Friction experiments were conducted utilizing samples tested in a humid atmosphere (ambient laboratory humidity or argon bubbled through water) and samples that were carefully dried and tested in an atmosphere of very dry argon (H2O<1 ppm). Dry surfaces consistently yield a friction coefficient of 0.85 to 1.00 compared with 0.55 to 0.65 for the tests in a humid environment. Time-dependent increases in the friction coefficient and large-scale stick-slip instability are characteristic of surfaces exposed to atmospheric water, but they are eliminated with careful drying. Direct dependence of frictional strength on slip speed persists under both dry and humid conditions.