A Basic Comparison of Milling Media and Preparation Methods

Moderators: richardh08, Boophoenix, Lloyd

Post Reply
User avatar
Pyro-Gear
Site Admin
Posts: 3034
Joined: Wed Jul 10, 2013 8:24 am

A Basic Comparison of Milling Media and Preparation Methods

Post by Pyro-Gear »

A Basic Comparison of Milling Media and Preparation Methods for Black Powder
By David Forster, May 2017


Background:

I have been experimenting with black powder preparation methods for some time now. Most of my efforts lately have been with powders that were made by 'supermilling' the charcoal, and then combining it with milled potassium nitrate and rubbermakers sulfur. The sulfur was used 'as is' from the bag. With this method, I have been able to match or exceed the performance of Goex commercial black powder. My method takes a little longer than milling the 3 components together, but is useful in situations where running a potentially explosive mill safely is not an option.

With my method, the charcoal is first prepared for milling by running it through a garbage disposer. This reduces it to -10 mesh or so. It is then milled for 6 hours with stainless steel media. The potassium nitrate is technical grade, milled for 2 hours, also with stainless steel media. The charcoal, potassium nitrate, and sulfur are mixed together by hand, and run through a 40 mesh screen twice. The resulting product is analogous to 'mill dust'. It can be used just as one would use mill dust, in any application.

There has been much debate about milling media sizes and types over the years. Many statements are made, but very few test results are available to the fireworker that wants more details before deciding for themselves what is the best way for them to prepare black powder. Most people simply mill the 3 components together with lead media. This method has been in use by amateur pyrotechnists for decades now, with very few accidents reported. It is worth noting that the accidents that are reported are usually not mill explosions. The (very few) accidents mostly occur during handling after milling. Some people mill their black powder with stainless steel media. It is thought that the harder media would grind better. At the same time, stainless steel is less dense than lead. Therefore, the impacts between media have less force behind them. It's possible that these 2 differences cancel each other out. Another reason one might choose stainless steel over lead is because lead can wear down and contaminate the powder. Most feel that this is not a big issue if the lead has been hardened with antimony.

When I promoted my screen-mixing method for making black powder, I was asked if my method could produce black powder of equivalent power to that which was milled as a 3 component mixture in the same amount of time. I was not able to answer that question, since I had made no direct comparisons between methods. My goal was to produce excellent powder with single-component milling, not to win a contest. This question and others have been nagging at me for some time, so I decided to do some more testing. Maybe I would win the contest!

Baseball testing is quite popular among powder-makers for evaluating their powders, and comparing them (hopefully favorably) with the powders made by others. A 10 gram sample of powder is loaded into a 3" (76mm) mortar, and used to launch a standard baseball weighing about 150 grams. A stopwatch is used to measure the total flight time. According to Ned Gorski, host of Fireworking.com, a total flight time of 7.5 seconds will put a 3" shell to a proper display height. I chose this testing method.

The Test Batches

For this series of tests, 6 batches of powder were prepared. Each batch weighed 750 grams. This is a typical batch size for the amateur pyrotechnic hobbyist. To approximate the well-known 75-15-10 formula, I used 563 grams of potassium nitrate, 112 grams of charcoal, and 75 grams of sulfur. The powders for the test pucks (explained later) were drawn from these larger batches. My reasoning here was that I wanted to do things as closely as possible to what I felt other pyrotechnic hobbyists would do.

The powders were milled in Rebel 17 jars, which have a capacity of 5 litres each. The jars have heavy black rubber liners, which dampen the noise- and probably the milling efficiency as well. On my mill, these jars turn at 58 RPM. My mill holds 2 jars. To obtain maximum accuracy, I ran the test batches in pairs, and switched jar positions halfway through the milling time. The total milling time for each batch was 2 hours, 15 minutes. The mill is remotely located and barricaded as any black powder mill should be.

The first pair of tests was to compare the performance of black powders milled as complete mixtures, with the only difference being that one batch used 35 pounds of 1/2" hardened lead spheres, and the other used 30 pounds of 1/2" 304 stainless steel spheres.

The second pair of tests was done to compare screen-mixed powders to each other- and the above pair. In one batch, the charcoal was milled for 2 hours, 15 minutes with the lead media mentioned above. The batch size was 500 grams. In the other 500 gram batch, the charcoal was milled for the same time with the stainless steel media also mentioned above. The potassium nitrate for both these tests was separately milled for 2 hours with 30 pounds of 1/2" stainless steel spheres. The sulfur was used 'as is', straight from the bag. After milling the charcoal and potassium nitrate, the batches for testing were made up by screen-mixing all 3 ingredients together twice through 40 mesh.

