Identify scrap metal with three tests: a magnet separates steel, which pulls strongly, from non-magnetic aluminium, copper, brass, zinc and most stainless steels; colour and weight then separate those, with aluminium about a third the weight of steel for the same size, copper reddish-orange and heavy, and brass yellow and heavy; and a spark test on a grinder sorts steels by carbon content. Do the spark test outdoors in eye protection and never where fuel vapour might be present.
Part of Building Material Recovery
Different metals have different properties and uses, and several look alike under paint and grime. Simple tests with a magnet, a file and a grinder sort most of them without equipment.
The Magnet Does Most Of The Sorting
Magnet test. Carry a magnet, such as a speaker or refrigerator magnet (parent article).
- Sticks strongly: steel or iron. Structural, magnetic and forgeable.
- Sticks weakly: some stainless steels, which are useful for food and water contact because they resist corrosion.
- Does not stick: aluminium, copper, brass, zinc and most stainless. These are non-ferrous, each with distinct uses.
A galvanised sheet is steel under a zinc coat, so the magnet still pulls.
Colour and weight (parent article).
- Silver-grey, heavy and magnetic: steel.
- Silver-grey, light and non-magnetic: aluminium, about one third the weight of steel for a piece of the same size.
- Reddish-orange and heavy: copper, an excellent conductor and very valuable.
- Yellow and heavy: brass, a copper-zinc alloy good for bearings, fittings and low friction.
- Bluish-grey and of moderate weight: zinc, or galvanised steel with its zinc coating.
- Very heavy and soft grey: lead, found in roofing, old plumbing, weights and radiation shielding.
Scratch a bright spot with a file or a knife, since paint and corrosion hide the colour. Lead is soft enough to mark with a knife.
Spark test (steels). Hold the metal against a grinding wheel or stone and watch the sparks (parent article). See Grinding Wheels for wheel use.
- Long, straight, orange sparks: low-carbon mild steel, easy to work, bend and weld.
- Bushy, branching sparks: high-carbon steel, hard, holding edges, used for springs and tools.
- Few, short sparks: stainless steel.
- No sparks: non-ferrous metal such as aluminium or copper.
What You Need
- A magnet, a file and a knife.
- A grinder or a coarse stone, used outdoors.
- A scale or the feel of weight by hand, against a known steel piece.
- Eye protection and gloves.
Method: Magnet, Scratch, Weigh, Spark
Step 1: Magnet. Hold it to a clean spot. Strong pull means steel.
Step 2: File a bright patch on any piece you cannot place. Note the colour.
Step 3: Compare weight with a piece of known steel of similar size.
Step 4: Spark test the steels, outdoors, in a dim place so you can see the sparks. Compare against pieces you already know.
Step 5: Label and sort, and mark doubtful pieces for another test.
Common Mistakes
- Trusting colour on a painted or corroded piece. Scratch it.
- Assuming all stainless behaves alike. Some grades are weakly magnetic and some are not, so test several pieces.
- Spark-testing in a closed shed. Sparks and fuel vapour do not mix.
- Mixing high- and low-carbon steel. Their uses differ.
Safety
Grinder sparks ignite vapour, and wheels burst
The spark test throws sparks and metal fragments. Do it outdoors, away from fuel, solvent vapour, gas and dry vegetation, wearing eye protection and gloves. Check that the wheel is uncracked and stand to one side when starting it. Zinc coatings give off harmful fume when heated or ground in an enclosed space, so test galvanised pieces in the open air. Treat lead as toxic: do not grind or heat it, and wash hands after handling.
What’s Next
- Learn next: Cutting & Shaping Metal — the right tool for each metal
- Combine with: Copper Wiring & Plumbing — the most valuable metal in the walls