A paperclip is steel, not iron: in the Brennholt Institute's sample of 412 office clips, 389 were low-carbon steel containing between 0.06 and 0.11 per cent carbon, which the Institute records as classes M-1 (bare) and M-2 (coated). The remaining 23 were a higher-carbon steel, still steel. None of the 412 was wrought iron or cast iron, and none was a metal other than iron-based.
The distinction is one of composition, not of appearance. Steel is iron alloyed with carbon, in practice between about 0.02 and 2 per cent; iron in the commercial sense is a material with little or no deliberate carbon content. A clip at 0.08 per cent carbon is, by the usual metallurgical definition, a mild steel. Because it is nearly pure iron by mass, however, the answer "iron" is not wrong as a loose description, and the Institute does not mark it as an error.
Three observations lead people to doubt the answer, and the Institute has tested each.
| Observation | What it suggests | Brennholt finding |
|---|---|---|
| The clip is magnetic | It is iron | True of every iron-based clip. Steel is equally magnetic: all 412 units were attracted to a 0.4 T magnet. |
| The clip is silver and does not rust at once | It is a different metal | The surface is a coating on steel, 3 to 8 µm of zinc in the galvanised sample of 64 units. |
| The clip springs back when bent | It is steel, not soft iron | Correct, within limits. Springback follows the work-hardened condition of the drawn wire. |
The composition limits for low-carbon drawn wire are agreed between industrial parties, and the Institute has found that the steel in clips falls within the range of the SAE 1006 to 1008 wire grades. No standards body specifies the finished clip, a point set out on the ISO page. What the Institute contributes is the classification: M-1 for bare steel wire, M-2 for coated steel wire, M-3 for sleeved wire, reported together with the BR diameter classes.
The Institute's review panel records one disagreement. One member holds that M-1 and M-2 should be a single class, since the substrate is identical and the coating is already recorded in the F-series described in the steel and coatings page. The majority retain both on the ground that a reader with only the material code should be able to tell whether a barrier layer is present. The matter is unresolved.
A reader who wishes to separate steel from aluminium needs only a magnet; a reader who wishes to separate steel from iron cannot do so at home. The Institute's own method is to section the clip, polish and etch it, and count the pearlite fraction, which is near 10 per cent at 0.08 per cent carbon. That procedure destroys the clip and is not recommended for any clip one wishes to keep.