which consists of aluminum, nickel, and cobalt. Light-magnetic alloys include stainless steel and Monel
Metal (nickel and copper). All other metals and alloys are nonmagnetic.
(2) Spark testing. This test identifies some metals by characteristic sparks that are thrown off
when the specimen is held against a high-speed grinding wheel. The spark streams may vary from a few
tiny sparks to a shower of sparks. Skill in spark testing takes practice. When possible, compare the
sparks thrown off by the unknown specimen with spark streams from known samples. Standard samples
of known specifications should be maintained for comparison purposes. When testing, hold the
specimen with a firm, even pressure against the top of the grinding wheel. To free a wheel of metal
particles retained during previous uses, clean the grinding wheel surface frequently.
(3) A high-speed bench grinder is recommended for spark testing. Use a 6- to 8-inch wheel,
minute. Always wear goggles when spark testing. Metals and alloys that produce a spark on the grinder
include aluminum, brass, cadmium, copper, gold, lead, zinc, and antimony. Stainless steels and high-
temperature alloys with iron- and nickel-based compositions will produce characteristic sparks (Figure
5-13). As a general rule, the more iron in a specimen, the lighter the spark. As the percent of iron
decreases and the percent of nickel increases, the spark will darken.
Figure 5-13. Typical spark streams
c. Chemical (Acid) Spot Testing. When metals cannot be identified by visual examination or
mechanical testing use an acid spot test. Acid spot testing is done by placing one or more drops of acid
on the metal surface of a specimen, observing the surface's reaction to the acid, and comparing the color
reaction to a specific table of acid spot-test reactions. Clean a small surface of the metal with an emery
cloth, file, or grinding wheel before spot testing. Some of the acids needed for testing are-
Nitric acid (concentrated).
Hydrochloric acid (concentrated).