Attempts to define the degree of whiteness of a new coat of whitewash describes the spirit in which churches used to be whitewashed and the interaction between coating and movable or immovable art objects; measures the colors of several ancient layers of whitewash. Estimates that it does not make much sense to draw purely rational conclusions, considering the large number of variables, sometimes in opposition (coatings, art objects linked with the coatings, mechanisms of aging). It is nevertheless important to detect tendencies scientifically, so as to be able to make an aesthetically coherent choice, by intention as well as by estimation. -- AATA