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Razor Blade and Glass

I was unpleasantly surprised to read Gary Lange's comments on using a razor
blade to remove algae from a glass tank:

> Date: Mon, 9 Apr 2001 20:27:41 -0500
> From: "Gary Lange" <gwlange at mindspring_com>
> NEVER, in caps, use a razor blade on a tank, or any scraper that contains
> razor blade.  Even the "experts" gouge the glass surface while trying to
> clean off the algae!

because I've been doing just that for some time with a razor blade mounted
on the end of a dowel rod.  I will suspend that method until others confirm
or reject Gary's dictum.

However, I did look at the Mohs scale of hardness to see if it shed any
light on the subject.  Unfortunately, the various versions I found on the
internet provided somewhat ambiguous answers.  Neither steel or glass is one
of the ten defining minerals that carry hardness numbers from 1 to 10.

As examples of material hardness, though, the majority indicated that glass
was a bit *harder* than a knife blade (glass = 6-7, knife blade=5.5), which
means the knife shouldn't scratch the glass.  One version, however, gave the
knife a hardness of 6 and glass 5, in which case the blade could scratch the

Do we have a metallurgist in the group?

John T. Fitch
E-mail: JTFitch at FitchFamily_com
Web Page: www.fitchfamily.com