Ben, I've no idea but rubber absorbs water, is affected by corona, and I'd guess it's black due to carbon/graphite for u.v. protection. Don't use rubber except for protection! ... Sulaiman > Original Poster: "Benjamin W. Bouxsein" <bbouxse-at-comp.uark.edu> > > > Anyone know the dialectric constant of 30mil roofing rubber, or > electric > breakdown point? > > thanx, > ben >