Some companies offer to “rewire” houses that have aluminum wiring. Sometimes, what they actually do is install aluminum compatible switches/receptacle outlets and/or install “purple” wire nuts. They then add copper extensions to all the wires in order to connect them to copper devices/lighting fixture leads.
These aluminum compatible devices and these purple wire nuts (which contain an anti-oxidizing compound to reduce the likelihood of a galvanic reaction between the copper and aluminum conductors) are no longer approved by the Consumer Product Safety Commission for permanent repairs.
There are ONLY two approved, permanent, connection methods for joining aluminum and copper:
* COPALUM – Uses hydraulic press to join copper and aluminum together. Requires hiring an expert in the field of copalum.
* ALUMICONN – Involves connecting the wires together using “alumiconn” connectors. These connectors can be purchased off-shelf and installed by a licensed electrician.
Link to more information : https://en.m.wikipedia.org/wiki/Knob-and-tube_wiring
Why is Aluminum Wiring Dangerous?
“Aluminum will become defective faster than copper due to certain qualities inherent in the metal. Neglected connections in outlets, switches and light fixtures containing aluminum wiring become increasingly dangerous over time. Poor connections cause wiring to overheat, creating a potential fire hazard.”