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Making Black Powder Coated Rice Hulls

By Ned Gorski

Take just 20 minutes to read this article, print it out, and use it to make this firework. You will be amazed at how well it will work as long as you follow the steps we give you here. Try this project today and see if you don't agree.

Introduction from Harry Gilliam


Making black powder coated rice hulls (you'll also see these called "meal coated rice hulls") is one of the cornerstones of shell making.

The good news is that it's not rocket science. Learning how to make black powder coated rice hulls is very simple. And as you'll see in Ned's article, these homemade black powder hulls can do double duty for you sometimes as lift powder.

Ned's ball shells are consistent prize winners in fireworks competitions. Fact is, this article is really a collection of some of his important secrets to building his trademark, spectacularly-beautiful shells.

Once you learn to make meal coated rice hulls, you'll see why I consider them to be one of the black powder grades, just like 2FA and other sizes of BP.

--
February 25, 2012


Personally, I get great satisfaction from making nicely-performing large aerial fireworks ball shells.


(Click Image to Play Video)
12-Inch Ball Shell by the Author
(Photo by Norm Whetzel)

One of the key ingredients in a shell like that is the correct shell burst powder. The burst charge has to burn fast enough and be powerful enough to really "pop" the shell. I want it to produce a large and symmetrical display, but not so strong that it shatters the stars or blows them "blind" preventing them from lighting.

In small shells, granulated black powder (BP), or other types of burst powder, is used to break them. But in 4-inch shells and larger, black-powder-coated rice hulls--with or without a burst enhancer--create very pleasing shell bursts.

There are other "black" compositions which can be coated on rice hulls. There are also other carriers, like cotton seeds or cork bits. And the ratio of powder to carrier can also be varied to control the strength of the burst. For an extensive overview of those options, I'd recommend Takeo Shimizu's book, "Fireworks; the Art, Science, and Technique."

I consider making good black-powder-coated rice hulls to be one of the most essential fireworks-making skills. It's right up there with making good black powder and good stars. And, if I do say so myself, many folks, when they first see and handle my coated hulls, say "Man, those are nice, some of the best I've seen."

There are some tricks to making good BP-coated hulls. Read on.

It's not particularly easy or safe to open up a Chinese aerial shell. But if you did, you'd more than likely see some stars in there along with burst powder granules which look like long, skinny black footballs.

Rice hulls coated in meal powder
Black-Powder-Coated Rice Hulls

Those are black-powder-coated rice hulls, and they are the typical burst powder used in almost all ball shells and in some cylinder shells.

And, although it's not that common, some fireworks hobbyists, including yours truly, also use the BP-coated hulls for lift powder under those same shells.


What are rice hulls?


From Wikipedia: "Rice hulls are the coating for the seeds, or grains, of the rice plant. To protect the seed during the growing season, the hull is made of hard materials, including opaline silica and lignin. The hull is mostly indigestible to humans."

Rice hulls next to penny for size comparison
Rice Hulls

When rice is harvested and cleaned the outer coating, the hull, is removed from the edible rice grain. These hulls have a variety of uses, one of which is to be the "carrier" on which black powder is coated to serve fireworks purposes.

The individual hulls often look like small canoes. During the coating process, black powder with a little binder in it sticks to the exterior of the hulls often filling the inside of each "canoe."


Why coat the BP onto rice hulls?


In ball shells smaller than 4-inches or so, granulated black powder works just fine to burst the shells. But once we reach 4-inch shells and larger, granulated BP simply gets too heavy for that purpose.

A 12-inch single-petal ball shell has an area of approximately 450 cubic inches inside its stars that needs to be filled with burst powder.

But what kind of burst powder?

Well, commercial 2FA black powder has a density of 0.6 ounces per cubic inch. So if those 450 cubic inches were filled with 2FA burst powder, it would require 270 ounces of that powder: almost 17 pounds.

But in "Fireworks, the Art, Science and Technique," Dr. Shimizu recommends 85 ounces of burst charge in that shell--about 5.3 pounds. But the weight of 2FA burst powder is three times that.

Obviously we have to find some way of filling that volume with a burst charge that only contains about 85 ounces of black powder.

By coating the black powder on a "carrier," such as rice hulls, cotton seeds or cork bits, we can accomplish that desired reduction in the density of the burst charge used in the shell.

Why use rice hulls? Primarily because they are the easiest and cheapest carrier to get in the US.

Black powder coated on rice hulls, in a ratio of 7:1 by weight, results in a burst charge with a density of approximately 0.25 ounces per cubic inch. That is the actual amount of black powder per cubic inch, not including the weight of the rice hulls.

At that 0.25 ounces per cubic inch, the 450 cubic inches in the 12-inch single petal shell would contain 112.5 ounces of black powder bursting charge.

That's getting down into the range of Dr. Shimizu's recommendation. And, with a double-petal shell, where more volume is occupied by stars, and less by burst charge, the coated hulls would contain just the right amount of BP. Some of this depends on the size of the stars that are used, of course.

