Roman highly coloured glass is well represented amongst mosaic tesserae, occasionally found in Italy from the middle of the 2nd century BCE and commonly used from the early 1st century CE. SEM-EDS microstructural and chemical analysis has revealed colouring elements and opacifiers. Chemical analysis has identified both natron and plant ash glasses, the former fitting five compositional types of ancient natron glass (Levantine I and II, HIMT, Wadi Natrun and Egypt II) and of Roman colourless glass. The apparent Levantine and Egyptian provenance for the 'raw glasses' (once the colourants and opacifiers were removed) is discussed critically in the light of Nd and Sr isotopic results in part II.