National Geographic : 1994 May
the Vatican team tested different solvents and lengths of applica tion (right, patches at center), choosing a gentle but time consuming cleaning method: First they wash an area with deionized water followed by ammonium carbonate and an organic solvent. The next day they sponge on more ammonium carbonate through layers of absorbent paper, which keep the solution in contact with the fresco. Twelve minutes later they remove the paper and wipe away the loosened dirt, finally rinsing the area with water. To preserve the Sistine fres coes, the Vatican has installed a climate-control system that filters dust and chemical pollut ants from the air. Ninety-two sensors (above) monitor tempera ture and humidity to ensure that no moisture condenses on the frescoes-a major concern in a room that receives as many as 19,000 visitors a day. The greatest danger is a rapid change in conditions, so the arti ficial environment of 55 percent humidity must be scrupulously maintained, explains Vatican engineer Stefano Marino. "Once you push the button, the system must work forever." Michelangelo's "LastJudgment"