2005-02-28
Black Powder Moistening
Always controversial, making the hottest black powder has long been surrounded with rumour and superstitions. Many contend that adding water to BP only reduces its performance by allowing the oxidiser to crystallise out into larger particles. Other say it is critical to making good BP.
Whichever is true, BP made dry or moist is generally good-enough for the maker's purposes so the difference may be academic. None the less, I decided to do some basic experiments to see if there was any effect noticeable and get a feeling for its magnitude.
Method
100 g of 15:3:2 greenmix was prepared, using high quality charcoal and sulfur. The Potassium Nitrate was recrystallised three times and washed until it tested free from chlorides, calcium and sodium impurities. This was ball milled for about 10 minutes to make it homogeneous, then divided into half - parts A and B. The mill jar was optimised for ~50 g batches, so this initial milling wasn't very efficient, just enough to make the uniformly black with no specs of anything else visible.
Part A was divided into halves again, called lots #1 and #2. #2 was moistened with 5% water by mass and pushed through a sieve several times then spread onto paper and allowed to dry.
Part B was put back into the mill and milling continued for 3 hours after which it was divided into two lots #3 and #4. #4 was moistened with 5% water by mass and pushed through a sieve and dried in the same manner as #2.
Each lot was charged into 1/4" ID tubes 2" long (pyrotube), sufficient to give a 25 mm long powder column. The loading density was 1.2 g/cc, computed by careful measurements of the ID with callipers and weighing of the tubes before and after loading. Although charged by hand enough care was taken that the repeatability was better than 0.02 g, each tube containing almost exactly 0.9 g of composition.
Ignition was achieved with a short piece of zinc match. The burn was videoed and the burn time measured by observation of the audio spectrogram using Baudline.
Results
| lot | time (seconds) |
rate (mm/second) |
|---|---|---|
| #1 | 4.16 | 6.1 |
| #2 | 3.92 | 6.5 |
| #3 | 2.32 | 11.0 |
| #4 | 2.03 | 12.5 |
Note the banding in the spectrogram. It is not a function of burn time, so it is not a function of free-space in the tube above the composition. It is probably comb-filtering from standing waves in the testing area.
Discussion
There definitely seems to be a positive effect of dampening the meal and letting it dry. This does slightly granulate the mixture (and make it much cleaner to press into the tubes), but the grains are so soft they crush to nothing and the loading density was measured to be identical, so I doubt this is the cause of the improvement.
Only one lance of each composition was timed, this does bring into question the quality of the results. More trials for each lot should be performed and the results averaged. I am waiting on a funnel and wire set from Wolter to arrive before I do this. Longer burn times would allow more precise measurements.
I suspected the results would confirm positive effects from the dampening. I have long held the belief that water is the magic ingredient in BP. I had an open mind about the potential negative effects of crystal growth, but it is well known that oxidiser particle size is no where near as important as that of the fuel.
In the case of BP the Sulfur and Potassium Nitrate are in liquid form during the reaction reducing the importance of their particle size. Finer Potassium Nitrate is easier to melt and sticks closer to the fuel, so there is an effect, but it is not as pronounced as using more coarse charcoal.
This makes one wonder about the mechanism of the improvement. Many suggest the Potassium Nitrate is dissolved and soaks into the charcoal. I dispute this is the major effect. Without doubt some of the Potassium Nitrate dissolves, but at room temperature its solubility is about 400 g/l. 5% moisture by mass could dissolve at most about 2-3% of the Potassium Nitrate present in the composition.
I suggest the effect might be more physical movement of the Potassium Nitrate particles in and onto the charcoal by the surface tension of the liquid. As the liquid dries the small amount of dissolved Potassium Nitrate would crystalise out cementing the particles in place. The hydrophobic Sulfur would probably migrate to the surface of the KNO3/water droplets which might also be important, especially as Sulfur is important in the ignition process and fire transfer through BP.
The charcoal is somewhat hydrophobic too, so who is sticking to who and how might be a complex issue, perhaps the Potassium Nitrate crystals are in the core and a layer of Charcoal and Sulfur coats them, the crystallising Nitrate growing into the porous charcoal and holding it in place (remembering the needle-like Nitrate crystal growth habit).
It is notable that the dampened BP is somewhat darker than the original. This is a well documented observation that my model might explain. Unfortunately without a good microscope (preferably a SEM) study of the microscopic details after each operation is not something I can study.
10 comments.
Attachments
| title | type | size |
|---|---|---|
| video of lot #1 (greenmix) | video/x-msvideo | 1.261 Mbytes |
| video of lot #2 (dampened greenmix) | video/x-msvideo | 1.241 Mbytes |
| video of lot #3 (meal) | video/x-msvideo | 901.938 kbytes |
| video of lot #4 (dampened meal) | video/x-msvideo | 719.422 kbytes |