How do I tell what metal something is?

Getting this right matters more often than it sounds. It decides which drill bit and which blade, whether a fixing will corrode next to it, whether something can be soldered, brazed or welded, whether a paint will stick, and — in the case of lead — whether you should be handling it at all.

Safety

Step by step

1. Try a magnet first.It is the single most informative test and it takes a second. A magnet sticking means the metal is ferrous — iron or steel. A fridge magnet is enough; a small neodymium magnet is better because it also picks up the weak attraction of some stainless grades.
2. For a magnetic metal, look at how it rusts.Bare mild steel rusts readily and orange. Galvanised or zinc-plated steel resists rusting overall but rusts at cut edges and drill holes, because that is where the coating was removed. A magnetic metal that does not rust at all is likely a ferritic stainless, common in cutlery.
3. Tell cast iron from steel by weight, section and break.Cast iron is thick, heavy, grey and often has a slightly rough as-cast surface with mould lines. It is brittle: it chips and cracks rather than bending, which is why old cast iron gutters and soil pipes break rather than dent. Steel of the same size bends first.
4. For a non-magnetic metal, go by colour.Reddish brown is copper. Yellow is brass, an alloy of copper and zinc, which is what most valves, compression fittings, ironmongery and taps are made from under the plating. Bronze is browner and much less common in a house.
5. Use weight to separate aluminium from everything else.Aluminium is dramatically lighter than it looks, does not rust, forms a dull grey oxide instead, and is soft enough to scratch with a steel point. Nothing else in an ordinary house feels like it.
6. Recognise stainless steel by hardness and lack of rust.Heavy, hard, silver grey, and unmarked by weather. It resists a file more than mild steel does. Remember that the common A2 and A4 fixing grades are austenitic and only weakly magnetic or not at all — a magnet failing to stick is not proof it is not steel.
7. Watch for the two things that mislead.A coating hides the base metal: chromed brass looks like steel until you scratch a hidden face, and a painted steel gate looks like nothing at all. And a plated fixing looks like stainless until it starts rusting from the head.
8. Stop at a family, not a grade.These tests tell you it is brass, or stainless, or cast iron. They do not tell you the grade, the alloy or the strength, and no home test does. Where the grade matters — for corrosion resistance or for anything loaded — work from the specification.

Tips

• Scratch a hidden surface rather than a visible one. A small bright scratch on the back of a fitting tells you the base metal and does no harm.
• Copper in a house goes brown then, given long enough and weather, green. That green patina is the material behaving normally, not corroding away.
• A pipe you cannot identify in an older house is worth taking seriously before you touch it. Lead plumbing looks dull grey, is soft, and has swollen wiped joints rather than sharp fittings.

Common mistakes

• Treating a magnet failing to stick as proof of stainless — Aluminium, copper, brass and lead are all non-magnetic too, and they behave completely differently. Colour and weight are what separate them.
• Sanding or grinding an unknown old coating or metal — Old paint may be lead-based, old pipework and flashings may be lead, and grinding turns either into airborne dust. Identify before you make dust.
• Assuming a shiny tap is chrome-plated steel — Almost all are chrome-plated brass. It matters when you drill one, when you choose a fixing to sit against it, and when you decide whether it can be soldered.

If it doesn't work

Questions people ask