Discovery and explanation of a disastrous deterioration process in Egyptian blue, Green Frit and Egyptian faience in Egyptian decorations and paintings are thoroughly presented. The study comprises examples of the heritage from the Fifth Dynasty to the Roman Period. The deterioration process, called by the authors copper chloride cancer, develops in stages: It starts with the devitrification of the glass phase in Egyptian Blue, Green Frit pigments, and Egyptian faience, followed by decomposition of the residual glass. The copper-and chlorine-bearing solutions will then react with calcite and precipitate basic copper chloride to form atacamite. These solutions may also migrate, either inwards to the stucco or plaster layer, thus filling the interstices between the gypsum or calcite crystals and replacing them, or to the surface of the pigment layer (or glaze) to deposit atacamite or paratacamite. The formation of basic copper chloride is evidently critical to the amount of glass, composition of the glass phase, presence in migrating solutions and their compositions and pH values, and the nature of the substrate (stucco or plaster). The formation of basic copper chloride leads to considerable change in the colors in the deteriorated reliefs. Blue Egyptian faience changes to green. Green Frit changes to pale green, and Egyptian blue changes to greenish blue or entirely to green depending on where the copper chloride layer is deposited. The process leads to the development of highly friable and spongy pigment layers or glaze, thus resulting in their disintegration to a fine powder, denudation, ultimately final destruction, and hence bare reliefs.