Yes, most lenses are made of various plastics, but I was specifically talking about glass lenses. With plastic lenses scratches show up quickly and be pretty obvious when they happen, but with glass they gradually show up over a long period of time.
We frequently had to explain to people that almost anything that will scratch plastic lenses will also scratch glass lenses, it just takes longer and more times before you see them.
I thought water dissolves the granite. I recall having to memorize a long chain of reactions back in Earth Science classes, something about feldspars (or was is Marty Feldman)?
To pedantically reiterate what I’m getting from this thread: We have windshields with 150K+ miles on them never quite get ‘clean’. A light rain results in lots of smears and super-glare. Heavy rains are okay. New wipers didn’t really affect anything, even the fancy-shmancy brand.
So to revitalize the view, I’m going to use a foaming windshield cleaner and 0000 steel wool to clean (but not harshly scrub) the grime away.
Rock getting “shaped by water” is seldom attributable to just one single thing. Some of it is rock getting dissolved by the water. Some of it is chemical reactions with stuff in the water. Some of it is abrasion by particulate matter in the water. Some of it is wedging from water seeping into cracks and expanding when it freezes.
‘Not much’ is not the same as ‘not at all’.
A high pressure jet of pure water is commonly used in industry to cut metal, stone, glass and many other things, all of them harder than water. Of course, that’s an extreme case, but it’s actually going to be one end of a continuum. A less aggressive application will cause less wear, but there’s not going to be a point where the effect abruptly stops.
When it’s two solid materials rubbing against each other, the softer one wears more than the harder one, but the harder one does still wear some - it’s a continuous function, not a discrete one.
It doesn’t matter if the stone hits the pitcher or the pitcher pours out on the granite coutnertop under high pressure…
I restore antiques as part of my buisness. I have used steel wool and a soap slurry to clean all sorts of glass. For example railroad lanterns with 100+ years worth of soot and grime then left in a barn for a generation to be covered in pigeon crap. Don’t do it dry and you’ll be amazed.
Regarding Scotchbrite pads made by 3M there are at least 8 grades that I know of. The green one most people commonly encounter with the cleaning supplies is about 2/3 up the abrasive scale.
You are wrong. Glass cutters are made “of hardened steel or tungsten carbide” and indeed the specific glass cutter I own is made with a steel cutting wheel.
If I’m able to mark the glass with a glass cutter, then that’s proof steel is harder. You don’t need carbide to cut glass.
I don’t know where this notion is coming from that hard materials experience zero wear or abrasion from contact with anything less hard. If this were true, scissors, knives, scalpels, saw blades and drill bits would never go blunt unless used improperly.
That’s not true. Although pure water is used for soft materials, when cutting hard materials like metal an abrasive (some type of garnet, typically) is mixed into the stream. The nozzle must of course be harder yet to survive and is typically made from tungsten carbide.
One alloy of steel is harder than typical glass. High Speed Steel is already quite hard, but brittle to the point that it’s useless outside of tooling. There are other, even harder and more brittle steel alloys, of which your glass cutter is probably one. It has no resemblance to the steel used in steel wool.
My mistake - some of the search results for pure water jet cutting companies were listing stone and metal amongst the things they can cut, but the links, when followed, did not make good on this.
Nonetheless, all of the materials that **can **be cut with pure water, are harder than water, so the point still stands.
It doesn’t really reinforce your argument, though. Hard materials cut soft ones because (for a given applied pressure), the soft one deforms first. At high speed, though, you can exceed material strength by use of kinetic energy and momentum. A lead bullet can punch through a steel plate, but only when fired from a gun; the same bullet slowly squeezed into the plate won’t leave a mark. Anti-tank rounds punch through armor via a jet of liquid copper; obviously just pouring the copper on a tank would accomplish nothing.
Since the discussion here is low-speed applications, “kinetic hardness” (is that a thing?) isn’t really relevant.
As far as I know, blades tend to go dull from two main causes: first, the edge simply curls over; and second, all materials are contaminated to some extent with hard grit (like sand).
Is it therefore your assertion that in any interaction between two materials, the harder of the two experiences absolutely no wear at all? Because that’s a bizarre idea.
Are you asking theoretically or practically? If you eliminate all kinetic effects, all contaminant materials, any buckling effects (edges folding over, etc.), corrosion, inclusions, fatigue limits, and so on; and furthermore one material is significantly harder than the other (so that we aren’t arguing about degrees of deformation); then yes, I’d say the hard material will experience no wear at all.
