The effect of differing concentration of potassium nitrate on
deflagration rate of black powder
Shane Oh
Shanghai American School Pudong Campus, Shanghai, China
Submitted for review April 24, 2013
DISCLAIMER: The author assumes no liability for any personal injury, illness, or property
damage that may arise out of use of the contents of this document.
Acknowledgements
I would like to acknowledge Mr. James Happer and Ms. Shanneth Elliott, science teachers
at Shanghai American School, for their instructions and guidance in this investigation.
Abstract
Throughout the history of explosives, the ratios of explosives’ reactants were chemists’
primary interests. As black powder, discovered in the 9th century by the Chinese, had been the
central propellent for over 500 years, chemists were heavily urged to devise an optimal ratio of
reactants. Eventually, trial-and-error experiments led to the optimal ratio of 75% potassium
nitrate, 15% charcoal, and 10% sulfur ((Wakeman, R. (2003). Blackpowder to pyrodex and
beyond.)).
In this investigation, the effects of differing relative concentration (by weight) of potassium
nitrate on deflagration rate of black powder will be explored. Three different types of gunpowder
will be produced, with differing proportion of potassium nitrate by weight, 60%, 75%, and 90%.
The three different types of gunpowder will be poured into a container of specified
dimensions, then it will be ignited at one side. The arbitrary rate of deflagration, the mean speed
at which deflagration site travels across the container will be measured by analyzing frames from
a high-speed camera. Two-tailed t-tests for two means will be used to conclude whether there is
significant evidence that differing proportion of potassium nitrate has an effect on the
deflagration rate.
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1.0 Introduction
Chemical properties of dry mixed black powder ((Wallace, W. (1995). Fmx: The revised black
book. Paladin Press.))
• Explosive type: low
• Burn rate: 300m/s max.
• Chem. Formula: mixture of KNO3, S, and charcoal.
• Form: dry gray or black powder
• Deflagration temp.: 300ºC
• Water sensitivity: high
The combustion of black powder is modeled by the following chemical equation
((Bretscher, U. (n.d.). The recipe for black powder.)):
4 KNO3 + C7H4O + 2 S ! 2 K2S + 4 CO2 + 3 CO + 2 H2O + 2 N2
Gunpowder has been a very important element in the history of explosives. First discovered
in 9th century in China, black powder (also known as gunpowder) was used as propellent for
cannons, guns, and hand grenades, as it produces a significant amount of gases ((Bedard, A.
(n.d.). Black powder solid propellants.)). It has been in active use until 1850s, when newer
alternatives like nitrocellulose were discovered.
The black powder is a chemical mixture of sulfur, charcoal, and potassium nitrate: The
sulfur and charcoal act as fuels and potassium nitrate acts as an oxidizer ((Calvert, J. B. (2009,
September 4). Cannons and gunpowder.)). Unlike modern explosives, black powder’s ingredients
are not bonded at a molecular level. For this reason, black powder has slower deflagration rate
and lower brisance compared to modern alternatives.
I had decided to make my own black powder, as it allows me to manipulate the ratio of
ingredients. The produced black powder was not granulated, as granulating will produce granules
of varying sizes. Potassium nitrate and sulfur were obtained from a laboratory, and charcoal was
prepared by heating a vessel containing balsa wood ((Gorski, N. (n.d.). Charcoal making
secrets.)).
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2.0 Experimental design
2.1 Hypothesis
If the black powder has a higher concentration of potassium nitrate that provides oxygen
during the deflagration process, then the black powder will deflagrate faster. However, if the
concentration of potassium nitrate exceeds a certain threshold, the rate of reaction will decrease.
Comparing 60% and 75% black powder
µ75: mean burn rate in cm/s for 75% KNO3 black powder
µ60: mean burn rate in cm/s for 60% KNO3 black powder
H0 µ75 - µ60 = 0
Ha: µ75 - µ60 " 0
Comparing 75% and 90% black powder
µ75: mean burn rate in cm/s for 75% KNO3 black powder
µ90: mean burn rate in cm/s for 90% KNO3 black powder
H0 µ75 - µ90 = 0
Ha: µ75 - µ90 " 0
2.2 Procedure
Note: The procedure includes manufacturing black powder and testing the deflagration rate.
2.2.1 Preparing Black Powder ((Wallace, W. (1995). Fmx: The revised black book. Paladin
Press.))
