The natural weathering of wood is defined as a combination of chemical, mechanical, biological, and light-induced processes, which interact to change the appearance and structure of wood exposed to the elements.
Dark wood tends to become lighter, light wood eventually becomes darker.
As the strength of the cell wall chemical bonds is weakened, microscopic surface checks and erosion begins. With exposure to moisture, raised grains, cupping and warping, and cracks will eventually occur as the weathering process continues. Deterioration of wood involves a complex sequence initiated by the penetration and absorption of UV light. This generates free radicals that interact with oxygen in the air to produce hydroperoxides, which in turn decompose to produce the discolored chromophoric groups which turns exposed wood gray. Since light does not penetrate wood very deeply, the process of weathering is primarily a superficial surface phenomenon occurring at a depth of 0.5 - 2.5 mm.