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Separating Metal Particle Mesh Sizes for Making Fireworks


By Ned Gorski




Introduction


Often, in making fireworks stars, comets, fountains, rocket tails, etc. we can get the best spark effects when we use a fairly precise range of particle sizes.

But the metal powders we have available to us don't always match up to the particle size range we need. So, here is some information about how to screen and sort metal powders.

Two previously published Skylighter "Fireworks Tips" articles serve as a good foundation for this one:
These articles provide an in-depth exploration of metal-particle shapes, sizes, sorting, and uses, with references for follow-up study.

I won't repeat all the information in those articles, but will hit some highlights from them, and then expand on some important ideas.

This essay was inspired when I received a shipment of Skylighter CH3009 spherical titanium:

CH3009

Titanium, spherical, -40+300 mesh
[Ti-Al] silver spheres
94% titanium, 6% aluminum.

Approximate Particle Size Breakdown:
-40+100 mesh = 65%
-100-300 mesh = 35%

This is an industrial byproduct. May contain very small amount of flakes, needles, and slight amount of foreign matter; but will not affect its performance.

Produces bright white sparks. Works well in fountains, rockets, stars, comets, saxons and drivers. Mix this with larger particle sizes if you want long comet tails.

As described, this titanium is an "industrial byproduct." Skylighter occasionally purchases surplus materials such as this metal, which do not come with a lot of accompanying information. Such products are often not precisely sorted by particle size.

If there is a mixture of large and small particles in a drum of metal, they will tend to separate during shipping and handling. The larger particles will "float" to the top of the container, and the smaller particles will migrate to the lower part of the drum.

Since there is no cost-effective way to "remix" the particle sizes to achieve a consistent particle size distribution, these surplus metals often arrive to us hobbyists in a somewhat unpredictable range of different particle sizes.

When I opened my new tub of the spherical titanium, I could easily see there was indeed a mixture of large and small metal particles. Almost all of the particles looked like small balls, as would be expected in a spherical-shaped metal.

There were some large particles, what looked to be between 1/32-inch and 1/16-inch. Many of particles were smaller than that; in about the size-range of grains of salt. And some of the metal bits looked very small and almost "powdery."

-40+300 mesh spherical titanium from Skylighter.com
Skylighter CH3009 Spherical Titanium

As I said, this is not uncommon or unexpected, but it made me pause and ponder exactly what uses I could put this titanium to.


Metal-particle shapes and sizes


The metal powders we use in pyrotechnics are typically identified by the element or alloy they consist of, such as aluminum, titanium, ferro-titanium (an alloy of iron and titanium) or magnesium-aluminum (also called mag-al, and magnalium).

The shape of the particles is also usually identified: flakes, granules, atomized-spheroidal, atomized-spherical, or sponge (particles which actually look like small, hole-filled sponges).

spherical, spheroidal, granular and flake particle shapes
Different Metal-Particle Shapes (Sponge not Shown)

And the range of particle sizes present in the metal powder is also specified. This range can be listed as mesh-sizes, fractions of an inch, microns, or millimeters.

Of course, other chemicals such as charcoal or oxidizers may be classified by particle size in the exact same way the metal particles we are describing are.

This chart is from the "Fireworks Info Charts and Tables" on the Skylighter.com website:


Mesh to Micron Conversion Chart


U.S. MESH INCHES MICRONS MILLIMETERS
3 0.2650 6730 6.730
4 0.1870 4760 4.760
5 0.1570 4000 4.000
6 0.1320 3360 3.360
7 0.1110 2830 2.830
8 0.0937 2380 2.380
10 0.0787 2000 2.000
12 0.0661 1680 1.680
14 0.0555 1410 1.410
16 0.0469 1190 1.190
18 0.0394 1000 1.000
20 0.0331 841 0.841
25 0.0280 707 0.707
30 0.0232 595 0.595
35 0.0197 500 0.500
40 0.0165 400 0.400
45 0.0138 354 0.354
50 0.0117 297 0.297
60 0.0098 250 0.250
70 0.0083 210 0.210
80 0.0070 177 0.177
100 0.0059 149 0.149
120 0.0049 125 0.125
140 0.0041 105 0.105
170 0.0035 88 0.088
200 0.0029 74 0.074
230 0.0024 63 0.063
270 0.0021 53 0.053
325 0.0017 44 0.044
400 0.0015 37 0.037
625 0.0008 20 0.020
1250 0.0004 10 0.010
2500 0.0002 5 0.005

Keep in mind that when we say "mesh size," what we are referring to is the count of the number of vertical wires in one inch of screen.

