Willow Shell (4")

From PyroGuide

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A Weeping Willow shell is one of the most beautiful and complementing type of shells to any fireworks display, either professional or amateur. The only drawback is the extended amount of time each shell requires for completeing by the average pyrotechnician. This tutorial will show how these stars are made and how this type of shell is assembled.

[edit] Materials

Stars - We'll be making willow stars in this tutorial for this shell.
Shell hemispheres, 4" - Either paper or plastic - I prefer plastic
Time fuse or Spolette - 1/4" time fuse will work well
Break charge - I use meal-caoted corn cob
Lift charge - Pulverone Black Powder for lifting the shell
Hot glue gun - for sealing the fuse in
Plastic drinking straw - for fusing

[edit] Formula

I'll be using one of my favorite willow formulas, off of the Willow Stars page, #2:

Willow #2

Potassium perchlorate	30
Lampblack	30
Charcoal, Pine Airfloat	25
Potassium nitrate	10
Dextrin	5
Shellac	5

Note that any other willow formula can be used, so long as it gives a long lasting full tail. A standard chrysanthemum composition will not work as well because of its lack of lampblack and therefore short-lived tail.

[edit] Making Stars

The Willow Stars page highlights in more detail how to make willow stars, but the basics will be covered here.

Mixing the Composition
Weigh out all ingredients, and make sure they are very fine before seiving together gently. If any chemical is coarse, for example Potassium nitrate, grind the ingredient well in a coffee grinder or ball mill before weighing out. Do not grind oxidizers and fuels together except for potassium nitrate and Charcoal. Do not mill lampblack with any ingredient. It is in most cases already exceedingly fine and will stick to everything it touches and is very difficult to get off.

Wetting the Composition
Once mixed thoroughly, the composition must be wet with a binder solvent. Here we used dextrin and shellac, so you either have to wet with water/alcohol mix to activate the dextrin, or with ethanol or acetone to activate the shellac. I suggest using the water because of the hazards associated with acetone. If you're more comfortable with it, you can substitute the shellac with Red gum, in a ratio of 2:1 shellac to red gum. This will increase the burn time of the stars however, and should be dealt with accordingly. Red gum is not as quickly consumed as shellac, and will therefore require less to achieve the fuel level needed. As a binder, red gum will work very well to bind this type of composition. Use 70-30 water/alcohol to wet this composition.

Pressing the Composition
Once wet, the composition must be pressed through a screen Pulverone style, and let to dry for at least 24 hours in low humidity. 48 hours would be better in moderate humidity. Once dry, the composition needs to be broken back up into a fine powder. A Mortar and pestle works well for this, since the composition is still pretty soft. Once ground back to a powder, it is time to wet the composition again and press it into stars. Your star pump should be the correct size as indicated by the chart below on star sizing.

Shell inch(mm)	Burst Height feet(m)	Star Volume (inch sq)	Star size (inch)
3 (76)	400 (120)	1.534	1" x 1"
4 (102)	500 (150)	1.534	1-1/4" x 1-1/4"
6 (152)	700 (210)	2.651	1-1/2" x 1-1/2"
8 (203)	900 (270)	3.370	1-1/2" x 1-1/2"
10 (254)	1050 (320)	3.370	1-5/8" x 1-5/8"
12 (305)	1150 (350)	4.209	1-3/4" x 1-3/4"

Note that these star sizes are approximations based on several types of compositions as a whole. Different compositions and pumping pressure may increase or decrease burning time, requiring testing to be done before firing. The easiest and safest way to do this is to wet a small amount of composition and press stars or a single star out of it. When dry, you'll have to shoot it and calculate burn time in accordance with the size of your shell. Do not skip this important step as a too long burning star can cause forest and brush fires.