1. It's possible. Another possible cause is the interaction between the insulation and the conductor.
2. The copper wire is oxidized. The copper wire itself is not pure and has poor oxidation resistance. Oxidation occurs during annealing, which is exacerbated by storage. Moisture or oil contamination during stranding of the conductors, or excessive preheating during processing, can cause oxidation.
3. It could be primarily a problem with the copper wire itself, or perhaps the annealing was not done properly. Other possible causes are also possible.
4. Yes, we often overlook problems with the copper wire itself.
5. Thinking the problem lies with the copper wire means the antioxidant on the copper wire surface is too poor.
6. It seems that the copper wire's oxidation resistance is not very strong during drawing and annealing.
7. I'm not sure how you softened the copper wire. If you used a can softener, the temperature might have been too high when it came out of the can.
