You will often see chemicals in fireworks formulas that look like these:
Aluminum, -325 mesh
Aluminum, -325 mesh, spherical, 22 micron
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 through 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.