How to Make Flashing Fireworks Strobe Pots
IntroductionStrobe pots are among the simplest of fireworks devices and are easy to make. They can really add some of that low-level variety to a pyro-display that so helps to keep an audience's attention.
"Hey, here's something different," they'll say to themselves as they stop, settle in, and start to pay attention.
How do these pyrotechnic "twinklers" work?It is not necessary, of course, to have a scientific understanding of strobes in order to make them. Like baking a loaf of bread, chemistry is not necessary. All you need is a recipe, the right ingredients, and a feel for the proper ways to manipulate those ingredients.
But, for the scientifically minded, there are a few informative resources, which explore the strobe phenomenon in depth. In the 1979 edition of Pyrotechnica, Number 5, Robert Cardwell, the editor and publisher of the Pyrotechnica series, wrote an article, Strobe Light Pyrotechnic Compositions: A Review of Their Development and Use.
In this essay, Cardwell explores the historical development of strobing compositions and presents quite a few different formulas.
Dr. Takeo Shimizu, in Fireworks, the Art, Science and Technique (FAST), originally published in 1981, writes about "Twinklers," which is how he refers to strobing stars. He presents an outline of the development of these strobing compositions, progressing during the second half of the 1900's.
Specifically Shimizu writes, "In Germany, U. Krone and F. W. Wasmann suggested that a twinkle composition consists of two kinds of compositions mixed with each other, i.e. a smolder composition and a flash composition-Ammonium perchlorate smolders when it is mixed with a small quantity of magnesium. This can be used for the smolder composition. A mixture of magnesium and sulfate flashes when it is heated to a high temperature. This can be used as the flash composition."
So, interestingly, a strobe composition is actually a mixture of these two types of comps, a smoldering one and a flashing one. When the mixture is lit, the first one begins to smolder. When the heat rises high enough, the flash comp ignites and emits a flash of light and heat. Then the mass returns to the smoldering state until the heat rises high enough to repeat the flash.
In some compositions, magnesium-aluminum (magnalium) is used instead of the magnesium. Magnesium requires a coating to prevent it from prematurely reacting with the oxidizer in the comp.
Additionally, sometimes barium nitrate or other oxidizers are used instead of ammonium perchlorate.
In 1987, John "Skip" Meinhart offered some details about his noteworthy strobing star formulas in Pyrotechnica XI. Except for Shimizu's White formula, and Skip's Pink formula, all the rest of the formulas use magnesium as the metallic fuel ingredient.
In the 1992 Pyrotechnica XIV edition Jennings-White explores Blue Strobe Light Pyrotechnic Compositions. Up until that point in time, blue strobes had not been explored in depth because of some unique problems associated with the chemical mixtures required to produce that color in a strobe.
All of this information ought to be able to keep you reading until late into the night if you are so inclined.
Making strobe potsI won't be focusing on making strobing stars in this project, but only simple, ground-effect strobe pots.
I'm also not going to be making any of the formulations, which contain magnesium. As I said, using that metal requires a special coating process because it does not form an oxidized protective layer on its own, as do aluminum or magnalium.
There seems to be some debate as to whether or not magnalium needs to be treated and coated when it is used in compositions containing ammonium perchlorate. Meinhart states, "I have had success using magnalium powders that have not been treated with potassium dichromate. In practice I have often used treated metal powders, but this does not always seem to be necessary."
Whereas in Hardt's Pyrotechnics, Barry Bush notes that the formulas he cites which contain magnalium or magnesium in combination with ammonium perchlorate do "require the metal powders used to be treated with potassium dichromate." Shimizu also specifies treated magnalium, and details the methods of treatment in FAST.
Shimizu does state that if there is any reaction between magnalium and ammonium perchlorate, which would be encouraged by the presence of water, it would only be a slow reaction in which the metal is affected gradually.
I have used untreated magnalium in these formulas, with no problems. One sign of an unwanted reaction would be the heating-up of the composition as I'm working with it, so I always pay attention to see if that is occurring. I avoid adding any water to such a composition. I also don't store these devices for long periods of time, which could produce a slow reaction of the ingredients, especially in the presence of moisture.
So, I think I'll make simple white and pink strobe pots. The white formula is the most commonly cited one:
White Strobe Composition
|Chemical||Percentage||16 Ounces||450 grams|
|Ammonium perchlorate||0.57||9.15 ounces||257.1 grams|
|Magnalium*||0.24||3.8 ounces||107.1 grams|
|Barium sulfate||0.14||2.3 ounces||64.3 grams|
|Potassium dichromate||0.05||0.75 ounces||21.5 grams|
* Shimizu specifies 80-mesh, whereas other sources specify 100-200-mesh. The mesh of the metal is known to vary the flash rate of the strobe, so some experimentation is in order. Initially, I'll be using 200-mesh magnalium, Skylighter #CH2073.
