MAKING AN 8” DOUBLE-PETALLED
AERIAL FIREWORKS SHELL
By Ned Gorski
Introduction
In the above photo, a double-petalled shell is illustrated. This pattern is created when two spherical courses, or layers, of stars within the shell display concentrically.
I like to think of any shell as a pattern shell. Essentially, in a well constructed shell, the pattern that is displayed in the sky is an enlarged version of the pattern the stars formed inside the shell.
So, if I want two concentric spheres of stars to display in the sky, one smaller than the other, I need to construct the shell so that there are two concentric spheres of stars inside the shell, and in the same relative size proportions that I want to see in the starburst.
I have seen several different methods of multiple-petal shell construction demonstrated.
In Fireworks, the Art, Science and Technique, Dr Shimizu illustrates a traditional Japanese method of constructing these shells. The inner petals (IP’s) are constructed with the stars held inside string-wrapped, tissue-paper spheres. Between each layer of stars is a layer of burst charge. The outer petal (OP) stars are held within a typical ball shell casing.
I have not tried this method due to its difficulty and complexity.
One master ball-shell maker in the US, Jim Widmann, has written about techniques he has used in the past for the manufacture of this type of shell. He has used inner petal shell casings which are similar to the outer casing, except that the inner ones are perforated with hundreds of ¼” holes to allow the burst gasses to permeate the whole shell before it bursts. Jim has spiked these inner petal casings with string during construction.
I have built shells using this method, sometimes with excellent results, but there has been a frustrating inconsistency of these results for me.
A simple method of multiple-petal construction is employed by others, and is often seen if Oriental shells of this type are carefully “dissected”. This technique can be seen on Pasfire.com in two articles: Jeff Doty’s Atomic Pattern Shell how-to, and in the 10” Double Petal Shell Autopsy.
It is this last, simple, method which I want to try out, and which I will describe in this article.
The Stars for the Shell
For the shell I’ll be building in this project, I’ve prepared two different types of stars in advance.
My outer petal stars will be ¾” pumped and primed, Winokur 13, gold-glitter stars. The 39 different glitter formulae, that you’ll see referred to as I did above, are found in Dr. Robert Winokur’s treatise in Pyrotechnica Number Two, The Pyrotechnic Phenomenon of Glitter.
I like to start with a homemade, black powder meal as a base for most glitter compositions. Therefore I’ve modified the original formula a bit as follows:
Winokur 13 Gold Glitter80 ounce batch
0.6homemade BP meal48 ounces
0.05potassium nitrate4 ounces
0.03sulfur2.4 ounces
0.1antimony trisulfide8 ounces
0.09 sodium bicarbonate (baking soda)7.2 ounces
0.06atomized aluminum 4.8 ounces
0.04dextrin3.2 ounces
0.03Magnalium (200 mesh)2.4 ounces
(I used dark-pyro antimony trisulfide, but Chinese-needle may be used. I used the 22 micron atomized aluminum from Skylighter.
To make a 48 ounce batch of my homemade black powder meal, I simply screen 36 ounces of KNO3, 7.2 ounces of airfloat charcoal, and 4.8 ounces of sulfur, through a 100 mesh screen. Then I add enough denatured alcohol to make a damp putty ball, and screen the putty through a ¼” screen onto kraft paper. I allow the granules to dry and then screen them through a 12 mesh screen or a similar mesh screen, kitchen colander. I end up with a meal which ranges from fine dust through 12 mesh granules.
I then screen the rest of the glitter ingredients individually through a 40 mesh screen and mix them together with the homemade meal. I shake the comp in a bucket with a lid to thoroughly integrate it, and then I spray on 5% water (4 ounces for this 80 ounce batch) with a trigger garden sprayer until the moisture is thoroughly mixed with the dry ingredients.
The gold glitter stars were then pressed with my ¾” star plate and hydraulic press, as described in Skylighter Fireworks Tips #92, and they were primed as described in #93.