Of course it’s hard to eliminate all this stuff. But I would suggest that there’s no reason to believe that ordinary steel wool is harsher on glass than other things that we rub on glass, like paper towels. Rub a paper towel on glass long enough and you’ll rub right through it since the dust in the air will act as an abrasive. It’s just not a significant effect.
Which would be why I said both
[ul]
[li]“[w]hile the comparison between various hardness scales are a little funky…”, and[/li]
[li]“Yes, there are a small handful of weird alloys that have been reported to have (converted) Mohs values above 6.5”[/li][/ul]
The properties of complex alloys (particularly exotic steels) are not well suited to the Mohs scale - which, in fairness, was devised two centuries ago as a qualitative test of mineral hardness. At some level its rather like asking “how sharp is a laser?”
Wait, what? :dubious: Chromium and nickel alloys are (almost always)harder and exhibit* increased* “toughness” vs the unalloyed steel samples. (Metallurgists will have a field day with the “almost above” - those who can provide an explanation for the exceptions without using the phrases “slip planes” or “grain boundaries” are invited to do so… 
).
(Ed: Summarized for quoting clarity)
Your first and third links are, respectively, from a engineering student’s lab page and a blog post on cell phone screen scratching - perhaps more authoritative sources would be appropriate. Your second link notes:
[ul]
[li]5.5 Knife blade[/li][li]6-7 Glass[/li][li]6.5 Iron pyrite[/li][li]7+ Hardened steel file[/li][/ul]
Which would appear to largely in line with what I’ve quoted so far.
N.B. Yes, if you insist on testing the hardness of your windshield using a steel file you are likely to be unhappy. Caveat actor. 
Both
The kinetic effects you’re talking about are just another way of saying “do this a lot, do it energetically and do it all in a short space of time” - it’s the same transaction that causes power tools to work faster than hand tools, even when the mechanical effects underlying them are the same.
No material is impervious to wear. Nature does not deal in absolutes, but rather, by degrees.
It doesn’t matter whether it’s chalk vs cheese or sapphire vs steel - when two materials come into moving contact, they both wear. The harder one wears less; the softer one wears more. You could wear a diamond down to dust by rubbing it with feathers, but it would take a very, very long time and a vast supply of feathers.
It may be that in many practical situations, the difference in hardness between the two materials means wear to the harder one is immeasurably small within normal timescales, but the bald fact that X is harder than Y does not imply that X is impervious to wear from interaction with Y. It’s not like rock-paper-scissors.
I very much disagree with this. The kinetic effects are effectively because the bulk modulus of common liquids and solids is quite high, and grows with increasing pressure. Under normal circumstances this is irrelevant since the soft material will just flow out of the way. But when the forces are applied over a short period of time, momentum ensures that the material can’t deform quickly enough, and then it just becomes a fight between the compressivity of one material vs. the other. This effect simply wouldn’t happen at low speeds.
I’m quite certain that you could not wear a diamond down to dust by rubbing it with feathers. A single rubbing would dislodge zero atoms. You could do this 10[sup]100[/sup] times and it would change nothing. Hardness is a threshold effect, and if you’re under that threshold the atoms just snap back into place.
OK, injudicious use of the word ‘just’ on my part, and point conceded, sort of…
However…
I don’t believe it’s correct to say that these effects ever scale down to an abrupt zero; they are essentially statistical in nature, so at the lower end of the scale, less than one isn’t zero, it’s ‘not every time’, and we can have things like sunlight evaporating diamonds - no single photon is guaranteed to dislodge any single carbon atom, but every once in a while, one does. Not zero times per interaction, just less than one.
Well, sure… at the quantum level just about anything can happen. Diamond at some non-zero temperature will eventually evaporate away on its own since sometimes the jiggles will line up just right and eject one of the atoms. But these are exceedingly improbable events most of the time.
And yes, touching a feather to a diamond could, in principle, add just enough of a jiggle to slightly increase the probability of an ejected atom. But it’s still going to be a very, very low probability since the carbon bonds are very strong and the maximum force the feather (keratin, I suppose) could apply is a small fraction of that. So it would still need a strong statistical fluctuation for anything to happen.
So you aren’t wrong, exactly, but you might have misapprehensions about the nature of these probabilities. They become exponentially less likely as the magnitudes of the forces get farther apart. When they’re close, say within a factor of two, then there is some possibility of mutual wear (see this article for an example, where rhenium diboride at 48 GPa can sometimes scratch diamond at 70-100 GPa). But this drops off rapidly enough that you’re soon in the “many lifetimes of the universe” territory as the forces get farther apart.