Barry Bush has an interesting note in Pyrotechnics concerning this formula. This formula "may be given a faster frequency by replacing the barium sulfate with anhydrous magnesium sulfate. The resultant fast strobe is sometimes called a "shimmer effect." I'll have to try this sometime in aerial-shell strobe-stars, since it is an effect I have admired in commercial shells.
Additionally, the flame created by this "white" composition is brilliant, but it does have a very slight green tint caused by the barium. Barium normally produces very green flames with the addition of a chlorine donor such as parlon or saran. Another experiment would be to include small amounts of these chlorine donors to shift the color of the white strobe pots to green.
The pink strobe pot composition is as follows:
Meinhart Pink Strobe Composition
|Chemical||Percentage||16 ounces||450 grams|
|Ammonium perchlorate||0.57||9.15 ounces||257.2 grams|
|Magnalium, 200 mesh||0.15||2.45 ounces||68.6 grams|
|Strontium sulfate||0.11||1.85 ounces||51.4 grams|
|Strontium carbonate||0.08||1.2 ounces||34.3 grams|
|Parlon||0.04||0.6 ounces||17.1 grams|
|Potassium dichromate||0.05||0.75 ounces||21.4 grams|
All the chemicals (except the magnalium, which I don't put through fine screens) are fine enough to pass through a 100-mesh screen. If they are not, they are milled individually in a blade-type coffee mill.
Note: Ammonium perchlorate does not play well with potassium nitrate. The combination forms ammonium nitrate, which is very hygroscopic, attracting moisture out of the air like crazy, rendering any mixture or composition containing it wet and useless. Don't grind either of these chemicals in a coffee mill which has been used on the other chemical, unless the mill has been thoroughly cleaned with soap and water.
Warning: Potassium dichromate is toxic and a known carcinogen. A good respirator and rubber gloves are required when working with this chemical, and when using it in pyrotechnic compositions. Don't breathe this stuff or get it on your skin.
All the chemicals for a given formula are weighed out individually and are passed through a 20-mesh screen 3 times to thoroughly mix them.
Then the composition is mixed with enough nitrocellulose lacquer (Skylighter #CH8196P) to create thick putty, similar to Play-Do. I did not dilute the lacquer, but used it right out of the can, as-is. The one-pound batches required 3 ounces, by weight, of the lacquer.
I started mixing the composition in a plastic tub with a paint stir-stick, and finished by kneading it with gloved hands.
Mixing Nitrocellulose Lacquer Into Strobe-Pot Composition
The dough is then pushed with gloved fingers into paper tubes to create strobe pots. I start this process by pushing the tube into the composition-putty to get the filling started.
Large pots can be made with 1.5-inch ID tubes, cut into 1.5-inch long sections. Or, smaller pots can be made with 3/4-inch ID tubes, cut into one-inch long sections, or even longer. While thicker-walled parallel tubes, like rocket tubes can be used, strobe-pot tubes do not need to be super-strong, so spiral-wound tubes like Skylighter #TU2142 or TU2053 can be used.
Large diameter strobe pots would be appropriate for large displays and venues. Smaller ones are nice in backyard size shows. Varying the length of the paper tube will adjust the total burn-time of the strobe pots, so their duration can be dialed in for specific uses.
For this project, I think I'll make mostly 3/4-inch ID by 1-inch long strobes to determine how well they are working and how long they burn, plus a few other sizes to see how they perform, too.
Batch of "Flashing Fireworks" Made With One-Pound of Composition
Once the composition has been stuffed into the paper tubes, they are placed on their sides and set aside on a tray to dry out in the open air. N/C lacquer releases acetone and other highly flammable solvents as it dries, and I don't want these vapors collecting in my shop as this occurs.
Toward the end of the tube filling, the remaining strobe-putty started to dry out and became difficult to consolidate into the tube. I added just a touch of acetone to the composition to re-dampen it.
It took 3 or 4 days for these pots to dry completely. When I tried to burn them before they were completely dry, they did not burn with a regular strobing-action, but with a more continuous flame.
Priming the strobesThe dry pots will light well if they are ignited with a piece of Visco-fuse or with a propane torch. But if I want them to ignite reliably with a fast-fuse or quickmatch line of fuse, then I need to prime them.
A black powder prime containing potassium nitrate cannot be used on these compositions because of the incompatibilities between the nitrate and the ammonium perchlorate.
In FAST, Dr. Shimizu lists a different prime specifically for this use.
