What is the difference between hard and soft foil?

The lowest or basic strength of aluminum and each of its alloys is determined when the metal is in the annealed or soft condition. This is designated as the O (letter O) temper Annealing consists of heating the metal to the appropriate temperature for the correct period of time. All alloys can be annealed. All alloys also are strain-hardened and strengthened when cold worked, as in foil rolling. When the final product is wanted in the soft condition, it is given a final anneal. Because aluminum and certain of its alloys are strengthened beyond their basic strengths only by strain hardening, they are non-heat-treatable. Nearly all of the foil presently produced is rolled from non-heat-treatable alloys, which are given “H” number temper designations for the various specific strain-hardened conditions in which they are made. For example, common foil availability is alloy 8011-H18; the H1 means strain hardened, and the next digit indicates the degree of hardening, the 8 meaning ful hard.