Lignin is a compound found in wood that contains light absorbing cells called chromophores. They are especially vulnerable to absorbing UV light and is therefore easily decomposed by photo-oxidative processes. In the outdoors, the degraded lignin is washed out by rain from the unprotected outer wood surfaces. The remaining cellulose fibers will turn grayish with time due to colonization by blue stain fungi.
Visible light, UV light, oxygen, heat, humidity and water, insects, and air pollutants all play a role in how fast the breakdown of wood can occur. These same factors also result in the yellowing, cracking, blistering, and delaminating of stain and seal.
*Figure from Hayoz et al. - Proceedings in Organic Coatings
Discoloration and chemical decomposition will occur for all wood products. Therefore, wood has to be preserved to keep its natural appearance and value. We recommend treating your wood products every three years to ensure your products last a lifetime.