[D0ct0rSt0N3]

I've been looking into black powder and ball mills for a research paper, and I read that black powder becomes more combustible when put in a ball mill filled with lead balls.
My question is HOW does crushing the substance more make it more reactive? I'm genuinely curious about the science behind it

[Corribus]

For one: a general feature of particulate matter is that as the particle size decreases, the reactivity increases. This is because the specific surface area increases and oxidation reactions happens at the surface. (Consider the analogous example grain dust is a serious explosion hazard in farm silos.)

[jeffmoonchop]

The act of ball milling is putting a lot of energy into the system. The constant milling will heat the system more the longer you mill, possibly getting over some activation energy barriers. And Corribus is right, the particle size will reduce at the same time.

[Borek]

The act of ball milling is putting a lot of energy into the system. The constant milling will heat the system more the longer you mill, possibly getting over some activation energy barriers.

Sounds like an ignition, not like making something easier to combust.

As Corribus wrote: reaction in solids takes place mostly on the grain surfaces, the finer the dust, the higher the surface per gram, the faster the reaction.

[rjb]

A ball mill represents a fairly safe way of grinding and mixing/homogenising the components of black powder.

Grinding reduces the particle size of the 3 components of BP (as well as ensuring that they are well mixed) and this has a really profound effect on the rate of reaction as well the energy required for ignition as stated by previous contributors. By reducing the size of particles, you are not only increasing the surface area available for combustion (which is essentially a surface phenomenon), but you are also bringing the individual particles (the fuel and oxidiser) closer together. These factors serve to reduce the energy required for ignition and increase the rate of reaction.   

A really nice example of how particle size affects rate of reaction and minimum ignition energy is demonstrated in thermites. The best known thermite is a mixture of Aluminium and Iron Oxide powders which will undergo a highly exothermic (but comparatively slow - at least in terms of pyrotechnics) redox reaction which has application in joining/welding of railway track amongst other things. Classical thermite takes a lot of energy to initiate... You can hold a match or lighter to it and it does nothing, you can whack it with a hammer or tap dance on it all day long and again it does nothing. It normally takes a lot of energy to get the stuff going and something like magnesium ribbon (which burns at about 2200C) is normally required. 

However, if you are able to reduce the particle size of the reactants, then thermites take on very different (and perhaps even slightly malevolent) properties. By reducing particle size to below about 100nm you produce what are known as metastable intermolecular composites (MICs) or nano-thermites which react extremely rapidly and are a lot more sensitive to initiation than common or garden 'classical' thermites. Interestingly, rate of reaction and ease of initiation are factors that can be tailored to a given application partly by increasing or reducing particle size, thereby allowing the production of MICs which are (for example) percussion sensitive. A nice example such an MIC is aluminium-bismuth oxide which has found application in primers for centre fire ammunition and is marketed (by Federal) under the Catalyst(TM) Trade name.

I hope that helps and my apologies for getting a bit carried away with my response

R