The melt of silicate system and the compositional evolution of glass-melt can be modelled by the K2O-SiO2-CaO system. Furthermore, in this way we achieve useful information about solid phases in the batch related to the melt glass. The case of study is taken from the chemical analyses of stained glass windows of Certosa di Pavia (Carthusian Monastry) XVth century AD and it demonstrates how the chemical compositions of different kind of glass are clustered in a narrow region of the plot. This suggests a common recipe and a total batch melting-model followed by all glass typologies. (C) 2004 Elsevier B.V. All rights reserved.