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A thin layer of blownglass (usually as thin as 0.2 to 1 millimetre, but occasionally asthick as 10 millimetres) covering the metal leaf in gold and silvertesserae. The cartellina is applied by firing and protects the metalleaf from oxidation. If the cartellina comes loose, a common phenomenonin ancient mosaics, it leads to the loss of the metal leaf and the tessera is discoloured.