The third pair of tests was an afterthought. Since I have various sizes of 304 stainless steel media, I thought I would compare the performances of 2 more powders, milled as a complete mixture, but with smaller media. The first batch used 30 pounds of 3/8" media, and the second batch used 30 pounds of 5/16" media.

At this point I would like to address the issue of lead contamination. Different people have different thoughts about this subject. I wanted to quantify the level of contamination in the batches that used lead media, so it could be considered as a possible variable while reviewing the results I will eventually get to. As mentioned previously, these are hardened spheres, and cast by an experienced man for the purpose of milling black powder. The quality is very good. The powder milled with lead in the first pair of tests gained 5 grams of lead/antimony weight, going from 750 grams to 755 grams. When milling the charcoal for the second pair of test batches, the 500 grams of charcoal milled with lead gained 20 grams. This was ascertained by starting with immaculate mill jars and cleaning them after milling as painstakingly as possible to collect every last bit of powder. The powder that remained on the media and jar surfaces afterwards was insignificant. It is fairly well-known that milling charcoal by itself is very hard on lead milling media. With stainless steel, this issue (if it is one) does not exist.

Preparations:

To achieve maximum accuracy of results, the materials must be uniform to start with. The charcoal I chose for these tests was my own homemade narrowleaf willow charcoal. It was run through a Waste King garbage disposer-based charcoal grinder. This produced a mixture of particle sizes, all the way from 10 mesh down to airfloat. All charcoal used for these tests came from a single bag. Each scoop of charcoal was brought up through the mass from the bottom of the bag, ensuring that I wasn't using just coarse stuff for one test and fine stuff as I got to the bottom of the bag. The potassium nitrate was technical grade, from Norway. The sulfur was rubbermakers sulfur, referred to as H10.

There can be significant variations in black powder performance, based on preparation methods. I chose a method of preparation that is used in the commercial production of black powder to get the most consistent product I could for my comparisons. Commercially, the powder is generally dampened slightly, and then pressed into 'pucks' (or other shapes) using high pressure. I'll just call them pucks. These pucks are quite dense, between 1.7 and 1.8g/cc. The pucks are then mechanically broken up, and then graded for size. At some point they are dried.

I used 6 batches of powder for this series of tests. Each batch weighed 300 grams. The individual powder details will be given later. Each of the batches was spritzed with 8% additional (24 grams) of water. The water was worked in by hand, and the powder was then run twice through a 20 mesh screen. The mixtures were left covered for a couple of hours to 'temper'. Tempering allows the moisture to distribute more evenly throughout the powder, and shouldn't be rushed. After tempering, the powders were briefly mixed again by hand. Each batch of moistened powder was pressed in charges of 51.0 grams. Some liquid was expressed during pressing, which was done at 5 tons of force, using a hydraulic press. The total time to press each puck was about 5 minutes. Each puck lost about 2.5 grams of liquid, and ended up at about 1/2" (12.7mm) thick.

After drying, the pucks were 'corned' and graded. I don't have a black powder factory, so my hillbilly method involved a Louisville Slugger, a piece of PVC pipe, and a test cap. The Slugger was modified, having the end of a meat tenderizer hammer mounted to the tip. The pucks were broken up, and run through a 4 mesh screen, into a box. What remained on the screen was returned to the pipe, given a few more downward hits with the Slugger, and re-screened. This process was repeated numerous times until all 6 pucks had been broken down to pass 4 mesh. After that, the 'powder' was agitated on a 12 mesh screen for at least 2 minutes, to ensure that all small particles were screened out. This 4-12 mesh cut is my homemade version of 2FA. 2FA is a slow black powder (due to the large particle size) used to lift larger shells. About 2/3 of each batch was graded to 2FA. If I were stronger or more aggressive with the Slugger, that amount would have been considerably less.

All batches of pucked, corned, graded powder were catalogued, and stored together for at least a week before official testing began. Only the 2FA fractions were used in this group of tests. The remainders of all powders were further graded to 12-16 mesh to simulate 1Fg commercial powder. Using this grade instead of 2FA adds 1-2 seconds to the total flight time of the baseballs. The -16 mesh powder was saved for other uses.