Referring to the chart, you can see that a 4-mesh particle, which would just fit through a 4-mesh screen (4 wires-per-inch running each direction), would measure 0.187 inches, 4760 microns, or 4.76 millimeters. That is the size of the openings in that screen.

The dimensions are simply conversions of each other: there are 25.4 millimeters in one inch, and there are 1000 microns in a millimeter.

It might seem at first that the particles that just fit through the 4-mesh screen ought to measure 0.25-inches, since there is a wire every 1/4-inch in the screen through which they are being sorted. But the wires take up some space, making the spaces in the screen a little smaller (0.187-inch square actually) instead of 1/4 -inch square.

Some interesting points of reference are that:
  • A typical grain of salt is about 50-mesh, 0.0117-inch in size.

  • The shaft of a standard straight-pin would just fit into a hole in a 24-mesh screen (0.025-inch).

  • The head of that straight pin would fit nicely in the holes of a 12-14 mesh screen (0.07-inch).

  • An average human hair would be about the same size as the holes in a 200-mesh screen.
Combining all the descriptions of a metal powder, type of metal, particle shape, and size of particles, a typical description might look like:

Aluminum, Atomized-spheroidal, 325-mesh

Very often a range of particle sizes is specified for a particular metal powder, such as the 65% of the CH3009 that is specified as -80+100 in the above product listing.

What that means is that 65% of the metal particles will pass through an 80 mesh screen (-80), but will be retained on a 100-mesh screen (+100). This is a valuable distinction to remember: a (-) sign means "passing through," and a (+) sign indicates "being retained on," or not passing through.

Metal particles retained by screen sorting
Metal Particles Retained-On (+) and Passing-Through (-)
a Sorting Screen

When you think about it, this sort of "size range" makes sense. But it would be pretty difficult to sort out only 4-mesh particles, for example, using only one size screen. One would have to visually verify that only particles which just snugly fit through that 4-mesh sorting screen would be set aside.

But, if 3-mesh and 5-mesh screens were used, only particles which would pass through the 3-mesh screen would be set aside (-3). Then those particles could be sifted on the 5-mesh screen, and only the particles which would not pass through that screen (+5) could be isolated. This batch of particles would then be designated as -3+5, with an average particle size of 4-mesh, which is what we were shooting for.

Do you see how that works?

This type of screen-sorting is only practical for particle sizes down to about 325-mesh. But it is impractical to try to make sorting screens with more than about 325 wires per inch. Heck, I don't know how in the world they make screens with wires that fine and which still have a gap between them.

Below that 325-mesh size, the particles have to be sorted by more advanced techniques, such as measurement with a microscope. For that reason these metals usually have their average particle size listed in microns, rather than a mesh-size-range.

12-micron or 20-micron atomized aluminum is often seen specified. You can see from the chart that those sizes would be down in a hypothetical 625-1250-mesh size range.


Uses for various metal-particle sizes


Often, in older texts such as Weingart's Pyrotechnics, formulas for pyrotechnic compositions might list "mixed aluminum," "granulated charcoal," or "fine charcoal," for example. Only experimentation with various mesh-sizes can dial-in the correct mesh-size of these ingredients to achieve the desired effect.

But most modern formulas recommend actual mesh-sizes for the ingredients, where there may be some different options available. We might see the admonition "all ingredients able to pass 40-mesh screen" for example (-40-mesh). And many glitter formulas specify 325-mesh atomized (spherical or spheroidal) aluminum.

In general, the finer metal powders, such as 325-mesh atomized aluminum or -200-mesh magnalium, come with a fairly homogenous range of particle sizes which are assumed to be accurate by the pyro-hobbyist. Most of us do not have 200-mesh or 325-mesh screens with which to sort or verify particle sizes. And even if we do, the task of screening such fine powders is not practical for us to do manually.

These fine metal powders are typically used as fuels in glitter formulas or colored-star compositions, and we simply experiment with the available mesh sizes until the desired effect is achieved.

