Protargol-S (silver proteinate) is used with the addition of copper metal.  The copper replaces the silver in the connective tissue, allowing a greater differentiation between the nerve fibers and the connective tissue.  The silver is reduced with hydroquinone to the visible metallic form.  The sections are toned with gold chloride, the gold chloride is reduced with oxalic acid, increasing the deposit of metallic gold on the sections.

Protocol: Staining of Nerve Fibers Protocol