Picture 1: Producing black powder
1. Prepare 75 u.b.w.(unit by weight) potassium nitrate, 15 u.b.w. charcoal (willow/balsa), 10
u.b.w. sulfur
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2. Place charcoal in a mortar. Grind well for 5 minutes. Pour the contents in a beaker.
3. Place sulfur in the same mortar. Grind well for 5 minutes.
4. Place charcoal in the mortar that contains sulfur powder, then add three drops of ethanol.
Grind well for 10 minutes.
5. Pour the sulfur/charcoal mix in the beaker. Do not clean the mortar.
6. Pour potassium nitrate in the mortar and grind for at least 15 minutes.
7. Pour the sulfur/charcoal mix in the mortar.
8. Mix until the mixture is homogenous.
9. Add three of ethanol. Grind for 10 minutes until you get fine powder.
10. Repeat steps 1~9 with 60% and 90% potassium nitrate.
11. Ensure that the black powder is properly ground by filtering the powder in a mesh with
1#1mm opening.
2.2.2 Testing deflagration rate
Picture 2: Deflagration of black powder. The flame is clearly higher than 5cm.
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1. Make 15 paper containers with dimensions 1.5#1.5#12.7 cm
2. Weigh 0.6g (±0.01g) of 60% black powder, and then pour it in the paper container. Ensure that
the powder is evenly spread out. Make four more such containers.
3. Repeat step 12 with 75% and 90% black powder.
4. Set up a camera and start recording a video.
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5.
6.
7.
8.
Place a paper container and a ruler on the ceramic plate, parallel to each other.
Light one end of the container.
Burn 14 remaining containers.
Using the video footage, calculate the speed at which deflagration site travels across the
container.
2.3 Variables
Independent Variable
• Proportion of potassium nitrate by weight
Dependent Variable
• Speed at which deflagrations site travels across the container.
Controlled Variables
• Air temperature (regulated by fume hood’s continuous air flow)
• Size of the container (regulated to 1.5#1.5#12.7 cm)
• Amount of black powder being tested (regulated to 0.6g)
• Spread of black powder in the container (uniformly spread out by inducing vibration)
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3.0 Data Collection
Table 1: Data for 60% KNO3 (KNO3: 7.5 u.b.w., C: 3 u.b.w., S: 2 u.b.w.)
Trial #
1
2
3
4
5
Length (cm)
12.7
12.7
12.7
12.7
12.7
Mass (g)
0.60
0.59
0.59
0.61
0.61
Frames (±1)*
159
210
217
186
226
Frames per second
30
30
30
30
30
Avg. speed (cm/sec)
2.3962
1.8143
1.7558
2.0484
1.6858
Table 2: Data for 75% KNO3 (KNO3: 15 u.b.w., C: 3 u.b.w., S: 2 u.b.w.)
Trial #
1
2
3
4
5
Length (cm)
12.7
12.7
12.7
12.7
12.7
Mass (g)
0.60
0.61
0.61
0.60
0.60
Frames (±1)*
77
79
73
69
73
Frames per second
30
30
30
30
30
Avg. speed (cm/sec)
4.9481
4.8228
5.2192
5.5217
5.2192
Table 3: Data for 90% KNO3 (KNO3: 45 u.b.w., C: 3 u.b.w., S: 2 u.b.w.)
Trial #
1
2
3
4
5
Length (cm)**
N/A***
5.6
5.0
6.4
3.8
Mass (g)
0.60
0.60
0.59
0.61
0.59
Frames (±1)*
N/A***
161
179
175
127
Frames per second
30
30
30
30
30
Avg. speed (cm/sec)
N/A***
1.0435
0.8380
1.0971
0.8976
Table 4: Overview data table for 60%, 75%, and 90% KNO3
Trial #
60% KNO3
1
2.40
2
1.81
3
1.76
4
2.05
5
1.69
Average speed, cm/s 1.94
75% KNO3
4.95
4.82
5.22
5.52
5.22
5.15
90% KNO3
1.04
0.84
1.10
0.90
0.97
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Frame counts were started when the flame reached a height of 5cm, and frame counts were
ended when the height was less than 5cm.
** The contents did not burn completely. The length of the reacted portion was measured.
*** The first trial of 90% KNO3 failed to ignite
Sample Calculations (60% KNO3 Trial 1)
Avg. Velocity = (length in cm) ÷ (#frames ÷ 30) = 12.7 ÷ (159 ÷ 30) = 2.40 cm/
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4.0 Data Analysis
Figure 1: dot plot of deflagration rate of 60%, 75%, and 90% KNO3 black powder.
