OB Nitrate Test

OB Nitrate Test

Richard Harrison, October 2014

Introduction

There has been some recent discussion regarding the quality of the potassium nitrate sold cheaply by Oliver Brown, in 8kg tubs. A number of people have used it to make black powder that has been variously reported as being moderate, to quite poor, in performance. As I have a tub that I had not so far used, I decided to investigate its properties, in an attempt to settle the issue.

I was a little surprised by the reported results, as I had always believed that the effect of using different nitrate samples would be relatively small, unless there were significant differences in their degrees of purity. I do, however, have to admit that I have never put this belief to the test.

I should start with a word of caution; I can only speak for what I found regarding the contents of the tub that I have, under the milling and testing conditions that I use. Given the widely differing reported performances, there may be other systematic differences that I have no knowledge of.

The Tests

The appearance of my sample is pure white and shows no visible signs of contamination. It is very lumpy and the lumps show no tendency to break up on handling. It appears to be somewhat sticky, and grains had a tendency to cling to the scoop that I used to extract the sample that I milled.

I made a standard 75/15/10 mix, with the nitrate taken straight from the tub, using the sulphur and a charcoal that I had previously run through my test procedure. As with my other tests, I milled the mix for a total of eight hours, extracting samples of the milled powder at fixed time intervals.

After taking the first two or three samples, I noticed that the powder was not as fluffy as it had been in my previous charcoal tests. That, coupled with the stickiness I had earlier observed, caused me to wonder if the nitrate was somewhat damp. Therefore, while the milling progressed, I took another 5gm sample and dried it at 50C for a few hours, weighing it at intervals, until the weight remained constant. As a control, I also took a 5gm sample of the ‘food grade’ nitrate that I had used in my earlier tests and submitted it to the same drying process. Both samples lost weight, the control sample by 0.5% and the test sample by 1.6%. On allowing the two samples to stand in the open air for 24 hours, the control sample more or less regained its original weight but the test sample showed a permanent weight loss of about 1.3%. I concluded that the nitrate under test was indeed damp, but that there was no sign that the dampness was caused by a hygroscopic contaminant.

As a further check on purity, I performed a quick flame test on both samples. They both showed the presence of sodium contamination, but in neither case was it so great that the yellow colouration entirely masked the violet, potassium flame colour. It isn’t exactly a conclusive test but, as far as I can tell, there seems to be no great difference in the amount of sodium impurity in the two samples.

Once milling was complete, I ran burn tests by hand-ramming each sample into a ¼ inch tube and measuring both the linear and mass burn rates. I compared the measured rates with those obtained from the earlier test of the same charcoal, but with ‘food grade’ nitrate.

The Results

Unsurprisingly, the general trends are similar in form to the earlier ones, but there appears to be a reduction of about 17% in the linear burn rate after milling for 8 hours (480 min). Bearing in mind that the new samples contained additional moisture, I thoroughly dried the last four samples, left them in the open air for 24 hours to reabsorb moisture from the atmosphere, and then re-performed the burn tests. As expected, drying the samples improved the linear burn rate, which ended up being only about 5% slower. The burn rate results are shown in the accompanying two graphs.

Interestingly, the mass burn rates for both the damp and dried samples are generally higher than those measured for the ‘food grade’ nitrate, but converge to identical values at a milling time of 8 hours. I can only surmise that the dry and damp powders mill in a slightly different way; the powder that was milled damp certainly appears to be somewhat denser – even after drying, as is shown in the third graph.

I should point out that I made some very slight changes to the milling process in the interval between making the two sets of measurements.My previous measurements indicate that these changeshave resulted in a small improvement in the milling efficiency but, as far as I can tell, have no significant effect on the final burn rates. The graph of mass burn rates appears to agree with this finding.

Some time ago I measured the variation in burn rates with powder density. I found that the mass burn rate was fairly insensitive to changes in density but that the linear rate was much more dependent. If I apply this measured correction to the linear burn rate results for the dried sample, the burn rates from the old and new tests appear to show a similar behaviour to that of the mass burn rate measurements – a higher rate at low milling times, but eventual convergence.

Conclusion

Allowing for the possible differences between milling damp and dry mixtures, including the significant variation in density, I believe that the two sets of measurements are broadly equivalent, to within the limits of the possible experimental errors.I have to conclude that – apart from a certain amount of moisture, which can be removed by adequate drying before use – I can find no significant difference between the two nitrate samples.