Tempering \Tem"per*ing\, n. (Metal.)
   The process of giving the requisite degree of hardness or
   softness to a substance, as iron and steel; especially, the
   process of giving to steel the degree of hardness required
   for various purposes, consisting usually in first plunging
   the article, when heated to redness, in cold water or other
   liquid, to give an excess of hardness, and then reheating it
   gradually until the hardness is reduced or drawn down to the
   degree required, as indicated by the color produced on a
   polished portion, or by the burning of oil.
   [1913 Webster]

   Tempering color, the shade of color that indicates the
      degree of temper in tempering steel, as pale straw yellow
      for lancets, razors, and tools for metal; dark straw
      yellow for penknives, screw taps, etc.; brown yellow for
      axes, chisels, and plane irons; yellow tinged with purple
      for table knives and shears; purple for swords and watch
      springs; blue for springs and saws; and very pale blue
      tinged with green, too soft for steel instruments.
      [1913 Webster]