75% and 90% distributions were symmetric, but the 60% distribution was slightly skewed to
right. The mean deflagration rate of 75% black powder (5.15cm/s) was higher than 60% black
powder (1.94cm/s), followed by 90% black powder (0.97cm/s). The spread of 60% distribution
($: 0.289cm/s) was the largest, followed by 75% ($: 0.272cm/s) and 90% ($: 0.121cm/s)
4.1 Sources of Error
The distributions of the data were approximately normal except for 60% KNO3 black powder,
whose distribution was slightly skewed right. The following list suggests why there were some
deviations.
• Making of black powder
o Mass of the ingredients may have differed
o The time spent on grinding the ingredients may have differed, therefore affecting
the surface area.
• Making the paper container
o Dimensions of the container may have differed
o The spread of powder in the container may not have been uniform.
• Burning
o Air temperature in the testing area may have differed
o The flames from the lighter could’ve affected the burning speed
o The burning of the paper could’ve accelerated the burning speed
• Analyzing the video
o Wrong determination of the time at which the BP started/ended burning.
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4.2 Qualitative analysis
Black powder with 60% potassium nitrate was black in color, while black powder with
90% potassium nitrate was gray. This is because potassium nitrate is white and charcoal is black.
It was also apparent that excess potassium nitrate would stall the black powder from being
ignited. It was harder to ignite 90% KNO3 BP than it was to ignite 60% KNO3 BP.
Black powder with 60% potassium nitrate has excess carbon, therefore, leaves a black
residue. Black powder with 90% potassium nitrate has excess KNO3 concentration, therefore,
leaves a white residue. Black powder with 75% potassium nitrate has the proper ratios of
reactants, therefore, leaves little or no residue and burns the fastest.
4.3 Statistical analysis (2-sample t-test for means)
4.3.1 Comparing 60% and 75% black powder
Conditions for inference are met, except for 60% distribution’s slight skewness. Proceed with
caution.
µ75: mean burn rate in cm/s for 75% KNO3 black powder
µ60: mean burn rate in cm/s for 60% KNO3 black powder
H0: µ75 - µ60 = 0
Ha: µ75 - µ60 " 0
t = 18.069 (d.f. = 7.970), p-value: 9.436#10-8
Because the p-value is smaller than any reasonable % level, we reject H0. There is sufficient
statistical evidence that the mean burn rate in cm/s for 75% KNO3 black powder is significantly
larger than that of 60% KNO3 black powder.
4.3.2 Comparing 75% and 90% black powder
Conditions for inference are met.
µ75: mean burn rate in cm/s for 75% KNO3 black powder
µ90: mean burn rate in cm/s for 90% KNO3 black powder
H0 µ75 - µ90 = 0
Ha: µ75 - µ90 " 0
t = 30.745 (d.f. = 5.766), p-value: 1.292#10-7
Because the p-value is smaller than any reasonable % level, we reject H0. There is sufficient
statistical evidence that the mean burn rate in cm/s for 75% KNO3 black powder is significantly
larger than that of 90% KNO3 black powder.
5.0 Conclusion
The two statistical analysis reveals that the deflagration rates of 60% and 75% black
powder differs significantly, and that the deflagration rates of 75% and 90% black powder differ
significantly. I also discovered that having a higher concentration of potassium nitrate causes
deflagration rate to decrease more than having a lower concentration of potassium nitrate.
Chemical imbalance of the ingredients in 60% and 90% KNO3 black powder is thought to
be the primary cause of their slower deflagration rate.
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References
Akhavan, J. (2011). The chemistry of explosives. (3rd ed.). Royal Society of Chemistry.
Bedard, A. (n.d.). Black powder solid propellants. Retrieved from
http://www.astronautix.com/articles/blalants.htm
Bretscher, U. (n.d.). The recipe for black powder. Retrieved from
http://www.musketeer.ch/blackpowder/recipe.html
Calvert, J. B. (2009, September 4). Cannons and gunpowder. Retrieved from
http://mysite.du.edu/~jcalvert/tech/cannon.htm
Gorski, N. (n.d.). Charcoal making secrets. Retrieved from
http://www.skylighter.com/fireworks/how-to-make/homemade-charcoal.asp
Wallace, W. (1995). Fmx: The revised black book. Paladin Press.
Wakeman, R. (2003). Blackpowder to pyrodex and beyond. Retrieved from
http://www.chuckhawks.com/blackpowder_pyrodex.htm
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