So, we coat black powder on rice hulls to lower the density and weight of the burst charge.

The resulting black powder coating is also much thinner than 2FA granules would be, so it burns much more rapidly than the solid granules would. That results in a very rapid pressure spike, and a strong bursting of the shell, with large star-pattern size and good symmetry to it.

Some experimentation is required in the long run to dial in the perfect burst charge and shell construction for optimum shell bursts. Burst powder strength, the amount of it in the shell, the amount of paper pasted on the shell, etc.--all contribute to the performance of a shell.


The black-powder to be coated on the rice hulls


Standard, finely milled, full strength, 75:15:10 (potassium-nitrate:charcoal:sulfur) black powder (or Meal-D if you can buy it), plus 5% dextrin binder, is ideal for coating rice hulls.

These projects detail the ball milling of such powder:

Quick and Easy Black Powder Ball Mill

Make Black Powder the Easy Way

How to Use a Ball Mill Safely and Effectively

Ball milling your black powder is the only way I know of to make the powder fine enough to nicely coat the rice hulls, and powerful enough to make good burst charge.

Skylighter's TL5005 ball mill will make 4.2 ounces of BP/binder at a time, and Skylighter's TL5010, one gallon ball mill will handle a 21-ounce batch.

We will be coating 21 ounces of BP onto 3 ounces (dry weight) of rice hulls, resulting in a 7:1 ratio of BP to hulls. So, we'll need one of the batches from the large mill, or 5 of the batches from the small one.


Black Powder to Coat 3 Ounces of Rice Hulls


Chemical Factor Percentage 4.2 Oz. 21 Oz.
Potassium nitrate 0.75 75% 3 oz. 15 oz.
Charcoal, airfloat 0.15 15% 0.6 oz. 3 oz.
Sulfur 0.10 10% 0.4 oz. 2 oz.
Dextrin +0.05 +5% 0.2 oz. 1 oz.

Multiply the "ounce amounts" by 28.4 to obtain "gram amounts".

If you are using the small ball mill, make five of the 4.2-ounce batches to have a total of 21 ounces of milled BP.

Commercial airfloat charcoal from mixed hardwoods will work satisfactorily for BP-coated rice hulls, used as lift or burst powder. Some homemade charcoal made from "hotter" woods will work even better and more efficiently.

Weigh and ball mill your chemicals together, observing all the safety precautions in the tutorials above. Remember, finely milled BP dust is extremely flammable. Handle and store it with caution.


De-dusting the rice hulls that are to be coated.


Say what?

Our goal is to get nicely coated rice hulls, as in the photo earlier in this piece. Rice hulls come with complete, canoe-looking individual hulls, as well as lots of finer bits, pieces, and dust, too.

If you leave the fine pieces in there, they will also get coated with BP, and you'll end up with a mix of large coated hulls, as well as smaller grains, too. We don't want that.

So, take a couple handfuls of the rice hulls and bounce and shake them on a 20-mesh screen or kitchen colander. After a little sifting, the fines will have been separated from the larger, complete rice hulls. Discard the fines.

Now weigh out 3 ounces of the de-dusted hulls to be coated with BP.


(Click Image to Play Video)
De-Dusting 3 Ounces of Rice Hulls


Wetting the rice hulls


Pre-wetting the rice hulls makes it much easier to get the black powder to cling to them.

Put 3 ounces of de-dusted hulls into a plastic tub and completely cover them with hot water. Let them soak for about 5 minutes.

Pouring hot water into plastic tub with de-dusted rice hulls
Soaking De-Dusted Rice Hulls in Hot Water

Now dump the wet hulls into an old pillowcase, and sling the load 'round and 'round until no more water is coming out of it. (Do not tell your significant other that you did this. But you can tell her you found a new salad spinner, if she wonders why the pillowcase is wet.)

Pouring wet rice hulls into a pillowcase to be dried
Pouring the Wet Rice Hulls into a Pillowcase

Pour the damp hulls out of the pillowcase and into a big flat-bottom, round tub. Something about the size of this Rubbermaid cake-keeper works nicely.

Plastic rubbermaid cake conatainer
Round, Flat-Bottomed Cake Container


(Click Image to Play Video)
Damp Rice Hulls Dumped from Pillowcase into Container


Coating the damp rice hulls with black powder


Using a small paper cup, sprinkle about 5 ounces of your black powder with dextrin mix on top of the damp rice hulls.

Pouring 5 ounces of black powder with dextrin onto damp rice hulls
Adding Black Powder to Damp Rice Hulls

Put the lid tightly on the plastic container, and shake the rice-hull and black-powder contents 'round and 'round to coat the hulls with the BP.

Shaking container of damp rice hulls and black powder
Shaking the Container to Coat the BP onto the Rice Hulls

Open the container. The contents should still look pretty wet. Add another increment of the black powder, close the tub, and repeat the shaking.

