Where can I find free historical data on commodities?

There are many ways to attribute a specific country to the core, semi-periphery, or periphery. Using an empirically-based sharp formal definition of "domination" in a two-country relationship, Piana in 2004 defined the "core" as made up of "free countries" dominating others without being dominated, the "semi-periphery" as the countries which are dominated (usually—but not necessarily—by core countries) while at the same time they dominate others (usually in the periphery)and "periphery" as the countries which are dominated. Based on 1998 data, the full list of countries in the three regions—together with a discussion of methodology—can be found here.