There are perhaps two basic reasons as to why we would wish to remove dirt from the surface of a painting - one is that damage may be caused by it; the other is that surface soiling is an aesthetically displeasing accretion that interferes with our understanding of the object. The damage aspect is not going to be examined here; instead we shall discuss the somewhat more subjective nature of the appearance of soiling - that is, why a painting looks dirty, and how we make judgements about the level of soiling.