[SEL] Oil Question

[JIM KIRKES]

I was recently at a show next to a hot air engine with lots of linkage.  The owner and I got to talking and we wound up with a oil question.  
when the linkage is oiled it later weeps out black even though their is no combustion nearby and it is in a clean location.  The things that were lubed were a steel pin through a brass link. Why does the oil turn black when dirt does not appear to be a factor?  

Jim and Diane Kirkes
Hemet, CA
jd.kirkes at verizon.net