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Gum
From WikiPedalia
Tan, semi-translucent rubber, commonly natural rubber. This was once commonly use on the sidewalls of tires and on brake hoods. Natural gum rubber lacks the additives which would help defend it against ozone and other chemicals. Gum rubber tends to crack and deteriorate into a sticky mass over time. Some tan rubber is not natural gum rubber, and is more immune to this deterioration. See skinwall, blackwall, gum hoods.
