that's interesting. i thought they are completely made out of rubber. thank you.

When they made plug wire they had two layers of rubber one white (first layer) and one black top layer. The two compounds were the same except for the carbon black. Back in the early days of rubber car tires they were white not black, and didn't last very long then they started using carbon black. They also add diatomaceous earth to the mix as a filler. The DE is or was also used in toothpaste and scrubbing powder like Bon-Ami.

 

When they made plug wire they had two layers of rubber one white (first layer) and one black top layer. The two compounds were the same except for the carbon black. Back in the early days of rubber car tires they were white not black, and didn't last very long then they started using carbon black. They also add diatomaceous earth to the mix as a filler. The DE is or was also used in toothpaste and scrubbing powder like Bon-Ami.

thank you