Aluminium wire/wiring in houses is not that common. Though information states that homes during the early 1950's and late 1970's made use of them. This is because they oxidise and this increases their resistance and leads to overheating. Pig-tailing with copper will solve this plus AL stamp on the plastic covering indicates aluminuim wires.
Aluminum in smaller size house wiring was outlawed in the 70s but in bigger sizes as out door transmission lines or the lines entering your house are aluminum due to the cost of copper. Smaller size aluminum wiring has a habit of expanding an contracting when it gets hot so the connections come loose after time and cause arcing which leads to faulty wiring with house fires. Now older house have tube and knob wiring which is usually Grandfathered in until there is some remodeling done on the house and then an electrical wiring upgrade is required by Building Codes and now all electrical switches and receptacles are required to be used only with copper wire.
So copper pigtails can be used with aluminum wiring. Now also if you have a house with aluminum knob and tube wiring, chances are the house doesn't have a grounding system so plug have two slots instead of newer 3 point plugs and these house usually have asbestos insulation and lead paint too
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