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Jul 05 2010

Pique the Geek 20100704: The Science of Fireworks

(midnight. – promoted by ek hornbeck)

This has been sort of a recurring theme for me the past few years for the installment nearest Independence Day.  You can hit my profile and find the earlier entries in this series.

This time, I intend to focus on the single greatest improvement in technology (other than the development of black powder) that has made modern, highly colored fireworks possible.  Until relatively recently the only colors available were white, yellow, and a dull red, with very faded out, compared to today, other colors.

First some theory, then some facts.  Please follow.

Color in fireworks is produced, for the most part, in one of two ways.  The first way is for the firework act as a blackbody radiator, meaning that the color of the firework depends on the temperature of the burn.  Like heating steel with a torch, as it gets hotter it begins to radiate visible light, first a dull reddish glow, then orange, to yellow, to white hot.  You can see the same thing with an electric kitchen stove element, which gets brighter and less red as you pump in the electricity.

The incandescent light bulb takes this to the extreme, producing a yellowish white light.  However, it is not possible to produce a “pure” color with a blackbody radiator because of the fact that the distribution of radiation covers a broad spectrum.  The reason that you can get a fairly good red from one depends on the wavelength sensitivity of the human eye.  When something is glowing red, it actually is glowing over a wide range of wavelengths, most of them infrared that the human eye can not see.  A rattlesnake can, though.

Thus, except for the red, a blackbody radiator can not provide a “pure” color.  By the way, blackbodies are of extreme interest from the point of view of physics, because the behavior of their radiation was not explained by classical physics.  Max Plank explained the actual observations of blackbody radiators by postulating the quantization of energy, and that set the basis for modern particle physics.  A blackbody radiator is pretty well defined as an object that has no intrinsic color of its own, the color of the light that it emits being only a function of its temperature.  Another term for the same thing is cavity radiation, since a hollow sphere with a little observation hole in it behaves the same way, if the radiation from it depends only on temperature.

Thus, you can see that it is not possible to get vibrant colors from a blackbody radiator.  That means that some other process has to happen to make fireworks brilliant.  That other process is that of atomic emission of light (sometimes molecular emission).  Interestingly, is a direct result of quantization of energy, first postulated from the study of blackbody radiation.

When electronically excited, all atoms emit electromagnetic radiation when the excited electron returns to its ground state.  Sometimes this electromagnetic radiation falls into the visible range for humans.  I should explain those terms, or none of the rest of this will make any sense.  The ground state is the most stable arrangement of electrons around an atom at a particular temperature, in our case ambient July outdoor ones.  An excited atom has had some type of energy input to take an electron to a higher energy state.  It can exist in the excited state for very long, so the electron returns to its lower energy state, thus giving off a photon (the particle that mediates the electromagnetic force).  In doing so, if the wavelength is between 400 nanometers and 800 nanometers (roughly), it can be perceived as visible light to humans.

For example, the familiar neon light glows orange red because it has a strong emission in that part of the spectrum as an excited electron falls back to the ground state.  But if you were to feel of a neon tube, it would be barely warm (except at the ends, where the electrodes are).  You could hold on to the center of a glowing neon tube for hours without harm, but an oven element of the same color would burn you instantly and severely.  Thus, the mechanisms of color production are quite different.  In a neon tube, a weak electrical current excites some of the electrons to an excited state, then they fall back to the ground state.  Heat is not required, but also can be a source of excitation.  Here is the line spectrum of neon.

Neon

Neon

Note that the line spectra that I present are photographs, and photographic film is, for the most part, more sensitive in the violet and becomes less sensitive towards the red.  The human eye, however, has its maximum sensitivity in the yellow-green, falling off rapidly towards both violet and the red.

The same thing happens with a compact florescent lamp, except that mercury is the element emitting light.  However, we can not see the very efficient 253.7 nanometer wavelength (is is in the ultraviolet), so a coating on the interior of the bulb absorbs the UV and then reradiates it at a wavelength that is visible.  Thus, a 23 watt CFL can give off as much visible light as a 100 watt incandescent one.  Try this:  touch a 23 watt CFL coil and then a 100 watt incandescent one.  You can hold the CFL essentially forever, and will be burnt badly with the incandescent one.

