Coated electrical insulating paper and method of making it

Electrical insulating paper bearing a surface coating of a material insoluble in liquid dielectrics and substantially free of alkali metals. The coating material is present to the extent of between 2 and 15 percent of the total weight of the coated paper. The coated paper is thinner than otherwise identical uncoated paper having the same dielectric strength, and the coated paper has a percentage increase in dielectric strength greater than the percentage increase in its weight due to the presence of the coating. The coating material is soluble in water within a first range of temperatures and insoluble in water within a second range of temperatures. Prior to application of the coating material to the paper, the material is washed in water within the second range of temperatures to remove alkali metals, and thereafter the material is dissolved in water within the first temperature range to produce an aqueous solution for application to the paper. The coating material may be methyl cellulose, starch, polyvinyl alcohol, or protein.