But, there are coarser particle-size metals available with which to achieve metal-spark effects: coarse ferro-titanium, aluminum, magnalium, steel, iron, or titanium are good examples. These metals are the ones most likely to be described as "surplus" or "industrial byproducts." And it is with these that we may very well encounter the ambiguous particle sizes and size ranges I was describing earlier.

Some devices, such as this Willow-Diadem Star formula, specify a broad range of metal particle sizes. A lot of the unique effect this star produces is due to the broad range of metal-particle sizes, as well as the different types of metal included in it.

Willow Diadem Star

0.39 charcoal airfloat

0.31 potassium nitrate

0.11 sulfur

0.07 dextrin

0.045 ferro-titanium, -30+60 mesh

0.045 ferro-titanium, -40+325 mesh

0.03 titanium sponge, -40+80 mesh

Different metal types, such as ferro-aluminum, as well as different mesh-sizes, could be interchanged and experimented with in this formula.

  • Metal mesh size can be dependent on device application
Some small devices, such as small "Fairy Fountains," small stars, or mini-rockets, would probably specify fine metal powders, typically in the -80-mesh size range.

Medium sized stars, rockets, fountains, or comets would work well with metal particles in a medium-sized range, say -20+40-mesh.

And large salutes, stars, or comets could use metal particles as large as +20-mesh up through +10-mesh. One of the types of sponge titanium that occasionally hits the market is called "Grape Nuts Ti" because it so resembles the coarse grains of that cereal.

So, the specific device in which we plan to use a metal might require a specific mesh-size-range of that metal. This would lead me to sorting and separating the mesh sizes of a metal such as the CH3009 spherical titanium I received.

  • Metal mesh size dependent on mechanical process used in device manufacture.
For some fireworks manufacturing processes, for example when pumping stars or comets, it is necessary to avoid certain metal mesh-sizes.

When using comet pumps or star plates, some metal particle sizes will work just fine, but other sizes will bind the pump, jamming the parts together and damaging the surface of the tools.

Comet and star plate pumped stars
Comet and Stars, Made with Comet-Pump and Star-Plate

It's not too hard to imagine that the "wrong size" metal particles could jam between the piston and the cylinder of such tools, damaging the tool surfaces, and causing the moving parts to jam together.

So, it's important to determine what size metal particles can be used with a particular pump, and what size to avoid.

safely pumping stars with metal particles
Safe and Unsafe Metal Particle Sizes Used with Star-Pump

Looking at the sketch above, we can see that the pump's piston has a diameter-clearance of "D" between it and the pump's cylinder. When the piston is centered in the cylinder, there is a clearance of 1/2xD all around it.

Note: The sizes of the clearances shown in the sketches are greatly exaggerated.

It should be obvious that metal particles which are as large as 1.5xD or larger cannot wedge between the piston and the cylinder. So they would be safe to use. There is a little safety margin figured into this rule, of course. A metal particle can be slightly smaller than 1.5xD and still be safe to use, but if it starts to get down in size near D, it will have a tendency to bind the pump.

Rule 1. Metal particles 1.5xD or larger are safe to use with a particular star pump.

If the metal particles start to get smaller than that, though, as with the particle that is the same size as D, they can certainly wedge between the piston and cylinder, binding the pump and damaging its surface.

The same is true for particles as they continue down in size to the 1/2xD particles shown. Those particles can accumulate together to bind the pump.

It's only when particles get down to the ones that are about 1/4xD that there is once again no danger of them binding and damaging the pump, even if they are dispersed all around the perimeter of the piston.

Rule 2. Metal particles 1/4xD and smaller are safe to use with a particular star pump.

Once again, there's a little safety margin built into this rule. A particle slightly larger than 1/4xD can be safe to use in the pump, but when it starts to approach 1/2xD in size, it can start to damage the pump.

So, it's metal particles with sizes between 1/4xD and 1.5xD that we want to avoid using with the star pump.

Rule 3. With a star-pump that has a diameter-clearance of D, do not use metal particles with sizes between 1/4xD and 1.5xD
(1/4xD < metal particles to avoid < 1.5xD)


So, when using a star pump, these rules are the way to determine which size metal particles are appropriate to use in this mechanical process.


Determining the diameter-clearance "D" for a particular pump or star-plate


Obviously, in order to apply these metal-particle-size rules for a particular tool, we need to determine the clearance "D" for that pump or plate. There is an easy way to do that, which does not require any complicated measuring tools.

