The thermal stability of electrical insulating varnishes is usually tested by evaluating the effect of elevated temperatures on the physical and electrical properties of a varnish film. The varnish film is tested in the form of a coating on 0.005 in. thick, 1 1/2 in. wide copper foil or as a coating on cotton or glass cloth. Asbestos paper and mica mat have also been used as supporting media for testing insulating varnishes. Since electrical insulating varnishes are composed of one or more organic components, they are subject to deterioration at elevated temperatures. Deterioration is the result of chemical and physical changes occurring in the varnish film during the heat-aging period. The chemistry of heat aging is very complex because of the many possible reactions that can occur. It is possible, however, to correlate certain types of chemical reactions with changes in the physical and electrical properties of a varnish film. There is considerable difference in chemical composition between the various commercial insulating varnishes. Varnishes made of synthetic resins usually form more heat-resistant films than varnishes made of natural resins. The silicone varnishes possess unusually high heat resistance and are chemically very stable, particularly in their resistance to oxidation. When the variables in the varnishes themselves are considered, it is evident that heat-aging tests must be interpreted with caution. Accelerated heat-aging tests are conducted at higher temperatures and under conditions such as total exposure which are not encountered in actual practice. Therefore, too great a reliance should not be placed upon short-time tests alone, but they should be supplemented by long-time tests. Probably the most valuable knowledge obtained from accelerated heat aging tests is the information furnished the design engineers concerning the effect of continuous operation at elevated temperatures upon the varnished insulation and observation of the shortness of the resulting life.