Skylighter Fireworks Tips Newsletter
November 20, 2007 -- Issue #87

HOW PARTICLE SIZE & SHAPE IS DEFINED

You will often see chemicals in fireworks formulas that look like these:


Do you really know what those particle sizes really mean? What is really being described? When they say "-325 mesh" and "22 micron", what's the difference? And why does it matter to you?

Well it can definitely help you to know how the particle "size" ratings get assigned to metal powders. Most of the size ratings come directly from the wholesaler or manufacturer. But every so often we buy surplus materials which may not come with any additional information about the manufacturer, the size or shape of the powder. Recently, we received a surplus lot of magnesium powder, including several drums with almost no information available from the seller. Before we can sell it to you, we need to be able to tell you what it is, so you can figure out if it suits your purposes.

The first step in the identification process is a visual inspection. You may be surprised how much you can tell about a sample just by looking at it. By observing the flow characteristics of a powder, and how it feels between your fingers, you can approximate particle size and shape. If you have experience with metal powders, for instance, you can often tell if a sample is granular (rough feeling), or atomized (round particles, feels smooth, pours and flows quickly and smoothly). If you cannot feel any particles between your fingers, you can assume the powder is probably finer than 200 mesh, or even less than 325 mesh (written as "-325 mesh.")

The next step is to verify those assumptions though quantitative and qualitative testing.

To determine if a material is appropriate to be used in a given formula you'll need to know the particle's shape (morphology), size, and distribution (granulometry). Shape is easily determined under a microscope and classified as atomized (spherical or spheroidal), granular, or flake.



Particle size is reported in one of two ways: either by mesh size (large and medium particles, generally larger than 325 mesh) or by microns (very small particles).

Why use two measurements?

US mesh size describes the number of openings per inch in a screen. So if a material is listed as -60 mesh it will all pass though a 60 mesh screen (the minus sign in front of the 60 means that all particles are smaller then 60 mesh). Conversely, if the material is described as +60 mesh, it would mean that all particles would be retained on a 60 mesh screen and are therefore larger than 60 mesh.

But mesh sizes can only go so far. After a point the individual wires that make up the screen are so close together it is no longer practical to measure using screens. In practice, particles smaller than 325 mesh are usually described in microns. A micron is one thousandth of a millimeter, or one millionth of a meter. The unaided human eye can see particles of about 40 microns. Smaller than that, you need magnification.

There is no truly accurate conversion from mesh size to microns, because the wire thicknesses in screens vary all over the place. But approximate conversion tables are commonly used anyway. (In the table below, screen sizes of smaller than 600 mesh are shown, even though they don't exist in practice.)

U.S. MESH MICRONS 10 2000 20 841 40 400 60 250 80 177 100 149 200 74 325 44 400 37 625 20 1250 10 2500 5

More detailed conversion charts are available at: http://www.skylighter.com/fireworks/help/ Mesh_to_Micron_Conversion_Chart.asp

"Mass fraction analysis" is used to determine large-to-medium size particle distribution in a sample. The powder is sifted through a set of nesting screens, each with progressively smaller openings (higher mesh numbers). By measuring the percent of material that remains on each screen, we can classify a material by its size distribution.

If you were to sift Skylighter's #CH2080 Magnesium-Aluminum (described as 180-325 mesh) through a stack of 180 mesh, 200 mesh, and 325 mesh screens, a mass fraction analysis yields a particle size range that looks like this:


If the 180 mesh size was critical to your firework formula, you can interpret this to mean that 26% would remain on the 180 mesh screen (larger then 180 mesh) and 74% would pass through it (be smaller than 180 mesh).

Mass fraction by sieve analysis is a very helpful method of classifying coarse-to-medium particles, but what about the really small stuff?

When the average particle size is around 50 microns, sieve analysis is no longer practical, and doesn't adequately describe the particle sizes. Several methods are commonly used to measure really fine stuff: gravitational sedimentation, laser light diffraction, optical light microscopes, scanning electron microscopes (SEM) and transmission electron microscopes (TEM). The most accessible method to an amateur is an optical light microscope.

So how is a particle measured with a microscope? Do you need some kind of tiny ruler? As funny as that might sound, that's exactly how it's done. The microscope can be fitted with a gizmo called a reticule micrometer. After it is calibrated, it can be used to measure the size of individual particles in a powder sample right down to 1 micron.

But just because you can measure it doesn't mean it's a simple task.

Sure, measuring spherical material is fairly straightforward. After all, you're really just measuring the diameter of little balls. But what about flake, granular, and spheroidal samples? Digital imaging and software can drastically decrease the time needed to perform measurements and reduce error rates. But it appears that most if not all of the automated equipment measures any particle shape as if it is spherical. Because of this, there is not really a standard method for assigning a particle size.

Selecting the method seems to be based mostly on what you'd like your results to state. Below is an imaginary particle and three circles representing different measurement methodologies.



In the first example the measurement is across the smallest dimension of the particle. This method might be used to describe the particle in terms of its reactivity by describing the particle in the smallest possible size. Method B might be used conversely-to describe the particle's largest dimension. Arguably the most accurate methodology would be using example C, where an average size is calculated.

