I was in the rubber business for several years, and our lab had well over
ten thousand different compounds of natural and synthetic rubber, each
compound consisting of around ten to twenty ingredients.  If you walked
through the plant you would see tubing, sheeting, balls and every
imaginable shape of black rubber parts.

They all looked like they were made of the same smelly stuff.  They
weren't.  Our player piano tubing was a combination of synthetic rubber
(for ozone resistance) and natural rubber (to resist compression set --
it stays springy enough to keep grabbing the nipple).

It is a combination of a ton of practice and a huge memory, lots of
testing (high temperature to mimic long time) and science.  Our rubber
chemist started before roads were paved!  A good chemist knows what to
put in a compound to make it resistant to different environments.

Allan Merralls