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Sparks when grinding hard carbon steel
Please understand that, not being a chemist or metallurgist, I’m speaking out of my expertise and this is just my attempt to explain what is happening here.  I would more than welcome and hopefully someone here can offer edification, clarification or refutation.
In this post, Mr. Mark spoke of creating a lot of sparks when grinding hard carbon steel.  I responded in a following post by stating that I have never encountered sparking during basic sharpening with a Kally.  Mr. Mark then suggested that I try the tip of a file.  I did, and indeed, even the lightest grinding produced glowing sparks much like burning a sparkler.  Even just barely touching the file to the 150 grit Cubitron belt created sparks.  It’s rather remarkable.  So much so that it seemed that if only one abrasive particle on the belt touched the file a corresponding spark was produced.  Try it.  You’ll like it!
So, what’s gong on here?  It would be logical to assume that there must be something about the steel that increases friction so dramatically as to generate enough heat to melt steel to the point of glowing red hot with even the most minor and ephemeral contact with abrasives.  That makes sense, but I think that explanation is erroneous and there is another cause of the sparking.
Oddly, very fine particles of iron are pyrophoric.  Pyrophoric materials spontaneously ignite when exposed to oxygen rich environments such as air, due to heat generation caused by extremely rapid exothermic oxidation.
I’m sure many of us have witnessed flash rusting of freshly ground iron.  They don’t call it flash rusting for nothing!  Rust starts to form almost instantly if the material is not protected.  In this case, as soon as flash rusting starts it slows dramatically because the instantaneous coating of rust (oxidation) isolates the surface from exposure to oxygen in the surrounding environment.
Grinding during sharpening produces extremely small iron particles.  When grinding, not only is the protective layer of oxidation stripped from the outer surface of the blade by the abrasive, but when particles are torn from the edge the freshly exposed “underside” of these particles, also not protected by a layer of oxidation, is exposed.  This creates the maximum surface area relative to the particle size possible that is available for oxidation.  Even the small additional heat generated by grinding facilitates the oxidation process and the particles burn in the air.

So, I did some searching around and found a pretty good explanation of what I think is going on:

I always found it odd that iron and steel can actually burn.  My first exposure to this was when I was a kid with a chemistry set.  I remember being surprised to see iron filings burn when sprinkled into the flame of an alcohol burner, and equally fascinated to find out how easily steel wool can be ignited with a lighter.  I guess I shouldn’t have been considering I loved setting strips of magnesium on fire.  I highly recommend igniting magnesium as entertainment for kids.  Hours of fun there!  Oddly I still have all my fingers and vision in both eyes.
VERY interesting Mr. Grepper. Thanks for the explanation. Someone gave me a fire starter years ago. It was a piece of deer antler with a magnesium rod embedded in it, a small piece of steel file sticking out the end and a piece of hacksaw blade. With the hacksaw blade you scraped some shavings of magnesium onto your tinder and then drug the saw blade against the file creating a shower of sparks. The magnesium acted like a match set to gasoline.
Friction certainly gets metal hot, but I believe sparks are a different thing altogether. Friction produces heat, but heat isn't fire. You can get a bunch of hot steel particles without any sparks, which is common with lower speed grinding and softer steel. When you see a spark it's because a steel particle is actually "on fire", but it burns from reacting to the oxygen in the atmosphere. Yes, more pressure (which is a lot like "more friction") produces more sparks. Perhaps more friction makes the particles hotter, which leads to easier combustion. To be honest I'm not exactly sure what role friction plays, although I can tell you I don't see friction mentioned in relation to sparks. There is something called "oxidation potential" or "reduction potential" that seems germane to the topic, although it gets lost in translation to me. 

Check out "Spark Testing" on Wiki. It's one of the most complete guides to different sparks I've seen. It's fun to actually observe the variety of sparks, and learn the subtle differences. An angle grinder is probably the best tool for spark testing because the speed and coarseness of the abrasive produces sparks from merely touching practically anything.   

Files are hard, so the particles coming off are very small. Small particles have a lot of surface area (to react with oxygen) compared to their mass, so they immediately burst into flame even at low speed. The softer the steel the larger the particles, and large particles need more speed (perhaps more friction...?) to oxidize fast enough to light up.

The alloying elements of the steel have a lot to do with the sparks too. Chromium doesn't oxidize (rust or burn) very readily compared to iron, which is why stainless steel particles aren't reactive enough to oxidize quickly enough to burst into flame compared to plain carbon steel. 

This is a very interesting topic to me, so I'm willing to go out on a limb to throw my 2¢ into the ring. I have no formal training to back any theories I'm mentioning, but I've seen a lot of sparks.  Smile

ps- Magnesium fire starters are a lot of fun to me too, Mr. Bud. Wink
Yeah, it is interesting and it seems there are two disparate considerations; frictional heating and pyrophoric combustion.

The proverbial flint and steel fire starting thing is an excellent example. When you think about it, well, at least when I think about it, the copious volume of sparks produced by striking flint on steel doesn’t quite make sense because so little work is done in relation to the voluminous amount of sparks produced.  That quick strike of flint and steel can’t possibly cause enough mechanically induced frictional heating to produce such a glowing red hot shower of sparks.

I just did a quick search and of course the flint/steel stuff is all over Internet land.

Here you can watch the particles combust and burn in the air. Pretty cool. It starts about 1:20 into the video:
I watched the video Mr. Grepper and this old dog just learned something new. I would have never imagined that is how it works. I'm really amazed. Sparks don't even necessarily mean that anything on the metal being ground has gotten hot. Like I said, amazing!
Ditto with Bud, Grepper. Great video and great subject matter. We've learned something.
Thank you Grepper.

I wanted to know why many sparks occur when sharpening high carbon steels with belt sharpener.
I think this from Mark Reich above may speak to your question Mr. Sharpco.

"Files are hard, so the particles coming off are very small. Small particles have a lot of surface area (to react with oxygen) compared to their mass, so they immediately burst into flame even at low speed. The softer the steel the larger the particles, and large particles need more speed (perhaps more friction...?) to oxidize fast enough to light up."

And Merry Christmas to everyone!

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