No matter what method is used, the results would normally be presented to you, the buyer, as an average size (3 micron), a particle range (3 to 15 micron) or a frequency distribution (30% <5 micron, 10% 5-10 micron, 60% 10-15 micron), or some variation thereof.

So why does particle size or shape matter?

Many amateur fireworks makers only consider particle shape and size when a formula calls for a specific material. Even fewer consider particle size distributions. The shape and size of a particle has a huge impact on its reactivity. Flake particles have a large surface area that can be in contact with an oxidizer when compared with a spherical particle. Granular particles often have sharp edges that can ignite more easily than the smooth, round edges of an atomized powder.

Selecting powder with a different particle size or shape can create a wide variety of changes in the pyrotechnic effect, from hang time of a spark to delay of strobing. Even controlling the burn time can be accomplished by altering the particle size and shape.

Look what happens when we change particles in a real example.

The glitter formula below calls for -325 mesh spherical aluminum. Skylighter sells 3 aluminums that are -325 mesh spherical. One is further described as 5 micron (CH0100), one is 12 micron (CH0103) and another is 22 micron (CH0105).

D1 Glitter Formula
Chemical	Percent
Potassium Nitrate	53%
Sulfur	18%
Charcoal (airfloat)	11%
Aluminum (-325 mesh, spherical)	7%
Sodium Bicarbonate	7%
Dextrin	4%

Using the 5 micron aluminum did not produce a usable glitter. Instead it produced a bright star with an unattractive, dense, short-lived flitter-like tail. This aluminum was simply too reactive and started burning both in the flame envelope as well as after, creating poorly defined flashes.

The 12 micron aluminum produced a wonderfully dense, but short tail of fairly evenly-spaced flashes. Because the particle size distribution was within a fairly small range (mostly 6-18 microns), the glitter effect appeared fairly closely behind the star.

The 22 micron produced the best effect of all, creating a long tail that maintained good distribution of flashes over its entire length (with a few long delay pops). The 22 micron contains particles over a very wide range with most particles appearing between 5 to 38 micron.

It is clear from the results of the test above that tracking the average particle size and shape may not be enough to reproduce a specific effect, tracking the particle size distribution (if you know it) may also be worth noting in your formula book.

Brian Paonessa
Skylighter, Inc.

For more information see:

Mesh Sizes and Microns
Skylighter Fireworks Tips
November 26, 2002 -- Issue #44


Metal Particle Shapes: What They Mean
Skylighter Fireworks Tips Newsletter
December 2, 2005 -- Issue #68



Skylighter has two liquid flame coloring products back in stock! These chemicals are great for coloring campfires throughout the long winter months, making colored liquid fires and ghost mines. Don't know what a ghost mine is? Read how to make these beautiful colored fireballs: Ghost Mines Project.

Potassium Iodide. White caked crystals. Potassium Iodide is used to make colored liquid fires and ghost mines. Potassium Iodide makes a purple-to-violet colored flame.
Potassium Iodide #ZCH8225 $15.70 per pound

Lithium Chloride. A white, granular powder. Lithium Chloride is most frequently used to make colored liquid fires, ghost mines or for coloring the flames in fireplaces this time of the year (along with the ever-popular copper chloride). Lithium chloride makes a red-to-magenta colored flame.
Lithium Chloride #CH8178 $19.36 per pound



Thanksgiving is only a few days away, a day when everyone becomes gluttons and some of us remember what we're thankful for. Here at Skylighter we're always thankful for fireworks and the pyro community so we thought we'd add something to your list of things to be thankful for, a 25% off sale!

Thanksgiving is only a few days away, a day when everyone becomes a glutton and some of us remember what we're thankful for. Here at Skylighter we're always thankful for fireworks and the pyro community. So we thought we'd add something to your list of things to be thankful for, a 25% off sale!

This holiday, don't just lounge around on your couch waiting to eat your own weight in food. Go online and order some pyro supplies! Order something special for your pyro friends for Christmas or get your supplies for those shells you've been meaning to build for New Years. But my advice is to order immediately. There are two good reasons:

And they are the two bonus items that we only have a limited supply of.

Bonus #1 is a set of ten, 3-inch ID paper mortar tubes and bases worth $45.47. These tubes have a slightly thinner wall thickness than the 3" paper mortars we sell. They'd make fantastic mines. Think the flying fish fuse mine kits are fun? These suckers are more than 3 times the size of those tubes, 3 times the size 3 times the fun! We only have 48 set of these available. The first 48 takers are gonna get them. After that, you're outa luck.

Bonus #2 is for the first 30 customers to order before midnight, Thursday, November 22, 2007. They will get a second bonus: 2 lbs. of Ferro-Titanium worth $45.96.

This isn't one of the ferro-ti's we normally carry, this is a special item, very coarse, and great for long hang time sparks. We only have 30 containers and once we're out, we're out.

So don't miss this chance to get more than just a massive turkey ingestion coma this Thanksgiving. Head over to Skylighter right now to get 25% off everything and pick up 90 bucks worth of bonus freebies when you do.

But be sure and read the whole special offer and fine print for the 25% Off Everything special at http://www.skylighter.com/offers/29.