The Testing Method

For each of the 6 batches, 3 tests were done. Each test used 10.0 grams of 2FA-sized powder grains. A triple beam balance was used to ensure accuracy in weighing. The powders were all contained in identical plastic condiment cups after being weighed out. The cups came with snap on lids, so I punched a small hole in each lid to accommodate a piece of long thin black match, which I taped into the cups. I then snapped the lids on. A short visco leader was taped to the protruding match for lighting.

The launch tube was a standard 3" fiberglass mortar with a weighted base. The mortar was fitted with a bubble level, which could be removed after the mortar was in place and pointing straight up. The baseballs were painted black to make them easier to see in the sky. The individual lidded cups of powder were lowered into place by the black match, and a stick was used to make sure they were seated in the bottom of the launch tube. The baseballs were each lowered into place with a piece of sticky tape, which was yanked off when they were seated on the cups of powder. All tests were done the same afternoon. A standard stopwatch graduated in 1/5 second increments was used to record the total baseball flight times.
Let the Testing Begin!

1) The first powder to be tested was milled as a complete mixture with 1/2" hardened lead media for 2 hours, 15 minutes. These are the flight times: 10.4 seconds, 10.45 seconds, and 10.5 seconds. The average flight time for 75-15-10 milled with 1/2" lead spheres was 10.45 seconds.

2) This complete mixture was milled with 1/2" stainless steel spheres. These are the flight times: 10.1 seconds, 10.2 seconds, and 10.2 seconds. The average flight time for 75-15-10 milled with 1/2" stainless steel spheres was 10.17 seconds.

3) This screen-mixed powder was made with the charcoal that had been pre-milled for 2 hours, 15 minutes with 1/2" hardened lead spheres. The potassium nitrate was milled for 2 hours with 1/2" stainless steel media. These are the flight times: 8.9 seconds, 8.9 seconds, and 9.3 seconds. The average flight time for screen-mixed powder made with lead-milled charcoal was 9.0 seconds.

4) This screen-mixed powder was made with charcoal that had been pre-milled for 2 hours, 15 minutes with 1/2" 304 stainless steel spheres. The potassium nitrate was pre-milled as mentioned above. Here are the flight times: 9.1 seconds, 9.3 seconds, and 9.1 seconds. The average flight time for screen-mixed powder made with stainless steel-milled charcoal was 9.17 seconds.

5) This powder was milled as a complete mix with 3/8" 304 stainless steel spheres. These are the flight times: 9.5 seconds, 9.3 seconds, and 9.7 seconds. The average flight time for 75-15-10 milled with 3/8" stainless steel spheres is 9.5 seconds.

6) This powder was milled as a complete mix with 5/16" 304 stainless steel spheres. The times are: 10.2 seconds, 9.3 seconds, and 9.7 seconds. The average flight time for 75-15-10 milled with 5/16" stainless steel spheres is 9.73 seconds.

Observations, Conjecture, and Conclusions

Well, the results are in. The most popular medium for milling black powder is also the best. The amount of lead contamination per batch of complete black powder milled with hardened lead is negligible, and certainly didn't slow the powder to any significant degree. The stainless steel was so close in performance that the difference isn't even noteworthy. Of course, lead is known to be non-sparking. Stainless steel is harder, and could spark. If hard, gritty contaminants made their way into the mill jar, an ignition would seem much more likely if stainless steel was used to mill complete black powder. Hardened lead is obviously the most prudent choice in this application.

The next pair of results clearly shows 2 things. The first is that in apples to apples comparisons, milling black powder as a complete mixture makes a more powerful product than if the components were milled singly. The second thing is that although the screen-mixed powder is weaker, it is more than acceptable for general use. In this round, the stainless steel beat out the lead, but as above- not by much. The contamination from milling charcoal with lead was certainly significant. The performance was quite good, considering that 4% of the charcoal weight in the 75-15-10 was displaced by the contamination. Even though the powder was good, the flight times were the lowest of all powders tested. I certainly won't continue to mill plain charcoal with any type of lead media. For this application, stainless steel is a better choice.

The last pair of tests was done as an afterthought, since I happen to have full mill jar charges of 2 other sizes of stainless steel media. I thought it would be interesting to see how media size alone would affect the performance of milled black powder. When we compare the results of batches 2, 5, and 6, no obvious relationship seems to exist. The largest (1/2") media gave the longest times, but the smallest media (5/16") didn't give the shortest times. The mid-sized (3/8") media took that position. I have some thoughts on this.

