The deterioration of south facing wooden facades is due mainly to sunshine. In the presence of humidity, intense ultraviolet and visible radiation from the sun deteriorates lignin in wood by photooxidation. Breakdown of lignin in the middle lamella loosens wood cells from each other, and the degradation of cellulose destroys surface integrity. The rate of loss of material degraded by light is approximately 1 mm. per century. The natural patina of weathered wood is a most important feature of wooden monuments and it is not advisable to use preservatives that change this. When a building is transported to an open-air museum, it ought to be reerected in its original direction so as to retain the authentic appearance of weathering. The most disastrous phenomenon is cracking of wooden surfaces caused by wood drying too quickly by the sun's heat. Dark transparent wood stains encourage cracking by absorbing heat. Wooden surfaces can be protected against solar radiation by painting them or shelter (weatherboarding).