Solid copper crush washers compress to create a tight seal between two connecting parts. Sealing washers are commonly used as drain plug gaskets or in applications that require prevention from fluid (oil, fuel, water, etc.) leaks.
As a soft metal, copper fills small irregularities in contact surfaces. Corrosion resistant in damp and wet environments, copper is nonmagnetic and electrically conductive. Once compressed / installed, these gaskets are not reusable.