For my inner petal, I have some 3/8” red stars from another project which I will use. These stars were rolled and sized very precisely before they were primed. This precise sizing helps to insure that a petal will appear round in the sky, and that all the stars burn out at the same time which keeps that round appearance through the end of the “flowers” display.
¾” Gold Glitter Outer-Petal Stars and 3/8” Red Inner-Petal Stars
Constructing the Shell
I have previously described my ball shell construction techniques in Skylighter Fireworks Tips, #’s 93 and 94, Parts 3 & 4 of Nice Shells in 2 ½ Days.
Due to space considerations, I won’t go into as much detail concerning those basic construction techniques here. I would simply refer the reader to those articles for a more in-depth discussion of those methods.
In this article I want to focus on the details that are unique to the construction of this double-petal shell.
I pound a spolette time fuse with enough powder to give me a delay of 4 seconds as the fuse burns. I glue this fuse into a ½” hole that I’ve punched into one of the 8” casing hemispheres, and I also glue a passfire tube into the inside of the shell. This tube has blackmatch in it which is nosed with masking tape and tied with string.
Then I install the outer-petal glitter stars, hot-gluing the rings of stars in at the equators of each hemi, and then loosely filling in the rest of the stars.
I then line these stars with a layer of tissue paper, and start to fill the hemis with burst powder. For this shell I’m using black powder on rice hulls for the burst. I’ve tumbled the coated hulls in dry slow-flash booster, which I’ve detailed in the 4” Plastic Ball Shell article in Fireworks Tips #99. I am only using the amount of booster that will cling to the hulls once they’ve been tumbled in it.
Now I cover a 4” shell casing hemisphere with tissue paper, and begin to use it as a form with which to form a depression in the burst powder. I fill and compact the hulls below the former until it is sitting with its upper edge even with the upper edge of the 8” casing. It is very important that the rice hulls be packed as tightly as possible below and around this depression-former.
I’m very careful to keep the 4” hemi centered within the 8” hemi so that when the two halves are mated, the inner-petal stars align into a perfect sphere.
Once I have really worked the hulls tightly into place with my fingers, and the burst is full up to the level of the tops of the casings, I carefully unfold the tissue paper inside the 4” hemi. I press this tissue paper open flat against the rice hull burst. Then I carefully remove the 4” former.
When doing these steps with the 8” hemi that contains the passfire tube, I use a 4” former which has a hole punched in it just large enough to allow it to fit down around the passfire tube.
I then take these two shell halves outdoors and spray the tissue-lined depressions with a quick-drying, clear, spray-lacquer. I cover the passfire tube and blackmatch with a plastic baggie while doing this to prevent it from being “painted”. This lacquer, once it dries, stiffens the tissue paper into a semi-rigid casing.
Now I line these tissue depressions with stars just as I would a regular shell casing. I hot-glue the equatorial stars in, and then fill the rest of stars in loosely and carefully.
These layers of inner-petal stars are then lined with a layer of tissue paper, and these voids are tightly filled with more of the burst powder. I carefully fill them until they are filled to just slightly above the plane of the edge of the 8” casing.
The goal here is to have everything just slightly overfilled so that when the two shell halves are mated and joined, the “innards” become to tightly packed that there is absolutely no rattling when the shell is shaken.
Now I carefully clip off the excess tissue paper in each layer with scissors, avoiding the burst powder as I do so, and I fold and compress each of the three layers inward.
Then I hot-glue a tissue paper disc to the outer petal stars in order to cover the whole shebang.
Now I have two of these beautiful, tightly packed and sealed hemispheres, ready to be joined. I gently flip one hemi over onto the other and use strap clamps to bring the two halves together as I tap on them with the handle of my rawhide mallet. I seal the equator with strips of masking tape once they two halves are really mating up well.
Then it’s just a matter of pasting the shell, and lifting-and leadering it, as described in the articles cited above.
With this particular shell I used 1.25” x 35 lb, gummed, kraft-paper, tape to wrap the shell, instead of pasted paper strips.
Completed Double-Petal Aerial Fireworks Shell
Puttin’ Her up in the Air
Here’s a shot of the shell displaying in the air.