Date: Wed, 26 Aug 1998 17:12:51 -0700
From: Charles Falco
On August 26 Harold wrote:
At 07:20 PM 8/26/98 -0400, you wrote:
>> physics ruins restorations.
>
>And sometimes new underwear.
>
>One o' the early substances used for mirrors was quicksilver, which
some
>folks call mercury. Tends to hang with a fella'
>
>Listen, while you're on this metal reflectivity kick, why don't you
>advise as to how come stainless, chromium, and other alloys change
colour
>(ok, change the colour(s) they reflect) when they get hot as hell.
>
>For example, when the exhaust pipes turn blue, what gives? Are
we
>looking at some sort of compound between the chrome and stuff in the
>air, or what?
The colours you're seeing on formerly-hot pipes are the result of an interesting phenomenon: the destructive interference of specific wavelengths/colours of light due to the formation of semi-transparent oxides that are of roughly comparable thickness to the wavelength. Note that the visible spectrum covers ~7000 Angstroms (red) ---> 4500 Angstroms (violet), and that 10,000 Angstroms = 1 micrometer = ~39 micro-inches. As a rough approximation that's good enough for the present discussion, light with a wavelength that is approximately 4x the thickness of the oxide will be absorbed by the film rather than be reflected. Such a film is called a "quarter-wave anti-reflection coating." Thus, if an oxide is, say, ~1750 Angstroms thick, light near the red end of the spectrum (7000 A = 4 x 1750) _won't_ be reflected, leaving mostly blue in the light bounced back to our eyes.
>Howcome you (I) can remove discoloration of Stainless steel by
>mechanically gouging it off with a wire brush, but if I try to remove
>the bluing from chrome, the chrome comes off with it?
The reason is the head of your stainless steel fastener is ~1/4" thick, so polishing away a few micro-inches of oxide removes a negligible amount of the fastener. However, decorative chrome plating is only a few micro-inches thick (~500-1000 Angstroms) to begin with, so by the time it's blue or yellow, all that's left of the plating is a transparent film of chrome oxide anyway.
>The exhaust bluing thread hasn't been around for a week or two, maybe
>this time it could present some science.
Amal jetting doesn't kill chrome, physics kills chrome.
Charles Falco
Date: Tue, 7 Jul 1998 09:24:42 +0000
From: neoludd
Jim- do you loosen all the bolts/nuts at once, then retorque in stages to spec, or back off the nuts/bolts one at a time & retorque to spec? 90% of my engine rebuilding experience is automotive; lots more bolts, lots higher torque.
>and Jim Mason replied:
>I always loosen to break the static friction before retightening.