Kraft Paper Insulation

Kraft Paper is a special paper used in electrical insulation.  Kraft paper is just plain paper from which the alkaline salts have been removed.  It is usually impregnated with mineral oil, jelly or wax to improve its dielectric properties.  
Kraft paper is made thermally resistant by applying a diamond patterned epoxy resin to both sides.  This is known as thermally upgraded Kraft paper which finds wide application in oil filled transformers for coil layer insulation. 

Paper capacitors are also made using Kraft paper as the dielectric.