are still popular all over the world. China remains the leading exporter of fireworks, using many of the same color-producing compounds discovered in the early 19th century (strontium, barium, copper, sodium, etc.). The formula for black powder (10% sulfur, 15% charcoal, and 75% saltpeter), perfected in 1550, is also still used- almost 500 years later! Many governments around the world (both state and federal) strictly regulate the sale, possession, and use of fireworks.
Spectranalysis:
No, "spectranalysis" is not (yet) a recognized word in the English language. But if doctors can perform a urinalysis, then I don't see why pyrotechnicists and chemists cannot similarly perform a "spectranalysis." I have coined the term for the purposes of this project. Just as urinalysis examines the components of urine to better understand the workings and condition of the human body, so our "spectranalysis" will examine the spectral components of certain metal compounds, allowing us to better understand them and why they yield the colors they do.
Light, as the human eye perceives it, is merely a fraction of the electromagnetic energy spectrum called the visible spectrum. Anyone who has seen a rainbow or daylight through a prism is familiar with this visible spectrum, and may remember its component colors by the pneumonic "ROYGBV," for "red, orange, yellow, green, blue, and violet." When white light is observed, one's eyes are experiencing every wavelength of the visible spectrum- each of those component colors equally.
Applied to fireworks, colored flames are observed when the wavelengths of the visible spectrum are not transmitted equally. For example, a red flame appears red because the human eye perceives an overwhelming concentration of wavelengths on the red end of the visible spectrum instead of an equal balance of all wavelengths. Why and how though does this occur? How is light energy and color produced from a metal anyway?
All combustion reactions require three ingredients: ignition, a fuel source, and oxygen (as an oxidizer). The ignition, typically the flame from a match or the spark from an electric sparker, provides the energy necessary to initiate the reaction. The fuel source, like the wood on a campfire, is that which is combusted, or "broken down," in the reaction. The oxidizer catalyzes the process. Because elements (for all intensive purposes) cannot be broken down any further, and because many of them (like sodium and potassium) are too volatile in their elemental state to justify unnecessary handling anyway, fireworks use metallic compounds which can be handled and broken down molecularly, in order to achieve the colors we see. When these metallic salts are superheated, their electrons are excited out of their respective energy states, and in falling down to a lower energy state, release "photons"- light energy. The color produced- the wavelength- of each falling electron, is determined by how far it falls. The color the observer perceives is essentially the "average" color all these electrons produce together, due to the additive nature of light.
Every element has a different number of electrons, and subsequently, a different electronic arrangement. Although two compounds may appear to produce the same color when combusted, its emission spectrum is actually as unique as a fingerprint.
Let’s look at a few of these "fingerprints" using some traditional and still-popular metal salts used in modern fireworks.
When reading these emissions spectra, note:
1. The number of bands.
2. The location of the bands relative to the rest of the visible spectrum, and
3. The thickness/brightness (indicates overlapping) of these bands.
Also, the emissions spectra are arranged in order: ROYGBV. As you scroll down, watch a trend in the bands as their congregation moves from one end of the visual spectrum to the other.
Finally, note that these emissions spectra are subject to some variation as these (and other) metals can be combined with more than just one different element (i.e. sulfates, silicates, phosphates, etc.) to form a metallic compound. The element therefore is followed by the general hue it tends to produce.
Strontium: Reds and magenta
Notes: This spectrum, with its presence of blue wavelengths, is indicative of a magenta hue.
Calcium: Light oranges
Notes: Bands present all across spectrum which, if equally displayed, yields white light. But presence of orange-red wavelengths strong enough that an orange hue is produced.
Sodium: Yellows
Notes: Wavelength emission very localized in the spectrum. Safety glasses with didymium lenses (often used in glass blowing) effective in blocking out precisely those wavelengths so that less intense wavelengths may be observed.
Barium: Yellowish greens
Copper: Greens and Blues
Notes: Copper chloride produces a decent blue after a period of burning. Most copper compounds, however, produce only a green hue.
Potassium: Violets
In seeing and understanding these emission spectra, one can better comprehend the chemist's use of flame tests for qualitative analysis. If a new element is discovered, its emission spectrum is compared to those of other elements to determine its approximate position on the periodic table. One can also see how this information would be useful to the modern pyrotechnicist developing fireworks that are constantly increasing in complexity. If for example, he wished to create a firework that would explode into one color and then burn continuously all the way across the spectrum before fizzling out, it would be useful for him to know which chemical compounds produce the closest colors so as to plan his burn sequence accordingly. It is hard to believe that this science, as exact as it is becoming, has evolved from some of our earliest ancestors' quest to find the "perfect" toxic toffee formula. But despite this almost laughable irony, the history of fireworks is an evolution- a growth and a change- and there is certainly no shame in progress. Since the dawn of time, man has depended on fire and his ability to control it for survival. Although we as a species have come a long way since our hunter-gatherer days, our ability to control fire, whether for survival, religious, protective, or entertainment purposes, will remain a hallmark of our human identity.
http://center.acs.org/applications/ccs/application/index.cfm?PressReleaseID=2189&categoryid=4
"Practical Pyrotechnics"- a page from the Netherlands where fireworks are greatly frowned upon and where government regulation is EXTREMELY strict: http://www.wfvisser.dds.nl/indexEN.html
http://members.ozemail.com.au/~gmillar/
Adams, John, and Esther Foer. Fireworks in America. N.p.: n.p., 1985.
Kaboom. Dir. David Dugan. Videocassette. PBS/NOVA - Windfall Films, 1997.
Acknowledgements
Dr. Bordley, for help acquiring chemicals, preparing the flame tests, and being available to answer questions.
David Sprehn, for further explaining the nature of electrons and inspiring me to someday change my sleeping habits.