Stainless steel and metal tools

A metal tool is in a different mechanical regime from every stone on this site. Metals are ductile: loaded past their limit they deform permanently instead of cracking, so a steel tool that takes a serious impact bends or dents where a stone tool breaks. The identification question also changes shape. It is rarely which metal, and usually whether the object is solid metal at all.

What stainless steel is

Steel is iron with carbon. Stainless steel adds chromium — conventionally at least around a tenth of the alloy by weight — which reacts with oxygen to form an extremely thin, adherent, self-repairing passive layer of chromium oxide on the surface. That film, not the iron beneath it, is what resists corrosion, and because it re-forms wherever it is scratched, the resistance survives damage.

The grades relevant to small consumer objects are the austenitic ones, commonly designated 304 and 316. Both are chromium-nickel alloys; 316 adds molybdenum, which improves resistance to chloride attack. That difference matters in salt environments and is close to irrelevant in most others.

Two properties are worth having straight:

Hardness is modest. Austenitic stainless is in the region of 5.5 on a Mohs comparison — softer than quartz, softer than either jade, and comparable to window glass. A steel tool scratches easily against harder stone. This is also why a steel blade used as a scratch tester sits in an ambiguous band, and why the scratch test’s reference points are approximate.

Density is high. Steel is roughly 7.9 grams per cubic centimetre. Nephrite is near 3, jadeite a little above 3, quartz about 2.65. A solid steel object is therefore two to three times the mass of a stone object of the same dimensions, and this is the single most informative observation available about a metal tool.

“Stainless” is not “rustproof.” The passive layer can be defeated: chloride exposure causes localised pitting, and contact with plain steel can transfer particles that rust in place and look like the stainless has failed. Corrosion resistance is a property with conditions, not a guarantee.

What ductility means in an object

Stone failure is fracture: a crack starts at a stress concentrator and runs. Metal failure is plastic deformation: the material yields, changes shape and stays changed.

The consequences are almost opposite.

A dropped metal tool dents; a dropped stone tool may shatter. For a thin flat object this is a large practical difference and it is the entire material argument for metal.

A thin metal section bends rather than snapping. It may fatigue and eventually crack after repeated bending, which is a different failure mode with a different timescale.

Edges deform rather than chip. A metal edge rolls or burrs; a stone edge flakes.

Thermal shock is not a concern. Metals conduct heat far too well to build the steep internal gradients that crack brittle materials, and they yield slightly instead of fracturing. The mechanism that breaks quartz in hot water has no purchase on steel.

Plated castings, which is the real question

The substitution in the metal tool market is not one steel for another. It is a cast base metal with a thin decorative plating, standing in for solid stainless.

Zinc alloy — die-casting alloys based on zinc — is cheap, melts at low temperature, casts crisply into detailed moulds, and is much softer and weaker than steel. It is then electroplated with chromium, nickel or another finish to look like stainless.

The tells are consistent:

Weight. Zinc alloy is around 6.6 to 6.8 grams per cubic centimetre and hollow castings are lighter still, so a plated casting is usually noticeably lighter than solid steel of the same size — though still heavier than stone.

Plating wear at edges and high points. A plated surface is microns thick. Where it wears through, a differently coloured base metal shows: yellowish for brass, grey-white and dull for zinc. This is the same class of evidence as a coating on stone — thin films betray themselves at the boundary.

Casting features. Mould parting lines, sprue marks ground flat, and slightly soft internal corners. Sheet or bar stock that has been stamped and ground shows different traces.

Sound and feel. A solid steel piece rings dully and feels inert; a hollow casting sounds hollow. This is an impression rather than a measurement.

WHAT YOU CAN TELL — a metal tool

  · Two to three times the mass of a stone piece
      of the same size → consistent with solid steel
  · Metal appearance but only slightly heavier than
      stone → plated casting or hollow construction
  · Different-coloured metal showing at worn edges
      → plating over a base metal. Conclusive
  · Mould parting line or ground sprue mark
      → cast, not machined from stock
  · Dents and bends rather than chipping → ductile
  · Which alloy or grade it is
      → NOT DETERMINABLE at home. Needs elemental
        analysis such as X-ray fluorescence

The magnet, and why it settles less than expected

A magnet is the test people reach for, and its logic is often reversed.

Austenitic stainless steels — the 300 series — are essentially non-magnetic or only weakly magnetic; cold working can raise their response somewhat. Ferritic and martensitic stainless steels, and ordinary carbon steel, are magnetic. Zinc alloy, brass, aluminium and copper are non-magnetic.

So a strong magnetic response indicates carbon steel or a ferritic or martensitic stainless, and no response is consistent with austenitic stainless, with several non-ferrous metals, and with a plated zinc casting. In other words the informative outcome is the one people read as a failure, and the reassuring outcome is the ambiguous one. Weight discriminates better than magnetism throughout.

Other metals in the market

Titanium is claimed occasionally. It is genuinely light for a metal — roughly 4.5 grams per cubic centimetre, well under steel — and corrosion resistant. A tool of genuine titanium would feel conspicuously light for a metal, which makes the claim testable in the crudest possible way.

Copper and brass tarnish. Copper develops a darkening oxide, brass dulls, and both are much softer than steel. Tarnish that returns after cleaning indicates a bare copper alloy rather than a plated finish.

Anodised aluminium is very light — around 2.7 — with a coloured oxide surface layer that abrades to show bare grey metal.

Naming the material is straightforward at the level of family, and that is the level at which synthetic, imitation and composite distinctions matter here too: a plated casting is a metal imitation of a metal object.

The limit

You can distinguish solid metal from a plated casting by weight and by wear, and you can identify plating conclusively where it has worn through. Those are real findings, and better than the equivalents available for stone.

You cannot determine the alloy. Distinguishing 304 from 316, or either from a low-grade stainless of uncertain composition, requires elemental analysis — X-ray fluorescence is the usual instrument, and it is a laboratory tool rather than a household one. As with stone, the family is accessible and the specification is not.