The dielectric properties of inorganic materials above 600 F are extremely sensitive to minor amounts of surface contamination. The contamination can cause effects which appear as degradation of insulation resistance. Sources of this contamination include fingerprints, vacuum grease, machine oil, organic binders, vacuum pumping oils, and volatile elements from metals. Careful selection of materials and rigid adherence to cleanliness techniques are required to evaluate dielectric materials correctly at high temperatures in vacuum. Pressure, radiation, and time have a negligible effect on the properties of inorganic insulations in clean vacuum conditions.