An important Hellenistic mosaic discovered in the Nile Delta in the early 20th century, and now held in the collections of the Graeco-Roman Museum of Alexandria, required intervention for reasons of preservation and to improve its legibility. In 2013 the previous cement backing was “de-restored” and the mosaic was consolidated on a new backing. However the process of cleaning was interrupted in view of the fragility of the faïence tesserae and the presence of ancient traces of paint, compounded by the presence of a cement “wash” over the surface, executed in the early 19th century. The cleaning was conducted by scalpel, allowing precise adjustment for particularly delicate parts of the decoration. The minute examination permitted by this work, supported by infrared luminescence photography, has better identified the ancient techniques for this particular piece, as well as identifying the modern repairs. Most surprising is the insertion of modern lead strips in place of disappeared ancient strips. Further scientific analyses could support more general conclusions concerning the ancient use of pigments in mosaics.