The Preece test is a procedure to determine the thinnest areas of zinc in newly galvanized components. It is not used on weathered or old components or to determine the relative weight of a coating. The test involves immersion of a test sample in a copper sulfate solution in intervals of one minute in order to dissolve the zinc coating. After each immersion and cleaning the sample is visually evaluated to see if the zinc is removed. When the copper is plated out on the steel, the test is completed and the relative thickness of the coating is known.
The Preece test is no longer in common use; electronic and magnetic instruments are used to determine the thickness of the zinc coating.