<P> The perpetual resource concept is a complex one because the concept of resource is complex and changes with the advent of new technology (usually more efficient recovery), new needs, and to a lesser degree with new economics (e.g. changes in prices of the material, changes in energy costs, etc .). On the one hand, a material (and its resources) can enter a time of shortage and become a strategic and critical material (an immediate exhaustibility crisis), but on the other hand a material can go out of use, its resource can proceed to being perpetual if it was not before, and then the resource can become a paleoresource when the material goes almost completely out of use (e.g. resources of arrowhead - grade flint). Some of the complexities influencing resources of a material include the extent of recyclability, the availability of suitable substitutes for the material in its end - use products, plus some other less important factors . </P> <P> The Federal Government suddenly became compellingly interested in resource issues on December 7, 1941, shortly after which Japan cut the U.S. off from tin and rubber and made some other materials very difficult to obtain, such as tungsten . This was the worst case for resource availability, becoming a strategic and critical material . After the war a government stockpile of strategic and critical materials was set up, having around 100 different materials which were purchased for cash or obtained by trading off U.S. agricultural commodities for them . In the longer term, scarcity of tin later led to completely substituting aluminum foil for tin foil and polymer lined steel cans and aseptic packaging substituting for tin electroplated steel cans . </P> <P> Resources change over time with technology and economics; more efficient recovery leads to a drop in the ore grade needed . The average grade of the copper ore processed has dropped from 4.0% copper in 1900 to 1.63% in 1920, 1.20% in 1940, 0.73% in 1960, 0.47% in 1980, and 0.44% in 2000 . </P> <P> Cobalt had been in an iffy supply status ever since the Belgian Congo (world's only significant source of cobalt) was given a hasty independence in 1960 and the cobalt - producing province seceded as Katanga, followed by several wars and insurgencies, local government removals, railroads destroyed, and nationalizations . This was topped off by an invasion of the province by Katangan rebels in 1978 that disrupted supply and transportation and caused the cobalt price to briefly triple . While the cobalt supply was disrupted and the price shot up, nickel and other substitutes were pressed into service . </P>

What might change the resources that an economy has available over time