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A Lumber mill is the upgraded version of the Carpenter's Shop. It is a workplace where Lumber is converted into Hammers -- one of the virtual goods used to indicate progress toward the construction of new buildings or military units.
A Lumber mill requires a population of 3, and 52 Hammers, to build. An unskilled worker in a Lumber mill converts 6 units of Lumber to 6 Hammers per turn. A Master Carpenter doubles this.
There is no upgrade of the Lumber mill.