All elements have characteristic line spectra when they fall from an excited state to the ground state.  One very familiar to you is that of sodium, which makes a gas flame look yellow, as when a salty stew boils over its vessel.  As a matter of fact, the element helium was discovered by an analysis of the light from the sun before it was found on earth, hence the name, from the Greek helios, sun.  For fireworks, the following ones are important.  I have given a name for the element and its line spectrum as a picture after it, with some comments.

Barium

Barium

Note that barium has a very complex spectrum, but the important thing to take away is that the human eye is most sensitive in the green part of the spectrum.  There you see a big, fat line in the blue green area, and another, not as fat one in the yellow green area.  When decoded by our brains, those two lines overwhelm the others and make a nice, pure green.

Strontium

Strontium

Strontium also has a complex spectrum, but if you look closely you will see that the fattest bands are in the red part.  (The apparent “fatness” of the bands in the blue region is an artifact resulting that photographic film is much more sensitive towards the blue end of the spectrum, unlike human eyes, and that that same film is very much insensitive to red wavelengths).  The result is a very carmine red as we perceive it.

Sometimes lithium is used for a red, but I can not find a picture of its line spectrum.  It tends to be shallower than strontium because of several reasons.

Iron is often used for sparkle effects, and it does has a visible spectrum.  However, because that it is burning very hot, we mostly see the blackbody radiation from it.  You can see that it emits in the green, mainly, but that is not what we see.  We see yellow to white sparkles since the blackbody effect very much overwhelms the atomic emission.

Iron

Iron

Yellow is easy.  Sodium does it well.

Sodium

Sodium

Look at that HUGE, fat line in the yellow (not too far from the human maximum sensitivity for color perception).  This is one reason that potassium salts are used to make pyrotechnic mixes for fireworks, because the sodium yellow line would overwhelm otherwise.  Sodium salts are cheaper than potassium ones, but everything would look yellow.  Here is why.

Potassium has a complex spectrum, but the fattest lines are in the far blue and violet.  Hence, a potassium transition is barely visible to the human eye.  By using potassium salts, the colors of the other elements can be observed.  As a matter of fact, potassium nitrate is used in military infrared flares because it is not very bright in the visible (although cesium nitrate is used in the more advanced ones, but it horrifically ($100 / lb or more).

Potassium

Potassium

White is easy.  Burning aluminum, magnesium, or both provide a very bright white light, due to blackbody radiation.  They burn at a much higher temperature than does iron, so their atomic emission spectra are completely overwhelmed by their blackbody radiation.

Orange is also easy.  Calcium is the ticket.

Calcium

Calcium

That big, fat band of orange overwhelms the green ones, and it looks orange.  However, orange is not a very popular fireworks color, the yellow from sodium contamination in the raw materials is often used to make up for it, and the incandescent burning iron makes a better effect.  Calcium is rarely used in fireworks.

Blue is the hardest to achieve.  No common, nontoxic element has a strong blue line, and our eyes are insensitive to blue in comparison with longer wavelengths.  Arsenic was used many years ago (unfortunately, I have not been able to find an emission spectrum of arsenic), but was banned because of its toxicity.

Making a true, quite blue, fire is just about the ultimate reach of the pyrotechnician.  In the first place, the human eye is relatively insensitive to blue photons, so there have to be a lot of them to get a proper sensation.  Second, with no atomic line spectrum that gives an intense one, other means have to be found.

That means is the copper/chlorine molecular ion.  Copper, in its 1+ electronic state (aka, cuprous) and chlorine form a very unstable molecular ion with the formula of CuCl+.  When excited gently, it produces a very pure blue light (I could not find the spectrum for it) that is dazzling.  Only in the past few decades has a “true” blue been possible without arsenic.  That is usually made by taking a copper salt and mixing it with polyvinyl chloride plastic, and with a minimum amount of oxidizer to keep the temperature low enough so not to destroy the molecular ion.