There are a couple more things I'd like to present for consideration. First, there were 3 test flights done with batch #1 that do not appear here. The results were 9.1 seconds, 9.6 seconds, and 10.6 seconds. These were my first tests, and the spread on the times was so great that it did not appear that I was going to derive any valuable conclusions from my baseball testing. After thinking about it, I figured that maybe the spread was due to the fact that I used 3 different (painted) baseballs. The baseballs weighed between 149 and 152 grams. I repeated the tests using a single baseball, and got the results to be much tighter, as reported for batch #1 earlier.

After seeing how important it was to use the same ball for consistent results, I decided to use the same ball for all tests. By the third round of tests, the ball was looking a little battle-scarred. A bit of the stitching had melted away and let go, and a flap of 'leather' was sticking up a little on the ball. The wounds were patched with some carefully laid down masking tape, and the testing continued. The flights of the ball did become somewhat more erratic as the testing went on. It's reasonable to assume that the changes in the ball would affect the aerodynamics and therefore the accuracy of the test results. It's also a known fact that a baseball can be made to curve by the way it is thrown, and by the orientation of the stitching when thrown. It doesn't seem to be much of a stretch to think that the anomaly seen between the results of batches 2, 5, and 6 was due to irregularities in the ball.

A Final Word

While there were no earth-shattering revelations, these tests did prove one thing conclusively. It is not difficult to produce perfectly serviceable black powder using different types and sizes of milling media, whether the powder is milled as a complete mixture, or as single components. All the results in this round of testing were more than satisfactory. The spread between the lowest and highest flight times was fairly close.

The study of black powder has spanned centuries, because it is a complex and important product. It is my hope that this examination has been helpful, if only in a small way, in furthering our general understanding of black powder and its preparation.
Last edited by Boophoenix on Fri May 26, 2017 3:13 pm, edited 1 time in total.
Reason: Edited the formatting a little to be easier to read like the original document
User avatar
Boophoenix
Posts: 968
Joined: Wed Sep 09, 2015 11:49 pm

Re: A Basic Comparison of Milling Media and Preparation Meth

Post by Boophoenix »

I always enjoy following Dave's work. We discuss it sometimes prior to his postings. I learn a lot from those discussions and his reports. I even get surprised from time to time. One of the first debates I ever had with Lloyd when I first visited him is covered in the article. You guessed it, Lloyd was correct :lol:

Thank ya Dave for the hard work and sharing.
User avatar
Pyro-Gear
Site Admin
Posts: 3034
Joined: Wed Jul 10, 2013 8:24 am

Re: A Basic Comparison of Milling Media and Preparation Meth

Post by Pyro-Gear »

Yeah big thanks, Dave, I could not get the link to work when reposting so I just posted the lot!

Lloyd is very rarely wrong ;)
User avatar
Lloyd
Posts: 1902
Joined: Sun Aug 09, 2015 1:43 am

Re: A Basic Comparison of Milling Media and Preparation Meth

Post by Lloyd »

Thanks, guys. But in truth - like all 'experimenters' - I'm far more-often wrong than right. It's the mistakes that usually lead to some sort of discovery!

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
User avatar
richardh08
Site Admin
Posts: 2226
Joined: Wed Aug 07, 2013 1:54 pm
Location: Bedfordshire

Re: A Basic Comparison of Milling Media and Preparation Meth

Post by richardh08 »

That's an excellent study. Right up my street!

It seems clear to me that, provided you don't pursue the chimera of ultimate performance, it isn't that difficult to produce BP of more than adequate quality. That seems to be something that was well known in the past, but seems to have been forgotten in more recent times.
Even when I'm wrong, I'm convincing.
User avatar
Boophoenix
Posts: 968
Joined: Wed Sep 09, 2015 11:49 pm

Re: A Basic Comparison of Milling Media and Preparation Meth

Post by Boophoenix »

So true Richard! I think everyone either thinks someone messing with BP and Charcoal is always looking for the Holly Grail of Max performance. When they do or get that impression they forget commercial manufacturing uses bulk powders that someone else produces in a way that is easier to make consistently than to try to make the hottest and fastest.

Dave and I have been discussing things back and forth for a couple of year now at least. So long as I don't share to many of my idea or thoughts we have some very educational discussions. If I do get side tracked I have to be careful or Dave just says so otherwise I might skew his thought process he is working on at the time. He's way more focused in one direction than I am when he's got something on his mind.
Post Reply