Open the container, and use your gloved hand to wipe any excess BP off that's clinging to the tub bottom or walls.

The coated rice hulls will probably still look pretty wet. If so, add another increment of the black powder, and repeat the shaking.

At some point, probably after this third increment, the hulls will start to look more dark-gray than black, and the BP coating will start to look dusty, rather than solid. It's at that point that you should start spritzing on a little extra water with the fine-mist spray from a trigger garden-spray bottle.

Spray this extra water sparingly, directly onto the rice hulls--not onto the container's sides. After a few spritzes onto the hulls, shake and tumble the whole mass to expose un-spritzed hulls, then spritz again.

Wetting black powder coated rice hulls with a spray bottle of water
Spritzing More Water onto the BP-Coated Rice Hulls

Seal the container and shake the hulls to disperse the water throughout the mass of hulls.

Open the container, add another increment of BP, close and shake the tub.

Repeat this process of:
  • Spritzing on a little more water
  • Sealing and shaking the container to distribute the water
  • Adding another increment of black powder
  • Sealing and shaking the container to coat the hulls with BP
Do this until all the black powder has been added to the rice hulls.

All of this takes some practice and getting a feel for it, but it's really pretty simple and foolproof. Just make sure you don't add too much water. Spritz on just enough moisture to keep the BP-coated hulls black-looking, and to get all the BP to adhere to the surface of the hulls.

Once you have added all the black powder to the mass of rice hulls, spritz on a little more water so that the rice hulls each have a nice, solid, black-looking, non-dusty coating of BP. Tumble the hulls between spritzes of the water to completely distribute the water onto all the hulls.


(Click Image to Play Video)
All the Black Powder Coated onto the Rice Hulls


Drying the BP-coated rice hulls


Spread the black-powder-coated rice hulls out onto a drying screen where air can get to them from above and below. If you don't have a drying screen, newspapers will work, too. Airflow is what's most important.


(Click Image to Play Video)
Spreading the Coated Rice Hulls onto a Drying Screen

Place the loaded screen into a drying box, or in a dry, warm, breezy location. Depending on the temperature and humidity, the hulls will be dry and ready to use in a day or two. When they are dry, the coated hulls should feel hard and crispy--obviously dry.

Store the dry hulls in a sealed container in safe storage or a magazine.


Applying slow-flash-booster to dry BP-coated rice hulls


For many aerial shells, especially ones I want to burst hard, like round peony or chrysanthemum shells, I like to apply slow-flash booster to the dry BP-coated rice hulls. This booster heats up and speeds up the black powder burst charge, resulting in larger, rounder starbursts. But, it does not produce a distracting bright flash when the shell bursts.

The booster is never added to BP-hulls that are to be used as lift powder, though. That could easily result in blown-up mortars.

The slow-flash booster consists of 2:1:1, potassium-nitrate:bright-flake-aluminum:sulfur. 325-mesh bright flake aluminum (Skylighter CH0178) works well in it. The potassium nitrate should be finely milled by itself in a blade-type coffee-mill or individual-serving blender.


Slow Flash Booster


Chemical Factor % 4 Ounces
Potassium nitrate 0.5 50% 2 oz.
325-mesh aluminum 0.25 25% 1 oz.
Sulfur 0.25 25% 1 oz.

After milling the potassium nitrate until it is nice and fine, weigh out the correct amounts of each chemical, and place them onto a sheet of paper.

Roll the chemicals together, back and forth on the paper, until they are nicely mixed. This is called "diaper mixing."

Slow flash burst charge booster powder
Diaper-Mixing Slow-Flash-Booster Powder

Diaper mixing on paper is considered one of the safest ways to mix flash powder. It has low friction, low-no static electricity and you are not in direct contact with the mix.

Pour the mixed booster into a container, and keep it tightly closed in safe storage until it is used.

The booster is applied by dusting onto dry BP-coated rice hulls, adding 2% of the rice hulls' weight of the booster.

So, if you want to use all 24 ounces of the BP-coated hulls you just made, add 0.48 ounce of the booster to that batch of hulls. I'd just round that off to 0.5 ounce.

To coat dry rice hulls with slow-flash booster, simply add 2% by weight of the booster to the 24 ounces of hulls in a container. Close the container tightly and gently rotate it to get the dry hulls to pick up all of the booster powder on their surface.

To determine what 2% of a specific weight of rice hulls is, multiply the weight of the hulls by 0.02. For example, 2% of 8 ounces of coated rice hulls is 0.02 X 8 = 0.16 ounce of the booster to be added.


How much of the coated rice hulls should you use in different size aerial ball shells?


Once again, from Dr. Shimizu's "FAST," we get some approximate figures:

  • 4" shell, 3.5 ounces of hulls
  • 5" shell, 5 ounces of hulls
  • 6" shell, 10 ounces
  • 8" shell, 27 ounces
  • 10" shell, 70 ounces
  • 12" shell, 85 ounces


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