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I know that ceramic is harder (you can scratch glass with a piece of ceramic), might have something to do with it too.
I tried the ceramic thing because I was incredulous (this was way before the days of YouTube -- and yes, it was done legally on an old car an uncle was junking; he's the one who'd told me about it) and it doesn't require all that much effort. I hurled similar sized chunks of gravel at the window to compare with nowhere near the same results. So it's pretty surprising.
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