Fortunately, standard printer paper is typically 0.004-inch thick. This can be verified by measuring the thickness of a 500-sheet ream of the paper. A ream of the standard printer paper measures 2-inches thick. 2 divided by 500 equals 0.004, so each sheet is 0.004-inch thick. This can be verified for a particular sheet of paper by measuring its thickness with a micrometer.

thickness of paper
Measuring the Thickness of One Sheet of Paper, and 500 Sheets

1/4-inch-wide strips of the paper can now be used as feeler-gauges to determine the diameter-clearance of a star pump. Will one layer of the paper fit between the pump and the cylinder? How about two layers?

using paper to measure clearance around star pump
Measuring Pump Diameter-Clearance (D) with Paper Strips

You can see that my large brass pump has a diameter clearance of one strip of paper: D = 0.004 inch. The large coated-aluminum pump has a clearance of two paper strips: D = 0.008 inch. And 8 strips of paper will fit into the largest gap in my economy 1/2-inch star-plate: D = 0.032 inch.

I have quite a few different comet-pumps and star-plates, and every one, except for the two exceptions shown above, have D = 0.008 inch. So, I will use that dimension of D in the following examples.

Using rule #3 above, for a D = 0.008 inch, I would not want to use metal particles of (0.25) (0.008") < particles to avoid < (1.5) (0.008").

So, with that pump, I'll avoid metal particles 0.002" < particles < 0.012".

I can see from the Mesh-to-Micron Conversion chart above that: 0.002 inch is about 270-mesh, and 0.012" is about 50-mesh.

So, when using my pump, I don't want metal particles in the composition which are between 270-mesh and 50-mesh (+270-50 mesh). Particles smaller than 270 mesh would be fine to use, as would particles larger than 50-mesh.

And that is exactly the information I need in order to stay out of trouble when using metal particles in a composition which will be pumped with a tool which has a D = 0.008", as do most of my tools.

In reality, many of the compositions I use have fine 325-mesh metals in them. These would be fine to use in my tools. And as long as I sift my larger metal powders and remove any which will pass through the 40-mesh screen, the remaining large particles will also be fine to use.


Separating the various metal particle sizes


Now we're coming around back to the beginning that I started from. I mentioned at the beginning of this tutorial that I received the CH3009 spherical titanium, and that it contained particles from dust-size up through some which looked to be about 1/32-inch to 1/16-inch.

For some projects such as the Willow Diadem stars, that wide range of metal-particle sizes might work just fine as-is.

For other projects like Fairy Fountains, I want only very fine particles in the composition, and for other devices like large comets, I might want to use only the largest metal particles.

And, as shown in the last section, when using hard metals like the titanium, I want to verify that there are no particles in that +270-50 mesh size range.

How can I separate the different size particles that came in my tub of spherical titanium? Easy: by screen sorting.

First of all, a good Major Commandment is to "Never Use Good Screens to Sort Metal Particles." Metal particles will lodge in a flat, framed screen, almost permanently clogging more than a few of the openings in the screen, and potentially contaminating future screened chemicals or compositions when some of the metal particles dislodge from the screen.

In this sketch, repeated from above, you can see how a metal particle of the right (or wrong, depending on how you look at it) size can lodge in a screen opening. That particle on the right side of the sketch can be tough to remove from that opening.

Metal particles retained by screen sorting
Metal Particle on Right, Lodged in Screen Opening

Fortunately, there are good work-around alternative screens which are suitable for screen-sorting metal particles: inexpensive kitchen colanders (gravy strainers).

kitchen colanders used for screen sorting metals
Kitchen Colanders (Gravy Strainers) Used for Screen-Sorting

These colanders are inexpensive, can be dedicated to uses such as this screen-sorting, come in a variety of mesh sizes, and particles which lodge in their openings can be easily dislodged because of their rounded shape. Simply tapping the screen with the palm of the hand will knock most wedged-in metal particles loose very quickly.

Here are some useful tips to use when going to the kitchenware section of a department store to choose some of these colanders. Buy strainers which have the screen securely attached to the frame. Some of the cheaper versions have screens which can easily be pulled out of the frames, so they won't last long.

