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How to Identify Unknown Metal in the Shop

Guide · Updated October 28, 2026

Unmarked drops and scrap pile bar are cheap, but you need a rough idea of what they are before you pick speeds, try to weld, or heat treat. A few quick shop tests, used together, narrow it down: a magnet, the spark pattern on a grinder, a file, and a density check from weight and size. They cannot tell 4140 from 4340, but they will tell mild steel from tool steel, stainless from aluminum and brass from bronze in most cases.

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Step by step

  1. Look and clean: Clean a spot and note the color, surface finish and any mill marks or paint codes.
  2. Magnet test: Carbon, alloy and tool steel, cast iron and 400-series stainless are magnetic. Aluminum, copper alloys, titanium and most 300-series stainless are not.
  3. Density check: Weigh the piece and calculate its volume to get lb per in³.
  4. File test: Try a file on an edge. If it skates without cutting, the piece is hardened, roughly 60 HRC or more.
  5. Spark test: For magnetic ferrous metals, touch lightly to a grinding wheel and compare the spark stream with known samples.
  6. Confirm if it matters: For critical parts, get a positive material identification (XRF) or buy certified stock.

Density from weight and size

Density = weight ÷ volume. For a round bar, volume = 0.7854 × D² × L. Typical densities in lb/in³: aluminum 6061 about 0.098, titanium 6Al-4V about 0.160, gray cast iron about 0.26, carbon steel about 0.284, 304 stainless about 0.289, free-machining brass about 0.307 and copper about 0.323.

Worked example: a 1.000 in diameter bar, 6.00 in long, weighs 1.44 lb. Volume = 0.7854 × 1 × 6 = 4.712 in³. Density = 1.44 ÷ 4.712 = 0.306 lb/in³, which points to brass rather than bronze or steel. If it weighed 1.34 lb (0.284 lb/in³) and was magnetic, it would be steel. A non-magnetic bar near 0.29 lb/in³ is probably 300-series stainless.

Reading the spark stream

Use the same wheel and light pressure each time, and compare against known samples under the same light.

More carbon makes more bursts. Alloying elements such as tungsten and chromium dull the color and reduce bursts.

Chip and cut clues

Mild steel makes long stringy chips and is gummy; free-machining 12L14 or 1215 makes short, broken chips with a bright finish. 300-series stainless work-hardens and turns blue chips at moderate speeds. Gray cast iron makes powdery black chips. Leaded brass makes fine, broken chips and cuts very easily, while bronze is usually darker and tougher.

Common problems

Thinking stainless is never magnetic (cold-worked 300 series can be slightly magnetic, and 400 series is fully magnetic), spark testing on a dirty wheel loaded with aluminum, and trusting tests for parts that must meet a material spec. Never spark test magnesium or titanium dust in quantity; fine chips are a fire hazard.

FAQ

Is stainless steel magnetic?

400-series is magnetic. 300-series is mostly non-magnetic but can become slightly magnetic after cold working.

What does a file skating mean?

The material is hardened, roughly 60 HRC or above.

Can a spark test identify the exact alloy?

No. It separates families such as mild, high-carbon, cast iron and high-speed steel.

How do I measure the bar for density?

Use a mic for diameter; check the mic against gage blocks first. Build any stack free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.