Strong, pin-hole free paper, sometimes impregnated with synthetic resins and made from unbleached Kraft pulp. Electrical insulating paper must neither contain fillers nor conductive contaminants (metals, coal, etc.) nor salts or acids. Lava stone bars are used on rotor and stator to avoid any metal contamination. Cable papers, that are wound around line wires in a spiral-like fashion, are electrical insulating papers with a particularly high strength in machine direction. Electrical grade papers include cable papers, electrolytic papers and capacitor paper.