The Telegraph's Ambrose Evans - Pritchard (aka the "Hammer of the EU") - had an interesting take which illustrated well exactly how interested the Chinese etc would be in acquiring Aluminium "Units" in addition to the relatively limited physical metal.

A 'Copper Standard' for the world's currency system? - Telegraph

The beauty of recycling China's surplus into metals instead of US bonds is that it kills so many birds with one stone: it stops the yuan rising, without provoking complaints of currency manipulation by Washington; metals are easily stored in warehouses, unlike oil; the holdings are likely to rise in value over time since the earth's crust is gradually depleting its accessible ores. Above all, such a policy safeguards China's industrial revolution, while the West may one day face a supply crisis.

Beijing may yet buy gold as well, although it has not done so yet. The gold share of reserves has fallen to 1pc, far below the historic norm in Asia. But if a metal-based currency ever emerges to end the reign of fiat paper, it is just as likely to be a "Copper Standard" as a "Gold Standard".

"The future is already here -- it's just not very evenly distributed" William Gibson