There are a few different ways to determine the mesh size of a screen. If you take a ruler to the store with you, you can hold it against the screen. Carefully counting the number of wires in 1/8-inch, and multiplying that number by 8, will provide the number of wires per inch, which is the mesh number.

If a little packet of salt is poured into a 40-mesh screen, about 2/3 of the crystals will pass through, with the remaining 1/3 being slightly too large to pass through the screen.

The shaft of a straight pin will slide into the holes of a 24-mesh screen with only a slight amount of room to spare. That same pin will be able to wobble a bit in the holes of a 20-mesh screen. The head of that pin will be about the same size as the holes in a 12-14-mesh screen.

Not only can the various sized metal particles be quickly separated from each other with these kitchen screens, but a weighed quantity of the batch of metal can be separated, and the resulting "fractions" can be weighed and noted. In this way, the amount of each screen-separated fraction in the whole batch of the metal can be predicted.

For instance, we'll take 2 ounces of my CH3009 titanium. First we'll sift it through the coarsest screen, the 20-mesh gravy strainer.

sifting spherical titanium through a kitchen colander
Sifting 2 Ounces of CH3009 Spherical Titanium Through 20-Mesh Screen

We'll put the particles which are retained in the screen (+20) in their own dedicated paper cup which has that size marked on it.

Then we'll sift the particles which passed through the screen (-20) through the next-finer screen, the 24-mesh one. The particles retained on that screen are put in a cup marked -20+24.

A 40-mesh screen then is used to separate the particles into -24+40 size, and -40 size.

For some devices, as in the Fairy Fountains, I like to use only the finest titanium particles that I can sort out. So I have a small piece of 80-mesh screen which I've bent into a "bowl". I can use that screen to further separate the metal into -40+80 size, and -80 size.

sorting metal particles with an 8-mesh screen
Using a Piece of 80-Mesh Screen to Sort Metal Particles

The openings in this 80-mesh screen do clog up quickly when used this way, so it's a lot more difficult to do this last screen-separation. But, it's worth it to get the very finest particles for my fountains. And the screen pores unclog easily by flexing and tapping the screen.

Screened metal particles sorted into paper cups

sorted and labeled titanium particle sizes
CH3009 Titanium Sorted into Five "Fractions"

This particular 2 ounces of the titanium separated into these fractions:

+20 0.15 ounce 7.5%
-20+24 0.05 ounce 2.5%
-24+40 0.20 ounce 10%
-40+80 1.00 ounce 50%
-80 0.60 ounce 30%


Using the various metal-particle-size fractions


That fractional analysis of the CH3009 titanium is handy to know, and it's very useful to know how to separate that metal into different mesh sizes, too.

How the metal sizes are to be used will determine which screen sizes to select for use, and what specific separations to make with the screens.

For my Fairy Fountains, I really only like to use very fine spherical titanium. So, I'll simply sift some of the metal through my 80-mesh screen until I obtain the amount of that -80-mesh-size that I need for the batch of fountain fuel that I plan on mixing.

For my large comets and salutes, I like to use the coarsest titanium I can lay my hands on. From the above chart I can see that 10% of my metal will not pass through the 24-mesh screen. That +24 mesh size is a good size for these larger devices, so I think I'll sort out all of that large mesh metal out of the tub I received.

And for star and comet compositions which will be pumped with my comet-pumps or star plates, I don't want to use metals with particle sizes smaller than 50-mesh. If I sort all the remaining metal through my 40-mesh screen, I can eliminate the metal that is finer than 40-mesh (-40), and will end up with a fraction which is all between 40-mesh and 24-mesh (-24+40), which is a nice consistent size for many metal-spark-tailed stars and smaller comets.

The metal particles which are smaller than 40-mesh, and larger than 80-mesh (-40+80) might get used in sparklers or cut stars.

Earlier, I mentioned the Grape Nuts Titanium which I have a few pounds of. That metal came listed as -10+18 mesh, so it ought to be fine to use with my comet pumps. But, in truth, when I toss some of that metal in my 40-mesh colander, some fines do sort out which are smaller than the 40-mesh openings, and could potentially jam or damage my pumps. There are only less than 1% of those fines, but sifting them out before using that metal in a comp which will be pressed with my pumps, removes that danger and is a good precautionary measure.

So, that's it. That's how you can screen sort your metal powders and not only end up with consistently performing fireworks, but save wear and tear on your fireworking tools.

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