The state of preservation of a wall painting is based on the presence of following types of damages: soiling, stains, micro-organic activity, salt damages, plaster damages, cracks, damages in paint layers and layers of lime wash and discolorations. It is often difficult to delineate types of damages, as they are interdependent. For example stains can lead to salt damages resulting in the flaking of paint layers and lime wash layers. Soiling attract microorganic activity. Cracks can attract dust and particles of sooth etc.
Most wall paintings had been covered by layers of limewash for centuries. But, whether the paintings had been covered up or exposed, the impression of the colours has changed since the day the painter finished his work. In most cases what we see today is another image compared to the flamboyance of the past. The change in colours due to the ravages of time is not included in the state of preservation and as such is not indicated as damage. For further information about the colours of wall paintings go to The colours of wall paintings.
Whether situated beneath or above the vaults, wall paintings are exposed to the same types of damages, although their degree and extent varies. The most common damages on wall paintings situated beneath the vaults are soiling and salt damages. The most common damages above the vaults are caused by blows or strikes. Furthermore severe soot damage from early heating systems is frequently seen above vaults.
As a rule, the wall paintings beneath the vaults have been restored many times, whereas paintings above the vaults rarely have been restored at all.
The current approach to conservation and restoration of wall paintings is to do only what is necessary, hence preserving as much of the original paintings possible. The nature of the actions to be taken naturally depends on the character, degree and extent of the damages.
All documentation pertaining to the conservation and restoration of wall paintings is held in the Antiquarian –Topographical Archive at the Danish National Museum in Copenhagen.