How To Make a Firework Crack Ball

Up at Kellner's Demo nights one spring, I ran into the fireworks nut to be known to you only as Dan X. Dan had these nifty crack balls that go bang when you drop them or knock them together. Now I had seen smaller versions of these crack balls for sale as novelty fireworks. The ones I had seen before were red and blue and came from South America.

If you would like a more in-depth examination of this particular firework device, Warren Klofkorn suggests you take a look at Pyrotechnica 14 (BK0114), and the excellent article therein on "Firework Manufacture in Argentina" by Jorge Ferrando. That article contains a thorough explanation of the commercial manufacture of these crack balls in Sr. Ferrando's fireworks plant.

I want to preface this particular article with a note of caution. The mixture involves mixing potassium chlorate and sulfur, often described as a "no-no" in fireworks making.

So, my advice to all who want to try this one: DO NOT MIX THESE INGREDIENTS DRY. FOLLOW THE DIRECTIONS AND MIX THEM WET.

If you think I say this lightly, read on. Sulfur will cause a violent reaction when it is mixed dry with potassium chlorate and then either hit or rubbed on a hard surface. Try this: take a very small pinch of sulfur and a very small pinch of potassium chlorate and place them on top of each other on a rock, or a concrete surface. Then with a hammer, rub the two chemicals together on the hard surface. You will be amazed at the loudness of the result. But only do this with really small quantities or your eardrums will live to regret it.

Thanks, Dan X., for all the entertainment and for the following article.

Crack Balls

I like to try to figure out how commercial fireworks products are made. Here's a case in point. I found these two little colored crack balls, which, when hit together, make a bang. You would think that if you coat a stone with something that will detonate or burn it would cause a chain reaction and burn all the way around, but it doesn't if you do it using the formula below.

Here's the fairly safe one:

Ingredient Parts by Weight
Potassium chlorate 60%
Sulfur 35%
Dextrin 2%
Powdered glass 3%

The formula will work without the glass, but not as well. The glass helps it to detonate. I had some glass beads from a bead blaster. I used my mortar and pestle to grind it into a fine powder. You could do the same with broken Christmas tree ornaments or any other thin glass using a cheap coffee and spice grinder from Wal-Mart.

I screen the sulfur, dextrin, and glass together, mixing thoroughly.

Never screen potassium chlorate with sulfur. Always use a different screen for the potassium chlorate!

Then I screen the potassium chlorate separately, using a different screen, used only for the potassium chlorate. Do not mix this with the previous ingredients. Keep it separate. Make sure it the potassium chlorate is reduced to a very fine powder, getting rid of any lumps.

First mix all the ingredients EXCEPT the potassium chlorate together with enough distilled water to create a slurry. It should be thick, but still runny, about the consistency of Elmer's Glue. Then stir in the potassium chlorate. If the mix becomes too thick, add a little more water. Add food coloring if you want. Then get one of your kid's paintbrushes and paint it on a smooth round stone that you can find at your local creek. Be sure the stone is fairly round, definitely not flat. If you don't want to paint the rock, you can dip the rock in the slurry. Let dry very well. If you are as impatient as I am, you could use a fan to speed things up a little. These things have to be very dry before you use them.

Don't apply too thick a layer of slurry you don't want to put it on very thick. Sure, it's a lot louder but it will most likely chip off if it's too thick. If you put it on thin it's not as loud, but you get a lot of bangs for your buck. Use common sense when doing this because the rocks will actually start to fly! Try to use as round a rock as you can find. I made one crack ball with a rock about the size of a racquetball, but flat on one side. It landed on the flat spot and bounced straight up about 30 feet. No joke. The coating was only about 3/32 inch thick, so be very careful!

--Daniel X.

Crack Ball Update

Some questions were raised after I wrote the first part of this article, and I thought some clarification might be in order.

When I'm working with any pyro formulas, I wear a long-sleeve cotton shirt with the cuffs buttoned. Depending on the formula, I also use gloves when mixing. Safety glasses are a must, but in some cases a face shield is much better. Keep your work area clean and clutter-free, because chain reactions can happen very easily and can ruin your day. Also, work outside if you can and NEVER test any pyrotechnic item indoors. Always test your device in a proven, safe firing site, away from everything.

When you are working with a new formula or device, THINK SMALL! Make the bare minimum amount of a formula that is required for your test project. For example, I make 20-gram batches of crack ball composition at a time. And I use a 35mm Kodak film canister to mix it in. This is generally enough composition to coat 3 tennis ball size rocks. When I'm finished, I burn the film canister outdoors. This prevents any accidents from dried potassium chlorate-sulfur mixes.

When you're mixing these chemicals, remember to add the oxidizer LAST! I've been refining the process a little. First, I screen the sulfur, dextrin and glass powder together, mixing it well. And remember to ALWAYS use a different screen for the chlorate! I use a 40-60 mesh screen to screen the chlorate, so that when I am finished, it is very fine dust. This helps obtain the proper consistency. Once you have everything mixed wet, the correct consistency will be similar to paint. It doesn't glob or run while coating the rock.

Here's a helpful hint for drying: The next time you go out for pizza save those little plastic 3-legged tables that keep the cheese from sticking to the top of the box. If you turn them upside down, they make a great drying rack. Another good drying device is to use short paper tubes, the kind Skylighter sells for making small paper shells. Again, I burn them when I have finished using them.

I did some more testing. After the product is thoroughly dry, I thin some nitrocellulose lacquer with acetone and put a couple of coats on the rock. This allows you to apply thicker coats without it chipping off. It also protects the composition from moisture.

Remember that it's very important to use rocks that are as smooth and as round as possible. The thicker and heavier the coating, the louder and higher the rock will bounce. A case in point: One of the experimental crack ball rocks I was working with had a total coating thickness of nearly 1/8 inch. When I tested its performance, not only was it extremely loud, the rock bounced right back up into my hand! My ears were ringing. Remember, ANY pyrotechnic device can be dangerous, especially during the experimental stages. After observing that rocks bounce that high, I wanted to see if I could get it to fly anywhere but straight up, I could not. Even spinning the rock didn't change the result it still bounced straight up.

They normally don't jump at all but they do bounce when you roll them. If you toss one out in front of you, every time it bounces it detonates. You can get 3 to 4 good bangs out of a toss.

Bigger rocks or thicker coatings may not always be safe. So keep them no larger than the size of a tennis ball, and don't go overboard with the coating. Play safe and have fun impressing your friends.

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