Ignition Composition for Twinklers
|Potassium perchlorate||0.74||11.85 ounces||333 grams|
|Red gum||0.12||1.9 ounces||54 grams|
|Charcoal, airfloat||0.06||0.95 ounce||27 grams|
|Potassium dichromate||0.05||0.8 ounce||22.5 grams|
|Aluminum*, flake 100-325||0.03||0.5 ounce||13.5 grams|
*Skylighter #CH0174 aluminum would fit the bill in this prime.
After making sure all the individual chemicals (except the aluminum) will pass through a 100-mesh screen, I weighed them out individually and mixed them together by passing them through the 20-mesh screen three times.
I weighed out 1 ounce of the dry strobe prime composition, and added 1 ounce (by weight) of the nitrocellulose lacquer. This created a wet prime comp that had a consistency between that of honey and peanut butter.
I used a wood stick to apply this wet prime to one end of each strobe pot, and quickly pushed that wet end into some dry strobe composition for the final prime layer.
Priming Strobe Pots for Easy Ignition
I perform one final operation to finish the individual strobe pots. I hot-glue a paper disc onto the bottom of each pot. This prevents sparks and/or slag from dropping and igniting the bottom of a twinkler prematurely as the pot burns. It also facilitates mounting the pots to a board when a show is being set up.
Paper Discs Hot-Glued to the Bottom of Strobe Pots
Mounting and fusing strobe pots for use in a fireworks displayOnce the individual strobe pots have been completed, they can be mounted to a board and fused for easy installation out in the field prior to a fireworks show.
To do this, I simply hot-glue the pots to a board at the desired spacing. I find a spacing of 4 feet on-center to work well. Then a run of quickmatch or tape-covered fast fuse is used to fuse all the pots together. A "window" is opened up in the quickmatch-pipe, and the bared black-match is taped on top of the strobe pot with 3 wraps of masking tape.
Strobe Pots Hot-Glued to a 1x2 Board, and Fused Together with Quickmatch
The quickmatch can be ignited by a piece of Visco-fuse, or an electric igniter can be employed, per the information in How to Make Electric Matches and Wiring Fireworks and Firing Systems in a Fireworks Display.
Three Strobe Pots Ready to Be Electrically Fired
ResultsI burned white and pink strobes made with the 200-mesh magnalium. The white one burned for 15 seconds with a very fast strobe rate of about 10 flashes per second. The pink one actually looked red, burned for 23 seconds, and flashed about 4 times per second.
Warning: These strobe pots burn with an extremely brilliant flame and light. It is best to avoid looking directly at them to prevent eye damage. Placing the pots where their light can reflect off of a structure or trees makes their effect visible without having to look directly at them.
Strobe Smolder Phase, White Strobe Flash, and Red Strobe Flash
Click below to see a video of the white and red strobes.
Red and White Strobes with 200 Mesh Magnalium
I liked the performance of the pink/red strobe, but the white one flashed too rapidly for my taste.
So, I made a new batch of each color using 60-mesh magnalium. I know that using a larger granulation of the metal will slow down the burn time and also its strobe frequency.
Burning these new strobes produced the following results:
White strobe with 60-mesh magnalium, burned for 25 seconds, and flashed 1.5 times per second. I found this to be a very pleasing strobe frequency.
Red strobe with 60-mesh magnalium burned for 27 seconds, flashed at a rate that varied from slow to fast. This pot just couldn't seem to find a groove and settle into it.
Check out the video of these two types of strobe pots:
Red and White Strobes with 60 Mesh Magnalium
Of the four variations I prefer the white strobe pots made with the 60-mesh magnalium, and the pink/red twinklers made with the 200-mesh magnalium.
Although I made mostly 1-inch long twinklers, I also made some larger ones. Two-inch long ones, made in the 3/4-inch ID tubes, burned as follows:
- 2-inch white strobe with 60-mesh magnalium burned for 40 seconds with about 2.5 flashes per second,
- 2-inch long red strobe with 200-mesh magnalium burned for 40 seconds with flashes varying from slow to fast again.
White Strobes Set to Music
I do like what these simple, low-level ground devices can contribute to a fireworks show.
- Aluminum, flake 100-325 (CH0174)
- Ammonium Perchlorate (CH5000)
- Barium Sulfate (CH8030)
- Charcoal, airfloat (CH8068)
- Disks, 7/8" or 1-3/4" (DK0701 or DK1600)
- Magnalium, 200 mesh (CH2073)
- Nitrocellulose Lacquer (CH8196P)
- Parlon (CH8210)
- Potassium Dichromate (CH5525)
- Quickmatch (GN3001), if available
- Red Gum (CH8230)
- Rubber Gloves
- Screen, 100-mesh (TL2009)
- Strontium Carbonate (CH8310)
- Strontium Sulfate (CH8313)
- Super-Fast Paper Fuse (GN1205), if not using Quickmatch
- Tubes, 3/4" or 1-1/2" ID (TU2053 or TU2142)