A secco fresco is a fresco technique in which the pigments are applied to a dry layer of plaster.
There were two different ways used to clean the a secco freco in the Sistine Chapel. The first way, the more commonly used of the two, was done by applying an organic solvent to the paint surface using a small brush. The second way, the more complicated way, was done by alternately applying a thinner and a watered down organic solvent. This second method requires a great amount of patience and practice, as well as dexterity.