Why are my aluminum parts turning black?

Now when I machine aluminum the parts are turning brownish black!

Have I contaminated my coolant?
I have been using the synthetic stuff from Enco.

It would be nice to color my aluminum parts, but it is not a uniform discolorization, its only where the coolant puddles or runs over the parts!

I tryed to remove the stain using scotchbrite under running water, but it is not an easy task! This also leaves the parts frosted, and not as shiny as my other parts!

Any ideas, or should I just pump it out and scrub the sump inside the mill's casting? (Not looking forward to that)

Eric

Last month I setup my Bredgeport clone for flood coolant, and have been using it daily! Last week I was milling some bronze, it turned my coolant from clear/yellow to clear/green!Now when I machine aluminum the parts are turning brownish black!Have I contaminated my coolant?I have been using the synthetic stuff from Enco.It would be nice to color my aluminum parts, but it is not a uniform discolorization, its only where the coolant puddles or runs over the parts!I tryed to remove the stain using scotchbrite under running water, but it is not an easy task! This also leaves the parts frosted, and not as shiny as my other parts!Any ideas, or should I just pump it out and scrub the sump inside the mill's casting? (Not looking forward to that)Eric

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