Now to mention the breakthrough in fireworks material.  Whilst black powder (a mixture of potassium nitrate, carbon, and sulfur) is usually used as the lifting mix in aerial fireworks, black powder burns at much too of high of a temperature for good color development.  Remember what we have discussed about exited states of atoms.

To make anything burn, there are three requirements.  First, oxygen (for normal burning, but other electronegative elements can also suffice), fuel, and a high enough temperature to keep the reaction going.  Unfortunately, potassium nitrate requires such a high temperature that things being burnt by its oxygen often become ionized rather than just excited.  That means rather than just exciting an electron to a higher state, the electron is completely removed from the atom.  When a new one finds its way back, the electromagnetic radiation that results is usually in the ultraviolet, and we humans can not perceive it, but some insects can.

That is why fireworks are almost totally dependent on potassium chlorate.  This material begins to emit oxygen at a very much lower temperature than the nitrate does, and makes possible the vivid colors.  Potassium nitrate does not decompose until 560 degrees C, but potassium chlorate, properly sensitized with sulfur, begins to decompose at around 120 degrees C.  Thus even in colored fireworks without a lifting charge, you can still smell burning sulfur.  Colored smokes are similar in that chlorate is used with sulfur to keep the temperature low enough not to decompose the dyes that color the smokes.  I love the smell of burnt powder!  It is pretty easy to make good greens, reds, and whites with it (most of the brilliant whites are from atmospheric oxygen, anyway, and are blackbody effects), but to get a good blue is extremely challenging.

That cuprous chloride molecular ion is very fragile, and even chlorate can ionize it, washing out the color.  Even with the elemental ones, temperature control is essential.  Fortunately, we have ways to make them behave.

One of the most important parts of a firework to develop good color is a coolant.  This is where science and art intersect, because it is impossible to say with any certainty what combination will work well.  The purpose of a coolant is to slow down the reaction between the fuel, the oxidizer, and the colorant to make the most intense display of brilliance.  Many chemicals have been used as coolants, but only a few work very well.

Sodium bicarbonate is a good one, since it releases carbon dioxide and water when it nears 100 degrees Celsius.  It is also a sweetener, since its weak basic properties neutralize the acids given off as the other materials decompose, making the fireworks more stable.  However, it does not last forever and finally can be exhausted as fireworks age.  Hint:  do not keep fireworks for more than a couple of years.  Not only does their performance suffer, they become, in some cases, dangerously unstable.

Another sweetener has to do with a series of complex organic molecules, often used in “smokeless” powder.  These are also sacrificial in that they neutralize the decomposition products of the propellant.  Most of them are not coolants, however.

We call them sweeteners because the acidic products of self decomposition are sour to the taste.  Caution to those of you who load your own cartridges, just use new powder.  Even 10 year old powder may be dangerously depleted in sweeteners.

You ask, “Doc, how do you know all of this?”  I answer that before I was five years old, my dad had taught me to reload shotgun shells (that was back in paper shell days).  I got good at it, and when the plastic ones came out, dad and I jiggered his rig to reload them, too.  Then came the sabot “shot cup”, made of plastic that replaced the compression and spacer wads that were made of wood fibre.

Well, I also became a scientist and found my way into making smoke munitions for hiding troops for the Army as a civilian.  I then had access to the best literature on the subject.  I also developed defensive “sting” grenades as well, and have been hit by many rubber balls testing them.  Yes, I kept on my protective glasses.

Well, that is it for tonight.  Eat your Independence Day food (mine happens to be a hamburger with all of the trimmings, the former Mrs. Translator’s baked beans, and perhaps some French fries (I eat them very rarely)).  I wish only the best for everyone reading, and hope that you have an excellent evening.

Well, you have done it again!  You have wasted many einsteins of photons reading this colorless post.  And even though Mike Huckabee admits that he is going to run in 2012 when he reads me say it, I learn much more by posting this series than I could possibly ever hope to teach.  Too bad that he has rectal/cranial inversion.  Please keep comments, recs, questions, and corrections coming!  No science or technology subject is off topic here, and I love to interact with folks who comment.

Warmest regards, and Happy Independence Day!

Doc

Crossposted at Docudharma.com and Dailykos.com

 

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