Why outlet makers warn against aluminum wires?

>>"I have copper wires going from the box into the wall but the ends coming out of the wall are for some reason dull silver colored.That is why i assumed they're aluminum.Can it be some other material?How can i find out?Thanks."<<
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If your home was built in the 50s, or ealier, it is possible the conductors are tinned copper.

With the circuit turned off you can take a knife and scrape the tinned coating off exposing the copper conductor.

Some homes built in the middle to late 70s were wired with AL romex.
Jim

Here is a little reading material regarding AL terminations and AL to copper terminations.
Lit is a little old, but still to the point.

http://www.inspect-ny.com/aluminum/piclib02.htm