Name: Corned BP
| percent | parts | component |
|---|---|---|
| 75.00% | 15 | Potassium Nitrate |
| 15.00% | 3 | Charcoal (airfloat) |
| 10.00% | 2 | Sulfur |
The chemicals are ball milled together, as for making Blackpowder Meal. The fluffy meal powder is then dampened with 3-8% of its mass of water and pressed in a suitable die with a hydraulic press until a cake of density near 1.7 g/cc is formed. Some water will be pressed out of the cake in this process.
The cake is allowed to dry until it sounds like china when tapped, then broken up in a mortar and pestle and sieved into different particle size fractions for different purposes.
An additional step of glazing is optional, but is used commercially in sporting powders. The powder is tumbled with a small amount of graphite to polish and glaze it. Such powders are denoted by the "g" type extension.
Unglazed powders (so called blasting powders) are denoted with the "A" extension, but may be tumbled without graphite to polish them slightly.
The GOEX specifications are based on 3% held and 12% pass for each mesh size range.
Sporting Powders:
-0.5+4 mesh Whaling
-6+12 mesh Cannon/Life Saving
-10+20 mesh Saluting
-12+15 mesh Fg
-16+30 mesh FFg
-20+50 mesh FFFg
-40+100 mesh FFFFg
Blasting Powders:
-0.3+5 mesh FA
-4+12 mesh 2FA
-10+16 mesh 3FA
-12+20 mesh 4FA
-20+50 mesh 5FA
-30+50 mesh 6FA
-40+100 mesh 7FA
Meal Powders:
-40 mesh Meal D
-100 mesh Meal F
-140 mesh Meal XF
BP is generally specified as 75:15:10 to make the parts into percentages.
Everyone has their own secrets for making 'hot' BP.
It is generally agreed that adding moisture before pressing is critical to the corning process. Using alcohol alone or other solvents results in weak grains that break up easily and consequently results in a high yield of corning dust.
The use of moderate alcohol solutions for moistening is more debated. As some liquid escapes in the pressing process and the oxidiser particle size is critical for performance it is often argued that anything that lowers the potassium nitrate solubility in the moistening solution is a good thing. In practice little is actually lost even with pure water, but recrystalisation is a potential cause of lowered performance.
I use straight water, and it seems to work just fine.
The charcoal is also often discussed. Low density timbers seem to make the best BP charcoal, Willow is considered one of the best. Balsa works very well too, but the yield is low. Bark in general is terrible, and usually has a high ash and grit content, so logs for BP charcoal are debarked before use. It isn't quite as simple as just cooking up willows sticks though, the reactivity of the charcoal also depends on the temperature it was pyrolysed at and the duration of the process. It is possible to "cook" the goodness out of charcoal for BP. The more "graphitic" and the less bound hydrogen and oxygen contained in the charcoal the worse it will be for BP.
While at first it would appear activated charcoal would be perfect because of its high surface area, its relative purity renders it unsuitable for high performance BP. The reactivity of charcoal for BP use is often assumed to come from the benzoyls and other cyclic hydrocarbon radicals bound in its structure.
High resin timbers generally work poorly as well, but are often used in BP based effects compositions for long duration sparks. Pine makes poor lift BP, but great rocket or driver propellant for example.
Commercial production of BP is said to source its charcoal from mixed hardwoods and compensates for variations in the feedstock by blending different particle size fractions and/or BP batches to achieve consistent results.