Testing a Metal’s Various Properties
Study the properties of your sample to determine whether it's a metal. In some cases, it can be easy to mistake nonmetal minerals for metals. Keep in mind that metals are typically strong, dense, and malleable, and often have some degree of luster or shine. By contrast, nonmetal minerals tend to be lightweight and brittle, with a dull coloration or finish. Try tapping your sample a few times with a hammer. Nonmetal minerals are easily broken, while metals will hold up to repeated strikes. Additionally, nonmetal minerals are insulators rather than conductors, meaning they don't transfer heat or electricity well. One simple way to test the conductivity of a mineral is by making it part of a circuit with a battery pack on one end and an electric bell or light bulb on the other. If your sample is a metal, the electricity will pass through it and activate the bell or bulb.
Hold a magnet up to the metal to see whether it’s ferrous or nonferrous. If you feel an attraction, the metal you’re attempting to identify is most likely ferrous. Generally speaking, ferrous metals are magnetic, while nonferrous metals are non-magnetic. Ferrous metals are those that contain iron, including raw iron, carbon steel, pure nickel, and various iron alloys. Many common metals are nonferrous, including lead, aluminum, lead, copper, gold, silver, and zinc.
File a section of the metal to gauge its hardness. Run a carbon steel file over an inconspicuous part of your sample. If the file slides easily over the surface of the metal, it means that it’s too hard for the file to leave a mark. If the file bites into the metal or shaves off small pieces, however, it’s most likely a softer type, such as copper or lead. Certain metals, like lead, aluminum, magnesium, and alkali metals, are so soft that you can actually cut them with a knife. It can be difficult to leave so much as a scratch on harder metals like iron, stainless steel, titanium, and various alloys.Tip: Filing a metal sample can also reveal whether it’s the same substance all the way through or it's been coated with another exterior metal.
Feel for a weight difference in similar-looking metals. If you have two or more unidentified sample metals that you can’t tell apart, pick them up and compare how they feel in your hand. Lead, for example, will be noticeably heavier than iron, and stainless steel will have more heft than tin or aluminum. In order to accurately compare the weight of your samples, it’s important that they be close in size.
Conducting More Specialized Tests
Run a Rockwell Hardness Test to find the metal’s exact hardness. The Rockwell Hardness Test is the most scientifically accurate and consistent means of testing the hardness of a given metal. To carry out this test, load your sample into a Rockwell Hardness Test machine, which will make a pair of indentations in the same site along the sample's surface using a sharp test point. The softer the metal, the deeper the final indentation will be. Once the test is complete, the machine will automatically generate a numerical Rockwell Hardness Rating for your sample. Compare this number to a chart listing the ratings of various metals to determine what type of metal your sample most likely is. A rating of 20-25 on the “B” scale, for example, corresponds to aluminum. There is also another type of hardness test known as the Brinell Hardness Test. This version uses a rounded metal test point rather than a sharp one, but the basic principle is the same.
Perform a spark test if you’re an experienced welder or technician. Hold a piece of scrap metal against an automated grinder with enough pressure to create a steady stream of sparks. Once the sparks are flowing, examine their length, shape, and color to make a determination about the type of metal that produced them. Wrought iron sparks, for example, have long, straight shafts and burn a bright yellow-white, while the sparks from high-carbon steel are pure white and form erratic branching patterns. Other machinable metals have their own unique spark profiles that can aid in their identification. If you have access to the necessary equipment and experience working with different metals, a quick spark test can tell you almost everything you need to know about the material you have on your hands. Before you begin grinding your test metal, pull on a pair of thick, durable welding gloves and some safety goggles to protect yourself from flying sparks.
Use nitric acid to test for real gold or silver. Fill an eyedropper with a small amount of pure nitric acid. Then, file down a small, out-of-the-way spot on your metal, apply 1-2 drops of acid, and wait a few seconds. Real gold won’t react at all to the acid, while real silver will turn a creamy white where it makes contact. If the exposed area turns green or another color, it’s most likely a different type of metal. You can purchase nitric acid online from laboratory supply stores that carry test chemicals. Remember: not all that glitters is gold. A metal that looks like gold could just as easily be brass, pyrite (a mineral sometimes referred to as “fool’s gold” for its resemblance to the metal), or some sort of composite. Silver also tarnishes when exposed to the elements, producing a finish similar to the patina that forms on brass.Warning: Nitric acid is mildly corrosive, and could cause irritation if it touches your skin. Be sure to wear gloves and safety goggles, and set up your materials in a well-ventilated space.
