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What is it about dirt getting into a saw blade (as opposed to, say, sawdust, rock chips, etc (presuming the right blade is being used, of course)) that will kill a saw blade?

I've seen it happen to sawzall-type blades and chainsaws - but don't see what is so "special" about dirt that would cause those issues: especially when the self-same blade can be worked through PVC, pitch-encrusted trees, etc with little-to-no (or "normal") wear/damage.

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"As opposed to rock chips". Rock chips should always do more harm than dirt. But they are a whole lot less common and easier to avoid. For example with a chainsaw, dirt is in general going to cause the most wear. But metal and rock is far more instantly catastrophic, it is just less common and easier to avoid. –  Jonathon Wisnoski Jul 31 at 15:47

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up vote 28 down vote accepted

"Dirt", really soil, is made up of components that varies by locale, but almost always contains a significant amount of silica and other silicates, in the form of sand or finely ground quartz. It also contains other hard minerals in addition to softer organic materials.

Silica and silicates are the same materials that are used in abrasives, such as sandpaper and grinding compounds.

Running a sharp metal blade through abrasives dulls them.

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The key measure here is scratch hardness: quartz has a Mohs hardness of 7, while tool blades usually have a hardness around 6. PVC, wood, etc. don't even register on the scratch hardness scale. –  Mark Jul 9 at 8:52
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@Mark That's exactly what I was going to say (if I knew any science). –  bib Jul 9 at 12:40
    
Same for chain saw blades when you "accidentally" push them into the ground. –  user19121 Jul 9 at 20:41
    
Basically, saws made for wood are only good for wood, and trying to cut metal, rock, or dirt with them will cause problems. It is just that dirt is so much more common. –  Jonathon Wisnoski Jul 31 at 15:01

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