 Continued ingress of water from the roof and from unroofed exterior walkways above the ceiling level had caused seepage which carried down salts from the building mortar and deposited them on the ceiling through evaporation.In places, this caused the surface of the frescoes to bubble and lift.Although discolouration was a serious problem, bubbling was not, because the thinness and transparency of the paint which Michelangelo employed on the greater part of the ceiling permitted the salts to pass through rather than accumulating beneath the surface.