Analyzing a Metal’s Appearance
Note the deep gray color of iron. Iron, the quintessential ferrous metal, tends to have a moderate or dark gray hue throughout. Pure and minimally-processed iron may look somewhat dingy or display hints of brown. Most types of iron exhibit an angular, crystalline structure when filed or broken. This is especially true if your sample hasn’t been processed or blended with other metals. Items like household fixtures, tools, electric motors, and old-fashioned furniture and appliances often contain some percentage of iron. Cast iron has a Rockwell Hardness Rating of 86.
Look for the characteristic silver shine of stainless steel. Unlike unprocessed ferrous metals, stainless steel is notable for its bright, pale-gray color that gleams in the light. It may even be somewhat reflective, depending on how it’s been treated. Stainless steel almost always has the same unique dull silver tone, despite there being several different kinds. If you have a nonferrous metal with a silvery hue, there’s a good chance that it’s either tin or aluminum. Stainless steel is commonly used for cookware, eating utensils, household appliances, building materials, and storage containers. Stainless steel has a Rockwell Hardness Rating of 88.
Pay attention to a reddish tint that might suggest copper. You can usually recognize copper by its distinctive red-orange color, which may appear more brown if your sample is particularly old. It’s the same metal that’s used to coat the outside of U.S. pennies. If you’re still not sure whether the metal you have is copper, place it outside for a few days. Copper turns green as it oxidizes. Thanks to its exceptional conductivity, copper is a go-to metal for manufacturers of pipes, electrical wires, roofing materials, and internal components for computers and other electronics. Copper has a Rockwell Hardness Rating of 10.
Check for a yellow hue to differentiate brass and bronze from copper. If your metal is more yellow than red or orange, it’s possible that it’s either brass or bronze. Both of these metals are copper alloys, which means they look very similar to copper. However, they generally have more of a golden or hay-colored cast. Bronze is often slightly darker than brass, as it contains a higher proportion of copper. Brass and bronze are used everyday to make things like musical instruments, pipe fittings, and manifolds. Brass ordinarily has a Rockwell Hardness Rating somewhere between 55 and 65, while the HRC of bronze can range from 42 to 78. Bronze is an alloy made of copper and tin that's used to make items like Olympic medals, bells, and sculptures. Brass is an alloy of copper and zinc that's used to make items like musical instruments, plumbing, and ammunition casing.Tip: As bronze ages, it develops a patina, or a multicolored film-like buildup that occurs when the metal reacts with the air. The presence of a patina is a tip off that you’ve got genuine bronze as opposed to copper or brass.
Single out lead by its deep gray color, weight, and softness. Lead looks a lot like silver, only darker and with a dull, washed-out finish. It’s also very heavy for its size, and is soft enough to rub off on other objects. Try dragging the edge of your metal against a piece of paper. If it leaves a streak, it’s probably lead. One surefire way to find out whether a given metal is lead is to evaluate a small sample using a lead testing kit, which you can pick up at your local hardware store. Follow the provided instructions carefully—most kits display a red or pink color when lead is detected. It’s a good idea to wear gloves anytime you’re handling a metal that you think might be lead, as the metal is highly toxic. While not as common as it used to be, lead can still be found in certain industrial materials, such as electrical sheathing, car batteries, and soundproofing. In the past, it was a popular ingredient in everything from interior paint to children's toys. Lead has a Rockwell Hardness Rating of 5, the lowest of all metals.
Learn to recognize aluminum by its pale color and light weight. Aluminum is most often a very light gray color, without the sparkle of silver or the dullness of lead. Since it can be tough to distinguish it from other silvery metals, a quick weight check can come in handy. Aluminum is one of the lightest metals there is—small pieces may feel practically weightless in your hand. Aluminum is also nonferrous, which means that a magnet won’t stick to it. Another remarkable characteristic of aluminum is that it doesn’t rust. If all else fails, leave your metal outside for several days or weeks. When you come back to collect it, look for traces of red or brown that might point to oxidation. Mailboxes, ladders, trash cans, metal fencing, bicycle frames, staples, and hubcaps are all made from aluminum, which is lightweight and holds its shape well when molded. Aluminum has a variable Rockwell Hardness Rating of 20-40, depending on how it's processed and whether it's combined with other metals.
Identify titanium by comparing it to other gray-colored metals. Titanium is darker than aluminum, but lighter and shinier than lead. Like most metals, it’s nonferrous, so it won’t have an attraction to even the strongest magnet. And, since it's extremely hard, you won't be able to scratch it with a file. Inspect your metal closely for grade markings. Scraps of processed titanium may be stamped with a string of numbers and letters indicating their exact composition. Titanium is prized for its high strength-to-weight ratio, which makes it a perfect addition to building materials, aircraft components, golf clubs, jewelry, eyeglass frames, and even artificial hip and knee joints. Titanium has a Rockwell Hardness Rating of 80, coming in just behind cast iron and stainless steel.
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