What Makes Grease Turn Black?

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CCI

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Jul 15, 2009
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New Mexico USA
I've been using Chevron Delo #2 (Blue) grease to pack wheel bearings on the motorcycle for about 15 years now, and seeing better bearing life than any other grease that I have used. By the time the tire is worn enough to change, the grease is black and thinned out, but the appearance of the bearing suggests that it is still working.

What makes the grease go from blue to black? Is the color change in any way indicative of a change in the lubricating properties?
 
Is the metal turning the grease black? Kinda like when you rub a white towel on aluminum it turns the towel black.
 
Oxidation, water getting past the seal, raceway damage.

Since the appearance of the bearing didn't alarm you, I have to agree with oxidation from overheat.
 
If you have decent seals, the main reasons for color change is microsize metal particles from wear and oxidation from heating.
 
You are using an excellent grease. I would not change. Those who mention oxidation (or moisture) causing the grease to turn black are correct. I always just grease u-joints until blue (or red with Spicer-spec Mystic) appears around all 4 yokes. Of course with the wheel bearings, your gonna clean all the old black grease out of the wheel and repack the bearings.
 
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