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sions that require more information than may be obtained from other thickness- gauging methods. Information concerning porosity, surface roughness, grain structure, and adhesion may be gathered from the specimen prepared for a thickness test. ASTM B 487 is a standard method outlining the conditions for accurate results. The wavelength of light limits the resolution of the light microscope to about 10 microinches. The scanning electron microscope utilizes the shorter wavelength of electron waves to measure metallurgical specimens with a 4 microinch reso- lution. ASTM B 748 is the standard test method for this procedure. Attachments are available for the metallurgical microscope that allow it to be used as an interferometer. The method is mentioned, because its accuracy with thin coatings can be within ±5% of the true thickness. Conditions necessary for this procedure are contained in ASTM Standard Method B 588. THE SPOT TEST This test was developed as a rapid and inexpensive thickness test for chromium coatings on nickel and stainless steel. The test has an accuracy of ±20% for coat- ings up to 1.2 µm thick. A wax ring is outlined on the part to be tested. A drop of hydrochloric acid is placed in the ring, and the time between the onset and ces- sation of gassing is recorded. ASTM B 556 provides a standard guide for the per- formance of this test. X-RAY FLUORESCENCE This method is similar to beta backscatter in that the area to be tested is the target of radiation, and the energy emitted from that surface is measured. In this method, the radiation used is X-rays produced by an X-ray tube. The radiation measured is secondary emissions from the interaction of the X-rays with the coating and substrate. Unlike beta backscatter, the emissions measured are specific for each metal. Among the unique characteristics of this method are the following: 1. No physical contact with the measured surface is required, thereby protecting that surface. 2. Extremely small areas may be measured. 3. Since the emissions are specific for each metal, alloy compositions may be determined. 4. With proper calibration, intermediate coatings may be measured in a multilayer system. Thickness may be measured in the range 0.25 to 10 µm, depending on the met- al being measured. With proper calibration, the thickness may be determined to within 10% of its true value. ASTM B 568 outlines a standard method for this mea- surement system. Due to the noncontacting characteristic and the rapid test time, this procedure is used to continuously monitor thickness on continuous-coil plat- ing machines and automated plating machines. STANDARDS Thickness standards are required for calibrating thickness gauges. In most cas- es, the standards should be plated from a similar plating solution and on the same base metal as the product to be tested. 496