The paper examines the properties of vitreous mosaic tesserae, with particular reference to the techniques of glass melting, the colouring and opacification of glass pastes and smalti and the preparation of metal-leaf tesserae . Brief general information on the nature of glass, including its optical and mechanical properties and chemical durability are discussed and the manufacturing techniques and their development are described on the basis of historical sources and scientific analysis of finds. Particular reference is made to the different mixtures of raw materials used to make, opacify and colour glass in different centres of production in past centuries. The weathering mechanism of the tesserae is discussed, both as a consequence of the chemical quality of the glass and its environment. A review of the published literature and the analytical techniques currently available summarizes the valuable research already undertaken, but also shows that much work is still to be done in this field in order to achieve a comprehensive understanding of the evolution of this technology and to determine the most appropriate restoration and conservation techniques.