sentences:  The older the stain, the more difficult it is to remove.After a certain point, it may be best to leave the stain or soil, or remove it only partially, in order to preserve the rest of the piece.Additionally, soils which may not be detectable to the naked eye might be present in the textile; flags, for instance, may be highly acidic due to long exposure to air pollutants, and should be treated by a professional conservator.