In the 1960s and 70s, single-strand (solid) aluminum wiring was sometimes substituted for copper branch-circuit wiring in residential electrical systems due to the increased price of copper. After a decade of use by homeowners and electricians, inherent weaknesses were discovered in the metal that lead to its disuse as a branch wiring material. Aluminum will become defective faster than copper due to certain qualities inherent in the metal. Neglected connections in outlets, switches and light fixtures become increasingly dangerous over time. Poor connections may cause wiring to overheat, creating a potential fire hazard. Due to implications in numerous house fires, this once common practice is no longer permitted in new installations in most jurisdictions. Don't panic if you find aluminum wiring in your home! Aluminum wiring can be just as safe as copper wire as long as it was installed correctly, as it is very unforgiving if it incorrectly installed.
Aluminum wiring expands when it warms up, contracting when it cools down and reacts differently than copper wire after several warm/cooling cycles. After each cycle aluminum tends to lose more of its tightness (often referred to as "cold creep"). Combined with aluminum's tendency to oxidize when in contact with certain metals, it can lead to dangerous problems. When aluminum oxidizes it heats up more to conduct the same amount of electricity, which then causes more oxidation. Due to this cycle, eventually the wires may start to get very hot, melt the attached fixture and possibly cause a fire. Aluminum wiring should be evaluated by a qualified electrician who is experienced in evaluating and correcting aluminum wiring problems.
If you are considering purchasing a home with aluminum wiring, or have discovered it later, be sure to hire a licensed electrician or inspector to check over the wiring for any potential problems.