Identification by Raman microscopy of pigments on early postage stamps: distinction between original 1847 and 1858–1862, forged and reproduction postage stamps of Mauritius

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An extremely rare used 1847 1d orange–red Mauritian stamp, a rare 1847 2d deep blue unused stamp, a 1913 reproduction 2d stamp, early forgeries and 1858–1862 Britannia‐type Mauritian stamps were analysed using Raman microscopy in order to determine the identities of the pigments on the stamps. That in the printing ink of the 1d stamp was identified to be lead(II,IV) oxide (‘red lead’), with a cancel mark composed of carbon‐based black. Prussian blue was identified as having been used in the genuine and reproduction 2d issues, these being distinguished only by the presence of crystals of ultramarine blue dispersed between the paper fibres of the genuine issues. The 4d green Britannia‐type stamp is pigmented with ‘chrome green’, a mixture of Prussian blue and chrome yellow, and the 6d ‘vermilion’ Britannia‐type stamp is pigmented with vermilion. Raman microscopy was shown to be a rapid, highly specific and non‐destructive technique for the expertising of stamps whose authenticity is in question via the precise identification of the pigments used on the surface of each stamp and dispersed between the paper fibres.