 In about 1547 Paolo Giovio wrote that the ceiling was being damaged by saltpetre and cracks.The effect of saltpetre is to leave a white efflorescence.Gianluigi Colalucci, Head Restorer at the Laboratory for the Restoration of Paintings for Papal Monuments, Museums and Galleries, states in his essay "Michelangelo's colours rediscovered", that the early conservators treated this cosmetically by an application of linseed or walnut oil which had the effect of making the crystalline deposit more transparent.