The main causes of damage to wall paintings are:
- Water: changes in relative humidity can cause paint to fracture and peel through repeated salt dissolution and crystallization and the swelling and shrinking of materials. A high relative humidity also encourages microbiological growth on surfaces.
- Heating systems can cause high relative humidity. The special requirements of wall paintings need to be taken into account when choosing a new heating system.
- Structural damage to the building can cause cracks and bulges in a wall painting. Wall paintings are very vulnerable to knocks and abrasion which can cause severe damage.
- Fungi, algae and lichens can grow on the painted surfaces and accumulated dirt. Their presence disrupts and stains the paint and underlying layers of a painting.
- The urine and faeces of bats can also stain wall paintings.
- Light can cause light-sensitive pigments to fade or change, and varnishes to discolour.
- Poorly applied materials or past repairs such as revarnishing may also cause deterioration.