While making a splice into the 50 year old wiring on my Atlas, I skinned the old wiring to find dull brown, oxidized, copper under the insulation. A crimp connection would have high resistance, and a solder connection would be near impossible.
I removed the oxidation by fanning out the wire strands, and dipping the wire ends in cartridge brass cleaner, producing shiny copper in minutes. The active ingredient is citric acid. You can make your own by mixing 1 cup water, 1/2 cup vinegar, 1/4 cup lemon juice, and 1/16 cup salt.