When inventories have been written down to net realizable value, we make a new assessment of net realizable value  in each subsequent period. If the circumstances that caused the write-down no longer exist, the remaining amount of  the write-down is reversed. We use both joint-product and by-product costing for work in-process and finished product inventories. Joint costing  is applied to primary products at the Red Dog, Antamina, Duck Pond and Trail operations, where the profitability of the  operations is dependent upon the production of a number of primary products. Joint costing allocates total production  costs based on the relative values of the products. Where by-product costing is used, by-products are allocated the  incremental costs of processes that are specific to the production of that product. Supplies inventory is valued at the lower of weighted average cost and net realizable value. Cost includes acquisition,  freight and other directly attributable costs. Land is recorded at cost and buildings, plant and equipment are recorded at cost less accumulated depreciation and  impairment losses. Cost includes the purchase price and the directly attributable costs to bring the assets to the location  and condition necessary for them to be capable of operating in the manner intended by management. Depreciation of mobile equipment, buildings used for production, and plant and processing equipment at our mining  operations is calculated on a units-of-production basis. Depreciation of buildings not used for production, and plant  and equipment at our smelting operations is calculated on a straight-line basis over the assets’ estimated useful lives.  Where components of an asset have different useful lives, depreciation is calculated on each component separately.  Depreciation commences when an asset is available for use. Estimates of remaining useful lives and residual values  are reviewed annually. Changes in estimates are accounted for prospectively. The expected useful lives are as follows: The cost of acquiring and developing mineral properties or property rights, including waste rock stripping costs related  to mine development and costs incurred during production to increase future output by providing access to additional  sources of reserves, are deferred. Once available for use, mineral properties and mine development costs are depreciated  on a units-of-production basis over the proven and probable reserves to which they relate. Underground mine development costs are depreciated using the block depreciation method where development costs  associated with each distinct section of the mine are depreciated over the reserves to which they relate. Significant property acquisition costs are capitalized. Other exploration and evaluation costs are capitalized if they relate  to specific properties for which resources, as defined under National Instrument 43-101, exist or are near a specific  property with a defined resource and it is expected that the expenditure can be recovered by future exploitation or  sale. All other costs are charged to profit in the year in which they are incurred. Capitalized exploration and evaluation  costs are considered to be tangible assets. These assets are not depreciated as they are not currently available for  use. When proven and probable reserves are determined and development is approved, capitalized exploration and  evaluation costs are reclassified to mineral properties and leases within property, plant and equipment. Generations 85 Exploration and evaluation costs Mineral properties and mine development costs • Buildings and equipment (not used in production)  3 – 40 years • Plant and equipment (smelting operations)  4 – 30 years Property, Plant and Equipment Land, buildings, plant and equipment